Modulation of Magneto-Optical Properties of Metallic Nanostructures by External Stimuli

by Abid Siddique

B.Sc. in Electrical Engineering, May 2007, UET Peshawar, Pakistan M.S. in Electrical Engineering, May 2010, The George Washington University

A Dissertation submitted to

The Faculty of The School of Engineering and Applied Science of The George Washington University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

January 19, 2018

Dissertation directed by

Edward Della Torre Professor of Engineering and Applied Science

Lawrence H. Bennett Research Professor of Engineering and Applied Science

The School of Engineering and Applied Science of The George Washington University certifies that Abid Siddique has passed the Final Examination for the degree of Doctor of Philosophy as of December 19th, 2017. This is the final and approved form of the dissertation.

Modulation of Magneto-Optical Properties of Metallic Nanostructures by External Stimuli

Abid Siddique

Dissertation Research Committee:

Edward Della Torre, IEEE Life Fellow and Professor of Engineering and Applied Science, Dissertation Co-Director

Lawrence H. Bennett, Fellow of the American Physical Society and Research Professor of Engineering and Applied Science, Dissertation Co-Director

Robert J. Harrington, Fellow of IEEE and Professor of Engineering and Applied Science, Committee Member

Shahrokh Ahmadi, Professor of Engineering and Applied Science, Committee Member

Dr. Chidubem A. Nwokoye, Electrical Engineer, Naval Surface Warfare Center, Carderock Division, Committee Member

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Dedication

I dedicate my dissertation to my beloved parents, whose love, prayers, sacrifices, and never-ending inspiration for hard work remained a source of guidance throughout my life.

My brothers Atif Siddique, Waqas Siddique, Bilal Siddique, Aaqib Siddique and my lovely sister deserve my special appreciation for their unconditional support, love and encouragement.

I also dedicate my dissertation to my wife. Without her support, I couldn’t make this. Ayaan, Ziml, and Haniya—my beautiful kids—made my life interesting, full of happiness and gave me strength and purpose to walk through all the challenges during my research.

Finally, I would also like to dedicate my work to my late grandparents and to my dearest late cousin, Abdullah Imran, who left us so early.

Acknowledgments

First of all, I would like to thank Almighty Allah, the most Beneficent and the most

Merciful, for blessing me the knowledge, perseverance, intellect, love and strength to make this work possible.

I am highly indebted to my PhD advisors and mentors, Professors Edward Della

Torre and Lawrence H. Bennett for their persistent support, guidance, and encouragement throughout my research. Their professionalism and immense dedication to the work will always remain a source of inspiration for me.

I would also like to thank my dissertation committee members, Professor Robert

Harrington and Professor Shahrokh Ahmadi for their participation and input in my dissertation.

Dr. Chidubem A. Nwokoye, a colleague and member of my dissertation committee, deserves my special thanks and appreciation. We spent countless hours together in the wonderful MOKE lab and shared so many moments of success and frustration.

I would also like to thank all my fellow members at the Institute for Magnetics

Research, including Khurram Khattak, Hatem Elbidweihy, Amir Aslani, Mohamadreza

Ghahremani, and Ali Jamali for their invaluable help and discussions throughout research.

Last but not the least, I want to express my sincere gratitude to Higher Education

Commission of Pakistan (HEC), for putting their trust in me and providing an opportunity to achieve my education in USA.

Abstract of Dissertation

Modulation of Magneto-Optical Properties of Metallic Nanostructures by External Stimuli

Over the last few decades, with the advancement of high tech fabrication of devices, many new phenomena are observed that were not possible in bulk materials. The nanostructures like thin films are subject to extensive research with many applications in sight. One of such applications is non-volatile memory devices with high areal density and low power consumption. The irreversible tailoring of the mechanical, or electronic properties of nanostructures has been carried out previously, [KUM03], [VAL00], [TRI01], however, the reversible and dynamic control of the intrinsic properties like the magnetic are shown recently [WEI07]. These modifications are however limited by the thickness of the thin films used. The charge neutrality is disturbed to induce the reversible changes and is affected by the screening length. Here, in this research, we tried to explore the effects of thickness on the behavior of critical parameters like coercivity, saturation magnetization, squareness etc. of magnetic nanostructures. These field-induced variations are an alternative to the spin current-induced changes, which are currently employed for the reverse the magnetization in the memory devices.

Secondly, the quantum effects are significant in the nanomaterials and require deeper understanding. To explore the quantum of behaviour of magnons confined in the intermetallic nanostructures like CoPd, not too much has been done. Though, some theoretical aspects of magnons entanglement has been presented, [MOR05], yet experimental evidences are yet to be realized. Although, the spin-photon entanglement is actively researched in many semiconducting systems like quantum dots (QDs), yet the

magnon-photon entanglement in metallic systems is yet an area to be explored. We here discover the magnon-photon entanglement.

Thus, in a nutshell, the purpose of this research is as below

• To explore the dynamic and reversible control of magnetic properties of metallic

nanostructures like CoPd based on the thickness.

• To explore the quantum entanglement of magnons in metallic thin films under BEC

temperatures.

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Table of Contents

Dedication ...... iv

Acknowledgements ...... v

Abstract of Dissertation ...... vi

Table of Contents ...... viii

List of Figures ...... xi

List of Tables ...... xv

List of Symbols ...... xvi

Chapter 1. Introduction ...... 1

1.1 Motivation ...... 1

1.2 Scope ...... 2

1.3 Objectives ...... 2

1.4 Organization of Dissertation ...... 3

Chapter 2. Background ...... 4

2.1 Origin of Magnetization ...... 4

2.1.1 Ferromagnetism ...... 5

2.1.2 Antiferromagnetism ...... 6

2.1.3 Ferrimagnetism ……… ...... …..………………………...... 7

2.1.4 Paramagnetism ………………… .. …….………………………….. 7

2.1.5 Diamagnetism ……………………… ...... ……...... 8

2.1.6 Superparmagnetism ……………… .. ……………………………… 9

2.2 Electrochemistry ...... 10

2.2.1 The electrical double layer ……… ………………………………. 10

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2.2.2 Electrolytes ……………………………… ...... ……….11

2.2.3 Electrodes …………………… ...... ……………………………11

2.3 Entanglement ...... 11

2.4 Types of States ...... 12

2.4.1 Pure States…………………………… ...... …………………....12

2.4.2 Mixed States…………… ...... ………………………………….12

2.5 Quantum Decoherence ...... 12

Chapter 3. Experimental Methods ...... 14

3.1 MOKE Magnetometer ...... 14

3.2 Configurations of Moke ...... 16

3.3 Experimental Setup of MOKE System ...... 17

3.4 Experimental Procedure ……… ...... ……………………………………22

3.5 Vector Vibrating Sample Magnetometer(v-VSM).…… ...... ……………23

3.6 Scanning Electron Microscope (SEM)…………… ...... …………………24

3.7 Rutherford Backscattering Spectrometer (RBS)… ...... …..……………...26

3.8 X-Ray Reflectometry (XRR)…… ...... …………………………………...27

Chapter 4. Electrical Modulation of Magneto-Optical Properties ...... 28

4.1 Introduction ……… ...... ………..……………………………………….28

4.2 Experimental Setup ………… ...... ………………………………………30

4.3 Cyclical voltammetry … ...... …………………………………………….32

4.4 Experimental Procedure ………… ...... ………………………………….33

4.5 Electric-field control response in CoPt3 ……… ...... ……………………34

4.5.1 Results and Discussion …… ...... ………………………………36

4.6 Electric Field Control Response for Co/Pd …… .... …………………………38

4.6.1 Results and Discussion …… ... ……………………………………41

4.7 Conclusions and Future Work …………… ... ………………………………42

Chapter 5. Optical Modulation of Magneto-Optical Properties ...... 44

5.1 Introduction ……… .... ………………………………………………………44

5.2 Some Interpretations of Quantum Mechanics...... 45

5.2.1 Standard Interpretaion/ Copenhagen Interpretation...... 45

5.2.1 DeBroglie-Bohm Interpretation...... 45

5.2.3 Everett’s Interpretation/ Many-Worlds Interpretation (MWI)...... 46

5.3 Recent trends in Quantum Entanglement...... 46

5.3.1 Photon Entanglement: Creation Techniques and Applications...... 46

5.3.2 Entanglement in Atomic Ensembles...... 50

5.3.2.1 Ion Traps...... 50

5.3.2.2 Bose-Einstein Condensates...... 51

5.3.2.3 Magnons...... 52

5.4 Experiment...... 53

5.5 Results and Discussions...... 56

5.6 Conclusions...... 63

Chapter 6. Conclusions and Future Work ...... 64

List of Publications ...... 67

References ...... 69

List of Figures

Figure 2.1 Hysteresis loop for ferromagnets ...... 6

Figure 2.2 Model of the electric double layer ………… ... ………………………………10

Figure 3.1 Configurations of Moke. (a) shows Polar Moke, (b) depicts Longitudinal

Moke, and (c) shows Transverse Moke ………………………… ………………………17

Figure 3.2 Schematic illustration of the Moke setup (in polar geometry) … ...... ……….18

Figure 3.3 Moke magnetometer located in a completely dark room in moke laboratory at the institute for magnetics research, George Washington University, Ashburn campus ...... …………19

Figure 3.4 Multiple hysteresis loops of Polar kerr rotation for Co/Pt multilayer film

Performed at room temperature at Moke lab in GWU………………… . ……………….22

Figure 3.5 Vector Vibrating Sample Magnetometer [IMR13] …… ...... 24

Figure 3.6 Raith PIONEER two SEM at GW Nanofabrication and Imaging Center

[GW16] ...... …25

Figure 3.7 SEM layout [SEM17] ………………………………… ...... ………………26

Figure 4.1 Schematic illustration (top view) of the in-situ experimental setup that consists of a Moke apparatus, a potentiostat and a three-electrode electrochemical cell. [SHU12] ...... ….31

Figure 4.2 Electrochemical Cell with sample holder and Teflon Cap .. ………………….31

Figure 4.3 Cyclic Voltammetry plot for voltage sweep between +1.3v and -0.9v for

CoPt3 ...... ….33

Figure 4.4 Cyclic voltammetry plot for voltage sweep between +1 v and -1v for

Co/Pd...... 33

Figure 4.5 Image of CoPt3 sample ………………………………… ...... ……………….34

Figure 4.6 SEM image of Co/Pt showing the surface of sample…… ... …………………35

Figure 4.7 Rutherford Backscattering (RBS) spectrum of CoPt3 …...... 35

Figure 4.8 X-Ray Reflectivity (XRR) of CoPt3……………………… ..... ………………36

Figure 4.9 Polar Kerr hysteresis loops at 300 K under gate voltages 0V, −0.3V, −0.5V, and −0.8V [SHU12] …………………………………… ...... ……………………………37

Figure 4.10 Change in coercivity as a function of gate voltages. [SHU12]… . ………….38

Figure 4.11 SEM image of surface of CoPd sample……………………… .. ……………39

Figure 4.12 Normalized hysteresis loops (Kerr Rotation vs Applied field) under

Various biased voltages…………… . ……………………………………………………40

Figure 4.13 Shows the zoomed lines of hysteresis loops under different bias voltages near zero remanence. It clarifies the change in coercivities due to gate voltages .... ……40

Figure 4.14 Coercive field as a function of gate voltage Vg for CoPd. Triangles show the coercivity at various applied voltages ……...... 41

Figure 5.1 An SPDC scheme with Type II output [ZEI10] …… . ……………………….47

Figure 5.2 Quantum teleportation between the Canary Islands La Palma and Tenerife over both quantum and classical 143-km free-space channels. A, experimental scheme. B, set-up. In La Palma, a frequency-uncorrelated polarization-entangled.

