Materials Engineering, Characterization, and Applications of the Organic- Based Magnet, V[TCNE]

Materials engineering, characterization, and applications of the organic- based magnet, V[TCNE]

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Megan Harberts

Graduate Program in Physics

The Ohio State University

2015

Dissertation Committee:

Professor Ezekiel Johnston-Halperin, Advisor

Professor Jay Gupta

Professor Annika Peter

Professor William Putikka

Megan Harberts

2015

Abstract

Organic materials have advantageous properties such as low cost and mechanical flexibility that have made them attractive to complement traditional materials used in electronics and have led to commercial success, especially in organic light emitting diodes (OLEDs). Many rapidly advancing technologies incorporate magnetic materials, leading to the potential for creating analogous organic-based magnetic applications. The semiconducting ferrimagnet, vanadium tetracyanoethylene, V[TCNE]x~2, exhibits room temperature magnetic ordering which makes it an attractive candidate. My research is focused on development of thin films of V[TCNE]x~2 through advancement in growth, materials engineering, and applications. My thesis is broken up into two sections, the first which provides background and details of V[TCNE]x~2 growth and characterization. The second section focuses on advances beyond V[TCNE]x~2 film growth. The ordering of the chapters is for the ease of the reader, but encompasses work that I led and robust collaborations that I have participated in.

V[TCNE]x~2 films are deposited through a chemical vapor deposition process (CVD). My advancements to the growth process have led to higher quality films which have higher magnetic ordering temperatures, more magnetically homogenous samples, and extremely narrow ferromagnetic resonance (FMR) linewidths.

Beyond improvements in film growth, materials engineering has created new materials and structures with properties to compliment thin film V[TCNE]x~2. Though a robust collaboration with chemistry colleagues, modification of the molecule TCNE has led to the creation of new magnetic materials vanadium methyl tricyanoethylene carboxylate,

V[MeTCEC]x and vanadium ethyl tricyanoethylene carboxylate, V[ETCEC]x.

Additionally, I have lead a project to deposit V[TCNE]x~2 on periodically patterned substrates leading to the formation of a 1-D array of V[TCNE]x~2 nanowires. These arrays exhibit in-plane magnetic anisotropy, which is not observed in films of

V[TCNE]x~2.

Additional collaborations have also made significant progress in addressing one of the challenges for incorporating V[TCNE]x~2 into applications, which is the degradation of magnetic properties with exposure to oxygen. By working off of encapsulation technology which has been developed for OLEDs, we have shown that we can use a simple epoxy to extend the magnetic properties of V[TCNE]x~2 films from an order of hours to one month in ambient conditions.

iii

Acknowledgments

First I would like to thank my advisor and mentor, Professor Ezekiel Johnston-Halperin for his guidance and support throughout my entire graduate career, even before I was a formally member of his research group. I truly appreciate everything he has done to help shape me into the scientist I am today.

I would also like to thank Professor Arthur J Epstein for introducing me to the exciting, yet slightly unusual field of organics. His contribution to both the field of organics and to interdisciplinary materials research at OSU is extensive and I am honored to have been a part of his legacy.

I am extremely thankful for all of the staff members who have helped me complete my

PhD. I would especially like to thank Jenny Finnell and Kris Dunlap. Jenny helped work through all of my travel as well as many other things in the Epstein group and Kris is probably the most serious person concerned with making sure every graduate student in physics succeeds and I am honored to consider her a friend as well as a colleague. I would also like to thank all of the Nanosystems Laboratory staff including Denis

Pelekov, Billy Kelley, Laura Heyek, Asnika Bajracharya, and numerous undergrads who have been a tremendous help in the transition and operation of the organic cleanroom. I

iv would also like acknowledge Bob Wells who left this world too soon and who was truly one of my favorite people to work with.

Next I must thank the numerous people in both of my research groups who I have had the pleasure of working with. From the Epstein group I must thank of all those who came before me, especially those to taught me how to work in the Epstein labs. This includes:

Kadriye Deniz Bozdag, Jung-Woo Yoo, Chi-Yi Chen, June Hyoung Park, Austin Carter,

Bin Li, Chi-Yueh Kao, Raju Nandyala, and Vladimir Prigodin. I also want to thank all of the JH group students who have provided advice, expertise, and comradery including: Lei

Fang, Yi-Hsin Chiu, Kurtis Wickey, Yu-Sheng Ou, Justin Young, Michael Chilcote, and

Matthew Sheffield. I would also like to thank all of the undergraduate students that I have worked with and mentored including Calli Campbell, Cayla Nelson, Marisa McCaffrey, and Carissa Brown. Teaching you has helped me become a better communicator and a better scientist. Many, many thanks are in order for the two people with whom I have worked the most closely over the past 6 years, Howard Yu and Yu Lu. I am so grateful for our collaboration which has benefited us all more than if we had never worked together. I would also like to offer best wishes to Ian Froning and Andrew Franson as they continue on the organics work.

I would also like to acknowledge a few of my strong female mentors who have provided me with guidance and many unique opportunities. I would first like to thank Shawna

Hollen, she is one of the most amazing researchers I know and truly appreciate everything she has done to help me when nothing required her to. I also must acknowledge Amy Connolly and Nancy Santagata, co-founders of A Day in the Life in

Physics. My life changed forever when we started the blog and the doors that it has opened have truly shaped my graduate experience and helped me discover a passion for communicating science. I am also extremely honored to have had the opportunity to work so closely with both Amy and Nancy and learn from their experiences.

I am honored to have been a part of the incoming class of 2009, we rock! I am so grateful to all of my classmates for being supportive throughout graduate school and providing me with many lasting memories. I would especially like to thank the people who ate lunch with me every day and kept me sane-Ula Szfragua, Richelle Teeling-Smith, Mark

Patrick, Yaser Helal, and Nick Minutillo.

Finally, I thank my family. My parents instilled a love of science in me and I would not have made it this far without them. I am also honored to follow in the footsteps of my grandfather and wish he were still here to see me achieve this goal. Lastly to the person I owe the most thanks is my husband Dan. He has always supported me and made sacrifices to allow me to pursue my goals and I do not know what I would do without him.

Vita

2005...... Sandia High School

2009...... B.S. Physics and Mathematics, New Mexico

State University

2009-2010 ...... University Fellow, The Ohio State

University

2010-2012 ...... Graduate Teaching Associate, Department

of Physics, The Ohio State University

2012-Present ...... Graduate Research Associate, Department

of Physics, The Ohio State University

Publications

1.) M. Harberts, M. Chilcote, Y. Lu, G. Schmidt, E. Johnston-Halperin, "Formation of organic-based magnetic nanowires", in preparation. 2.) Y. Lu, H. Yu, M. Harberts, A. J. Epstein, E. Johnston-Halperin, “Vanadium [ethyl tricyanoethelene carboxylate]x : a new organic-based magnet”, Journal of Materials Chemistry C, 3, (2015) 7363. 3.) M. Harberts, Y. Lu, H. Yu, A. J. Epstein, E. Johnston-Halperin, “Chemical vapor deposition of an organic magnet, vanadium tetracyanoethylene, V[TCNE]x~2”, Journal of Visualized Experiments, 101, (2015) e52891. 4.) I. Froning, M. Harberts, Y. Lu, H. Yu, A. J. Epstein, E. Johnston-Halperin, “Thin-film Encapsulation of the Air Sensitive Organic Ferrimagnet Vanadium Tetracyanoethylene,” Applied Physics Letters, 106, (2015) 122403. vii

5.) H. Yu, M. Harberts, R. Adur, Y. Lu, P.C. Hammel, E. Johnston-Halperin, A. J. Epstein, “Ultra-narrow ferromagnetic resonance in organic-based thin films grown via low temperature chemical vapor deposition,” Applied Physics Letters, 105, (2014), 012407. 6.) Y. Lu, M. Harberts, C. Kao, H. Yu, E. Johnston-Halperin, and A. J. Epstein, “Thin film deposition of an organic magnet based on vanadium methyl tricyanoethylenecarboxylate”, Advanced Materials, 26, (2014) 7632. 7.) H. Yu, M. Harberts, L. Fang, K. Deniz Bozdag, C.Y. Chen, A. J. Epstein, and E. Johnston-Halperin, "Electrical transport in a hybrid organic/inorganic heterostructure," Proceedings of SPIE Spintronics IV, Aug 21, 2011.

Fields of Study

Major Field: Physics

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Table of Contents

Abstract ...... ii

Acknowledgments...... iv

Vita ...... vii

Publications ...... vii

Fields of Study ...... viii

Table of Contents ...... ix

List of Figures ...... xiii

Section 1: Introduction to the organic-based magnet, V[TCNE]x~2 ...... 1

Chapter 1: Introduction to organic-based magnetic materials ...... 2

1.1 Introduction to organic electronics ...... 2

1.2 Magnetism ...... 5

1.3 Organic-based magnetic materials ...... 14

1.3.1 Single molecule magnets ...... 14

1.3.2 Higher dimension molecule-based magnets ...... 17

1.3.3 V[TCNE]x~2 synthesis ...... 18

1.4 Properties of V[TCNE]x~2 ...... 19

1.4.1 Physical structure ...... 19

1.4.2 V[TCNE]x~2 magnetic and electronic properties ...... 20

1.5 Conclusions ...... 25

Chapter 2: V(TCNE) growth, optimization, and characterization ...... 27

2.1 CVD growth of V[TCNE]x~2 films ...... 27

2.1.1 Growth basics ...... 27

2.1.2 Film characterization ...... 29

2.1.3 DC magnetometry...... 30

2.1.4 AC susceptibility ...... 37

2.1.5 Electronic transport ...... 39

2.1.6 Carrier mediated magnetism ...... 40

2.1.7 Ferromagnetic resonance ...... 42

2.2 Optimization of CVD growth of V[TCNE]x~2 films ...... 46

2.1.1 New heater ...... 46

2.2.2 FMR diagnostic ...... 48

2.3 Conclusions ...... 50

Section 2: Beyond V[TCNE]x~2 films ...... 51

Chapter 3: Engineered organic-based magnetic materials ...... 52

3.1 New organic-based magnetic materials...... 52

3.1.1 Previous work ...... 52

3.1.2 Ligand modification ...... 53

3.1.3 Transport properties of V[MeTCEC]x ...... 56

3.2 New organic magnetic morphologies ...... 60

3.2.1 Organic magnetic nanowires ...... 60

3.2.2 Characterization of V[TCNE]x~2 nanowires ...... 64

3.2.3 Independence of anisotropy on growth parameters ...... 68

3.2.4 Anisotropy on improperly patterned samples ...... 71

3.3 Conclusions ...... 72

Chapter 4: Encapsulation of V[TCNE]x~2 using UV-cured epoxy ...... 73

4.1 Previous methods of encapsulation ...... 73

4.1.1 Customized airtight containers ...... 73

4.1.2 Parylene capping layers ...... 77

4.2 Encapsulation with epoxy ...... 77

4.2.1 Visual test of epoxy effectiveness ...... 79

4.2.2 Interaction between epoxy and V[TCNE]x~2 films ...... 80

4.2.3 Time dependence of the encapsulation process ...... 84

4.3 Conclusions ...... 87

Chapter 5 Applications and future directions for organic-based magnets ...... 89

5.1 Organic spintronics ...... 89

5.2 High-frequency applications ...... 93

5.3 Looking to the future ...... 94

References ...... 96

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List of Figures

Figure 1.1. Examples of commercial organic applications………………………………..4

Figure 1.2. Magnetic ordering schematic………………………………………………....6

Figure 1.3. Magnetic hysteresis loop………………………….…………………………..9

Figure 1.4. Energy diagram of a single domain particle…………………………………10

Figure 1.5. Schematic of spero- and sperimagnet………………………………………..13

Figure 1.6. Molecular picture of Mn12ac………………………………………………...15

Figure 1.7. Model of energy levels for Mn12ac molecule………………………………..15

Figure 1.8. Magnetization versus field of Mn12ac molecule……………………………..16

Figure 1.9. Schematic of a tetracyanoethylene molecule………………………………..19

Figure 1.10. Photos of V[TCNE]x~2 film deposited on Teflon tape……………………..20

Figure 1.11. Schematic of V[TCNE]x~2 structure……………………………………….21

Figure 1.12. Spin density distribution of (TCNE)-………………………………………22

Figure 1.13. An electronic model for V[TCNE]x~2 showing the location of the spins…..22

Figure 1.14. Magnetization versus temperature at different applied fields……………...23

