ABSTRACT MAGNETISM and ASSOCIATED EXCHANGE BIAS EFFECTS in Mn2ni1+Xga1-X HEUSLER ALLOYS and SELECTED Fe DOPED DERIVATIVES By

ABSTRACT

MAGNETISM AND ASSOCIATED EXCHANGE BIAS EFFECTS IN Mn2Ni1+xGa1-x HEUSLER ALLOYS AND SELECTED Fe DOPED DERIVATIVES

by Sutapa Biswas

A series of Mn based intermetallic bulk Mn2Ni1+xGa1-x (0 ≤ x ≤ 0.65) alloys and melt- spun ribbons of selected Fe doped Mn2-xFexNi1.4Ga0.6 (x = 0.25, 0.5, 1) derivatives have been investigated for their magnetic and exchange bias properties. The bulk alloys were prepared by arc melting and annealing techniques while the ribbons were prepared by arc melting followed by melt spinning. All samples, bulk and ribbons, showed similar crystalline properties. For x > 0.2 all the Mn2Ni1+xGa1-x bulk samples showed exchange bias properties that enhanced with increasing Ni content. The Mn2-xFexNi1.4Ga0.6 (x <0.1) melt-spun ribbons showed exchange bias properties when measurements were done under both zero-field cooled and field cooled cooling conditions. Scanning electron microscopy images showed that no grain formation occurred in the bulk samples but well-defined grains formed on the surface of the melt-spun ribbons. The size of the grains increased with annealing, which significantly change the EB properties of the ribbons. The experimental results are presented and discussed in this Thesis.

MAGNETISM AND ASSOCIATED EXCHANGE BIAS EFFECTS IN Mn2Ni1+xGa1-x HEUSLER ALLOYS AND SELECTED Fe DOPED DERIVATIVES

A Thesis

Submitted to the

Faculty of Miami University

in partial fulfillment of

the requirements for the degree of

Master of Science

Sutapa Biswas

Miami University

Oxford, Ohio

2020

Advisor: Prof. Mahmud Khan

Reader: Prof. Herbert Jaeger

Reader: Prof. Steve Alexander

This thesis titled

MAGNETISM AND ASSOCIATED EXCHANGE BIAS EFFECTS IN Mn2Ni1+xGa1-x HEUSLER ALLOYS AND SELECTED Fe DOPED DERIVATIVES

Sutapa Biswas

has been approved for publication by

The College of Arts and Sciences

and

Department of Physics

______Prof. Mahmud Khan

______Prof. Herbert Jaeger

______Prof. Steve Alexander

Table of Contents

CHAPTER 1 INTRODUCTION 1 CHAPTER 2 THEORY 4 2.1 Atomic Origin of Magnetism ……………………………. 4 2.2 Classification of Magnetic Materials …………………….. 5 2.2.1 Diamagnetic Materials ……………………………… 5 2.2.2 Paramagnetic Materials ……………………………... 6 2.2.3 Ferromagnetic Materials …………………………….. 7 2.2.4 Antiferromagnetic Materials ………………………… 9 2.2.5 Ferrimagnetic Materials ……………………………...10 2.3 Exchange Bias ……………………………………………...10 2.4 EB in Nanostructured Materials ……………………………12 CHAPTER 3 EXPERIMENTAL TECHNIQUES 15 3.1 Sample Fabrication ……………………………………………..15 3.2 Magnetization Measurement …………………………………...16 3.3 X-ray Diffraction Measurements ……………………………….18 3.3.1 Basic Principles of XRD …………………………….. 18 3.3.2 X-ray Diffractometer ………………………………… 19 3.3.3 Sample Preparation for XRD ………………………… 21 3.4 Scanning Electron Microscope (SEM) ………………………….21 CHAPTER 4 RESULTS AND DISCUSSION 24

4.1 The Structural and magnetic properties of Mn2Ni1+xGa1-x………24

4.1.1 Structural properties of Mn2Ni1+xGa1-x………………...24

4.1.2 Magnetic and EB properties of Mn2Ni1+xGa1-x………...26

4.2 The magnetic and structural properties of Mn2-xFexNi1.4Ga0.6 melt-spun ribbons………………………………………………………………..29

4.2.1 The magnetic properties of Mn2-xFexNi1.4Ga0.6 as-prepared melt-spun ribbons……………………………………………..30

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4.2.2 The magnetic properties of the annealed ribbons………….34

4.2.3 The crystalline properties of Mn1.75Fe0.25Ni1.4Ga0.6 ribbons …………………………………………………………………...36 CHAPTER 5 CONCLUSION 39 REFERENCES 40