[XIA12] ……… ...... …………….49

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Figure 5.3 Illustration of Experimental Setup [LIA17] ………………… ………………50

Figure 5.4 A linear Paul trap [BLA08] ………………………………… ……………….51 Figure 5.5 Picture of two Co/Pd samples cut from same sample and placed 3.5mm

apart, edge to edge ………………………………………… .. ………………………….55

Figure 5.6 Block diagram of experimental setup. L1 & L2 are laser sources, P1 is a linear polarizer, P2 is a photoelastic modulator (PEM) crystal, E is an electromagnet,

M1-3 are mirrors, A is an analyzer, V is vacuum cryostat, C is variable frequency chopper, and D is a laser diode detector …………… ...... ……………………………….55

Figure 5.7 Kerr rotation signal time responses at coercivity field (≈970 Oe). A1 represents measurement without external modulated laser (baseline). A2 represents measurement with external laser with modulation frequency of 1 kHz. A3 represents measurement with external laser with modulation frequency of 2 kHz…… ……………57

Figure 5.8 Kerr rotation major hysteresis loop with and without external laser modulation frequencies at room temperature………………… ...... …………………….58

Figure 5.9: Kerr rotation hysteresis major loops. D1 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 60 K. D2 depicts when laser

L2 is switched on and modulated laser beam is incident on sample S1 at 60 K. D3 depicts when laser L2 is switched off at 60K………………………………………… .... 60

Figure 5.10 Kerr rotation hysteresis major loops. D4 depicts when laser L2 is

xiii

Switched on and modulated laser beam is incident on sample S2 at 9.5 K. D5 depicts when laser L2 is switched on and modulated laser beam is incident on sample S1 at 9.5 K. D6 depicts when laser L2 is switched off at 9.5 K ...... 61

Figure 5.11: Normalized Remanent Kerr rotation against temperature. D1 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 9.5 K. D2 shows when laser L2 is switched on and modulated laser beam is incident on sample S1 at 9.5 K. D3 represents when laser L2 is switched off at 9.5 K.

D4 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 60 K. D5 shows when laser L2 is switched on and modulated laser beam is incident on sample S1 at 60 K. D6 represents when laser L2 is switched off at 60 K ...... ……….62

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List of Tables

Table 4.1 Percent change in coercivity of CoPd with applied gate voltages ……………42

List of Symbols

BEC Bose-Einstein Condensation

Rb Rubidium

Cs Cesium

Co/Pd Cobalt Paladium multilayered thin film

Co/Pt Cobalt Platinum multilayered thin film

QD Quantum Dots

MOKE Magneto-Optical Kerr Effect

3d Elements with outer electrons in 3d orbital

4f Elements with outer electrons in 4f orbital

s Spin Magnetic moment

l Orbital magnetic moment gs g-factor due to spin of electron ms Spin quantum number

B Bohr Magneton h Planck’s constant me Mass of electron gl g-factor of angular moment

Tc Curie Temperature

Ms Saturation Magnetization

Hc Coercivity

Mr Residual Magnetization

TN Neel Temperature

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 Magnetic Susceptibility

YIG Yttrium Iron Garnet

BaFe Barium Ferrite

C Curie constant

N Neel Relaxation time

KB Boltzmann constant

LiClO4 Lithium perchlorate

Ag/AgCl Silver/Silver Chloride Reference electrode

CV Cyclical Voltammetry

↑ Spin up

↓ Spin down

 Bra Notation

 Ket Notation

H Hilbert Space

휌̂ Density Matrix

푝푖̇ Probability of ith state

VSM Vibrating sample Magnetometer

XRR X-Ray Reflectometry

RBS Rutherford Backscattering

SEM Scanning Electron Microscope

휀̃ Dielectric Tensor

k Kerr Rotation

k Kerr Ellipticity

xvii

AD Areal Density

SPDC Spontaneous Parametric Down Conversion

BBO Beta Barium Borate

KDP Potassium Dihydrogen Phosphate

xviii

Chapter 1: Introduction

1.1 Motivation

Enormous research in the field of nanotechnology over the last few years driven by both the fundamental and technological interests has opened the new realms in science.

New possibilities emerging from the transition of scientific focus from bulk to nanomaterials created new challenges in completely understanding the underlying principles and exploring full potential of future devices. The behavior of magnetic nanomaterials differs significantly from their bulk counterparts. One of the key applications of modern magnetic materials is in the memory devices. With growing demands of production and push for the miniaturization of devices, the researchers look for new areas to increase the areal density by exploring new storage media and creating novel and efficient read/write techniques. Spin-polarized currents are the major source of changing the bits in the domain walls of metallic nanostructures. However, the voltage control magnetization switching is an attractive alternative as this concept is more energy efficient.

The initial motivation was to explore the materials and techniques for creating the electrical-controlled magnetic switching.

Later, after working in the Magneto-Optical Kerr Effect (MOKE) lab with various magnetic materials at room and low temperatures enticed our group to explore the quantum entanglement aspects of the metallic nanostructures with magnetic properties. We already have shown the presence of Bose-Einstein condensation of magnons and developed direct and indirect measurement of BECs in magnons, the possibility of investigating the quantum entanglement in magnons experimentally gained the priority. The entanglement in Bose-

Einstein condensates is actively pursued in materials like Rubidium (Rb) and Cesium (Cs) etc., however in solid magnetic nanostructures, the entanglement of magnons has not been demonstrated experimentally yet. Theoretical aspects like the scenarios of bipartite and multipartite entanglement in magnons are discussed though. The spatial entanglement in magnons under their BEC temperatures was an exciting prospect that lead the second half of our research.

1.2 Scope

The scope of this research to investigate the magneto-optical properties of thin film ferromagnets (Co/Pt and Co/Pd) by applying external optical and electrical fields. The choice of the system and its synthesis is critical as it determines the expected outcomes.

Thin films with varying thicknesses enabled to systematically analyze the impacts of the external stimuli. The magnons-photons interaction in the MOKE magnetometer at room and low temperatures provides an insight into the magnon entanglement that augurs well for the quantum memory and quantum communication networks. The systemic analysis of interaction of entangled states of photons and magnons is though not in scope of this dissertation but its importance is significant.

1.3 Objectives

The objective of this dissertation is to explore the effect of electric field and optical field to the magnetic behavior of the materials. The thickness and anisotropy of the multilayered thin films determines the overall magnetic characteristics and the penetration of the surface charge or in other words, the screening length, affecting the charge neutrality of the 3d and 4f materials, thus causing the variation in the magnetic properties like,

coercivity, saturation magnetization, remanent magnetization etc. Also, the BEC of magnons is by nature the entangled state of matter spatially, though, the internal states may not be in the entanglement with each other. The experimental demonstration of entanglement found in the magnons under BEC is also a major objective of this dissertation.

1.4 Organization of Dissertation The organization of this dissertation proposal is as follows. Chapter 1 presents the motivation of the research. Chapter 2 describes the few concepts including the basic concepts of magnetism, electrochemistry, and the quantum mechanics. In third chapter, experimental equipment is explained. Moke magnetometer along with cryostat and Gamry potentiostat are the main apparatus used in the research. Chapter 4 presents the result of experiments involving the electrical modulation of the magnetic properties of thin films.

Electrical modulation was achieved by creating the electrochemical double layering across the surface of thin films. Next, in the 5th chapter, quantum entanglement in the thin films at cryogenic temperatures is explored. The system showed the entanglement of magnons confined in the Co/Pd multilayer thin films. Finally, in the last chapter, conclusions and the future experiments are proposed.

Chapter 2: Background

2.1 Origin of Magnetization

The origin of magnetization in the magnetic materials remained hidden from the physicists until the development of quantum mechanics in twentieth century. All the materials in the nature are made up of atoms and their corresponding motion of electrons is responsible for the magnetic description of those materials. The magnetic moment of the atom is due to the net magnetic moments of the electrons. The magnetic moments of electrons are associated with their angular momentum and spin. The spin magnetic moment is due to the intrinsic spin of the electron and is given by

s= gsmsB (2.1)

Where, gs is the g-factor of the spin of electron and has the value of 2, ms = 1/2 is the spin quantum number associated with that electron, and B is the Bohr magneton and is given by eħ/(2me). Here ħ= (h/2π) and h is the Planck’s constant.

The magnetic moment originating due to the orbital motion of the electron is given by

l= -gl m lB (2.2)

Where, gl is the g-factor of angular momentum, ml is the angular quantum number and B is Bohr magneton. The total magnetic moment an ith atom is the resultant of all the magnetic moments of the electrons present in it and is given by

푖 = ∑ⅈ̇  푖푠 + ∑ⅈ̇  푖푙 (2.3)

Magnetization can be classified into several different categories based on the net behaviour of the magnetic moments.

2.1.1 Ferromagnetism

Ferromagnetism is the spontaneous magnetization of the material due to the parallel alignment of the magnetic moments of the atoms inside a magnetic domain. The net alignment of magnetic moments inside the magnetic domain is in the same direction and with the application of an external magnetic field, all the domains align in the direction of field and maintain that magnetization even after the field is removed. This magnetization is temperature dependent and by increasing the temperature, the spins are thermally agitated and start to lose their order. They retain their order below the Curie Temperature

(Tc) and become disordered at temperatures above it. Iron, Nickel, Cobalt and many of their alloys, Heusler alloys are prominent ferromagnets.

Ferromagnets show strong hysteresis, which is the measure of magnetization versus the applied magnetic field and its plot called as hysteresis loop, tells various properties like, coercivity, saturation magnetization, squareness etc. When a magnetic field is applied, magnetization reaches its maximum value known as saturation magnetization (Ms) at saturation field and maintains its value even after increasing further field. When the field is reversed and reduced to zero, there still exist some magnetization known as remanent magnetization or residual magnetization (Mr). The magnetic field required to demagnetize it is called coercivity (Hc) and it determines the hard or soft nature of ferromagnet. This irreversibility of the magnetization is called as hysteresis and is measured by the plot between the magnetization versus the applied magnetic field also called as hysteresis loop.

The typical hysteresis loop for the ferromagnetic materials is shown in Figure. 2.1.

[MYE97]

Figure 2.1: Hysteresis loop for ferromagnets

2.1.2 Antiferromagnetism

In the antiferromagnetic materials, the spins in one plane are aligned in one direction but are antiparallel to the neighboring planes. This results in no spontaneous magnetization of the material at temperatures below the Neel temperature (TN) and above this temperature, the material becomes paramagnetic. The magnetic susceptibility for an antiferromagnet is maximum at TN and at lower temperature, the susceptibility depends upon the direction of the applied magnetic field. In the case of an applied field parallel to the magnetic moment, the susceptibility decreases from the maximum at TN to the zero at

T = 0K. If the applied field is perpendicular to the moments, the susceptibility will be independent of temperature below θN. and is given by =1/. In the paramagnetic region,

(T>TN), the magnetic susceptibility is given by = (C/ T+), where C refers to Curie

constant of specific sublattice. Chromium (Cr), Manganese oxide (MnO), and Iron manganese (FeMn) etc. are some typical antiferromagnetic materials.

2.1.3 Ferrimagnetism

Ferrimagnetism is the special case of antiferromagnetism, in which the spins in the two adjacent sublattices arrange in antiparallel direction but their net magnetic moments are unequal, thus resulting in some net magnetization. These materials show spontaneous magnetization and weak hysteresis. Above Curie temperature, TC, the magnetic order is lost and are phase transitioned to paramagnetic state. At lower temperatures, the magnetization of each sublattice varies differently and anyone can dominate the other at particular temperature, thus their susceptibilities do not follow Curie Weiss law [CUL09].

Examples include iron garnets like yttrium iron garnet (YIG) and ferrites like Barium ferrite

(BaFe).

2.1.4 Paramagnetism

Paramagnetism is found in the materials with their atoms having unpaired electrons in their electronic configuration and their dipole moments are randomly oriented and do not interact with each other magnetically. This result in no spontaneous magnetization but in the presence of an external magnetic field, the magnetic moments tend to align in the direction of magnetic field and the material gets magnetized showing hysteresis. However, once the field is removed, the magnetic order is lost. The magnetic susceptibility of paramagnetic materials is temperature-dependent as the alignment of the magnetic moments is affected by different temperatures. As the temperature is increased, the thermal

agitation will cause the susceptibility to decrease and the alignment is weakened. This behaviour is described by Curie Law and mathematically,

 = C/T (2.4)

Curie law is a special case of Curie-Weiss law. It integrates temperature constant (θ) describing the interaction between magnetic moments derived from Weiss theory for ferromagnetic materials.