Figure 1.15. Historical comparison of TC values for V[TCNE]x~2………………………24

Figure 1.16. Magnetization as a function of applied field……………………………….25

Figure 2.1. Schematic of the custom-built CVD reactor………………………………...28

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Figure 2.2. Film thickness dependence based on CVD reactor location………………...29

Figure 2.3. Schematic of a DC SQUID…………………………………………………..31

Figure 2.4. Voltage dipole response of sample in the MPMS…………………………...32

Figure 2.5. Uncorrected magnetization versus applied field for V[TCNE]x~2……..…….33

Figure 2.6. Corrected magnetization versus applied field for V[TCNE]x~2………….….34

Figure 2.7. Magnetization versus temperature for V[TCNE]x~2…………………………35

Figure 2.8. Updated comparison TC values for V[TCNE]x~2……………………………36

Figure 2.9. Zero-field cooled & field cooled sweeps...... ……………………………….37

Figure 2.10. AC susceptibility for V[TCNE]x~2…………………………………………38

Figure 2.11. Electronic transport for V[TCNE]x~2……………………………………….39

Figure 2.12. Connection between magnetic and electronic properties for V[TCNE]x~2...41

Figure 2.13. Schematic of Bruker EPR…………………………………………………..44

Figure 2.14. FMR spectrum for V[TCNE]x~2……………………………………………45

Figure 2.15. Angle dependence of FMR spectra………………………………………...46

Figure 2.16. Picture of second generation CVD heater………………………………….47

Figure 2.17. Temperature profile for the V[TCNE]x~2 second generation heater………..48

Figure 2.18. Map of out-of-plane FMR spectra for V[TCNE]x~2 films………………….49

Figure 3.1. Molecular schematic of MeTCEC and ETECE molecules………………….53

Figure 3.2. Magnetization versus temperature of V[MeTCEC]x & V[ETCEC]x films.....54

Figure 3.3. Magnetization versus applied field of V[MeTCEC]x & V[ETCEC]x films....55

Figure 3.4. . Electronic transport for V[MeTCEC]x……………………………………..56

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Figure 3.5. Percent change in the resistance as a function of time for V[MeTCEC]x…...57

Figure 3.6. Resistance as a function of temperature for an V[MeTCEC]x film………….58

Figure 3.7. Magnetoresistance for an V[MeTCEC]x film………………………………..59

Figure 3.8. SEM images of patterned SiO2 substrates………………………………...…61

Figure 3.9. SEM images of V[TCNE]x~2 nanowires……………………………………..62

Figure 3.10. SEM image of thick V[TCNE]x~2 on a patterned substrate………………...63

Figure 3.11. Comparison of film growth direction with respect to pattern……………...64

Figure 3.12. FMR spectra for angular rotation of V[TCNE]x~2 nanowires……………...65

Figure 3.13. DC magnetization of V[TCNE]x~2 nanowires……………………………...67

Figure 3.14. FMR spectra for in-plane of V[TCNE]x~2 nanowires………………………68

Figure 3.15. Center field values for FMR on thick and thin films…………………….…69

Figure 3.16. Center field values for FMR on varied groove spacings…...………………70

Figure 3.17. SEM image and FMR on improperly patterned substrate………………….71

Figure 4.1. Model of air-free can for optical and electrical cryostat measurements…….75

Figure 4.2. Picture of airtight sample mount which fits in a PPMS……………………..76

Figure 4.3. Schematic showing the epoxy process………………………………………78

Figure 4.4. Time series of V[TCNE]x~2 films in air……………………………………...79

Figure 4.5. FTIR spectra for V[TCNE]x~2 with and without epoxy……………………..81

Figure 4.6. Temperature versus magnetization on bare and epoxied V[TCNE]x~2……...82

Figure 4.7. Magnetization versus applied field on bare and epoxied V[TCNE]x~2……...83

Figure 4.8. Temperature versus magnetization on epoxied V[TCNE]x~2 over time……..85

Figure 4.9. Magnetization versus applied fieldon epoxied V[TCNE]x~2 over time….…..86

Figure 4.10. Magnetic parameters as a function of time………………………………....87

Figure 5.1. Results from hybrid V[TCNE]x~2 tunnel junction………………….……….91

Figure 5.2. Polarization response for a spin-LED with a V[TCNE]x~2 film…………….92

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Section 1: Introduction to the organic-based magnet,

V[TCNE]x~2

The focus of this thesis is centered around the organic-based magnetic material vanadium tetracyanoethylene, V[TCNE]x~2. This section provides an introduction to the history of organic materials, as well as the background physics necessary to understand the unique properties of organic-based magnetic materials. Chapter 1 discusses the state of the field of organic-based magnetic materials which include single-molecule magnets, as well as bulk molecular magnets. It also contains results of studies on V[TCNE]x~2 through the first decade of the 21st century. Chapter 2 provides an introduction into my work on

V[TCNE]x~2 films. This includes details about the growth and the relevant characterizations of films. Advancements I have made in the growth have led to films with improved qualities shown through comparison to past work. The overall goal of this section is to provide an understanding of the history and properties of the organic-based magnetic material, V[TCNE]x~2 as of 2015.

Chapter 1: Introduction to organic-based magnetic materials

This chapter provides an overview of organic-based magnetic materials with an emphasis on the room temperature organic-based magnet, vanadium tetracyanoethylene,

(V[TCNE]x~2). It is split into four sections, the first which provides an overview of the field of organic electronics as a whole. The second section provides a background to the concepts in magnetism necessary to understand organic-based magnets. Section 1.3 discusses the history of field of organic-based magnetic materials and finally section 1.4 provides a historical background to the material V[TCNE]x~2.

1.1 Introduction to organic electronics

The field of “organics” is comprised of carbon-based materials that form small molecules and polymers. Prior to 1977 organics were considered to be of limited use for electronics since they were primarily insulating materials; however the first report of electrical conductivity in polyacetylene opened the field of organic electronics for which Heeger,

MacDiarmid, and Shirakawa were awarded the Nobel Prize in chemistry in 20011. Since the discovery of conducting polymers, the global organic electronics market has grown to

$16.45 billion in 2014 and predicted to grow to $75.82 billion by 20202.

While organic-based electronics materials are unlikely to overtake traditional materials such as silicon they do offer advantages not found with inorganic materials. One of the main drivers of development is cost. Organics have the advantage of being low-cost to manufacture because they can be produced in a typical chemistry lab rather than an expensive cleanroom facility. Another factor in their low cost is that they are comprised of widely available elements. Additionally, organics are easily chemically modified to create variations of materials with different properties. The ease of deposition of thin films through evaporation or spin coating makes incorporation into devices attractive.

Further, organics and organic-based materials are mechanically flexible and lightweight and therefore can be used to create completely flexible circuits and devices3.

As of 2015 the main commercial applications of organics is in light emission as components of organic light emitting diodes (OLEDs) which are used in display screens for household electronics and cell phones. The basic structure is a multilayer where electrons and holes are generated from a cathode and anode, respectively, by applying a voltage to the system. The electrons and holes travel through the organic layers to form excitons which decay and emit light with a wavelength determined by the material(s) in the device. Significant research to develop OLEDs for the full range of colors has led to many commercial successes, especially in the case of OLED TV's which are outperforming their traditional LED counterparts in image quality and power consumption4. Additionally paper thin display screens (figure 1.1a) and curved displays, which OLEDs make possible, are entering the market.

The other area of organic electronics which has experienced commercial success is organic photovoltaics (OPV). These devices work in reverse of OLED: light enters the device, the electrons and holes are separated and move to the cathode and anode where they generate a current. While organic materials are far less efficient than the best photovoltaic devices based on inorganic materials, their low cost means that they may be more cost efficient. They also offer advantages such as flexibility which make them attractive for novel applications.

Figure 1.1 a) An LG OLED wallpaper display that is .97 mm thick5 b) Picture of a flexible OPV6.

The area of organic-based magnets is at an earlier stage in development due, in part, to the shorter amount of time for materials progress. The success of OLEDs and OPVs points towards a future that incorporates organic-based magnetic materials in commercial applications.

1.2 Magnetism

Before going into detail about organic-based magnets, this section provides a brief introduction into the background on magnetism necessary for understanding the magnetic properties in these systems.

Magnetic ordering in materials generally arises from exchange interaction between unpaired electrons. This exchange interaction for electrons is given by the following

Hamiltonion:

퐻 = − ∑푖<푗 퐽푖푗푺푖 ⋅ 푺푗 (1.1) where Jij represents the exchange integral which describes the strength of the interaction between spins, and S is the total spin of each atom. When it is energetically favorable for spins to couple in the same direction, the material is called ferromagnetic and Jij is positive. In the case where spins couple in opposite directions the material is antiferromagnetic and Jij is negative. When the exchange interaction is larger than the energy from thermal excitations, magnetic ordering occurs. The Curie temperature, TC, is the magnetic ordering temperature for a specific material.

Below TC a ferromagnet will exhibit an overall net magnetization because the spins are aligned as shown in figure 1.2. However, generally for antiferromagnets, there is no net magnetization, below TC because the oppositely aligned spins cancel each other out.

There are materials which have different spin sites that anti-align where the total moment on the two different sites is unequal and therefore the total spin does not cancel out. This is called a ferrimagnetic material.

Figure 1.2 Schematic showing how the spins align for different types of magnetic ordering.

The degree to which a magnetic material responds to an applied magnetic field is called the magnetic susceptibility, χ where

푴 = 휒푯 (1.2)

Above the Curie temperature, ferro- and ferrimagnetic systems behave like a paramagnet, where χ is positive. Paramagnets do not experience exchange interaction between spins and so the magnetization is determined by how easily the spins align and the size of an applied field. In the paramagnetic region for ferromagnets, the susceptibility is described by the Curie-Weiss law:

퐶 휒 = (1.3) 푇−푇퐶 with the Curie constant C, specific to each material. This is a reasonable description for

T>>TC. Close to the Curie temperature the susceptibility is described by:

퐶 휒~ 4⁄ (1.4) (푇−푇퐶) 3

6 due to fluctuations of magnetic moments which are not described by the mean field theory used to derive the Curie-Weiss law.

Below TC the actual field felt by each spin can be described as

푯푇 = 푯 + 푁푤푴 (1.5) where there is an addition to the applied field, H. This addition is the molecular field felt by the spins which is described by a constant Nw multiplied by the total magnetization, due to each spin experiencing the average magnetization of the system. By putting this value of the effective field, HT into the most general description of magnetization as a function of temperature:

푀 = 푁푔휇퐵퐽 퐵퐽(푥) (1.6) where N, is the number of atoms per unit volume, J is the total angular momentum, g is the electron g-factor, 휇퐵 is the Bohr magneton, and BJ is the Brillouin function defined as:

2퐽+1 2퐽+1 1 푥 퐵 = coth 푥 − coth (1.7) 퐽 2퐽 2퐽 2퐽 2퐽 for

퐽푔휇 퐻 푥 = 퐵 (1.8) 푘퐵푇 the behavior for ferromagnet in the spontaneous magnetization region can be determined.

Setting H=0 the following equations can be extracted.

푀(푇) = 퐵 (1.9) 푀(0) 퐽

푀(푇) 푘퐵푇 = 2 2 2 푥 (1.10) 푀(0) 푁푁푤푔 휇퐵 퐽 which must both be satisfied simultaneously. 7

The maximum magnetization possible is for T→0 given by

푀(0) = 푁푔휇퐵퐽 (1.11).

Above T=0 the spontaneous magnetization has been found experimentally7 to deviate from its saturation value by an amount proportional to T3/2 known as the Bloch law which describes the magnetization as :

3 푀푆(푇) = 푀푆(0)(1 − 퐵푇2) (1.12) where MS is the saturation magnetization and B is a fitting parameter.

In addition to the short-range exchange interaction described above, there is also a much weaker dipolar interaction. In ferro- and ferrimagnets, the competition between the short- range exchange interaction and weaker dipolar interactions results in the formation of macroscopic to microscopic regions of ordered spins, called domains, whose net magnetization point in different directions. Within a single domain, all of the spins are ordered, but the spins near the edge of the domain experience exchange interactions with spins from neighboring domains that are aligned in different directions. The exchange interaction at these domain walls is short-ranged and affects only a few spins, but the formation of domains lowers the overall dipole energy of the system, making domains energetically favorable.

When a large enough external magnetic field, H, is applied, all of the domains within the ferro- or ferrimagnet will align. In a measurement of the magnetization as a function of the applied field, this is observed as a saturation of the magnetization, Ms. After removing

8 the applied field, the material remains mostly ordered; the value of this ordering at zero field is called the remanence, Mr. To restore the unmagnetized state, a field in the reverse direction has to be applied. The result of this irreversible behavior is a hysteresis loop like the one shown in figure 1.3.