List of Tables

Table 4.1 HEB, Hc, and Ms values obtained at 10 K obtained under ZFC conditions for the as-prepared ribbons …………………………………………………………….33

Table 4.2 HEB, Hc, and Ms values obtained at 10 K obtained under FC conditions for the as-prepared ribbons …………………………………………………………….33

List of Figures

Fig. 1.1 Positions of the Heusler alloy elements in the periodic table. .………………….1

Fig. 2.1 Behavior of diamagnetic and paramagnetic materials in the presence of external magnetic field and their temperature dependence………………………………………...6

Fig. 2.2 Magnetic dipole moment configuration of a paramagnetic material before and after the external field is applied………………………………………………………….7

Fig. 2.3 Magnetic hysteresis cycle in ferromagnetic materials. Non-linear response of magnetization (M) in applied field (H)……………………………………………………8

Fig. 2.4 Antiparallel spin alignment in antiferromagnetic materials……….………...... 9

Fig. 2.5 Magnetic moments of ferrimagnetic materials………………………………….10

Fig. 2.6 Shifted hysteresis loop with exchange bias effect………………………………11

Fig. 3.1 Sample in (a) bulk material form (b) nanostructured ribbons form…………….16

Fig. 3.2 PPMS dewar and probe a) PPMS probe b) PPMS sample region with the cross section……………………………………………………………………………………17

Fig. 3.3 Bragg’s law diagram showing the path of incident and diffracted beam……….19

Fig. 3.4 Schematic diagram of X-ray tube used in laboratories………………………….20

Fig. 3.5 Components of a diffractometer and their positions during the experiment……………………………………………………………………………21

Fig. 3.6 Schematic diagram of SEM with the basic components of the instrument…………………………………………………………………………22

Fig. 4.1 Room temperature XRD patterns for selected Mn2Ni1+xGa1-x bulk samples. The peaks marked by “*” are related to the Pnnm structure………………………………….25

Fig. 4.2 SEM images Mn2Ni1+xGa1-x with x =0.00 (left) and x=0.50 (right)…………….26

Fig. 4.3 Temperature dependence of the dc magnetization of selected Mn2Ni1+xGa1-x samples measured at 1 kOe……………………………………………………………….27

Fig. 4.4 Magnetization as a function of Magnetic field measured at 5 K under ZFC and FC conditions for Mn2Ni1+xGa1-x (a) x = 0.0, (b) x = 0.20, (c) x = 0.35, and (d) x = 0.50……………………………………………………………………………………….28

Fig. 4.5. Magnetic field dependence of the dc magnetization for the Mn2Ni1+xGa1-x (x = 0.55, 0.60) samples measured at 5 K under ZFC and FC conditions…………………….29

Fig. 4.6 Magnetic field dependence of the dc magnetization obtained at 10 K under ZFC condition for the as-spun ribbons samples ………………………………………………31

Fig. 4.7 Magnetic field dependence of the dc magnetization at the lower field region obtained at 10 K under ZFC condition for the as-spun ribbons samples ……………………………………………………………………………………………31

Fig. 4.8 Magnetic field dependence of the dc magnetization obtained at 10 K under FC condition for the as-spun ribbons samples ……………………………………………….32

Fig. 4.9 Temperature dependence of magnetization for Mn1.75Fe0.25Ni1.4Ga0.6 as-spun ribbon measured at magnetic fields of (a) H = 50 Oe and (b) H = 1 kOe ……………………………………………………………………………………………34

Fig. 4.10 Magnetization vs applied field for annealed Mn1.75Fe0.25Ni1.4Ga0.6 ribbons measured at 10 K under a) FC and b) ZFC protocol……………………………………..34

Fig. 4.11 Magnetization as a function of temperature for annealed Mn1.75Fe0.25Ni1.4Ga0.6 ribbons measured at an applied magnetic field of 1 kOe………………………………….35

Fig. 4.12 Room temperature XRD pattern for as-spun nanostructured ribbon sample

Mn1.75Fe0.25Ni1.4Ga0.6…………………………………………………………………….36

Fig. 4.13 SEM image for Mn1.75Fe0.25Ni1.4Ga0.6 before annealing. The line (in blue) shows about 4 grains in the nanostructure formation……………………………………………37

vii

Fig. 4.14 SEM images for annealed sample Mn1.75Fe0.25Ni1.4Ga0.6. Free surface showing the grains formation (on the left), wheel surface (on the right)…………………………..37

viii

Acknowledgements

I would like to thank Dr. Khan for his help and guidance with this thesis project. I would also like to extend my gratitude to our research group members for their help and support throughout the completion of the project.