 = C/(T-) (2.5)

In the above equation for Curie-Weiss law, θ may have either a zero, negative or positive value. For θ = 0, the material is paramagnetic and obeys Curie law. When θ is positive, the material is only paramagnetic above certain temperature called as Curie temperature (Tc) and the materials is ferromagnetic below it. When θ is negative, the material is paramagnetic above the transition temperature and is antiferromagnetic below it. This transition temperature for the materials is called as Neel temperature (TN) and the sublattices aligned themselves in antiparallel direction. This law is not universal; however, it becomes valid when the material is in its paramagnetic state. Some common examples paramagnetic materials are Platinum, Aluminum, Lithium etc.

2.1.5 Diamagnetism

Diamagnetism is inherently a part of all the materials. In the absence of an external magnetic field, the net magnetic moment of the atoms is zero as there are no unpaired electrons and no magnetic order is found here. However, in the presence of field, the orbital motion of the spinning electrons precessing at a particular frequency, creates an opposing but weak magnetic field resulting negative magnetization and negative susceptibility.

Diamagnetic contribution is always hidden in the materials but is corrected in the magnetic characterization of the materials. Copper, Mercury, Silver, Carbon, and water are some common diamagnetic materials. The superconductors are considered the perfect diamagnets with =-1.

2.1.6 Superparamagnetism

Superparamagnetism is a form of magnetism that arises in the weakly interacting ferromagnetic and ferromagnetic nanoparticles. In 1949, Neel [NEE49] discovered that at very small particle size, the magnetization of the particle can flip without the presence of any magnetic field due to thermal agitation. For the particle to retain its magnetization, it has to maintain its threshold energy level. However, due to thermal fluctuation, it gets enough energy to overcome its barrier and switch its magnetization. The time between two flips is called the Néel relaxation time, N and is given by,

N= 0 exp(KV/KBT) (2.6)

Where, KV is the energy barrier associated with the magnetization flip, KB is the

Boltzmann constant, and T is the temperature. 0 is the attempt time.

In the absence of an external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Néel relaxation time, their magnetization appears to be in average zero: they are said to be in the superparamagnetic state. In this state, an external magnetic field can magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than that of paramagnets.

2.2 Electrochemistry

2.2.1 The Electrical Double Layer

When a metallic material is submersed in an electrolyte, it forms a capacitive double layer at the surface. This metal/electrolyte interface is generally known as electrochemical double layer and charge is induced at the its surface was first proposed by Hermann

Helmholtz in 1879 [RIE87]. Figure 2.2 shows the interaction of these metal/ electrolyte interfaces. The accumulation and concentration of the ions at the surface and their interaction with their each layer and electrolyte is explained by various models like

Helmholtz model, Gouy-Chapman model [PLI08], and Stern model.

Figure 2.2 Model of electric double layer

2.2.2 Electrolytes

A liquid electrolyte can be defined as a conducting liquid medium usually with a large ionic and low electronic conductivity. The conductivity comes from dissolved conducting ions in an aqueous or non-aqueous solvent. Examples include Lithium perchlorate, ionic liquids etc. Another category of electrolytes is solid electrolytes, which is further classified into three subgroups, i.e., crystalline solid, glassy solid, and polymer electrolytes. Na-- alumina, Li3PO4-Li2S-SiS2, and LiPF6, LiBF4 are few of the examples of each respectively.

2.2.3 Electrodes

There are usually three kinds of electrode employed in an electrochemical cell. These include the working electrode, also known as indicator electrode, is the electrode under analysis. In our workstation, the ferromagnetic thin film is the working electrode. The second type of electrode is called counter electrode and it is usually much larger in size than working electrode and it prevents the passing of current through reference electrode which is the third type of electrode. The reference electrode is an electrode with known and stable potential. Silver/Silver Chloride reference electrode with electric potential (E=

+0.225 V) and standard calomel electrode with electric potential (E= +0.242 V) are commonly used reference electrodes.

2.3 Entanglement

Entanglement is the correlation between the distinct states of the physically separated systems. The phenomenon was first discovered by Einstein, Podolski, and Rosen, in 1935 [EIN35] and later significantly worked by Schrodinger [SCH35] and Bell [Bel64], has created enormous interest among the scientists over the last century with many exciting applications in the sight. The quantum states entangled in the system can be determined by

finding the correlation between the physical properties like spin, position, momentum, or polarization. For instance, in a bipartite system, if one of the entangled particles is spin up

↑, the other particle will be in the knowledge of its state. Thus, if that particle flips it state, the entangled particle will also flip its state accordingly, irrespective of the distance between them.

2.4 Types of States

2.4.1 Pure State

The quantum state of the system is called as pure state, if one has the complete knowledge that in which state the system is in. It is usually represented by a unit vector known as ket,   in complex Hilbert space H. It can also be linearly mapped from

Hilbert space, H, into a field of complex numbers, C, by dual vector, known as bra .

2.4.2 Mixed State

The quantum system is said to be in mixed state, if we have either no or little knowledge about its presence in a certain state. It could be the mixture of pure states and is generally represented by the density matrix,

푁 휌̂ = ∑ⅈ=1 푝푖̇훹푖̇ 휙ⅈ (2.8)

Where,  훹푖̇ is the set of pure states and N can have any value irrespective of dimensions of

Hilbert space.

2.5 Quantum Decoherence

Quantum decoherence is the interaction of the quantum system with the external environment causing it to lose its coherence. To keep the coherence in the system, it must be completely isolated. The wavefunction representing the physical state of the system will have some definite phase relation under coherence and if it experiences some

environmental interactions, the superposition is lost and it decays to collapsed state. The amount of entanglement is also sometimes determined by the ability of the system to resist decoherence.

Chapter 3: Experimental Methods

Here in this chapter, I will briefly discuss the experimental equipment used during research. A home-built Magneto-Optical Kerr Effect (MOKE) magnetometer installed in the MOKE lab at the Institute for Magnetics Research (IMR) at GWU is the major apparatus used for various experiments. The equipment was modified and supplied with cryostat, potentiostat and other necessary tools to study the variation of magneto-optical properties of thin films under external stimuli at room and low temperatures. The samples synthesized under various fabrication techniques were also characterized by the Vibrating

Sample Magnetometer (VSM) at room and cryogenic temperatures. The surface morphology of the systems under study was analysed by X-ray reflectometry (XRR),

Scanning Electron Microscopy (SEM) and Rutherford Backscattering Spectrometry

(RBS). The brief description of above mentioned instruments will also be presented here.

3.1 MOKE Magnetometer

Theoretical Background:

Magneto-Optical interaction was first investigated by Michael Faraday in 1845, when he observed the linearly polarized light transmitted through the medium of glass in the presence of magnetic field was elliptically polarized [FAR46]. Later, in 1876, John

Kerr, found the similar change in the polarization state of the light upon reflection from the magnetic material inside the magnetic field [KER77]. The net magnetization of the samples was found in proportional to these magneto-optical effects and was related to their dielectric tensors [HUN67]. Landau and Lifshitz [LAN60], described the dielectric tensors for the propagating medium responsible for the magnetic behaviour as below.

휀푥푥 휀푥푦 0 휀̃ = [−휀푥푦 휀푦푦 0 ] (3.1) 0 0 휀푧푧

Here, the off-diagonal components, xy, of the dielectric tensor 휀̃ are correlated to the magnetization of the sample and diagonal components do not contribute to the magnetic response [HUN67], [REI84]. xy, the off-diagonal components, induce the change in the rotation, (Kerr rotation, k), and the ellipticity, (Kerr ellipticity, k) of the polarization plane of the incident linearly polarized light. The complex angle representing both the rotation of plane and change from the linear to elliptical polarization of the plane is given by

ⅈ휀̃푥푦 k= k + ik = (3.2) √휀̃푥푥(1−휀̃푥푥)

Both the Kerr rotation and Kerr Ellipticity, k and k, can be deduced by the intensity of the reflected light at the photodetector. The intensity of light I(t) as a function of time is given as,

I(t) = Io [1+2k cos (Ao ωt) - 2k sin (Ao ωt)] (3.3)

where Io is the DC component of the detected light, Ao is the retardation amplitude of photoelastic modulator (PEM), and ω is the angular frequency of oscillation of PEM.

Using Fourier series to expand above equation as,

I(t) = Io [1+2k Jo (Ao) - 4k J1 (Ao) sin(ωt) + 4k J2 (Ao) cos(2ωt) +……] (3.4)

Neglecting the higher terms of the series, k and k can be calculated as the ratios of the measured rms AC and DC terms to avoid any fluctuations in the intensity of the light as follow,

√2 푉1푓 k= (3.5) 4퐽1 푉퐷퐶 and

√2푉2푓 k = (3.6) 4𝑗2푉퐷퐶

Moke magnetometer utilizes the above principle to study the magnetic properties of the ferromagnetic nanostructures like thin films. The hysteresis loops delineated by sweeping the magnetic field across the sample and measuring the corresponding Kerr rotation depict the magnetic properties like coercivity, saturation magnetization, squareness, etc. Also,

Moke can be used to do the aftereffect, first order reversal curves (FORC), minor loops measurements as well.

3.2 Configurations of Moke

Kerr rotation and Kerr ellipticity can be measured in three geometries based on the orientation of magnetization with respect to the reflecting surface and the surface of incidence as shown in Figure 3.1. In polar mode, the magnetization vector is almost perpendicular to the reflection surface and parallel to the plane of incidence. In longitudinal mode, the magnetization vector is parallel to both the reflecting surface the plane of the sample and the plane of incidence. In transverse mode, the magnetization vector is perpendicular to the plane of incidence and parallel to the reflecting surface. The polar mode is used to measure magnetic thin films that exhibit perpendicular anisotropy, whereas the longitudinal and transverse mode are used to study magnetic thin films that exhibit in- plane anisotropy. Generally, the Kerr effect obtained in transverse mode is smaller than that obtained in Longitudinal mode. Since the CoPt and CoPd multi-layered thin films

employed in this dissertation exhibit perpendicular magnetization, the polar mode is primarily used in the characterization of the sample’s magnetic properties.

Figure 3.1: Configurations of Moke. (a) shows Polar Moke, (b) depicts Longitudinal

Moke, and (c) shows Transverse Moke

3.3 Experimental Setup of MOKE System

The schematic illustration and picture of the MOKE setup located at the magneto- optics lab of the Institute for Magnetics Research, George Washington University is shown in Figure 3.2 and Figure 3.3. The description and functionality of the components of the

MOKE system is as follows:

ELECTROMAGNET LASER 2 OPTICAL CHOPPER

PHOTOELASTIC LASER 1 POLARIZER MODULATOR

CRYOSTAT ANALYZER. PHOTOELASTIC MODULATOR CONTROL DETECTOR AC POWER OUTPUT SIGNAL CONDITIONING UNIT PHOTODIODE DETECTOR BIPOLAR POWER SUPPLY DC OUTPUT DETECTOR SIGNAL

O/P CH 1 LOCK-IN AMPLIFIER DIGITAL MULTIMETER SR530 HP 34401A GAUSSMETER O/P CH 2 LAKESHORE 450

ADC 3 ADC 2 GPIB: ADDRESS 14 ADC 1 LOCK-IN AMPLIFIER COMPUTER EG&G 7280 GPIB: ADDRESS 10

SCHEMATIC DIAGRAM OF POLAR MOKE SETUP WITH CRYOSTAT INSTALLED INSIDE THE ELECTROMAGNET

Figure 3.2: Schematic illustration of the MOKE setup (in polar geometry)

Figure 3.3: MOKE Magnetometer located in a completely dark room in MOKE Laboratory at the institute for Magnetics Research, George Washington University, Ashburn Campus

Laser 1

It is an Aerotech (Model LSR5P) Helium-Neon laser and used a source of monochromatic light. The output power is 5 mW, wavelength= 632.8 nm and the beam diameter= 2 mm.

Laser 2

A 650nm red laser light with a beam diameter of 2mm.

Polarizer and Analyzer

The Glan-Thompson polarizing prisms are used to increase the degree of linear polarization from 500:1 from laser source to 100,000:1 to acquire accuracy desired for Kerr measurements.