Figure 1.3 Magnetic response to an applied field for a ferro- or ferrimagnet8.

The parameters TC, Ms, and the coercive field (the width of the hysteresis loop for M = 0) can be used to compare magnetic materials and help determine their usefulness for various applications.

When magnetic particles are small enough that they do not contain domains they exhibit superparamagnetism. In a classical picture the particle can either be in a spin up or spin down state, which in the absence of applied fields are equal in energy. The energy of this system is described by:

1 퐹 = 퐾푉(sin 훼)2 (1.13) 푇 2

9 where K represents the anisotropy constant for the material, V is the volume of the particle, and α is the angle between the magnetization and the easy axis. Therefore when the spin is either aligned or anti-aligned with the easy axis (훼 = 0 or 훼 = 휋), the energy

1 is minimized, but these two states are separated by an energy barrier of height 퐾푉 as 2 shown in Figure 1.4.

Figure 1.4 Energy diagram of single domain particle where α is the angle with respect to the easy axis.

A perturbation to this system, such as thermal fluctuations can provide the energy for the spin to flip from one low energy state to another. This spin flip is characterized by some time, τ, which depends exponentially on the temperature, T, and the sample size following an Arrhenius behavior:

퐾푉 푘 푇 휏 = 휏0푒 퐵 (1.14)

-9 -10 Here τ0 represents the spin flip attempt time which is typically on the order of 10 -10

7 seconds and kB is the Boltzmann constant. Based on the timescale for measurements, τm, of these particles, a blocking temperature, TB, can be defined as:

퐾푉 푇퐵 = 휏푚 (1.15). 푘퐵 ln 휏0

Above TB there is enough thermal energy for the spins to flip and achieve equilibrium quickly and therefore no hysteresis is observed. Below TB the magnetic response to an applied field exhibits irreversible behavior leading to hysteresis. This special case of magnetism is important for some forms of molecular magnetism.

The other important factor to consider for molecular magnetism is the role played by disorder. Structural disorder can lead to variation in exchange interactions between different pairs of spins. Below a certain temperature, often referred to as the freezing temperature, a disordered system responds very slowly to changes in external conditions.

In ordered systems, equilibrium is reached very quickly compared to experimental measurement times so nonequilibrium effects can be ignored.

There are two types of disordered systems, those with and without frustration. In frustrated systems, the frustration arises when a system cannot minimize its energy of each spin pair in the network at the same time. A system which exhibits both disorder and frustration is referred to as a spin glass. Spin glasses exhibit random, but cooperative freezing of spins due to competing interactions such as ferro- and antiferromagntic interactions.

Random anisotropy magnets, on the other hand, lack frustration, but exhibit disorder due to uniform exchange with anisotropy distributed randomly across different sites. This random magnetic anisotropy can be modeled with the following Hamiltonian

2 2 퐻̂푅퐴푀 = −2퐽 ∑푖<푗 푺푖 ⋅ 푺푗 + 퐷푟 ∑푖(풏̂푖 ⋅ 푺푖) + 퐷푐 ∑푖(푵̂ ⋅ 푺푖) (1.16) where the first term represents the Heisenberg exchange with a strength of J, the second is a random uniaxial anistropy term of strength Dr for a local orientation of 풏̂푖, and the final term is for the coherent (uniform, non-random) uniaxial anisotropy term for a fixed

9 orientation 푵̂ with a strength of Dc. .

This random anisotropy can be found in systems with all different kinds of magnetic exchange, but the cases of interest here are for those with ferro- and ferrimagnetic exchange. The case of ferromagnetic exchange between spins in a system with a random anisotropy that is large compared to the exchange is called a speromagnet. It has a coherent anisotropy that is small, leading the spins align with the anisotropy axis at each site. The short range ferromagnetic order with spin orientation, shown in figure 1.5a, can resemble a spin glass. In the case of two sub-networks with one or both of them is frozen in random directions the system is called sperimagnetic, analogous to ferrimagnetism, shown in figure 1.5b.

Figure 1.5 a) Schematic of a speromagnet b) Schematic showing two subnetworks with frozen disorder, called a sperimagnet9.

Both superparagmagnets and disordered magnetic systems exhibit time dependent phenomena at low temperatures which are slow enough to be experimentally observed through application of a frequency dependent magnetic field.

When this frequency dependent filed is applied at low frequencies the result is similar to the application of the static magnetic field. The induced moment in this case is given by:

푀퐴퐶 = (푑푀⁄푑퐻) × 퐻퐴퐶 sin 휔푡 (1.17) where HAC is the applied driving field, 휔 is the driving frequency, and 휒 = 푑푀⁄푑퐻, the magnetic susceptibility, is the slope of the M(H) curve10. For higher frequencies the magnetization lags behind the drive field by some phase factor, ϕ resulting in

푖휙 푀퐴퐶 = (푑푀⁄푑퐻) × 퐻퐴퐶 sin 휔푡푒 (1.18).

The susceptibility can now be considered as two components: an in-phase component 휒′ and an out-of-phase component 휒′′ where 13

휒′ = 휒 cos 휙 (1.19)

휒′′ = 휒 sin 휙 (1.20)

The in-phase component is similar to the slope of the M(H) curve, while the out-of-phase component indicates a dissipative process in the sample such as irreversibility and relaxation in a spin glass. This can also help determine transition temperatures such as freezing temperature or blocking temperature.

1.3 Organic-based magnetic materials

Following this introduction to the field of organic electronics and the physics concepts necessary to understand magnetism in molecule-based systems, this section provides an introduction and historical background for some of the organic-based materials which have been discovered to-date that exhibit magnetic ordering. This section starts with the lowest dimensional form of magnetism, single molecule magnets and moves into 3-D magnetism observed in powders and thin films.

1.3.1 Single molecule magnets

In addition to the formation of magnetic hysteresis through the classical picture of superparamagnetism, it is possible for spins to tunnel quantum mechanically across the barrier shown in figure 1.4. This was first observed in an organic-based material

11-15 [Mn12O12(CH3COO)16(H2O)4] , abbreviated as Mn12ac, which is now considered to be

16, 17 a member of the class of single molecule magnets (SMMs) . Mn12ac has a total spin

S=10 which comes from the magnetic core of Mn4+(S=3/2) which is ferrimagnetically exchanged with the eight Mn3+ (S=2) atoms as shown in figure 1.6.

4+ Figure 1.6 Molecular picture of Mn12ac with the Mn atoms shown in green and the Mn3+ atoms shown in red.18

These molecules can crystallize into a tetragonal lattice with very weak exchange interaction between molecules. Therefore the spin energy on each molecule can be modeled as a double well potential with 10 levels like the one shown in figure 1.7.

18 Figure 1.7 Model of the energy levels of a Mn12ac molecule.

When a magnetic field is applied parallel to the easy axis it tilts the potential wells causing those on one side to move up and on the other side to lower. For discrete field values of

퐻 = 푁퐷 (1.21) 푁 ⁄푔휇퐵 where N is an integer number, D is the uniaxial anisotropy energy, g is the electron g- factor, and 휇퐵 is the Bohr magneton, the energy levels of the two sides align and tunneling can be observed. This can be seen as steps in the magnetic hysteresis like the one shown in figure 1.8.

Figure 1.8 Magnetization of a Mn12ac crystal which has been normalized by the saturation value for different temperatures in magnetic field swept at 10 mT s-1 applied in the direction of the easy axis.18

There is now an entire class of these single molecule magnets. However, the record

17 highest observed TB for a SMM as of 2015 is 13.9 K . There is excitement about using

SMMs for quantum information storage, but work is currently being done to identify new compounds with higher blocking temperatures, with a goal of 푇퐵 > 77 K.

1.3.2 Higher dimension molecule-based magnets

Introduction of intermolecular exchange interaction to SMM in the simplest form results in the creation of single chain magnets (SCMs) where a molecular chain is defined as having: uniaxial anisotropy and spin, non-zero interactions between spins along the chain, and small inter-chain interactions17. SCMs are typically formed from chains of

SMMs and also exhibit maximum blocking temperatures of 14 K. Therefore these materials are beneficial in the study of molecular magnetic interactions, but have limited practical applications. In the case where a magnetic molecular system experiences interactions in a 3-D network, behavior such as a ferromagnetism is observed and the material can be treated like a bulk magnetic material.

+ - Bulk magnetic ordering was first observed in the organic [Fe(C5Me5)2] [TCNE] (TCNE

19 = tetracyanoethylene) below its TC of 4.8 K . Since this discovery there has been an

8, 20, 21 entire class of molecule-based magnets synthesized , the majority of which have TC values well below room temperature. In 1991, the first organic-based magnet with room

22 temperature ordering, V[TCNE]x~2 was created . Since its discovery, some compounds with near room temperature magnetic ordering have been created by replacing [TCNE] with other acceptor molecules23, 24. Additionally, numerous compounds which replace the vanadium ion with other magnetic ions such as cobalt, iron, and manganese have been

25-27 created, but do not exhibit room temperature ordering . To date, V[TCNE]x~2 is the only TCNE-based organic magnet with room temperature ordering. The rest of this chapter will focus on the properties of the room temperature magnet V[TCNE]x~2.

1.3.3 V[TCNE]x~2 synthesis

The first report of V[TCNE]x y(CH2Cl2) was made by reacting TCNE with V(C6H6) or

22 V(CO)6 in a solution of (S= CH2Cl2, acetonitrile, tetrahydrofuran) . This results in the creation of a black powder with magnetic ordering up to 350 K. The material is observed to have an amorphous structure and be extremely reactive to oxygen. Following this discovery, thin film forms of V[TCNE]x~2 were synthesized using a chemical vapor deposition (CVD) process. Films of thicknesses ranging from tens of nanometers to a few microns can be deposited. The CVD grown thin films are more air stable than the pyrophoric powder, but they still decompose in air resulting in a loss of magnetic properties.

28 Thin films of V[TCNE]x~2 exhibit higher TC values compared with the powder, due to the presence of residual solvent molecules in the powered form that create structural disorder 29. Additionally, a thin film form offers the opportunity to integrate the material into heterostructures and devices. CVD grown V[TCNE]x~2 has been incorporated as one of the magnetic layers in hybrid organic/inorganic spin valves30, 31 and all organic spin valves32. It has also been used as a polarizing layer in an active organic/inorganic semiconductor heterostructure33.

In addition to the CVD process, two other methods have been developed to make thin films of V[TCNE]x~2. Carlegrim et al. have developed a physical vapor deposition (PVD) process which uses pure vanadium in a system under ultra-high vacuum to get oxygen-

18 free thin films34. These extremely thin (1-10’s of nm) films exhibit magnetic ordering in atmospheric conditions, which is attributed to denser packing making it more difficult for oxygen to diffuse into the film. The other synthesis method is called molecular layer

35 deposition (MLD) . In this process, the chamber is opened to the two precursors, V(CO)6 and TCNE separately so a single layer of material is deposited in each step. This also results in very thin film (typically less than 100 nm) which also exhibits better air stability compared to powders and CVD grown films. Li et al. took advantage of the larger coercivity observed in MLD films compared to CVD grown films to create an all- organic spin valve32 using each material as one of the magnetic layers.

1.4 Properties of V[TCNE]x~2

1.4.1 Physical structure

V[TCNE]x~2 is comprised of vanadium ions and tetracyanoethylene (TCNE) molecules.

The structure of the TCNE molecule is an ethylene base with the hydrogens replaced by cyano groups as shown in figure 1.9

Figure 1.9 Schematic of a tetracyanoethylene molecule.

X-ray photoemission spectroscopy reveals a composition of VC16.9N8.8O1.1 which is in

36 reasonable agreement with V[TCNE]2 (VC12N8) . Vanadium atoms coordinate with

TCNE molecules in a distorted octahedral environment. Each vanadium atom can coordinate with up to six TCNE molecules with a V-N bond spacing of 2.084 (5) Å at

300 K and 2.076 (4) Å at 10 K37. This was determined by extended x-ray absorption fine structure (EXAFS) analysis because V[TCNE]x~2 does not exhibit long range order.

Therefore, tools such as x-ray diffraction and transmission electron microscopy are not useful in providing structural information. This lack of crystal structure is what gives the material mechanical flexibility while maintaining the magnetic ordering. This can be physically shown by the pictures in figure 1.10 which shows a piece of Teflon tape coated with a V[TCNE]x~2 film attracted to a magnet.

Figure 1.10 A series of photos showing a V[TCNE]x~2 film deposited on the flexible substrate, Teflon tape, attracted to a magnet.