Chapter 1 Introduction

Magnetism and magnetic materials have greatly contributed to advancing the modern field of science and engineering. The functionality of the society that we live in today greatly relies on the discovery of new materials with novel magnetic properties. For example, the increase of the use of permanent magnets in numerous electronic devices and wind power generation demands the development of new, more powerful and cheaper permanent magnets.1 The successful development of new materials require extensive research. Therefore, in this thesis, a series of intermetallic materials, known as Heusler alloys, have been investigated with the aim of contributing to the ongoing research on magnetic materials.

Fig. 1.1. Positions of the Heusler alloy elements in periodic table.2

Heusler alloys are a group of intermetallic compounds that were first reported in 1903 by Fritz Heusler. They are generally represented by the two formulas: (i) XYZ for half-Heusler alloys and

(ii) X2YZ for full Heusler alloys. In both representations, X and Y are transition metals or selected lanthanides and Z represents a main group element (see Fig. 1.1). These class of compounds have become a great interest due to their special feature of displaying a certain characteristic that is completely different from their constituent elemental properties. For instance, Cu2MnAl alloy exhibits ferromagnetic properties, but neither Cu, Mn, nor Al are ferromagnetic by themselves.

TiNiSn compound is semiconducting although, the constituent elements are conductors.2 The magnetic properties of Heusler alloys are greatly studied for their potential applications in spintronics devices such as magnetic tunnel junctions (MTJS), giant magneto resistive sensor

(GMR) devices, and many others. Since the discovery of Cu2MnAl alloy and its interesting properties at a time when quantum physics was not known, discovery of Cu2MnAl alloy caused a great research interest. All Heusler alloys share the L21 cubic structure at room temperature.

Heusler alloys are also known to exhibit exchange bias (EB) properties. The EB phenomenon was first overserved in 1956 by Meiklejohn and Bean in Co/CoO nanoparticles.3, 4, 5 The EB effect in a material is signified by the shift of its isothermal magnetic hysteresis loop along the magnetic field axis. The exploitation of exchange anisotropy started in 1970s, when it was used to tune the read head to a highest point of sensitivity in magnetoresistive recording head for the first time.6 Over time, a keen research interest in this phenomenon has been developed, which broadened its fundamental aspects and facilitated the existence of many technological applications including spin valve, and magnetoresistive sensors, and storages.

EB properties have been reported in several Mn based bulk material systems.7, 8, 9 As will be described in detail in chapter 2, it is broadly accepted that EB effect results from the exchange coupling between well-defined FM and AFM layers present in the material. For the bulk material systems, there is no well-defined boundary between FM/AFM materials which makes it a complicated system and different from the traditional systems where EB properties have been observed. Hence, understanding the EB mechanism in the bulk material system becomes more difficult. These phenomena occurring in bulk system are rare and have potential ranges of applications, including design of new permanent magnets. Therefore, investigation of EB properties in bulk materials is of great importance.

Although, EB effects have been reported for several Mn-rich Heusler alloys, not all Heusler alloys with excess Mn and co-existing FM/AFM interactions exhibit EB. Mn2NiGa is a Heusler alloy that exhibit the I4/mmm tetragonal structure at room temperature.10 The material exhibit a martensitic phase transformation at 328 K and has a Curie temperature of 588 K.10, 11 Ideally, the 4d atomic site is shared by the Mn (MnNi) and Ni atoms, and the remaining Mn (MnMn) and the Ga atoms occupy the 2a and 2b sites, respectively. However, due to antisite disorder the Mn atoms also occupy the 2b (MnGa) sites. While the MnNi-MnMn and MnNi-MnGa moments in

Mn2NiGa show anti-parallel alignment, the MnGa-MnMn moments couple ferromagnetically. However, the material does not exhibit any EB effect.10 The absence of the phenomenon was attributed to the relatively smaller size of the FM clusters in the compound. Mn2Ni1.4Ga0.6 (a 12, 13 Mn2NiGa derivative) on the other hand has been reported to exhibit EB effect. Coexistence of short range FM ordering and reentrant SG interactions were proposed to be the primary reason for the observation of EB in the material.

A study on the Mn2Ni1+xGa1-x system showed interesting phenomena. The parent compound of this series Mn2NiGa did not show any EB effect. But, as x increased, EB effect started to appear in the system (the results of this experiment will be discussed in chapter 4). A strong EB effect was observed in the material with x = 0.4. Motivated by these preliminary results, a series of Fe doped Mn2-xFexNi1.6Ga0.4 materials were synthesized and characterized for their EB properties.