PEM (Photoelastic modulator)

The Hinds Instruments (Model II/CF57) is used as Photoelastic Modulator (PEM). It uses

Calcium Fluoride (CaF2) crystal with the resonant frequency of 57 KHz. If the light is circularly polarized, the signal detected will be at the modulator frequency, i.e, (1f). At

(2f), the signal detected will be due to the linearly polarized light passing at the 45° to the modulator axis of the crystal. The measurement of these two components of the light correspond to the Kerr Rotation and Kerr Ellipticity in the MOKE measurements.

Detector

The Hinds Instruments, Model DET-90-004), Silicon photovoltaic diode detector is used to detect the signals arriving after generated from PEM and impinging the sample.

SCU (Signal Conditioning Unit)

The Hinds Instruments (Model SCU-90) Signal conditioning unit is used to intake the composite signal and separate the AC and DC components of it and then amplifies the signals.

1st Lock-in Amplifier

The AC component of the signal separated by the SCU is fed into the 1st Lock-In Amplifier which is Stanford Research Systems (Model SR5301). It enhances the signal to noise ratio.

2nd Lock-in Amplifier

EG&G Princeton Applied Research (Model 7280) digital lock -In Amplifier is used to communicate with various components of the entire system.

Digital Multimeter

Keithley (Model 197) is used to measure the DC signal from SCU

Electromagnet

The magnetic field is produced by the Varian (Model V-4004) electromagnet. The airgap between the poles caps can be varied from zero to 4.3 inches to obtain the desired magnetic field. The maximum field is about 0.85 Tesla. The magnetic field could be reversed by the means of Kepco (Model 100-2M) bipolar power supply.

Gaussmeter

The magnetic field present inside the poles caps can be measured by Lakeshore (Model

450). The field is detected by a Hall probe and is displayed on the gaussmeter.

Optical Chopper

Stanford (SR540) is used for optical modulation of laser.

Vacuum Cryostat

Janis (CCS 100/204) used for cooling the sample down to 9 K

Helium Compressor

SHI Cryogenics Group (Model HC-4E) is used in compressing helium gas needed in the low temperature experiments for obtaining cryogenic temperatures.

Temperature Controller

Lakeshore Cryotronics (Model 336) is used to control and acquire desired temperatures.

Gamry Potentiostat

Gamry (G 300/750); The characterization of magnetic properties of thin film under the stimuli of external biased electric field was carried out by incorporating the Gamry potentiostat in the Moke system.

GPIB (General Purpose Interface Bus) and PCI (Peripheral Component Interface) are used for digitally connecting the components of whole system and a VB (Visual Basic) program

is used to acquire the data. In electrochemical workstation, the data acquisition is done by

Gamry software packages like Gamry Echem Analyst.

3.4 Experimental Procedure

The MOKE apparatus can independently perform major and minor hysteresis loop and aftereffect measurements on thin film samples in polar, longitudinal and transverse geometries at room and low temperatures. It can also be used to measure surface charge induced variations in magnetic properties of thin films. Figure 3.4 shows the polar Kerr rotation of Co/Pt sample performed at room temperature. Standard calibration procedures were followed to eliminate excessive noise and distortion of hysteresis loop. However, it was noticed that the drifting of measured signal may still exist after calibration.

0.21 0.19 0.17 0.15 Kerr Rotation 0.13 0.11 0.09 0.07 0.05 -10000 -5000 0 5000 10000 Applied Field (Oe)

Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Trial 6 Trial 7 Trial 8

Figure 3.4: Multiple hysteresis loops of Polar Kerr Rotation for Co/Pt multilayer film performed at room temperature at Moke lab in GWU.

3.5 Vector-Vibrating Sample Magnetometer (v-VSM)

During the research, the newly installed vector-Vibrating Sample magnetometer

(Model 7410) from Lakeshore Cryotronics, Inc. at the Materials testing lab at the Institute for Magnetics Research (IMR) was frequently used to authenticate the results obtained by

MOKE magnetometer. The state-of-the-art v-VSM, shown in Figure 3.5 is a sophisticated device capable of measuring the magnetic moments of samples over a wide range of temperatures with higher precision [KHU15], [KHU17]. With the oven option and cryostat

(Janis Research Co., Inc. Model CCS 700/204) installed, the magnetic characteristics of the materials could be analyzed from 8K to 310 K. The instrument is supplied with a 10- inch electromagnet capable of producing the magnetic fields up to 3.1 T at room temperature by suitably adjusting the pole caps. A variety of samples including bulk materials, nanoparticles, thin films, etc. could be characterized for their magnetic behavior by installing them on the sample holder and then bringing inside the uniform magnetic field produced by the electromagnet equipped with bipolar power supply. The magnetized sample then generates its own magnetic field and is vibrated in the presence of detection coils. The current induced in these coils is proportional to the magnetic moment of the sample. By measuring the magnetic moments against the varying applied magnetic field

(M vs H) or against the temperatures (M vs T) curves exhibit the important magnetic characteristics of the samples.

Figure 3.5: Vector Vibrating Sample Magnetometer [IMR13]

3.6 Scanning Electron Microscope (SEM)

The surface of the magnetic systems used in the experiments were investigated by the SEM images produced by the Raith PIONEER Two Scanning Electron Microscope

(SEM), as shown in the Figure 3.6, installed at the GW Nanofabrication and Imaging

Center (GWNIC).

Figure 3.6: Raith PIONEER Two SEM at GW Nanofabrication and Imaging Center [GW16] SEM scans the high energy focussed beam of electrons produced by the electron gun and processed by the magnetic lenses inside a high vacuum chamber at the sample surface. As a result, there is emission of secondary electrons from the specimen. The low energy electrons (< 50 eV) emitted are detected by the Secondary Electron (SE) detector and the high energy electrons (>50 eV) are detected by the Backscattered Electrons detector (BSE).

The basic layout of SEM is shown in Figure 3.7.

Figure 3.7: SEM Layout [SEM17]

SE detector describe the surface topography of the sample upto 15 nm as the low energy electrons captured by the detector usually cannot below this depth. The BSE detector generates the signals based on the atomic number contrast and provide the composition of the sample as the elements with higher atomic number materials producing brighter signals.

The image produced is corrected for astigmatism, edge effects, or charging to display the required picture.

3.7 Rutherford Backscattering Spectrometer (RBS)

The sample structure and composition is verified by the Rutherford Backscattering

Spectroscopy performed at the Institute for Nanotechnology (INT), Karlsruhe Institute of

Technology (KIT), Germany. RBS, also known as high energy ion scattering (HEIS), impinges the high energy particles on the sample and the backscattering of these particles is dependent upon the structure and composition of the elements involved. The backscattered ions are detected with different energies due to the losses from the nuclei and electrons of the elements. The measurement count versus the energy plot determines the composition of the sample by comparing it with the known scattering from nuclei and electrons of materials. The position of peaks in the energy spectrum tells the element and the depth of the elements contained in the sample can be determined by the concentration of peaks heights.

3.8 X-Ray Reflectometry (XRR)

X-Ray reflectometry is another technique used to conclude the composition of the sample used in the experiments. X-Ray reflectometry was also carried out at INT,

Germany. X-rays are impinged upon the sample at a particular angle (angle of incidence is equal to the angle of reflection) and the intensity of the reflected beam is analyzed. The intensity of reflected beam is proportional to the elements at the surface and specifies its density.

Chapter 4: Electrical Modulation of Magneto-Optical Properties

In this chapter, first I will discuss brief review of literature and then our experiment with results and discussions and finally some future work.

4.1 Introduction

The shift from the longitudinal to perpendicular mode in the magnetic media in the last few years has seen an enormous growth in Areal Density (AD). Currently, the magnetic recording industry is striving to push the Areal Density (AD) of stored information beyond the superparamagnetic limit of 1 Tbit/in2. The progress is challenged by the superparamagnetic trilemma [PLU12], which becomes significant at smaller magnetic bits. One of the potential candidates to overcome it is the electrical tuning of the magnetic properties. Manipulation of the magnetization direction of the magnetic bit via electric current induced magnetic field has been widely used. However, as the bit size is miniaturized, the required electric current increases dramatically. Therefore, the current induced magnetic field cannot be used for high density magnetic storage devices. Spin transfer switching has attracted much attention as an alternative method of electric current induced magnetization switching in nanopillars of spin valves and magnetic tunnel junctions. The driving principle of this method is based on spin transfer torque, which is caused by the interaction between spin angular momentum of conduction electron spins and local magnetic moments [SLO96] [BER96]. Even though the required current reduces as the bit size is reduced, the electric current is still large for practical applications.

Voltage driven modulation is proposed as a desirable alternative method for electrical control of magnetic properties, which can circumvent the limitations in the other methods, as it does not require electric current flow. Recently, the use of electrostatic fields

applied at a solid-solid [BER96], [MAR09], [CHI08], [BER13], [DU06], [RON08],

[CHE06], [BOR05], [CHI12], [CHU08], [EER06] or liquid-solid [WEI07] interface to achieve dynamic and reversible control of material properties (tuning) has gained momentum. Such a method to control magnetic properties, via application of an electric field-effect gating principle, well known in transistors, is of fundamental interest in any application concerned with the manipulation, storage, and transfer of information by means of electron spin. It has been shown for many nanostructures that an applied electrostatic field, or surface charge, can bring about a noticeable magnetoelectric response for various ferro and ferrimagnetic materials [NIE00], [CHI03], [STO08]. As a spectacular experimental benchmark, one can consider reversible control over magnetic phase state in metals, which has been demonstrated in ultra-thin films (a few atomic monolayers) of

Pt/Co, Fe upon dielectric and electrochemical charging [MAR09], [SHI12].

The CoPt3 and Co/Pd multilayer thin films have strong perpendicular magnetic anisotropy (PMA) and are a type of materials of interest for the magnetic recording industry and are employed here. The response of the magnetization to electrochemical surface charging by the non-aqueous electrolyte was studied by a MOKE magnetometer at room temperature. The most significant difference between our systems reported here and those in [MAR09], [SHI12] is the film thickness. Ours are much thicker, in the range of 15 nm for Co/Pt and about 35 nm for Co/Pd. Yet, despite their larger thicknesses, the magnetic anisotropy of the whole film is affected by an applied surface charge. Normally, in metals a large carrier concentration limits the screening length of the external field to a few monolayers. This implies a new mechanism is necessary for the thicker sample.

4.2 Experimental Setup

We designed and built an in-situ experimental system for the measurement of surface charge induced variations in magnetic properties, which consists of a MOKE apparatus, a three-electrode electrochemical cell and a Gamry G300/750 Potentiostat as shown in Figure 4.1. The electrochemical cell consists of three electrodes, working electrode, i.e. the sample, counter electrode (Pt) and a reference electrode (Ag/AgCl) and a Teflon cap is shown in Figure 4.2. 1M Lithium Perchlorate (non-aqueous electrolyte,

99% pure Sigma Aldrich) dissolved in ethyl acetate (non-aqueous solution, 99% pure

Sigma Aldrich) was proven to provide large stability potential window and negligible reaction with the Co/Pt alloy and was selected as the electrolyte. MOKE configuration is set into Polar mode to characterize the magnetization of Co/Pt or Co/Pd multilayered thin films as they exhibit perpendicular magnetic anisotropy. The electrochemical cell installed on the non-magnetic sample holder is placed in between the poles of electromagnet in such a way that the incident polarized light from the laser falls perpendicularly on the sample inside the cell. The sample surface is charge electrochemically when the biased field is applied by the potentiostat against the Platinum (Pt) counter electrode and Ag/AgCl reference electrode. The application of biased fields at specific intervals during the hysteresis loop measured by MOKE apparatus is used to analyze the critical magnetic parameters of the sample. The formation of electrochemical double layer at the surface of sample induces significant changes in the magnetic system as opposed to using dielectric gating.