1.4.2 V[TCNE]x~2 magnetic and electronic properties

V[TCNE]x~2 is a ferrimagnet. Magnetic ordering in the structurally disordered material arises from an ferrimagnetic exchange between spins on the TCNE molecules and the

20 vanadium atoms. Although the material does not exhibit a long range structural order, the local ordering give rise to robust magnetic local order which is shown schematically in figure 1.11. Rotational freedom of the TCNE molecule leads to disorder in the system, which is most pronounced at lower temperatures. This disorder has been attributed to a spin glass phase38 based on experimental results, but recent theoretical calculations suggest that is disorder is a sperimagnetic phase in the material9 due to a lack of frustration in the system.

Figure 1.11 Schematic of V[TCNE]x~2 structure with spins to show the origin of ferrimagnetic ordering28.

Neutron diffraction studies on the TCNE molecule reveal that the spin is mainly located on the double bonded carbon atoms and the nitrogen atoms as shown in figure 1.1239, 40.

Figure 1.12 Spin density distribution of (TCNE)-39.

The total spin on each TCNE molecule is S=1/2 and on the vanadium ions it is S=3/2.

Stoichiometrically there are two TCNE molecules for every vanadium, so this results in a total spin of S=1/2. The spins on the TCNE molecules, donated from the V2+ ions are located in the π* orbital. The π* anti-bonding level can accept another electron of

41 opposite spin, which costs additional Coulomb energy , Uc, and leads to the splitting of

42 the band into oppositely polarized subbands . The t2g level of the vanadium atom lies between these two subbands43, as shown in figure 1.13.

28 Figure 1.13 An electronic model for V[TCNE]x~2 showing the location of the spins . 22

Electronically, V[TCNE]x~2 exhibits semiconducting behavior with a conductivity of

-4 -1 42 σRT~10 S cm at room temperature . The electrical resistance increases with decreasing temperature. An activation energy of ~0.5 eV can be extracted from a fit to an Arrhenius

42 plot . This is the value of the band gap in figure 1.13, with the t2g levels forming the valence band of the organic semiconductor while the unoccupied π*+Uc levels form the conduction band. Theoretical and experimental studies suggest that these bands are fully spin polarized30, 43-46.. The material also exhibits a linear magnetoresistance with no sign of saturation up to 32 T47.

The magnetization depends strongly on temperature with a typical magnetization as a function of temperature plot in Figure 1.14.

Figure 1.14 Magnetization versus temperature at different applied fields38.

The films physically breakdown when heated above 350 K so it is not possible to measure TC, but it can be extracted using a Bloch law, equation 1.12. While the Curie temperature for all forms of V[TCNE]x~2 has always been above room temperature, improvements in growth have yielded higher quality films with higher values of TC. The historical comparison of values is shown in figure 1.15.

Figure 1.15 Historical comparison of TC values for V[TCNE]x~2 from the following papers [A]22, [B]36, [C]48, [D]38

In addition to the temperature dependence, the field dependence results in a hysteresis loop like the one shown in Figure 1.16 with a coercivity of ~10 Oe and full saturation by

200 Oe.

Figure 1.16 Magnetization as a function of applied field measured at room temperature32.

1.5 Conclusions

V[TCNE]x~2 is a ferrimagnetic semiconductor with fully spin-polarized conduction and valence bands. It has a Curie temperature above room temperature and exhibits less disordered magnetism at room temperature compared to lower temperatures, which is attractive for incorporating this material into devices. Like other organics, it has advantages such as mechanical flexibility and ease of chemical tunability. This material continues to reveal unique features including photo-induced magnetism38, 49 and a complex ferromagnetic resonance spectrum38, 48. This represents the state of the field as of the first decade in the 21st century. The success of organic-based systems for light emission suggests similar potential for organic magnetic applications. The fully spin polarized nature of V[TCNE]x~2 makes it very attractive as a spin injection material in devices as been previously shown. However the variation in properties between growths and the lack of air stability presents a challenge for incorporating the material in commercial devices.

The following chapters will focus on advancements in the growth leading to higher quality films, material engineering to modify V[TCNE]x~2 to create additional organic- based magnets and to create new morphologies with different properties, encapsulation techniques to address the air sensitivity issue of V[TCNE]x~2, and finally potential applications and future directions based on these advancements.

Chapter 2: V(TCNE) growth, optimization, and characterization

This chapter focuses on the growth and characterization of V[TCNE]x~2 films created through a chemical vapor deposition (CVD) process. The first section focuses on the original chemical vapor deposition set-up and characterization of films by magnetometery, electrical transport, and ferromagnetic resonance measurements. The second section highlights work done to optimize the growth resulting in higher quality films with an emphasis on the role of temperature.

2.1 CVD growth of V[TCNE]x~2 films

2.1.1 Growth basics

V[TCNE]x~2 is deposited in a custom-built reactor whose schematic is shown in figure

2.1. The system is built out of glass with the deposition boats made of quartz. The entire system is located inside an argon glovebox with O2<1.0 ppm and H2O<3.0 ppm. The process starts by setting the temperatures. A silicone oil bath is set to 10° C for the

V(CO)6 precursor. The reactor is heated by a glass tube wrapped with heater wire. The spacing of the wire determines the gradient between the area where the TCNE precursor sits and reaction zone. Samples are loaded into the reaction zone by placing them on a

Teflon tape coated glass slide. The reactor is then assembled to look like figure 2.1.

Figure 2.1 Schematic of the custom-built CVD reactor located inside an argon glovebox50.

The precursors are weighed out in a ratio of 10:1 TCNE to V(CO)6 and loaded into the reactor inside the glovebox. Commercially purchased TCNE is purified by sublimation and stored in a -35°C freezer. The vanadium precursor is synthesized from

[Et4N][V(CO)6] and phosphoric acid which forms V(CO)6. This material is highly reactive and stored under an argon atmosphere at -35°C. The total amount of precursor depends on the desired film thickness which is determined by deposition time. A typical deposition is 75 minutes and uses 50 mg of TCNE and 5 mg of V(CO)6 and results in a film of thickness of ~700 nm. The thickness also varies depending on the sample location in the reaction zone. Figure 2.2 shows a thickness map for the reaction zone measured using profilometry.

Figure 2.2 a) Top view of the CVD reactor showing the location of the substrates. b) A map of the film thickness based on profiliometry measurements50.

The other important parameters for the deposition include a pressure of 30-35 mm Hg and flow rates of 56 sccm for the V(CO)6 and 84 sccm for the TCNE. The reaction is stopped by simply turning off the temperature controllers and closing the vacuum line.

The full step-by-step instructions for synthesis and purification of the chemical precursors and the CVD process can be found, along with a video, in an article published in the Journal of Visualized Experiments50.

2.1.2 Film characterization

Film growth can be characterized by several different methods. Each provides different information about the quality of the film. Early work on V[TCNE]x~2 utilized chemical composition studies to identify the stoichiometry of ~2 TCNE for every vanadium atom and the vanadium atoms bonding primarily with nitrogen36, 37, 51 . The following sections 29 detail magnetic and electronic characterizations that provide details about both the quality of the film and potential usefulness for device applications.

2.1.3 DC magnetometry

DC magnetometry shows the response of a material to an applied static field.

Measurements of V[TCNE]x~2 films were performed in a Quantum Design Magnetic

Properties Measurement System (MPMS). Films deposited on glass substrates are sealed in quartz tubes and mounted inside a straw. The sample is moved up and down through a series of superconducting coils. These coils are connected to a superconducting quantum interference device (SQUID). When a magnetic sample moves through coils it induces a current which the SQUID converts to a voltage.

A SQUID is comprised of two superconducting materials with two gaps between them, known as Josephson junctions. These gaps are close enough to allow tunneling of the

Cooper pairs that carry the current in a superconductor. Each superconductor has a random but constant phase, and if these phases differ, current will flow across the junction to minimize the free energy. Therefore, the current is proportional to the tunneling properties of a gap and the difference in phase.

Figure 2.3 Schematic showing a DC SQUID with two Josephson junctions separating the superconducting regions.

When there is no magnetic flux, no current will flow, and thus the difference in phase across both gaps is the same. When a magnetic field induces a current, there is a difference in phase across both gaps leading to a difference in current in each side of the loop. This current is converted to a voltage using a shunt resistance across the loop. The

SQUID is extremely sensitive to small voltages meaning that it can measure small magnetic moments.

Figure 2.4 shows what the voltage signal looks like as a sample with a magnetic signature passes through the coils.

Figure 2.4 Voltage dipole response of a sample passing through the pick-up coils52.

The MPMS can perform magnetization measurements at different applied fields and/or temperatures. A sweep of the field for V[TCNE]x~2 at room temperature results in a hysteresis loop like the one shown below in figure 2.5, however there are several corrections which need to be made to the data.

Figure 2.5 a) Uncorrected plot of magnetization versus field measured by an MPMS at 300 K. b) Full applied field range from -1 T to T showing the diamagnetic background signal.

Cotton is usually used to pack the samples which exhibits a diamagnetic (inversely linear with applied field) background signal at room temperature. This background signal can be fit and subtracted from the data. In addition, a correction to the field value must be made.

The superconducting magnet which applies the field usually contains some impurities that pin and immobilize magnetic flux, which remains behind when the magnetic field is removed after being set to a high value, leading to a non-zero field when the coils are swept from a high field to zero resulting in an offset in the reported field values. This offset can be determined and applied to the data53. The final correction to the raw data is a volume normalization which allows direct comparison between measurements of different samples. After applying these corrections the data looks like what is shown in figure 2.6.

Figure 2.6 a) Corrected plot of magnetization versus field measured by an MPMS at 300 K. b) Full applied field range from -1 T to 1 T.

From the hysteresis loop a typical coercivity of 5-30 Oe can be extracted for CVD

V[TCNE]x~2 films, with variation from growth to growth. The magnetization is typically saturated by 100-200 Oe. In addition to the magnetization as a function of field, the magnetization can be measured as a function of temperature. Figure 2.7 shows the magnetization versus temperature. The sample is measured with Happ=100 Oe.

Figure 2.7 Magnetization versus temperature in applied field of 100 Oe for sweeps of zero field cooled (ZFC) and field cooled (FC).

Fitting to the Bloch law, equation 1.12, in the linear portion of the curve yields a TC of

608 K,  = 6.7±0.1 × 10-5 K3/2 in this case. This value is lower than a simple linear extrapolation of the data. In comparison to historical data, however it is significantly higher than previously reported values as shown as the star in figure 2.8.

Figure 2.8 Historical comparison of TC values for V[TCNE]x~2 from the following papers

[A]22, [B]36, [C]48, [D]38, and [E]28.

In addition to determining the Curie temperature, the sample can be cooled with zero applied field and measured at Happ=100 Oe upon warming for a zero-field cooled (ZFC) curve and then cooled again with an applied field on for a field cooled (FC) curve as shown for a sample in figure 2.9.

Figure 2.9 Magnetization versus temperature in applied field of 100 Oe for sweeps of zero field cooled (ZFC) and field cooled (FC).

Divergence between the field-cooled and zero field-cooled curves is evidence of frozen- in disorder. This divergence may be evidence of a spin glass or sperimagnetic phase. For this sample the temperature dependence increases with temperature below 110 K and then decreases above that point which is also suggestive of a phase transition at low temperatures to a more disordered state.

2.1.4 AC susceptibility

To further explore the possibility of a spin glass or sperimagnetic phase at low temperature, which both exhibit slow relaxation times, an AC susceptibility measurement can be performed on the system yielding the in-phase, 휒′, and out-of-phase, 휒′′, susceptibilities introduced in section 1.2.

A spin glass is characterized by a cusp in the 휒′ versus temperature curve at the freezing temperature, but this location of this cusp shows a frequency dependence. AC susceptibility measurements of V[TCNE]x~2 exhibit a peak in the 휒′, but do not show a sharp cusp as can be seen in figure 2.10a. Additionally, a comparison of different frequencies does not move the peak to other temperatures suggesting that V[TCNE]x~2 is not a spin glass.

Figure 2.10 a) In-phase component of the susceptibility measured as a function of temperature for ac field of 3 Oe for drive frequencies of 33, 100, and 1000 Hz. b) Out-of- phase component of the susceptibility measured as a function of temperature for ac field of 3 Oe for drive frequencies of 33, 100, and 1000 Hz.

However the presence of a peak in the out-of-phase component as shown in figure 2.10b suggests dissipative behavior, which could be due to a disordered phase such as sperimagnetism. The location of the peak is well matched to the peak in the DC M(T) curves for the same growth, further suggesting evidence of a phase transition. The peak is fairly broad, but given the amorphous nature of the material, sharp transitions are not to 38 be expected. Recent theorectical work on V[TCNE]x~2 also confirms a sperimagnetic phase is likely9.