Figure 4.1: Schematic illustration (top view) of the in-situ experimental setup that consists of a MOKE apparatus, a potentiostat and a three-electrode electrochemical cell. [SHU12]

Figure 4.2 Electrochemical Cell with sample holder and Teflon Cap

4.3 Cyclical Voltammetry

The cyclical voltammetry was carried out to ensure a stable electrochemical potential window for the application of biased fields. The selection of electrolyte is crucial from two aspects; first, it possesses the stable and significant potential window to create a noticeable change in magnetic behavior of thin films and secondly, it should be colorless so that the visibility of laser and its shining on the sample inside the electrochemical cell placed between the poles of the electromagnet could be guaranteed. Shuo in his thesis

[SHU12] has calculated the electrochemical potential of 1 M LiClO4 electrolyte prepared in Ethyl Acetate. The calculation of Reduction half reaction and Oxidation half reaction of the chemical reaction revealed the window of 4.27 V. The fields were restrained between

1.23 V to -3.04 V and were swept at different scan rates, 5mV/s to 30 mV/s to assure enough electrochemical charging at the surface of the films. The positive fields were applied with special precaution to avoid any accumulation of hydrogen during the reaction as it could explode the whole apparatus. The compressed nitrogen gas was constantly passed through purging lines from the cap of electrochemical cell for this purpose. The cyclical voltammogram for both Co/Pt and Co/Pd samples are shown in Figure 4.3 and

Figure 4.4 respectively.

Figure 4.3: Cyclic Voltammetry Plot for voltage sweep between +1.3V and -0.9V for CoPt3

0.8

0.6

0.4

0.2

-0.2 Current(mA)

-0.4

-0.6

-0.8

-1 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Voltage (V)

Figure 4.4: Cyclic Voltammetry plot for voltage sweep between +1 V and -1V for Co/Pd

4.4 Experimental Procedure

An experimental procedure is used in the present study to systematically evaluate the effect of induced surface charge on critical magnetic parameters. We measured the hysteresis loop (Polar Kerr rotation vs. applied magnetic field) without potentiostatic control, i.e., the gate voltage, VG = 0. An electrochemical double layer naturally formed at

the interface between the metallic thin film and the electrolyte. Then we set the gate voltage to a non-zero value of interest within the potential window (for electrochemical double layer formation) and measure hysteresis loop. Thereafter, we calculated and analyzed the percentage variation of critical magnetic parameters, e.g. coercivity with the application of gate voltage. Afterwards, the same procedure was repeated with different bias voltages.

4.5 Electric-field control response in CoPt3

The first part of the research conducted by our group was on CoPt3 and was provided by R.F.C. Farrow. The picture of sample is shown in Figure 4.5 and the SEM image is presented in Figure 4.6. The Rutherford backscattering spectrometry (RBS) and

X-ray reflectometry (XRR) performed on the sample revealed the composition of the sample as shown in Figure 4.7. and Figure 4.8 respectively. The darker area in the film is

Si substrate and the CoPt3 thin film with <101> texture and (111) reflectivity has the thickness of 15nm was grown with perpendicular magnetic anisotropy in the shiny circles.

The film was capped with 5nm of Pt to stabilize against the surface oxidization.

Figure 4.5 Image of CoPt3 Sample

Figure 4.6 SEM image of Co/Pt showing the surface of sample

Figure. 4.7 Rutherford Backscattering (RBS) spectrum of CoPt3 performed at Institute for Nanotechnology, Karlsruhe Institute of Technology (KIT), Germany

Figure 4.8 X-Ray Reflectivity (XRR) of CoPt3

4.5.1 Results and Discussion:

When different negative gate voltages are applied at room temperature as shown in

Figure 4.9, the Co/Pt system exhibits different ferromagnetic behavior in hysteresis curves.

When the gate voltage changed from 0 V to −0.8 V, that is, the magnitude of negative gate voltage increased, significant changes in critical magnetic parameters were observed; the saturation magnetization, coercivity, and switching field distribution decreased, however, the perpendicular anisotropy clearly increased. The change in coercivity, ΔHc, as a function of gate voltage is shown in Figure 4.10.

Fig. 4.9: Polar Kerr hysteresis loops at 300 K under gate Voltages 0 V, −0.3 V, −0.5 V, and −0.8 V.[Shu12]

The change in coercivity, in particular, indicates that magnetization switching process can be modulated electrically. The magnetization reversal process starts with the nucleation of small reversed domains, followed by the subsequent domain wall propagation. The change in domain wall propagation results in modulated activation energy barrier, and therefore leads to the variation in coercivity. The change in coercivity in our experiment is bigger than what has been reported until now. We believe that our system is entirely different than others. Except for Weisheit et al. (Weisheit et al., 2007), all other reports have used dielectric gating in applying bias voltage. The surface charging obtained through electrochemical gating is two to many orders higher. This high surface charge probably triggers the filling of d-bands in the Co/Pt system and thus reducing the coercivity drastically. This band filling was predicted by Gleiter (2000). The large reduction in coercivity was also shown by Lin, Chang, Tsai, Shieh, and Lo (2014) when he increased the thickness of ZnO in Fe/ZnO from 10 to 320 nm by the application of same voltage. So, in our system, we also expected a high contribution of strain coupling.

Fig. 4.10: Change in coercivity as a function of gate voltages. [Shu12]

4.6 Electric Field Control Response for Co/Pd

A CoPd multilayer film prepared by molecular beam epitaxy (MBE) using e-beam evaporation was used. The sample composition was [Si (100)/SiO2/22nm W/ 10nm Pd /

(0.3nm Co/ 1nm Pd)15 10 nm Pd/ 1.5nm Fe] and annealed for an hour at 523 K showing strong perpendicular magnetic anisotropy. The SEM image for the sample is shown in

Figure 4.11

Figure 4.11 SEM image of surface of CoPd sample

Fig. 4.12: Normalized hysteresis loops (Kerr Rotation vs applied field) under various biased voltages.

Fig. 4.13: Shows the zoomed lines of hysteresis loops under different bias voltages near zero remanence. It clarifies the change in coercivities due to gate voltages.

1030

1020

1010

1000

990 Hc Coercivity

980

970 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 Gate Voltage V(G)

Fig. 4.14: Coercive Field as a function of gate voltage VG for CoPd. Triangles show the coercivity at various applied voltages.

4.6.1 Results and Discussion:

After applying the similar procedure to the previous experiment, here the observed magnetic response of the system to the electrical modulation is weaker. By applying the gate voltages of 0 V, -0.5 V, -1 V, and -1.5 V, the hysteresis curves showing the normalized

Kerr rotation, k versus the applied field is depicted in Figure 4.12 and is more clarified around zero remanence in Fig. 4.13. The system is affected here as expected but the change in coercivity is less drastic as compared to Co/Pt system. The changes in coercivity for various applied electrical fields can be clearly viewed by Figure 4.14. The percent change in the coercivity with the gate voltages is shown in the table 4.1.

Gate Voltage Percent Change in

Coercivity

-0.5 V 1.77 %

-1 V 2.47 %

-1.5 V 3.26 %

Table 4.1: Percent change in coercivity of CoPd with applied gate voltages

This change was expected as the thickness of the film used here is much larger, about 35nm.

The systems not only differ in thicknesses but also in their composition. Though both platinum [SID16] and palladium are paramagnetic, but platinum has the unfilled d and s- orbitals, that contributes differently to the magnetization of the whole film as compared to the filled d-orbitals of the palladium in the Co/Pd system. Thus, when the electrical field is applied to both the systems, charge neutrality is seen to be affected more in Co/Pt and we observe large changes in magnetic properties. Secondly, the thickness of the film has a complex contribution to the behaviour of the magnetic properties under bias electrical fields. As it has been shown by Lin et al. [Lin14], strain coupling due to the larger thicknesses of their sample creates significant magnetic response to the applied field. Here we encounter smaller response with increase in thickness, since the applied field could not create the desired charge neutrality due to the already filled d-bands.

4.7 Conclusions and Future Work:

The present investigation presents reversible variations in the critical magnetic parameters of a Co/Pt and Co/Pd multilayer thin films by manipulating the induced

electrochemical surface charge. Dynamic control of magnetization switching is realized by negatively charging the interface of the ferromagnetic thin film and the electrolyte at various stages of the magnetization reversal process. For future, the research could be extended to increase the electrochemical potential window of the electrolyte. This selection of the electrolyte based on larger electrochemical potential window at room and low temperatures can give more opportunity to disturb the charge neutrality of the magnetic systems and possibly more dynamic control. Currently, there are some ionic liquids that offer wider window upto 6 V and can be operated down to 230 K. In future, it is thought to try ionic liquids and new systems which are greatly modified by electrochemical double layer charging. The possibility of reversibly and dynamically controlling the magnetic properties of ferromagnetic metallic nanostructure, under the assistance of a small bias voltage, at room temperature offers new functionalities to spintronic and magneto-electric devices. This could enable the development of non-volatile magnetic storage devices with ultra-low-power consumption.

Chapter 5: Optical Modulation of Magneto-Optical Properties

5.1 Introduction

Quantum entanglement is one of the most peculiar phenomena in modern physics and is a resource to many potential applications like quantum teleportation, cryptography, and quantum communications. It shows the strong correlation between the systems of particles which cannot be described by the classical mechanics. Over the past few decades, significant research has been made on a variety of systems like atomic ensembles

[MOE07], [RIT12], [HOF12], [CHO05] or ion-traps [STU12] to demonstrate the photon- photon entanglement [WEB09], spin-photon entanglement [BER08], [XU08], [KRI12] etc. of different quantum states. The spin-photon entanglement is demonstrated in semiconducting nanostructures like quantum dots (QDs) with single spins interacting with incoming photons. The Kerr or Faraday rotation of the polarization of photons is observed in [ARN15] and it created a renewed interest in quantum magneto-optics by investigating the magnon-photon interaction and magnon entanglement in metallic nanostructures.

Magneto-Optical Kerr Effect (Moke) magnetometer has been previously used for the characterization of the magneto-optical properties of magnetic materials [CHI15],

[CHI16] at room and low temperatures but has not been utilized for the exploration of entanglement of magnons. Theoretically the fundamental aspects of entanglement of magnons were presented [MOR05], we here experimentally report the magnon entanglement in multilayer CoPd thin films at low temperatures which are spatially separated inside a cryostat in a MOKE magnetometer.

5.2 Some Interpretations of Quantum Mechanics The conceptual understanding of the quantum mechanics has always remained a challenge to the scientists. At the start of the twentieth century, the departure from classical interpretation of the nature to the quantum interpretation gained momentum and subsequently, efforts were made to explain the issues like realism, determinism, local realism, and completeness. Here are few of the interpretations are briefly discussed to get a background needed for the topic.

5.2.1 Standard Interpretation/ Copenhagen Interpretation

The Copenhagen Interpretation was formulated between 1925 and 1927 by Neil

Bohr and Werner Heisenberg. It is the set of principles defining the quantum nature of the systems. It is widely accepted interpretation, though it also fails to provide answers to the phenomena like entanglement, Schrodinger’s cat, and Double-Slit diffraction etc. The states of the systems describing the physical nature of them are represented by the wave- functions. Before an experiment, the system is assumed to be in the eigenstates of the wavefunction before it collapses to one of the eigenstates upon observing it.

5.2.2 De Broglie-Bohm Interpretation

This interpretation, also known as pilot-wave theory, is based on the theory presented in 1927 by Louis de Broglie and later rediscovered by David Bohm in 1952. It is also one of the examples of the hidden variables interpretation of Quantum mechanics.

The system is described by the particles which are guided by the wave-functions as put forward by the Schrodinger’s wave equation. The wave functions here do not give the description of a quantum system but rule the motion of the particles. The description is

completed by the knowledge of actual position of the particles. This is nonlocal in nature and it involves no collapse of wave-function.

5.2.3 Everett’s Interpretation/ Many-Worlds Interpretation (MWI)

Hugh Everett in 1957 [EVE57] proposed another idea about the interpretation of quantum mechanics. According to him, there are myriads of parallel worlds in the universe apart from the one we are living and every quantum experiment has its possible outcome in those worlds. This theory removes the randomness and the few paradoxes perplexing the physicists. It is considered as local, deterministic, and realist theory. The Interpretation was further refined by DeWitt, Deutsch etc. [DEU85].

5.3 Recent trends in Quantum Entanglement

Next, I will briefly discuss the latest advances in the entanglement of some particles.

5.3.1 Photon Entanglement: Creation Techniques and Applications

The creation of entanglement in the particles is both experimentally and theoretically presented over the last few decades. The prime contender for it is the photons and significant effort is applied in the understanding their behavior. The formalism and theoretical aspects developed and so as the techniques to create the entanglement in them.

Alain Aspect [ASP82] and others claimed the creation of photons entanglement using the cascade sources. They put the calcium atoms into a high energy level and the emission of a single photon is evaded and instead there is the decay into two photons, one after the other. The separation between the two is in nanoseconds and the correlation in the polarization of these photons suggest the creation of entanglement. However, this photon entanglement source is slow as the photons are in random direction and to bring them in

the desired place requires time. Alternatively, a new concept emerged known as spontaneous parametric down conversion (SPDC), where a high energy beam of light is impinged on the non-linear optical crystal like BBO (Beta Barium Borate) or KDP

(Potassium Dihydrogen Phosphate) resulting in the creation of few pairs of photons with double the wavelength of original beam. These pairs of photons were found to be in strong correlation with each other when their polarization was measured by the photodetectors and thus demonstrating significant entanglement. The schematics of the creation of entanglement by SPDC is shown in the Figure 5.1.

Figure 5.1: An SPDC scheme with type II output [ZEI10]

The objective of the entanglement is to create the quantum devices capable of providing the reliable and robust quantum computing, quantum teleportation, and quantum communication etc. The quantum communication distances have significantly increased over the years of progress from table-top experiments [BOU97], [BOS98], [FUR98] with

few centimeters of separation to the satellite-based experiments handling the successful communication over 1200 km [LIA17]. The loss of photons in optical fibers is the limiting factor in reduction of transmission distances. On the ground based experiments, the researchers from the Austrian Academy of Sciences and University of Vienna collaborated to transmit the quantum states from a station at La Palma (Jacobus Kapteyn Telescope JKT of the Isaac Newton Group) to Tenerife (Optical Ground Station of European Space

Agency) which are 143 km apart as shown in Figure 5.2. The photons were emitted by the

Ti:Sapphire based femtosecond laser and were entangled by the SPDC of type-II.

Figure 5.2: Quantum teleportation between the Canary Islands La Palma and Tenerife over both quantum and classical 143-km free-space channels. a, Experimental scheme. b, Set- up. In La Palma, a frequency-uncorrelated polarization-entangled. [XIA12] Most recently, the Chinese quantum satellite, Micius, was launched into the LEO orbit to have the quantum key distribution between the satellite and the ground-based stations at

Xinglong (645 km apart) and Nanshan (1200 km apart). The communication was verified for both uplink and downlink distribution of quantum keys. The future experiments are involving intercontinental distances. The design of their experiment is shown in Figure 5.3.

Figure 5.3: Illustration of experimental setup [LIA17]

5.3.2 Entanglement in Atomic Ensembles

Photon entanglement lead to the creation of quantum qubits that are perceived as the flying or dancing qubit due to the fast-moving nature of photons. With the development of new technologies, novel ideas surfaced for detecting the entanglement in other atomic ensembles and many latest experiments have shown the entangled states in the neutral atoms and ion traps. Here is the brief description about these ensembles for generation and manipulation of entanglement.

5.3.2.1 Ion Traps

The most common method to generate entanglement is the spatial confinement of the particles (ions) with entangled states is ion-trapping and recently has been successfully demonstrated [BLA08], [WIN13], [HAF08], [ROO14], [MAR16], [DUA10], [BLA12],

[LEI03] etc. The ion traps could be created by using the oscillating electric fields like in

Paul traps (linear or surface electrode) [LEI03], [MIE16], or may be by using the combination of both magnetic and static electric fields as in the case of Penning traps

[BOH16]. A Paul ion trapping setup is shown in Figure 5.4. Decoherence in the ion traps is minimal as their interaction is reduced with the environment and can be detected with high efficiency. Due to the coulombic repulsion between the spatially separated ions, the individual addressing of the ions is achieved. The coherence time in ion traps is higher and can reach upto few tens of milliseconds, thus allowing quantum gate operations to take place.

Figure 5.4: A linear Paul Trap [BLA08]

5.3.2.2 Bose-Einstein Condensates Bose-Einstein condensation creates an entangled state where all particles (bosons) have integer spins. There could be entanglement in the internal states of the atoms but spatially two regions of the condensates are always found in the entangled state independent of the distances between them. BECs have extremely weak coupling with the

environment making them less prone to decoherence and having higher coherence time, however making it difficult to manipulate the states for quantum processing purposes.

Experimentally, the entanglement in Bose-Einstein condensates have been shown by

[EST08], [RIE10], [GRO10], [LUC11], [HAM12] [BER13], [OCK13], [MUE14],

[MUE15], [KRU16].

5.3.2.3 Magnons

The magnons are the quasiparticles found in the magnetically ordered crystals.

These are the quanta of spin waves created or annihilated by the flip of the spin in their lattices. Like the real particles, magnons also carry energy and obey the Bose-Einstein statistics. The Bose-Einstein condensation (BEC) of magnons at low temperatures is presented both experimentally and theoretically [BEN14]. Recently, the interest in the presence of entanglement in magnons has attracted interest from the researchers [MOR05], but is mostly confined to the theoretical aspects of entanglement related to bipartite and multipartite entanglement scenarios. Here in our research, we have tried to show the evidence of entanglement in magnons confined in the nanostructured thin film ferromagnets. The magnons states are created by cooling the samples below their BEC temperatures (~ 35 K for CoPd) and then entanglement is investigated by the interaction of linearly polarized light. This magnon-photon interaction could be a stepping stone for future quantum memory and quantum communication networks.

Next, the details of the experimental setup and techniques applied for creating the magnon entanglement is discussed.

5.4 Experiment

The Co/Pd test sample was grown by molecular beam epitaxy on a silicon dioxide substrate using electron beam evaporation. We followed a similar fabrication procedure illustrated in the work by Nwokoye, et al. [CHI17] The test sample composition is: 22 nm

Si<100>/SiO2 substrate, 10 nm of Pd, (0.3 nm of Co and 1 nm of Pd)15, 10 nm of Pd and

1.5 nm of Fe. The sample was designed to exhibit a high perpendicular magnetic anisotropy. The sample was cut into two pieces as shown in Figure 5.5 which were separated and glued on a thin rectangular copper sheet by a thermally conductive adhesive

(Stycast 2850 FT/24 VL) capable of withstanding low temperatures down to 9 K and vacuum pressures lower than 1x10-5 torr.

The copper sheet with the two samples (S1, S2) was placed inside a vacuum cryostat of an automated cryogen-free low-temperature PEM-based MOKE system. Details of the experimental setup and procedures are described in [CHI15]. The block diagram of the setup is depicted in Figure 5.6. The design is used to study the correlation between magnons confined in two separated samples. The setup consists of two 638 nm red lasers

(L1 and L2), linear polarizer (P1), PEM modulator (P2), optical CCR cryostat (V), electromagnet (E), analyzer (A), optical chopper (C), three mirrors (M1, M2 and M3), and laser diode detector (D).

(1) At room temperature, polar Kerr rotation hysteresis measurement on the sample

S1 was recorded for applied magnetic fields ranging from -4 kOe to +4 kOe with L2 switched off. On the same sample S1, polar Kerr rotation aftereffect measurement with holding field at the coercivity field of approximately 970 Oe for 600 seconds was recorded.

This is needed so that we could decisively identify the effect of the modulated laser from

L2 on sample S1. Afterwards, L2 was switched on (allows modulated laser beam to be incident on sample S2) and both the polar Kerr rotation hysteresis and aftereffect measurements were repeated on sample S1 for various chopper modulation frequencies ranging from 0 Hz to 2 kHz. These measurements were performed in order to observe the coupling of the photon-magnon interaction in sample S2 onto sample S1 at room temperature.

(2) Keeping the temperature unchanged, the mirror M3 in Figure 5.6 was adjusted to redirect the modulated laser beam onto sample S1 in order to directly measure the influence of the modulated laser beam on the Kerr rotation of sample S1. Both the aftereffect and hysteresis Kerr rotation measurements were recorded.

(3) Following the procedure described in [CHI15], low temperature measurements of the Kerr rotation hysteresis were recorded on the sample S1 at 9.5 K and 60 K with the following scenarios: (i) laser L2 switched off (ii) laser L2 switched on and modulated laser beam incident on sample S2 (iii) laser L2 switched on and modulated laser beam incident on sample S1. The proceeding paragraphs discusses the results of the performed experiments.

Figure 5.5: Picture of two Co/Pd samples cut from same sample and placed 3.5mm apart, edge to edge.

Figure 5.6: Block diagram of experimental setup. L1 & L2 are laser sources, P1 is a linear polarizer, P2 is a photoelastic modulator (PEM) crystal, E is an electromagnet, M1-3 are mirrors, A is an analyzer, V is vacuum cryostat, C is variable frequency chopper, and D is a laser diode detector. [CHI17]

5.5 Results and Discussions

The Kerr rotation aftereffect measurements at room temperature and holding field of 970 Oe for scenarios A1-3 are shown in Figure 5.7, where A1 represents measurement with L2 switched off and there was no modulated laser on sample S1. A2 represents measurement L2 switched on and mirror M3 is adjusted to allow the modulated laser with modulation frequency of 1 kHz to be incident on sample S1. A3 represents measurement

L2 switched on and mirror M3 is adjusted to allow the modulated laser with modulation frequency of 2 kHz to be incident on sample S1. We find from Figure 5.7 that the frequency of the incident modulated laser introduces an amplitude offset to the Kerr rotation signal.

This offset is evident in the aftereffect measurements because the aftereffect records the

Kerr rotation signal over a time span. The hysteresis major loops measurement at room temperature are shown in Figure 5.8 and the curves depict no difference at the coercive field but a small difference at the remanence (zero applied field) and saturation field.

Figure 5.7: Kerr rotation signal time responses at coercivity field (≈970 Oe). A1 represents measurement without external modulated laser (baseline). A2 represents measurement with external laser with modulation frequency of 1 kHz. A3 represents measurement with external laser with modulation frequency of 2 kHz.

Figure 5.8: Kerr rotation major hysteresis loop with and without external laser modulation frequencies at room temperature.

The result of the low temperature measurements at 60 K are depicted in Figure 5.9.

As shown, D1 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 60 K. This curve contains no influence of the external laser beam from L2 on sample S1. D2 depicts when laser L2 is switched on and the modulated laser beam is incident on sample S1 at 60 K. The D2 curve measures the amount of coupling from sample

S2 onto sample S1. D3 depicts when laser L2 is switched off at 60 K. We noticed the following: (i) the Kerr rotation response with applied field increased significantly with a decrease in temperature, this effect of an increase in magnetization with temperature was first observed by Bloch [BLO30] and has been previously recorded in [CHI17], [DEL05].

(ii) At the coercivity field, all the curves D1-3 showed no difference between each other.

(iii) At all the applied field values, curves D1 and D3 showed an eligible difference and were almost identical while, curve D2 showed an increased value from the other curves.

The result of measurements with lower temperatures down to 9.5 K are shown in

Figure 5.10. The curve D1 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 9.5 K. This curve contains no influence of the external laser beam from L2 on sample S1. D2 depicts when laser L2 is switched on and the modulated laser beam is incident on sample S1 at 9.5 K. The D2 curve measures the amount of coupling from sample S2 onto sample S1. D3 depicts when laser L2 is switched off at 9.5

K. Again, we noticed the following: (i) the Kerr rotation hysteresis loop response retained its large area size with the decrease in temperature. (ii) At the coercivity field, all the curves

D4-6 showed no difference between each other. (iii) At all the applied field values except at coercivity, curves D4 and D5 showed an increased value in Kerr rotation from each another and both curves had higher Kerr rotation values compared to curve D6.