2.1.5 Electronic transport

In addition to the characterization of the magnetic properties, electronic transport measurements reveal details about the film. After V[TCNE]x~2 films are deposited on a glass substrate, samples are transferred through interconnected gloveboxes into a metal chamber where ~30 nm of aluminum and ~20 nm of gold are deposited on top of the sample to make two top contacts roughly 9 mm2 each. The sample schematic is shown in the inset of figure 2.11b.

Figure 2.11 a) IV plots for temperatures from 300 K (black) to 160 K (pink) in 10 K steps b) Resistance versus temperature fit from IV curves. Inset shows the device schematic.

A custom air-free sample puck is used to transfer samples to a Quantum Design physical properties measurement system (PPMS) without exposure to air. Current-voltage (IV) characteristics were collected as a function of temperature with a Keithley 2400 sourcemeter and are shown in figure 2.11a. The IV curves show Ohmic behavior and a resistance value can be fit to each temperature point. The resistance increases with decreasing temperature. For temperatures below 160 K the small values of current reach the measurement limit of the detection hardware. The resistance values plotted as a function of temperature are shown in figure 2.11b. These can be fit to an Arrhenius equation

−퐸푎 ⁄푘 푇 푅 = 푅0푒 퐵 (2.1) to extract the activation energy, Ea, of ~0.5 eV. This activation energy is the thermal energy required to excite charge carriers from the t2g level in figure 1.13 into π*+UC level. This is consistent with the energy gap between the t2g level and π*+UC level determined by resonant photoemission spectroscopy43.

2.1.6 Carrier mediated magnetism

It is interesting to note that the temperature region at which the free carriers begin to freeze out is also the same region where there is a transition in the magnetism. This can be seen most clearly in figure 2.12a which shows the resistance values (in red) on an

Arrhenius plot of magnetization versus inverse temperature. This plot highlights the correlation between the low temperature magnetic phase transition (indicated by a peak in

40 the magnetization) and the freeze-out of the free carriers (indicated by the saturation of the resistance).

Figure 2.12 a) Magnetization (black) and Ln (R) (red) versus 1/T. For magnetization, filled (empty) points show zero field cooled (field cooled) data taken at 100 Oe field. Inset: Same data on a smaller temperature scale. Blue region highlights area around 150 K. (b) Schematic showing the magnetization of the films. The center panel shows the local exchange with the red and blue arrows representing the spin of the unpaired electrons on the TCNE- and V2+ respectively and the regional net magnetization represented by a large black arrow. The long range behavior for low and high temperatures is shown in the two side panels.

A possible candidate for this secondary magnetic phase is indicated schematically in figure 2.12b. As described in Chapter 1, V[TCNE]x~2 is composed of local regions wherein the ferrimagnetic exchange between the vanadium t2g and π* orbitals on the

TCNE- dominates, shown in the center panel of figure 2.12b. The left hand panel of figure 2.12b reveals the implications of this model for the global magnetic ordering, yielding islands of order that are only weakly coupled and leading to a disordered

41 behavior. This model appears to describe the low temperature behavior of the magnetization in figures 2.7 and 2.9.

The right hand panel of figure 2.12b shows schematically the consequence of adding a

2+ thermally activated exchange pathway between the V t2g orbitals and the unoccupied

- π*+Uc TCNE orbitals. In this case, the thermally excited carriers are free to delocalize in the largely empty π*+Uc orbitals (for electrons) or t2g orbitals (for holes). While this thermally activated π*+Uct2g exchange has the opposite sign relative to the low temperature π*t2g interaction, both yield overall ferromagnetic ordering of the vanadium t2g spins. As a result, this new exchange pathway serves to enhance the magnetic ordering of the vanadium and to correlate adjacent islands of local order (as indicated by the schematic electron wave functions in the right hand panel of figure

2.12b). This enhanced ordering can be seen as a more square hysteresis loop at higher temperatures.

2.1.7 Ferromagnetic resonance

The magnetization dynamics in V[TCNE]x~2 can also be explored through ferromagnetic resonance (FMR), a spectroscopic technique which investigates the response of magnetic material to microwave excitation. In a ferromagnetic system due to a strong interaction between spins, the system can be treated as a single macrospin when all spins are saturated. When an excitation is applied to the spin it exerts a torque on the system and the magnetization begins to precess. Damping in the system pushes the magnetization back to equilibrium. This process is described by the Landu-Lifshitz-Gilbert equation54

푑푴 훼 푑푴 = −훾(푴 × 푯푒푓푓) + (푴 × ) (2.2) 푑푡 푀푠 푑푡 where γ is the gyromagnetic ratio, Heff is the effective magnetic field felt by the system, α is the Gilbert damping parameter, and Ms is the magnitude of the magnetization vector M.

The first term describes the torque and the second term describes the damping.

Resonance occurs when the torque, is applied at a rate such that each time the magnetization to completes one precession it experiences a torque and continues to precess without damping. The angular dependence of the resonance frequency for a thin film without crystalline anisotropy such V[TCNE]x~2 can be described as

휔 푟푒푠 = √(퐻 − 퐻 cos2 휃 )(퐻 − 퐻 cos 2휃 ) (2.3) 훾 푒푓푓 퐻 푒푓푓 퐻 for 퐻 ≫ 퐻푒푓푓 where θH is the angle of the applied external field with respect to the normal of the film plane28. In the case where there is shape anisotropy due to a thin film

퐻푒푓푓 = 4휋푀푠.

This resonance can be measured by monitoring the microwave absorption. V[TCNE]x~2 films were deposited on sapphire substrates (chosen because they have low microwave loss), sealed in EPR grade quartz tubes and loaded into a Bruker EPR Spectrometer. The set-up for the spectrometer is shown in figure 2.13, with the microwave bridge shown on the right. The microwaves pass from the source through the attenuator which controls the amount of power which reaches the sample. It then passes to the circulator which passes microwaves coming into port 1 out of port 2 into the cavity and then microwaves that come from the cavity into port 2 out of port 3 into the detector. The microwave bridge

43 also contains a reference arm which diverts some power to maintain a diode current of

~200 amperes which is the highest sensitivity range55.

Figure 2.13 Schematic a Bruker EPR Elexsys E500 with the microwave bridge details shown in the right panel55.

The V[TCNE]x~2 films were measured at room temperature with a microwave frequency of 9.63 GHz which is tuned between 9 and 10 GHz for optimal cavity performance.

Figure 2.14 shows a spectra for a sample mounted in-plane. The inset shows the spectra of the two precursors for V[TCNE]x~2 at scale expanded by 1000x. In successful growths

V[TCNE]x~2 exhibits an extremely narrow linewidth of 1.4 G. There are no features of

44 comparable sharpness or position in the spectrum for the precursors confirming that the signal is from the entire system.

Figure 2.14 Ferromagnetic resonance spectrum of a V[TCNE]x~2 thin film taken an applied microwave frequency of 9.388 GHz, with the static field in the sample plane (black). FMR spectra of the precursors TCNE (blue) and V(CO)6 (red). Inset: Zoomed in 28 view of the TCNE (blue) and V(CO)6 (red) FMR spectra .

In previous results at some angles the FMR spectra for V[TCNE]x~2 films show a single narrow feature, but at other angles the spectra is complex with many features48. In successful growth results, V[TCNE]x~2 films show a single feature at all angles as shown in figure 2.15a. The centerfield of these values can be fit to equation 2.5 with a value of

3 4πMs=95 G (7.56 emu/cm ), shown in figure 2.15b, consistent with the value obtained from DC magnetometery measurement of the magnetization versus field.

Figure 2.15 a) Room temperature FMR spectra as a function of angle from in-plane (90°) to out of-plane (0°). b) Angular dependence of center field extracted from Lorentzian fits to spectra taken at 300 K. Solid line is simulated angular dependence curve with 28 4πMs=95G .

The linewidth of this FMR feature is comparable to the highly utilized material, yttrium iron garnet (YIG), of similar thickness, and is substantially better than all other known thin film ferromagnets. This single feature is highly dependent on the growth conditions of the CVD film, especially the temperature of the CVD reactor. However the system described in 2.1.1 was not calibrated, and independent control of the TCNE and reaction zone temperatures was difficult to achieve in a systematic way, leading to inconsistent

FMR results.

2.2 Optimization of CVD growth of V[TCNE]x~2 films

2.1.1 New heater

As was mentioned in the previous section, the first generation heater did not allow for independent, systematic control of the two temperature zones, so a second generation system was developed to allow that control. The new heater uses two pieces of heater

46 tape, each controlled with a separate controller, to define two zones. The two zones are not thermally isolated so there is a gradient between them. Figure 2.16 shows a picture of the heater with the two zones (with the TCNE zone in blue and the reaction zone in green).

Figure 2.16 Picture of the second generation reactor heater tube with the two zones, the TCNE zone highlighted in blue and the reaction zone highlighted in green.

The CVD set-up does not allow for in-situ temperature measurement, so a temperature profile is taken before the reaction by measuring with a thermocouple along the bottom of the reactor at 1 cm intervals for each pair of set-points. A typical profile looks like figure

2.17 for set points of TTCNE=56.5°C and Treaction=49.5°C.

Figure 2.17 A typical temperature profile for the V[TCNE]x~2 second generation heater.

This measurement allows a direct comparison of the measured temperatures in the reactor, which may differ from the set points. An ideal profile has a relatively flat region for the entire TCNE boat and for the reaction zone.

2.2.2 FMR diagnostic

While taking a profile of the heater provides additional details about the reaction conditions for a growth, it does not reveal the quality of the growth. In section 2.1 there were several different characterization methods introduced, but FMR resonance is the one most sensitive to sub-optimal growth conditions, because a good growth exhibits a single

FMR feature. Furthermore, growth quality issues are most sensitive to comparisons of the out-of-plane spectra. Figure 2.18 is a map of the FMR spectra for films grown at several different set points.

Figure 2.18 A map of out-of-plane FMR spectra for V[TCNE]x~2 films grown at the set points (TCNE temperature, substrate temperature).

This map reveals that the best spectra correspond to the highest TCNE temperatures at a set-point of about 77°C. Another interesting feature is that for set points that are nearly equal there is almost no resonance feature at all, suggesting that a gradient is important for the growth.

2.3 Conclusions

Since its discovery significant progress has been made in advancing the quality of

V[TCNE]x~2 films as well as characterizing their properties. My work through careful control of parameters and innovating the heater has led to material with new functionality for high frequency electronics. However there is still significant room for further development of this materials system through continued innovation and optimization of the growth properties. Additionally, other types of characterization will help build on the understanding of the physics of this material, and possibly continue to discover new applications. Advances in V[TCNE]x~2 are not only limited to growth and characterization, as will be discussed in Section 2.

Section 2: Beyond V[TCNE]x~2 films

Having now established an understanding of thin film CVD V[TCNE]x~2 this section explores the advancements which go beyond characterization of thin films. Chapter 3 discusses work done to engineer the materials to introduce new properties. The first part of chapter 3 discusses the results from a collaborative project on chemical tailoring of the organic molecule to create additional organic-based magnets. The second part of chapter

3 overviews a project which I led to use substrate engineering to create new organic magnetic morphologies. Chapter 4 reveals the results of another collaboration which addressed one of the difficulties in working with V[TCNE]x~2 which is its instability in ambient conditions by introducing encapsulation techniques. Chapter 5 discusses some of the applications and potential future directions for this materials system.

Chapter 3: Engineered organic-based magnetic materials

One of the advantages of organic-based materials is the ease at which they can be tuned for different properties. This chapter discusses two methods for engineering organic- based magnetic materials. The first section focuses on chemical modification of the

TCNE molecule to create new magnetic thin films through taking advantage of collaboration between chemistry and physics. The second section discusses new morphologies in V[TCNE]x~2 that introduce properties not seen in standard films.

3.1 New organic-based magnetic materials

3.1.1 Previous work

After the discovery of the room temperature organic-based magnet V[TCNE]x~2 several other materials containing other transition metals, M, in analogues of the form

25-27, M[TCNE]x have been explored. This family includes M=Co, Ni, Cr, Fe, Nb, and Mo

51, 56 which are created using high vacuum/temperature chemical vapor deposition (CVD) or physical vapor deposition (PVD). However spintronics applications that incorporate these materials are severely limited by the harsh deposition conditions and lack of room temperature magnetic ordering. Parallel efforts have focused on replacing the TCNE with other organic single electron acceptors23, 24, 57-60. Some of these materials exhibit room

52 temperature magnetism, but they are all synthesized in solution as powders cannot be incorporated in heterostructures and devices.