The extracted remnant Kerr rotation temperature response is shown in Figure 5.11 and it clearly shows the comparison of all the curves D1-6. As mentioned in the previous paragraph, the presence of the external laser beam from L2 creates an offset in the Kerr rotation signal and we find that this offset is responsible for the increase in the Kerr rotation

(θK) in D2 and D5. This is an interesting new phenomenon which has not been reported before. This increase is due to the photon-magnon interaction and further investigation will be conducted on the phenomenon. Furthermore, we noticed that the Kerr rotation of D5 was greater than D6 but lesser than D4, thus, there is a coupling phenomenon occurring when the temperature is decreased to 9.5 K as opposed to when the temperature is at 60 K.

Low temperature magnetic measurements on the Co/Pd sample with similar nanometer dimensions has been reported to have a magnon Bose-Einstein condensation (BEC) phenomenon occurring at temperatures below 35 K.

Figure 5.9: Kerr rotation hysteresis major loops. D1 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 60 K. D2 depicts when laser L2 is switched on and modulated laser beam is incident on sample S1 at 60 K. D3 depicts when laser L2 is switched off at 60 K.

Figure 5.10: Kerr rotation hysteresis major loops. D4 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 9.5 K. D5 depicts when laser L2 is switched on and modulated laser beam is incident on sample S1 at 9.5 K. D6 depicts when laser L2 is switched off at 9.5 K.

9.5 K. D3 represents when laser L2 is switched off at 9.5 K. D4 depicts when laser L2 is switched on and modulated laser beam is incident on sample S2 at 60 K. D5 shows when laser L2 is switched on and modulated laser beam is incident on sample S1 at 60 K. D6 represents when laser L2 is switched off at 60 K.

The above observed phenomenon indicates the occurrence of entanglement of magnons confined in the thin films. As in case of two separate samples particularly, the indication is even stronger due to the distance between the two samples is much larger than any existence of coupling between them [BRU65] which is usually limited to nanoscale

distances. The magnetic field originating out of the samples to have any effect on each other is also not significant as both the samples are placed inside relatively very high field.

Also, it was made sure that L1 and L2 do not have any direct interference with each other, thus leading to any experimental artifacts. Clearly, at room temperature under same setup, there is no significant change in Kerr rotation, thus validating the fact that the two beams are separated. The increase in Kerr rotation due to L2 on S1 and S2 at lower temperatures, especially under BEC temperature, suggests the violation of Bell’s inequality [BEL64] and hence demonstrating the macroscopic quantum entanglement of magnons here. Since, the two samples are from same source, the surface morphology and the properties of them are largely same. Thus, under BEC, the magnons of the two samples are almost quantized in the same lower energy states and any excitation in one is affecting the other. The lower values of Kerr rotation in S2 compared to S1 is perceived as due to decoherence and lack of measurement precisions. Since the magnons are in the coherent state and not the vacuum state, the decoherence is getting into act here at 9 K.

5.6 Conclusions

We have here observed a weak entanglement of magnons confined in the CoPd thin films at low temperatures. The increase in Kerr rotation in the magneto-optical characterization of the thin film by optically modulating another laser in a MOKE magnetometer suggests the entanglement. The full understanding of the phenomenon will be further investigated in future. It seems to open new dimension in fundamental and applied research.

Chapter 6: Conclusions and Future Work

The magnetic behavior of various nanostructures like multilayered thin films was investigated during the course of research. Initially, the modulation of the induced surface charge resulted in the dynamic and reversible control of the magneto-optical properties in the multilayer thin film nanostructures. This phenomenon was observed previously in much thinner ultra-thin films with maximum thickness up to 5 nm. We here studied the behaviour in much thicker systems, i.e., Co/Pt with thickness of 15 nm and Co/Pd with thickness about 35 nm. The impact of applied biased voltages resulted in changes the critical magnetic properties like coercivity, saturation magnetization. These results are important from technological perspective, as it could possibly lead to the realization of non- volatile storage devices with higher areal density (AD) and low power consumption.

Next, the quantum entanglement of magnons confined in these thin films is explored. Though the magnon-photon entanglement has attracted researchers lately but is still more focussed on the theoretical aspects of it. Here, we have demonstrated experimentally the occurrence of entanglement in two spatially separated thin films inside a cryostat under their BEC temperature (~ 35 K for CoPd). Both samples were cut from same sample and under BEC temperature, the electrons tend to be in the same energy levels.

Consequently, the magnons created or annihilated by the flip of spins, were found in correlation with each other when are excited independently in one of the films. There was enhanced Kerr rotation found in one sample when the second sample is excited by another laser in an in-situ Moke magnetometer. The whole phenomenon is yet to be completely understood and it requires further research both theoretically and experimentally.

Since the materials behave differently at nanoscale than their bulk counterparts, so to comprehend the physics working behind such quantum effects needs further investigation.

After our initial experiment, few more modifications of various parameters will be important. Below are some of the modifications in the initial experiment,

• Using two samples with different BEC temperatures. Thus, measuring the quantum

effects of magnons in both samples when they are both under BEC, above BEC

and one of them is in its BEC temperature region but other is not. Thus, quantifying

the contribution of BEC temperature in quantum entanglement.

• Also by employing two homogenous samples fabricated under same conditions

and time, can possibly create exciting results when are separated enough and

cooled down to temperatures below their BEC temperature. Though, there is no

evidence how these two samples will be entangled and communicate with each

other, yet any correlation in the magnons is a possibility and asks for further

investigation.

• Employing another photodetector to measure the second laser simultaneously.

Thus, we can get a clearer sense of quantum effects in both samples. It can help

better understand how the entanglement is occurring in both samples. Is the

enhancement of Kerr rotation is found in both samples or there is decrease in Kerr

rotation in other sample?

• Measuring the entanglement as the function of distance between the two samples.

Though the entanglement is distance independent, the change in distances here can

explain the quantum decoherence occurring in our experiment.

• The quantum teleportation and quantum memory is realized in the atomic

ensembles connected via optical fibers by the atom-photon entanglement. The

magnons-photons entanglement will be critical in the creation of new quantum

memories and quantum repeater nodes. However, it requires further theoretical

models and experimental setups to achieve it. Our experiment here will be a good

starting point as it has already shown the presence of entanglement in magnons by

using the linearly polarized light. If we may use the light having entangled photons

either by using spontaneous parametric down conversion (SPDC) in our lab, the

possible entanglement of quantum magnonic states and photonic states may yield

interesting results like the storage of quantum information and their manipulation.

• Our second laser in the experiment has unpolarized red light. We can use the beam

splitter to use the same laser which has linearly polarized light, for exciting the

second sample. Thus, observing entanglement in the presence of linearly polarized

light.

• Using femtosecond laser installed in the lab, repeating the experiment can further

reveal the details of entanglement occurring at pico- and femtoseconds time

resolution. As entanglement of states usually lasts in micro or milliseconds, the

resolution at femtoseconds is expected to present a better depiction.

• We can perform the same experiment on ferrimagnets like Bi-YIG to measure the

Faraday rotation and entanglement.

• As an extension of first experiment, the change in electrolyte to ionic liquids can

give more electrochemical potential window to apply higher biased voltages with

better stability at room and low temperatures down to 230 K.

List of Publications

• Chidubem Nwokoye, Abid Siddique, Lawrence H. Bennett, Edward Della Torre

“Quantum entanglement of magnons confined in multilayered Co/Pd

ferromagnets”, International Journal of Magnetism and Electromagnetism 3:009

(2017)

• Abid Siddique, Chidubem Nwokoye, Lawrence H. Bennett, Edward Della Torre,

“Electrochemical tuning of magnetic properties in metallic nanostructures” In

preparation (2017)

• Abid Siddique, Shu Gu, R. Witte, Mohammadreza Ghahremani, Chidubem

Nwokoye, Amir Aslani, R. Kruk, Virgil Provenzano, Lawrence H. Bennett and

Edward Della Torre. “Electric Field-Controlled Magnetization Switching in Co/Pt

Thin Film Ferromagnets.” Cogent Physics (2016), 3: 1139435

• Chidubem A. Nwokoye, Lawrence H. Bennett, Edward Della Torre, Abid Siddique,

Frank A. Narducci, Mohamadreza Ghahremani, Khurram S. Khattak, “Low

Temperature Magneto-Optical Kerr Effect Experimental System with a Cryogen-

Free Sample Environment” International Journal of Nanoparticles and

Nanotechnology 1:001 (2015).

• Chidubem A. Nwokoye, Lawrence H. Bennett, Edward Della Torre, Abid Siddique,

Frank A. Narducci, “A New Technique for Measuring the Chemical Potential of

Magnons Confined in Nanostructures” International Journal on Magnetism and

Electromagnetism 1:001 (2015).

Others

• Mohammadreza Ghahremani, Amir Aslani, Abid Siddique, Lawrence H. Bennett,

and Edward Della Torre “Tuning the heat transfer medium and operating conditions

in magnetic refrigeration” AIP Advances 6, 075221 (2016)

• Khurram Khattak, Amir Aslani, Chidubem Nwokoye, Abid Siddique, Lawrence H.

Bennett and Edward Della Torre. “Magnetocaloric Properties of Metallic

Nanostructures.” Cogent Engineering 2: 1050324 (2015)

References

[ARN15] Arnold, C. et al.,” Macroscopic rotation of photon polarization induced by a single spin”, Nat. Commun. 6:6236 doi: 10.1038/ncomms7236 (2015).

[ASP82] A. Aspect, P. Grangier, and G. Roger, "Experimental realization of Einstein-

Podolsky-Rosen-Bohm Gedanken experiment - a new violation of Bell inequalities,"

Physical Review Letters 49 (2), 91-94 (1982).

[BEL64] Bell, J. S, "On the Einstein Podolsky Rosen Paradox" Physics. 1 (3) 195–200

(1964).

[BEN14] Edward Della Torre, and Lawrence H. Bennett, “Bose-Einstein

Condensation of Confined Magnons in

Nanostructures,” Journal of Modern Physics, 5, 693-705 (2014)

[BER08] Berezovsky, J., Mikkelsen, M. H., Stolz, N. G., Coldren, L. A. & Awschalom, D.

“Picosecond coherent optical manipulation of a single electron spin in a quantum dot”.

Science 320, 349–352 (2008).

[BER13] A. Bernand-Mantel, L. Herrera-Diez, L. Ranno, S. Pizzini, J. Vogel, D. Givord,

S. Auffret, O.Boulle, I. M. Miron and G. Gaudin, “Electric-field control of domain wall nucleation and pinning in a metallic ferromagnet”, App. Phys. Lett. 102, 12 (2013) 122406.

[BER13] Berrada, T, S. van Frank, R. B•ucker, T. Schumm, J.-F. Schaff, and J.

Schmiedmayer (2013), “Integrated Mach-Zehnder interferometer for Bose-Einstein condensates," Nat. Comm. 4, 2077.

[BER96] L. Berger, “Emission of spin waves by a magnetic multilayer traversed by a current”, Phys. Rev. B, 54, 9353-9358 (1996).

[BLA08] Blatt, R, and D. J. Wineland (2008), “Entangled states of trapped atomic ions,"

Nature 453, 1008.

[BLA12] Blatt, R, and C. F. Roos (2012), “Quantum simulations with trapped ions," Nat.

Phys. 4, 277.

[BLO30] F. Bloch, Z. Phys. 61, 206 (1930).

[BOH16] Bohnet, J G, B. C. Sawyer, J. W. Britton, M. L. Wall, A. M. Rey, M. Foss-Feig, and J. J. Bollinger (2016), “Quantum spin dynamics and entanglement generation with hundreds of trapped ions," Science 352 (6291), 1297-1301.

[BOR05] Borisov, P., Hochstrat, A., Chen, X., Kleemann, W. & Binek, C. Magnetoelectric switching of exchange bias. Phys. Rev. Lett. 94, 117203 (2005).

[BRU65] J. Bruyere, O. Massenet, R. Montmory and L. Neel, "A coupling phenomenon between the magnetization of two ferromagnetic thin films separated by a thin metallic film--Application to magnetic memories," IEEE Transactions on Magnetics, vol. 1, no. 1, pp. 10-12, Mar 1965.

[CHE06] Chen, X., Hochstrat, A., Borisov, P. & Kleeman, W. Magnetoelectric exchange bias systems in spintronics. Appl. Phys. Lett. 89, 202508 (2006).

[CHI03] Chiba, D., Yamanouchi, M., Matsukura, F. & Ohno, H. Electrical manipulation of magnetization reversal in a ferromagnetic semiconductor. Science 301, 943–945 (2003).

[CHI08] D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno,

Nature (London) 455, 515 (2008).

[CHI12] Chiba, D., Kawaguchi, M., Fukami, S., Ishiwata, N., Shimamura, K., Kobayashi,

K., & Ono, T. (2012). Electric-field control of magnetic domain-wall velocity in ultrathin cobalt with perpendicular magnetization. Nature communications, 3, 888.

[CHI15] Chidubem A. Nwokoye, Lawrence H. Bennett, Edward Della Torre, Abid

Siddique, Frank A. Narducci, Mohammadreza Ghahremani, and Khurram S. Khattak,

“Low Temperature Magneto-Optical Kerr Effect Experimental System with a Cryogen-

Free Sample Environment” IntL. J. Nanopartcls Nanotech., 1, 1-6 (2015).

[CHI16] Chidubem A. Nwokoye, “ Temperature Variation of Magneto-Optical

Propoerties in Nanostructured Ferromagnetic Thin Films” Dissertation (2016)

[CHI17] Chidubem A. Nwokoye, Lawrence H. Bennett, Edward Della Torre,

Mohammadreza Ghahremani, & Frank A. Narducci, “Magneto-optical and magnetic properties in a Co/Pd multilayered thin film,” Journal of Magnetism and Magnetic

Materials, 421, 230-233 (2017).

[CHO05] Chou, C. W. et al. Measurement-induced entanglement for excitation stored in remote atomic ensembles. Nature 438, 828–832 (2005).

[CHU08] Chu, Y. H. et al. Electric-field control of local ferromagnetism using a magnetoelectric multiferroic. Nature Mater. 7, 478–482 (2008).

[CUL09] B. Cullity and C. Graham, Introduction to Magnetic Materials, New Jersey: John Wiley,

2009.

[DEL05] Della Torre, E., L. H. Bennett, and R. E. Watson. "Extension of the Bloch T 3/2 law to magnetic nanostructures: Bose-Einstein condensation." Physical review letters

94.14 (2005): 147210.

[DEU85] D. Deutsch, “Quantum theory, the Church-Turing principle and the universal quantum computer”, The Royal Society of London A.400, pp 97-117, 1985.

[DUA06] Duan, C.-G., Jaswal, S. S. & Tsymbal, E. Y. Predicted magnetoelectric effect in

Fe/BaTiO3 multilayers: ferroelectric control of magnetism. Phys. Rev. Lett. 97, 047201

(2006).

[DUA10] Duan, L-M, and C. Monroe (2010), “Colloquium: Quantum networks with trapped ions," Rev. Mod. Phys. 82, 1209-1224.

[DW13] DWave systems website press release, [online], http://www.dwavesys.com/en/pressreleases.html#dwaveus_Google_NASA (2013)

[EER06] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–765 (2006).

[EIN35] Einstein, B. Podolsky, N. Rosen: "Can quantum-mechanical description of physical reality be considered complete?" Phys. Rev. 41, 777 (15 May 1935).

[EST08] Esteve, J, C. Gross, A. Weller, S. Giovanazzi, and M. K. Oberthaler (2008),

“Squeezing and entanglement in a Bose-Einstein condensate," Nature 455, 1216-1219.

[GRO10] Gross, C, T Zibold, E Nicklas, J Esteve, and M.K. Oberthaler (2010), “Nonlinear atom interferometer surpasses classical precision limit," Nature 464, 1165-1169.

[GW16] https://nic.gwu.edu/lithography

[HAF08] Haffner, H, C. F. Roos, and R. Blatt (2008), “Quantum computing with trapped ions," Phys. Rep. 469 (4), 155-203.

[HAM12] Hamley, C D, C. S. Gerving, T. M. Hoang, E. M. Bookjans, and M. S. Chapman

(2012), “Spin-nematic squeezed vacuum in a quantum gas," Nat. Phys. 8, 305.

[HOF12] Hofmann, J. et al. Heralded entanglement between widely separated atoms.

Science 337, 72–75 (2012).

[IMR13] https://www2.gwu.edu/~imr/facilities.htm

[KHU15] Khurram Khattak, Amir Aslani, Chidubem Nwokoye, Abid Siddique, Lawrence

H. Bennett and Edward Della Torre. “Magnetocaloric Properties of Metallic

Nanostructures.” Cogent Engineering 2: 1050324 (2015)

[KHU17] Khurram Khattak, E.D. Torre, “A Contrastive Analysis of Gd5 (Six Ge1-x) Based

Alloys Useful for Magnetic Refrigeration.” International Journal of Magnetics and

Electromagnetism 3, no 010 (2017).

[KRI12]. Kristiaan De Greve, Leo Yu, Peter L McMahon, Jason S Pelc, Chandra M

Natarajan, Na Young Kim, Eisuke Abe, Sebastian Maier, Christian Schneider, Martin

Kamp, Sven Höfling, Robert H Hadfield, Alfred Forchel, MM Fejer, Yoshihisa

Yamamoto,” Quantum-dot spin–photon entanglement via frequency downconversion to telecom wavelength”, Nature 491, 421–425 (2012).

[KRU16] Kruse, I, K. Lange, J. Peise, B. Lucke, L. Pezze, J. Arlt,W. Ertmer, C. Lisdat, L.

Santos, A. Smerzi, and C. Klempt (2016), “Improvement of an atomic clock using squeezed vacuum," Phys. Rev. Lett. 117 (14), 143004.

[LEI03] Leibfried, D, R. Blatt, C. Monroe, and D. Wineland (2003), “Quantum dynamics of single trapped ions," Rev. Mod. Phys. 75, 281-324.

[LUC11] Lucke, B, M. Scherer, J. Kruse, L. Pezze, F. Deuretzbacher, P. Hyllus, O. Topic,

J. Peise, W. Ertmer, J. Arlt, L. Santos, A. Smerzi, and C. Klempt (2011), “Twin matter waves for interferometry beyond the classical limit," Science 334, 773.

[MAR09] T.Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A.A.

Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, and Y. Suzuki, Nature

Nanotechnol. 4 (2009) 158.

[MAR16] Martinez, E A, C. A. Muschik, P. Schindler, D. Nigg, A. Erhard, M. Heyl, P.

Hauke, M. Dalmonte, T. Monz, P. Zoller, and R. Blatt (2016), “Real-time dynamics of lattice gauge theories with a few-qubit quantum computer," Nature 534 (7608), 516-519.

[MIE16] Mielenz, M, H. Kalis, M. Wittemer, F. Hakelberg, U. Warring, R. Schmied, M.

Blain, P. Maunz, D. L. Moehring, D. Leibfried, and T. Schaetz (2016), “Arrays of individually controlled ions suitable for two-dimensional quantum simulations," Nat.

Comm. 7, ncomms11839.

[MOE07] Moehring, D. L. et al. Entanglement of single-atom quantum bits at a distance.

Nature 449, 68–71 (2007).

[MOR05] Morimae, Sugita, and Shimizu, “Macroscopic entanglement of many-magnon states”,Physical Review A 71, 032317-1~12 (2005).

[MUE14] Muessel, W, H. Strobel, D. Linnemann, D. B. Hume, and M. K. Oberthaler

(2014), “Scalable spin squeezing for quantum-enhanced magnetometry with Bose-Einstein condensates,” Phys. Rev. Lett. 113, 103004.

[MUE15] Muessel, W, H. Strobel, D. Linnemann, T. Zibold, B. Julia Diaz, and M. K.

Oberthaler (2015), “Twist-and-turn spin squeezing in Bose-Einstein condensates," Phys.

Rev. A 92, 023603.

[MYE97] Myers, H. P., Introductory Solid-State Physics, 2nd. Ed., Taylor & Francis, 1997.

[NEE49] L. Néel, "Théorie du traînage magnétique des ferromagnétiques en grains fins avec applications aux terres cuites". Ann. Géophys. 5, 99–136 (1949).

[NIE00] Nie, X. & Blu ̈gel, S. Elektrisches Feld zur Ummagnetisierung eines dunnen

Films. European patent 19841034.4 (2000).

[OCK13] Ockeloen, C F, R. Schmied, M. F. Riedel, and P. Treutlein (2013), “Quantum metrology with a scanning probe atom interferometer," Phys. Rev. Lett. 111, 143001.

[OHN00] H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno, and K.

Ohtani: Nature 408 (2000) 944

[PLU11] M.L. Plumer, J. van Ek, W.C. Cain,”New Paradigms in Magnetic Recording” La

Physique Au Canada/ Vol. 67, No. 1(2011)

[REI87] P. H. Rieger, “Electrochemistry” Prentice-Hall International Inc. (1987) p. 70,

104.

[RIE10] Riedel, M F, P. Bohi, Y. Li, T. W. Hansch, A. Sinatra, and P. Treutlein (2010),

“Atom-chip-based generation of entanglement for quantum metrology," Nature 464, 1170-

1173.

[RIT12] Ritter, S. et al. An elementary quantum network of single atoms in optical cavities.Nature 484, 195–200 (2012).

[RON08] Rondinelli, J. M., Stengel, M. & Spaldin, N. Carrier-mediated magnetoelectricity in complex oxide heterostructures. Nature Nanotech.3, 46–50 (2008).

[ROO14] Roos, C (2014), “Quantum information processing with trapped ions," in

Fundamental Physics in Particle Traps, Springer Tracts in Modern Physics, Vol. 256, edited by Wolfgang Quint and Manuel Vogel (Springer-Verlag) pp. 253-292.

[SCH35] E. Schrödinger, "Die gegenwärtige Situation in der quantenmechanik", Naturwissenschaften 23: pp.807-812; 823-828; 844-849 (1935)

[SEM17] http://www.ammrf.org.au/myscope/sem/practice/principles/layout.php#detail

[SHI12] K. Shimamura, D. Chiba, S.Ono, S. Fukami, N.Ishiwata, M. Kawaguchi, K.

Kobayashi, and T. Ono, App. Phys. Lett. 100 (2012) 122402.

[SHU12] Shuo Gu, “Particle Size and Surface Charge Induced Variations in Magnetic

Properties of Metallic and Semiconducting Nanostructures” Dissertation (2012)

[SID16] A Siddique, Shu Gu, R. Witte, Mohammadreza Ghahremani, Chidubem

Nwokoye, Amir Aslani, R. Kruk, Virgil Provenzano, Lawrence H. Bennett and Edward

Della Torre. “Electric Field-Controlled Magnetization Switching in Co/Pt Thin Film

Ferromagnets.” Cogent Physics (2016), 3: 1139435

[SLO96] C. Slonczewski, “Current-driven excitation of magnetic multilayers”. J.

Magn.Magn. Mater. 159, L1-L7 (1996).

[STO08] Stolichnov, I. et al. Non-volatile ferroelectric control of ferromagnetism in

(Ga,Mn)As. Nature Mater. 7, 464–467 (2008).

[STU12] Stute, A. et al. Tunable ion-photon entanglement in an optical cavity. Nature 485,

482–485 (2012).

[WEB09] B. Weber, H. P. Specht, T. Müller, J. Bochmann, M. Mücke, D. L. Moehring, and G. Rempe,” Photon-Photon Entanglement with a Single Trapped Atom”, Phys. Rev.

Lett. 102, 030501 (2009)

[WEI07] Weisheit, M. et al. Electric field-induced modification of magnetism in thin-film ferromagnets. Science 315, 349–351 (2007).

[WIN13] Wineland, D J (2013), “Nobel lecture: Superposition, entanglement, and raising Schrodinger's cat," Rev. Mod. Phys. 85, 1103-1114.

[XU08] Xu, X. et al. Coherent population trapping of an electron spin in a single negatively charged quantum dot. Nat. Phys. 4, 692–695 (2008).

[ZIE05] M. Ziegler, “Computational Power of Infinite Quantum Parallelism”, International

Journal of Theoretical Physics, Vol.44, No.11, November2005.