3.1.2 Ligand modification

The TCNE molecule can be modified by replacing one of the cyano groups with an alkyl substitution to form methyl tricyanoethylenecarboxylate (MeTCEC) or ethyl tricyanoethylenecarboxylate (ETCEC) whose molecular strucutures are shown in figure

3.1.

Figure 3.1 Molecular schematic of TCNE molecules that are modified by replacing a cyano group with alkyl substitution. Methyl tricyanoethylenecarboxylate (MeTCEC) is shown on the left and ethyl tricyanoethylenecarboxylate (ETCEC) is shown on the right.

These molecules were reacted with V(CO)6 in the CVD reaction described in chapter 2 to form V[ETCEC]x and V[MeTCEC]x thin films. The resulting films were characterized through x-ray photoemission spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to reveal information about the composition and bonding28, 61. The

53 magnetic properties were characterized with the DC magnetometry. Figure 3.2 shows the magnetization as a function of temperature for both thin films.

Figure 3.2 a) Magnetization versus temperature of a thin film of V[MeTCEC]x at an applied field of 25 Oe for a field cooled (FC) and zero-field cooled (ZFC) sweep. b) Magnetization versus temperature of a thin film of V[ETCEC]x at an applied field of 150 Oe for a field cooled (FC) and zero-field cooled (ZFC) sweep.

28 The V[MeTCEC]x has TC of 317 K as extracted from a fit to the Bloch law , equation

1.12. Similar to what is observed in V[TCNE]x~2, there is a peak in the magnetization around 190 K suggesting a transition from a sperimagnetic phase to a ferrimagnetic one.

This peak is less broad compared with V[TCNE]x~2 which may mean that the phase transition between phases happens more sharply. Figure 3.2b shows the magnetization as a function of temperature for the thin film of V[ETCEC]x. These films exhibited a lower

TC around 161 K with a peak in the magnetization at about 90 K. Figure 3.3 shows the magnetization versus field for 5 K and 300 K for the V[MeTCEC]x and at 5 K and 100 K for the V[ETCEC]x. 54

Figure 3.3 a) Magnetization versus field for V[MeTCEC]x thin film at 5 K and 300 K. Inset shows a zoomed in field range. b) Magnetization versus field for V[ETCEC]x thin film at 5 K and 100 K. Inset shows a zoomed in field range.

The field dependence for the V[MeTCEC]x film showed a small coercive field of 10 Oe at 5 K and 20 Oe at 300 K which was larger than the 1 Oe field observed in solution

24 processed V[MeTCEC]x . The magnetization approaches saturation by 1 kOe. Hysteresis loops for V[ETCEC]x are shown at 5 K and 100 K (due to the lower Curie temperature) in figure 3.3b. V[ETCEC]x has a small coercive field of ~6 Oe at 5K and about 25 Oe at

100 K. It also reaches saturation by 1 kOe.

The ETCEC molecule has a larger alkyl chain compared to MeTCEC, which itself has a larger molecular structure compared with TCNE. Additionally ETCEC is the most difficult of the three acceptors to reduce. Electrochemistry reveals that the energy of the

π* orbital determines the bulk magnetic properties. Therefore, the relationship between reduction potential and ordering temperature explains why V[ETCEC]x has the lowest

60 TC, followed by V[MeTCEC]x, with the highest for V[TCNE]x~2 .

3.1.3 Transport properties of V[MeTCEC]x

V[MeTCEC]x exhibits room temperature magnetic ordering, making it attractive for incorporation in all-organic and hybrid heterostructures and devices. Therefore, the transport properties of this thin film are of interest. V[MeTCEC]x films were grown on glass substrates with top contacts of ~30 nm of Al and 40 nm of Au deposited through thermal evaporation. Samples were transported to the Quantum Design physical properties measurement system (PPMS) in the customized air-free sample can. Current voltage (IV) characteristics were measured in a two-probe geometry as shown in figure

3.4 for different temperatures with a Keithley 6430 Sub-femtoamp sourcemeter.

Figure 3.4 a) IV plots for temperatures at 275 K (pink) to 175 K (black) of thin films of V[MeTCEC]x b) Resistance versus temperature fit from IV curves.

The IV plots reveal Ohmic behavior at all temperatures and the resistance increases monotonically with decreasing temperature. Below 175 K the small values of current 56 reached the measurement limit of the detection hardware. Above 275 K the resistance increased rapidly, likely due to a combination of residual air contamination during transport and current annealing of the sample, as shown in figure 3.5.

Figure 3.5 Percent change in the resistance as a function of time for an V[MeTCEC]x film at different temperatures.

The increase in resistance for 300 K did not correlate with the electronic properties so only transport data below 275 K was considered. The data were fit to both a hopping model and an Arrhenius equation since these materials exist at the boundary between a low-mobility band conductor and high mobility hopping. For hopping transport, we chose a simple model,

푇 ( 0)푚 푅 = 푅0푒 푇 (3.1) where R0 is characteristic resistance, T0 is the characteristic temperature and m has a value between 0 and 1, which in principle determines the dimensionality of the transport and the hopping mechanism62. The hopping transport was fit to the data plotted in figure

3.6a. The fit yielded an extracted value of m=0.78. This value was not explicitly consistent with either nearest neighbor hopping (m=1), or Efros Shklovskii variable range hopping (VRH, m=0.5); however, the available temperature range was limited to less than a decade due to the competing constraints of carrier freeze out (T~190 K) and sample degradation (T ~275 K), limiting the precision of this determination.

Figure 3.6 a) A log-log plot of the natural log of the resistance versus inverse temperature with a fit to the hopping transport. b) Arrhenius plot of the resistance versus temperature.

Fitting the data to thermally activated Arrhenius equation 2.3 revealed an activation energy of Ea~0.56 eV shown in figure 3.6b. This value of Ea is similar to V[TCNE]2

42 (Ea~0.50 eV) suggesting a similar electronic structure .

Additionally, magnetoresistance (MR) measurements were performed at temperatures above and below the peak magnetization temperature of ~190 K. These results are shown in figure 3.7 and reveal an anomalous positive magnetoresistance that increased with

42, 63 increasing temperature similar to what was observed in V[TCNE]x~2 . The MR is defined as

(푅(퐻)−푅(0)) 푀푅% = 100 ∗ (3.2). 푅(0)

Figure 3.7 Magnetoresistance as function of applied field for a temperature above and below 190 K for an V[MeTCEC]x film.

For the MR at 200 K (above the freezing temperature of ~190 K) the slope was 0.39% and the slope was 0.16% at 140 K. This anomalous linear behavior with increasing MR

42, 63, 64 for temperatures close to TC is consistent with what was observed in V[TCNE]x~2 .

This is inconsistent with the predicted H2 dependence expected for Mott VRH in non- magnetic systems65.

3.2 New organic magnetic morphologies

3.2.1 Organic magnetic nanowires

Up until this point, V[TCNE]x~2 has been created in both the form of a powder and in the form of a thin film, but new structures, such as nanowires, can be formed through substrate patterning. Due to the nature of the chemical vapor deposition process, every clean surface inside the reaction zone where the two precursors react with each other is coated with V[TCNE]x~2. Therefore, when V[TCNE]x~2 is deposited on a patterned substrate, like the one shown in figure 3.8a, it deposits not only on the top of the ridges, but also on the sides. This eventually causes shadowing where material is no longer deposited between the ridges, and nanowires are formed on the top of the ridges. This process can be seen in figure 3.8b for a sample that was at the edge of the reaction zone.

On the far right, the thinner layer of material was conformal over the ridges, whereas on the left side, there was a gap where no material deposited due to shadowing.

Figure 3.8 a) Scanning electron microscopy (SEM) image of patterned substrate of SiO2 on Si. One of the ridges (period of 350 nm, trench width of 170 nm) is highlighted in red. b) SEM image of V[TCNE]x~2 grown on a patterned substrate. This sample was located at the edge of the reaction zone, showing the growth progression of nanowires.

Fully formed V[TCNE]x~2 nanowires look like the ones shown in figure 3.9. The view on the left shows a scanning electron microscopy image of a cleaved sample with

V[TCNE]x~2 deposited on a patterned substrate. Figure 3.9b shows the same sample viewed from the top. It is apparent here that even though the areas of the V[TCNE]x~2 are touching, they have maintained their form and have not re-coalesced into a continuous film.

Figure 3.9 a) Side view of an SEM image of V[TCNE]x~2 grown on a patterned SiO2 substrate. b) Top-view SEM image of the sample showing that the nanowires extend the length of the sample.

For very thin films, the growth was conformal, but there was a question if the films would re-coalesce for a thick enough deposition. Figure 3.10 shows a ~1.4 μm film in both the side view and top view. The film lifted off the surface of the substrate during cleaving. It appears from the view in figure 3.10a that the V[TCNE]x~2 was more like a film than individual nanowires; however, a top view of the film showed that it still maintained some of the structural order of the substrate.

Figure 3.10 a) Side view of an SEM image of a 1.4 nm thick film of V[TCNE]x~2 grown on a patterned SiO2 substrate. The film lifted off of the substrate during the cleaving process. b) Top-view SEM image of the sample showing that the film still maintains some of the underlying substrate structure.

In addition to looking at very thick films, the orientation of the grooves with respect to the growth chamber was explored. Figure 3.11 shows two samples from the same growth that were grown in different orientations with respect to the gas flow in the CVD reactor.

SEM images, as well as FMR measurements, show no differences between samples grown in different directions.

Figure 3.11 a) Schematic showing the samples labeled X and Y with respect to the gas flow inside the CVD reactor. b) SEM image of a sample grown with grooves perpendicular to the gas flow. c) SEM image of a sample grown with grooves parallel to the gas flow.

3.2.2 Characterization of V[TCNE]x~2 nanowires

DC magnetometry was performed on nanowires by vacuum sealing samples mounted on a quartz rod to maintain their orientation with respect to the applied field inside a quartz tube. Magnetization versus temperature showed behavior similar to that of control samples of films grown on standard glass and sapphire substrates, with an extracted Curie temperature above 600 K.

In thin films of V[TCNE]x~2, the lack of long range structural order means that there is no magnetic easy axis for thin films. This lack of in-plane magnetic anisotropy limits the applications of the material. However, anisotropy can be introduced by growing

V[TCNE]x~2 on patterned substrates. This was observed in the magnetization versus field measurement. Whereas for a thin film of V[TCNE]x~2 the magnetization does not depend on sample orientation in an applied field, for the nanowires, the hysteresis exhibited a

64 different shape for the case where the wires were mounted perpendicular to the applied field versus the case where they are parallel to the applied field, as shown in figure 3.12.

Figure 3.12 Magnetization as a function of applied field at room temperature for V[TCNE]x~2 grown on a patterned substrate. The magnetization is normalized by the saturation magnetization and plotted for Happ perpendicular to the substrate pattern (∆) and parallel to the substrate pattern (■). The inset shows the magnetization over the full field scale.

The "easy axis" is the orientation where the applied field is parallel to the wires. It saturates at a lower field compared to the "hard axis," where the applied field is perpendicular to the wires. The presence of the material between the ridges makes it difficult to determine the saturation field for the nanowires; however, qualitatively the behavior is consistent with the expectation that it is easier to align spins along the direction of the wires.

The magnetic anisotropy in the V[TCNE]x~2 nanowires can be further explored by measuring the in-plane angular FMR response of the sample. Rotation of the film in an external magnetic field results in a shift of the center field of the main resonance feature with angle. As can be seen in figure 3.13a, the main resonance peak shifts sinusoidally as a function of angle. There were some additional side peaks which were likely due to the response from the V[TCNE]x~2 present between the SiO2 grooves. A fit to the center field values is plotted in figure 3.13b along with a comparison to V[TCNE]x~2 grown on a sapphire substrate (open squares), which shows no center field shift. The black line on plot 3.13b shows a fit to equation 3.3:

휔 푟푒푠 = √(퐻 − 퐻 cos2 휃 )(퐻 − 퐻 cos 2휃 ) (3.3) 훾 푖푝 퐻 푖푝 퐻 where the applied field, H, is much larger than the effective field Hip, and θH is the angle of the applied external field with respect to the nanowire axis. We extract a value of Hip =

64±1.1 G.

Figure 3.13 a) Room temperature FMR spectra for rotation in-plane of V[TCNE]x~2 deposited on a patterned substrate. b) Comparison of V[TCNE]x~2 nanowire center field values and V[TCNE]x~2 film center field values.

For the case of an isolated single wire with no crystalline anisotropy, 퐻푖푝 = 2휋푀푆. As more wires are added in an array, the dipolar interaction between the wires should destroy this anisotropy reducing the value of effective field, Hip. When the wires become dense enough to resemble a film, the in-plane anisotropy should be reduced to zero.

In order to extract the value of Ms we fit the in-plane to out-of-plane rotation shown in figure 3.14. In this case, we fit the center field values to equation 3.4:

휔 푟푒푠 = √(퐻 − 퐻 cos2 휙 )(퐻 − 퐻 cos 2휙 ) (3.4) 훾 푒푓푓 퐻 푒푓푓 퐻 for 퐻 ≫ 퐻푒푓푓 where ϕH is the angle of the applied external field with respect to the normal of the film plane. We extract a value of Heff = 96±7.8 G for the patterned sample, which is consistent with a value of Heff = 101±1.8 G for a sapphire sample and the value

66 of Heff =4휋푀푠= 95 G reported in the literature . The larger error is again likely due to a resonance signal affected by both material in the wires and in between the grooves.

Figure 3.14 a) Room temperature FMR spectra for rotation in-plane (90°) to out-of plane (0°) of V[TCNE]x~2 deposited on a patterned substrate. b) Comparison of V[TCNE]x~2 nanowire center field values plotted on left axis and V[TCNE]x~2 film center field values plotted on the right axis.

If the introduction of anisotropy is due solely to shape anisotropy from the formation of

1 2 nanowires, then we should find that 퐻 ≤ 퐻 , but we observe 퐻 = 퐻 , which 푖푝 2 푒푓푓 푖푝 3 푒푓푓 suggests that shape anisotropy alone cannot account for the introduction of in-plane anisotropy. Additionally, if the in-plane anisotropy is due to shape effects, we would expect it to vary with growth parameters such as wire thickness and width.

3.2.3 Independence of anisotropy on growth parameters

Section 3.2.1 showed how the growth morphology changes for films of thicker than a micron, with a mostly re-coalesced film, but a comparison between the in-plane FMR 68 measurements for the thick film and a very thin film of ~50 nm, as seen in figure 3.15, showed that the anisotropy was independent of the V[TCNE]x~2 thickness.

Figure 3.15 In-plane FMR rotation center field values for samples of thin (~50 nm) and thick (~1.4 μm) films.

In addition to the thickness of the film, the spacing and periodicity of the SiO2 pattern was also explored. Figure 3.16 shows the center field values plotted as function of angle for two samples from the same growth: one with a period 350 nm and trench width of

170 nm, and the other with a period of 250 nm and trench width of 150 nm. Again both samples showed the same anisotropy, independent of the pattern of the SiO2 and therefore the resulting V[TCNE]x~2 nanowires.

Figure 3.16 In-plane FMR rotation center field values for samples with patterned substrates of a period of 350 nm/trench width of 170 nm (filled squares) and of a period of 250 nm/trench width of 150 nm (open triangles) from the same deposition.

While there appeared to be some sample-to-sample variation in the center field values, this did not correlate with any of the variables explored and was consistent with variation seen in V[TCNE]x~2 growths on sapphire substrates. The independence of the anisotropy on the thickness and groove spacing further suggests that shape anisotropy by itself is not responsible for the observed effect.

At this time, there are two possible candidates for the origin of anisotropy. One possibility is that there is an oxide layer that forms on the top of the V[TCNE]x~2 and experiences an exchange interaction with the film underneath. In thin films, this would only be observed in an in-plane to out-of-plane rotation. However, the formation of wires could wrap that oxide layer around to create a core-shell nanowire with an oxidized shell.

In this case this exchange interaction is no longer isotropic in-plane.

The other possibility is that the presence of the underlying structure is leading to ordering within the material. This has been observed with organic-based systems, where a patterned substrate can align what are normally disordered polymer chains67, 68.

3.2.4 Anisotropy on improperly patterned samples

It is interesting to note that samples where the SiO2 was not properly patterned, as shown in figure 3.17a, V[TCNE]x~2 still exhibited magnetic anisotropy as shown in figure 3.17b.

This magnetic anisotropy was on the same order as that observed for samples with fully developed nanowires. This points more towards the theory of structural ordering in the

V[TCNE]x~2, but additional studies are required to confirm structural changes.

Figure 3.17 a) SEM image of V[TCNE]x~2 grown on a substrate that was improperly etched. b) In-plane FMR rotation center field values for sample shown in (a).

3.3 Conclusions

Some of the properties of V[TCNE]x~2, including TC and magnetic anisotropy, can be controlled by engineering the material through chemical tuning and by structural modification. Materials with different TC values and low temperature magnetic transition temperature can be created through the replacement of a cyano group on the TCNE molecules. Additional magnetic anisotropy can be controlled by depositing V[TCNE]x~2 onto substrates patterned with periodic nanoscale features. The ability to control properties advances the possible applications of V[TCNE]x~2 in heterostructures and devices.

Chapter 4: Encapsulation of V[TCNE]x~2 using UV-cured epoxy

One of the major challenges to incorporating V[TCNE]x~2 into devices is the material’s sensitivity to ambient conditions. While CVD grown films of V[TCNE]x~2 do not combust in air like the solvent based powders22 the material’s properties degrade rapidly.

This degradation can be observed in several ways: visually the opaque black films become transparent, the electrical resistance increases to the point where it can no longer be measured, and the saturation magnetization decreases to zero. While materials advances may make the material more air stable,35 devices that take advantage of the properties of CVD grown V[TCNE]x~2 still require some type of encapsulation of the films.

4.1 Previous methods of encapsulation

4.1.1 Customized airtight containers

One method utilized for measuring devices containing films of V[TCNE]x~2 as well as bulk properties is to seal the entire sample in a customized airtight container. This allows the devices to be loaded inside the glovebox where they are grown and transported to the measurement tool. The drawback to this method is that it requires a different customized container for each measurement tool and also limits ex situ experimental probes.

One of the simplest methods for sealing samples is used for cavity FMR measurements,

SQUID magnetometry measurements in the MPMS, and AC susceptibility measurements performed in the PPMS. This is done by placing the sample in an electron paramagnetic resonance (EPR) grade quartz tube (available from Wilmad Labglass). The sample can be simply dropped inside the tube, but for measurements where the orientation is important and/or measurements such as those in the MPMS which move the tube up and down it is best to mount the sample to something to keep it in place inside the tube. The type of mount depends on the measurement being performed. For cavity FMR measurements, limiting the material inside the cavity which will lead to microwave loss is the most important factor. Therefore a small mount made of a material such as Macor® works well. For magnetometry measurements, discontinuities in short mounts can produce a voltage signal as measured by the MPMS when it passes through the coils. Therefore a long, uniform mount which is longer than the sample transport length is important. A long thin piece of quartz plate works well. The tube is then evacuated using a sealing manifold. Finally the tube is sealed using a blow torch to pinch off the tube.

While sealing samples inside a quartz tube works well for bulk measurements, it does not allow for electrical feedthroughs. In that case a more customized mount maybe required to encapsulate the sample. One of the first highly sophisticated custom mounts was developed for electro-optical measurements inside a cryostat show in figure 4.1.

Figure 4.1 a) A Solidworks model of the air-free sample mount designed for optical and electrical measurements in a cryostat. b) A photograph of the components of the air-free sample mount: the copper sample mount, aluminum can and cap. c) A photograph of a mounted sample inside assembled the air-free sample mount. The copper mount, sample, and strips of Kapton tape securing the sample are visible through the window69.

This measurement (which is discussed in more detail in chapter 5) required both optical and electrical access to the sample. However this mount is limited to use in an Oxford cryostat. To perform transport measurements in the PPMS, a modified version of the

PPMS puck which matches up with pins located in the PPMS is needed.

Figure 4.2 a) Side view and b) top view of airtight sample mount which fits in a Quantum Design PPMS.

Multiple iterations were required to create something that was both compatible with the

PPMS as well as airtight. There are many additional factors to consider including the difficulty of wiring inside a glovebox and heat transfer to the sample for temperature dependent measurements. For this mount, wires with indium solder on the ends provide contact to the sample. An indium seal is placed inside the middle part, and the top piece compresses the indium.

Designing a custom mount for each measurement tool is not always possible or practical, however. Direct protection of the film via a capping layer would allow for ease of measurements in different tools.

4.1.2 Parylene capping layers

Previously, attempts to encapsulate V[TCNE]x~2 used the material parylene to cover films grown by CVD70. Pokhodyna et al. found that they can extend the lifetime of

V[TCNE]x~2 films as measured by magnetic and electrical measurements from ~1 hour to

7 hours with a Parylene C coating. They could further extend the lifetime to 14 hours by overcoating the Parylene with a layer of gold. However, this method did not extend the lifetime by more than one day, likely due to the presence of pinholes in the parylene coating. Additionally, the process of coating V[TCNE]x~2 films requires some heating of the films which degrades their magnetic and electrical properties. Therefore, this method requires a trade-off between lifetime in air and optimized film properties.

4.2 Encapsulation with epoxy

The challenge of encapsulating organic-based materials to protect them from water and oxygen is not limited to organic-based magnets. As such there are many people working to address this problem, especially in the field of organic light emitting diodes (OLEDs) which are currently being used in commercial devices71-73. Building off the existing work in the OLED field, a commercially available epoxy can be used to encapsulate

V[TCNE]x~2. The UV-cured epoxy developed by Osilla, Inc. (E131) for use with OLEDs and organic photovoltaic (OPV) devices has been shown to be inert when involved in light emitting and solar cell organic devices; however, its effects on magnetic materials had not been explored.

Figure 4.3 A schematic showing the epoxy process.

The epoxy was drop cast from a syringe or pipette onto V[TCNE]x~2 films deposited on any substrate. A glass cover slide was pressed firmly on top of the sample to spread the epoxy across the sample surface. This provided a barrier to vertical diffusion of atmospheric oxygen as well as mechanically protecting the film. Curing was achieved with a white LED lamp for at least 1 hour. For a faster cure, a UV light box can be used.

This recommended process provided the most robust encapsulation, but for measurements which require access to the films, other methods can be used. For Fourier- transform infrared spectroscopy (FTIR) the cover slide can be excluded, leaving just the cured epoxy on top of the sample. Additionally, the epoxy can be used to seal only the edges around a sample and cover slide that are sandwiched together, leaving the

V[TCNE]x~2 in the middle for measurements such as neutron scattering. Several

78 characterizations are necessary to determine if the epoxy effectively preserves the magnetic properties.

4.2.1 Visual test of epoxy effectiveness

The first and simplest test of the epoxy’s effectiveness is visual inspection. Post growth films are generally black or purple in color and opaque. If a film is exposed to ambient conditions, it changes from opaque to transparent in about 1 hour.

Figure 4.4 Top row: A bare V[TCNE]x~2 thin film on a 1 cm by 1.5cm glass substrate, photographed on the day of growth (left) and after being exposed to air for 26 h (middle) and 670 h (right). Bottom row: An encapsulated V[TCNE]x~2 thin-film from the same growth, photographed on the same day as the unprotected film.

In comparison to the unprotected films which become transparent, the encapsulated films remain dark and opaque. After 30 days, there appeared to be some degradation around the edges which was likely from the slow diffusion of oxygen. Using ImageJ analysis software, the films were still 98% opaque after 26 hours and 92% opaque after 670 hours.

While this suggests that the epoxy is able to protect V[TCNE]x~2 films, further metrics are required to determine if the epoxy reacts with the films and whether it preserves the magnetic properties.

4.2.2 Interaction between epoxy and V[TCNE]x~2 films

To determine if the epoxy material interacted with the V[TCNE]x~2 films and affected its properties, FTIR spectroscopy and SQUID magnetometry measurements were performed.

V[TCNE]x~2 films were deposited on double-side polished Si (100) wafers for FTIR measurements. Both a control sample, and sample which has layer of epoxy, but no top cover were transferred in airtight containers to the FTIR spectrometer, which allowed for a direct measurement of the encapsulation process unaffected by degradation due to air exposure. A comparison of the two samples revealed similar spectra. The bare film had

-1 the expected absorption of the C≡N stretches νCN at 2217, 2194, and 2158 cm , which indicated strong bonding between the cyano group and the vanadium ion36. The encapsulated film had absorption lines within experimental resolution at values of 2214,

2194, and 2155 cm-1. The evidence from that measurement suggested there is no significant chemical interaction between the epoxy and the bulk film.

Figure 4.5 Top: FTIR results for a thin film of V[TCNE]x~2. Bottom: FTIR results for an encapsulated film of V[TCNE]x~2 with the backgrounds of the epoxy and silicon substrate subtracted.

In addition to exploring the chemical interaction with FTIR, the magnetic interaction between V[TCNE]x~2 films and the epoxy was explored through SQUID magnetometry.

For this measurement V[TCNE]x~2 films were deposited onto glass substrates. Both the bare control sample of V[TCNE]x~2 on glass and an epoxied sample were sealed in quartz tubes as described in section 4.1.1 and loaded into the SQUID magnetometer and measured sequentially. The magnetization as a function of applied magnetic field and temperature were compared for both samples. The magnetization was normalized by the sample volume, which was estimated to within 20% based on variations in sample thickness.

Comparing the magnetization versus temperature for the two films which were measured at an applied field of 100 Oe, showed that the epoxied film had a higher magnetization at all temperatures. It may be higher due to a difference in thickness between the two films, but it certainly supports the conclusion that epoxy does not degrade the film.

Figure 4.6 Temperature dependence of magnetization at 100 Oe applied field on bare V[TCNE]x~2 film in red and an epoxied film in blue. The inset shows a linear extrapolation on T3/2 scale to extract and estimated Curie temperature.

The Bloch law fit to the data as described in section 1.4.2 gives an extracted Curie temperature (TC) of 550 K for the bare film and 590 K for the epoxied film, which indicates the magnetic properties of the film are preserved in the presence of the epoxy.

Further, the magnetization versus field shows hysteresis loops for the two films to have similar coercivities and saturation magnetizations.

Figure 4.7 Room-temperature magnetic hysteresis curves of the same samples between ±200 Oe, normalized by remanent magnetization. Inset: the magnetic moments for those samples over the entire ±7T field sweep, normalized by sample volume.

A comparison of the hysteresis loops normalized by remanent magnetization in the range of ±200 Oe shows that the epoxy did not affect the coercivity of the film. The hysteresis loop of the encapsulated film was slightly more rounded which could either be due to film variation or be evidence of less uniform magnetization. Based on the small differences between V[TCNE]x~2 films with and without encapsulation epoxy it can be concluded that the encapsulation did not affect the bulk magnetic properties.

Both FTIR and SQUID comparisons show that the bulk films are unaffected by the presence of encapsulation epoxy, however these measurements are not sensitive to the interface between the film and the epoxy. Therefore, interactions at the interface cannot be ruled out. While future measurements which can access the interface can provide more conclusive evidence for interface interactions, the fact that both the coercivity and remanence, which can be sensitive to surface effects, are unaffected provides evidence

83 that any effects that may happen at the surface have minimal impact on the overall magnetic properties of the film.

4.2.3 Time dependence of the encapsulation process

Given the fact that the epoxy does not interact in a negative way with V[TCNE]x~2 films and has been shown to visually preserve films, the question remains on what time scale it the magnetic properties of the films are protected. This was determined by measuring the film magnetization as a function of temperature and field at several intervals over a 30 day period.

The temperature dependence of the magnetization is shown in figure 4.8 for an encapsulated sample at time periods of 0.25, 24, 180, 340, and 710 hours of exposure to ambient atmosphere. The sample was kept at room temperature between measurements.

For comparison, a bare film, which was sealed in vacuum and stored at room temperature and is shown in the inset, provides a baseline for the effects due to sample aging in the absence of environmental exposure. The data show that the encapsulated V[TCNE]x~2 film still exhibited a magnetic signal after a month. In comparison an unprotected film no longer showed a magnetic signal after 2 hours.

Figure 4.8 Temperature dependence of the magnetization at 100 Oe applied field for various time scales. Inset shows results for a bare film sealed in vacuum.

In addition to the temperature dependence, field dependence for the same time periods is shown in figure 4.9. The coercivity of the encapsulated film did not change until 340 hours (14 days), and the full applied field shown in figure 4.9b shows that the saturation magnetization remains unchanged until between 200 and 300 hours (8 to 12 days).

Figure 4.9 a) Field dependence of the magnetization at 300 K for various time scales. Inset shows results for a bare film sealed in vacuum. b) The same field sweeps from Fig. 4.9a shown over the entire region of ±7 T.

The time dependent trends for the Curie temperature, saturation magnetization, and remanence are shown in figure 4.10. The remanence dropped off immediately, whereas the other two parameters remained relatively stable for a period of time. The unchanged saturation magnetization suggests the total number of free spins was preserved initially, but the decrease in remanence suggests that there is a time-dependent decay which occurs exponentially with a decay time of 0.35 hours where the film transitions from long-range ferrimagnetic ordering to shorter-range sperimagnetic ordering. This change in magnetic behavior detected by the remanence could have been due to a decrease in either the number or mobility of free carriers. There was a second stage of decay where after 340 hours (14 days), there was a rapid decrease in saturation and TC. This decay, likely slow diffusion of oxygen into the epoxy around the edges, lead to a structural degradation, decreasing the total number of spins. This time dependence showed that the long-range

86 magnetic ordering was more sensitive to degradation than the short range ordering, which provides an additional challenge in encapsulating these materials.

Figure 4.10 Three magnetic parameters on an encapsulated sample: normalized saturation magnetization, normalized remanence plotted on the left axis and extrapolated Curie temperature plotted on right axis all with respect to time on a log scale.

4.3 Conclusions

In summary, exposure to ambient conditions degrades thin films of V[TCNE]x~2 leading to a decay and eventual disappearance of magnetic properties over a period of hours for unprotected films. This decay can be slowed through sample encapsulation. Physical encapsulation provides a robust method of protection, but requires customized containers for each type of measurement. A protective coating on top of the film can be a more flexible option. The most successful encapsulation method discovered thus far is to use

UV-cured epoxy which preserves the magnetic signal for a month, significantly longer than previous methods which were limited to less than 1 day. Further advances may be

87 able to additionally extend the lifetime of V[TCNE]x~2 films, but this work represents significant advancement and progress towards incorporation of organic-based magnets in devices.

Chapter 5 Applications and future directions for organic-based

magnets

The success of organic electronics points toward the potential for similar applications that incorporate organic-based magnets. This chapter discusses early spintronic devices utilizing the organic-based magnet, V[TCNE]x~2. Additionally, a comparison of the properties of V[TCNE]x~2 with the widely used magnetic material, yttrium iron garnet

(YIG) reveal the possibility of high frequency applications incorporating organics. The final concluding section highlights the progress of organic-based magnets and discusses some of the goals that could help make organic-based magnets as successful as other organic systems in commercial applications.

5.1 Organic spintronics

Spintronics is a field which studies active control and manipulation of electron spin degrees of freedom. The field has experienced commercial success in magnetic memory applications since the discovery of the giant magnetoresistance effect74. In addition, spintronics is an area of active research with a goal of using spin to transport information as a potential solution to the heating effects and other problems that occur as electronics are made smaller and smaller. Organics are considered particularly attractive for 89 spintronics applications due to their low spin-orbit interactions75. Most of the focus on incorporating organic materials into spintronics structures has emphasized spin transport through an organic spacer layer. However, the physics in the organic spacer layer is complex and not fully understood. Effects such as the organic magnetoresistance effect

(OMAR), the lack of a Hanle signal (considered definitive proof of spin injection), and artifacts from sample degradation complicate the initial claims of spin injection76.

V[TCNE]x~2 provides the opportunity to use an organic material in a different role, as a spin injector. The first result showing spin injection using a V[TCNE]x~2 layer was in a hybrid tunnel junction30. The device is shown in figure 5.1a had a ferromagnetic layer of epitaxial La2/3Sr1/3MnO3 (LSMO) with a thickness of 80 nm. The spacer layer was comprised of 1.2 nm of LaAlO3 (LAO) and 5 nm of the small molecule rubrene, with a final top layer of V[TCNE]x~2 as the top ferromagnetic layer. Figure 5.1b shows the resulting magnetoresistance signal for 0.5 V applied bias at 100 K.

Figure 5.1 a) Schematic view of a hybrid tunnel junction of V[TCNE]x/Rubrene/LAO/LSMO. b) The magnetoresistance curves of the structure shown in a) measured at 100 K with a bias field of 0.5 V. The blue data was collected on an upsweep and the red data on a down sweep. The magnetization of V[TCNE]x (green) and LSMO (dotted line) measured with SQUID magnetometry is also shown30.

Since this first result using a V[TCNE]x~2 film as a ferromagnetic layer, additional studies

31 have shown a spin valve with a V[TCNE]x~2 layer which works at room temperature and an all-organic spin valve with two forms of V[TCNE]x~2 with different coercivities as both of the ferromagnetic layers32. In addition to showing spin injection from an organic magnet into an organic spacer layer, V[TCNE]x~2 has also been used to inject spins into an inorganic structure.

A spin-resolved light emitting diode (spin-LED) is a structure which can be used to detect spin injection. Spin polarized carriers are extracted from a polarized source and injected into a quantum well. V[TCNE]x~2 was the polarized source and carriers were injected into an n-doped layer of AlGaAs/GaAs quantum well. These polarized spins recombined in the quantum well to emit polarized photons with circular polarization. The polarization 91 was analyzed as emission from heavy holes or light holes. Figure 5.2 shows that the electroluminescence tracked the magnetization of the V[TCNE]x~2 (shown in green)

33 demonstrating that the spins come from the V[TCNE]x~2 layer .

Figure 5.2 Polarization response for a spin-LED with a V[TCNE]x~2 magnetic layer (magnetization shown in green). The red triangles show the response from heavy holes while the blue show the response from light holes33.

This is the first evidence of direct injection of spin from an organic-based magnet. While comparisons with control evidence prove this is a spin signal is conclusively from

V[TCNE]x~2 the signal is significantly smaller than is expected for the fully spin- polarized material suggesting further progress may be made through interface engineering. These early results show the strong potential for incorporating V[TCNE]x~2 into various spintronics devices.

5.2 High-frequency applications

Signal processing devices are widely used today for military and commercial applications for radar detection, communications, and instrumentation. The devices which use magnetic materials for these applications include circulators, isolators, phase shifters, and patch antennas. Microwave planar devices require a magnetic material with low coercivity, high remanence, and a square hysteresis loop. Most of these devices use soft ferrites as the magnetic material. The most widely used material is yttrium iron garnet

(Y3Fe5O12, YIG). Single-crystal YIG possesses the narrowest FMR linewidth of all materials, and thus the smallest losses, ΔH≈0.1 G at 10 GHz77. Thin films of YIG have a

78 77 linewidth 1.1 G and an 4πMs of 1750 G . The Curie temperature is dependent on the film thickness, but ranges from 400-600 K79.

A comparison of V[TCNE]x~2 with YIG reveals a similar Curie temperature and FMR linewidth, but a smaller saturation magnetization. While V[TCNE]x~2 is unlikely to overtake YIG which has been well-established in microwave devices, it offers advantages that can allow for the creation of new types of devices. Narrow-linewidth YIG has to be grown on lattice matched substrates of gadolinium gallium garnet, GGG, at high

78 temperatures over 600 K in order to produce narrow linewidths. V[TCNE]x~2, on the other hand, can be deposited on a variety of substrates conformally at temperatures of

>60 °C. This opens the possibility of depositing V[TCNE]x~2 on pre-patterned circuits and/or flexible substrates.

5.3 Looking to the future

Even though organic-based magnets are an earlier stage of development compared with other organic materials used for applications, the progress which has been made in these materials suggests the potential for widespread applications that incorporate organic- based magnets. Many challenges for incorporation of V[TCNE]x~2 films in commercial applications still exist. However, significant progress has been made to address these issues including advancements in the CVD growth process to create higher quality films and a better understanding of the growth parameter relationship to film characteristics.

Additionally, the challenge of working with air sensitive materials that degrade in relatively short lifetimes is also faced by other facets of the organic electronics industry.

Therefore organic magnets can build off the work being done to address these issues which has already proved fruitful by using an epoxy developed for OLEDs to protect

V[TCNE]x~2 films. Finally the success to date in engineering V[TCNE]x~2 has created new materials and new morphologies which introduce new properties, such as in-plane anisotropy, which expands the potential applications for these materials. Additionally as the library of organic-based materials and structures expands, new applications can incorporate more than one of these different materials for further understanding and advancements.

However there is still significant progress to be made. While many advances in organic- based magnets may not be predictable, some of the goals for this materials system include: magnetic ordering in atmospheric conditions for lifetimes extending to months

94 or years, the discovery of additional organic magnetic materials that work at room temperature, demonstrated success incorporating V[TCNE]x~2 into a planar microwave device, and transfer of magnetic nanowires to other structures. Given the rapid expansion of electronics and communications in everyday life across the world-organic-based magnets are poised to become an active player in these new technologies.

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