The results of this study along with the experimental results on the Mn2Ni1+xGa1-x system is presented and discussed in this Thesis. The Mn2-xFexNi1.6Ga0.4 materials were synthesized in rapidly solidified ribbon form. When alloys are prepared in this form, grains with sizes ranging from few nanometers to several micrometers may form. It has been showed that the sizes of these grains strongly influence the magnetic properties of the materials. Therefore, it is interesting to explore the EB properties of Mn2-xFexNi1.6Ga0.4 where Mn is partially replaced by Fe to control the FM/AFM interactions.

The rest of the Thesis is written in the following format. A brief discussion on the basics of magnetism and EB effect is presented in chapter 2. Chapter 3 discusses the experimental techniques and in chapter 4 results and discussions are presented. Finally, chapter 5 presents the conclusion and plans for future work.

Chapter 2

Theory

2.1 Atomic Origin of Magnetism

Magnetism is a combined effect of spin and orbital angular momenta of an electron. Spin and orbital angular moments are quantum mechanical concepts which arise from the Schrodinger equation, which leads to quantization of atomic orbital angular momentum of electron. According to the Schrodinger equation there are four quantum numbers required to have a meaningful wavefunction. These quantum numbers are restricted to following integer values: n = 1, 2 ,3, ……. l = 0, 1, 2, ……… n-1 ml = -l, -l +1, ….. l-1, l n is the principal quantum number that represents the energy level of an electron. The orbital quantum number, l, defines the angular momentum of an electron, and the orientation of this angular momentum is represented by the magnetic quantum number, ml. Two additional quantum numbers are used to describe the state of an electron. The spin quantum number, denoted by s, has 1 the value . The magnitude of the spin orbital angular momentum of an electron is given by |S| = 2 3 √푠(푠 + 1) ħ = ħ. This is analogous to the orbital angular momentum |L| = √푙(푙 + 1) ħ. The 2 quantum number, ms , is the spin analog to the magnetic quantum number ml, and can take values 1 1 of - and + , only.6 2 2

Iron is always thought of as the perfect example of magnetic material that it either can be magnetized or demagnetized in the presence of an external magnetic field. The piece of iron that is under the influence of an external magnetic field retains the previous state of magnetization as the field is being removed. In the atomic level, each atom in the piece of iron contains a particle property known as magnetic moment which is influenced by the external applied field and the

4 magnetic moment orientation of its neighboring atoms. The magnetic moment originates from the combined effect of orbital and spin angular momentum of an electron. Thus, in macroscopic level magnetism is observed. Russell-Saunders scheme of spin-orbit coupling can be useful to determine the magnetic moment of an atom.14 In the scheme, the total angular momentum

ħ퐿⃗ 푎푡표푚 = ħ ∑푖 퐿⃗ 푖 , and the total spin angular momentum ħ푆 푎푡표푚 = ħ ∑푖 푆 푖 , are combined to obtain a total angular momentum, ħ퐽 = ħ퐿⃗ 푎푡표푚 + ħ푆 푎푡표푚. The magnetic moment of the atom is directly proportional to the total angular momentum and is determined by 휇푎푡표푚 = − 푔휇퐵퐽 , 퐽(퐽+1)+푆(푆+1)−퐿(퐿+1) where g is the splitting factor and is determined by g = 1+ . 2퐽(퐽+1)

2.2 Classification of Magnetic Materials

Magnetic materials are generally categorized by the nature of their response to an external magnetic field, H. Three parameters are generally looked at when discussing a magnetic material. 휇 The magnetization, 푀 = 푡표푡푎푙, is defined as the net magnetic dipole moment per unit volume of 푉 the material. When a material is placed in a magnetic field, the total field inside the material is expressed as, 퐵 = 휇0(푀 + 퐻); 휇0 is the magnetic permeability of free space. The third parameter 푀 is the magnetic susceptibility, which is expressed as, 휒 = , and is a measure of the tendency of 퐻 a material being magnetized in an external magnetic field.

Magnetic moment of a free electron can originate from three principle sources: the spin of an electron, the orbital momentum about the nucleus, and the change in orbital momentum due to applied magnetic field. The first two effects are responsible for paramagnetic contribution and the third effect gives rise to diamagnetic effect. Diamagnetism is the weakest magnetic phenomenon of them all. It occurs in all atoms which have no net magnetic moment present and either their outer electron shell is completely full or empty. In other materials, the diamagnetism effect is overshadowed by the stronger effect of ferromagnetism or paramagnetism. This section will focus on introducing different types of magnetic materials.

2.2.1 Diamagnetic Materials

All materials exhibit a diamagnetic response when there are no other magnetic behaviors present. Diamagnetic susceptibility is negative that is, magnetization decreases with increasing magnetic

5 field. Diamagnetism originates from the orbital motion of the electrons. The circulating electrons around an orbit acts like a current loop. The applied magnetic field aligns in the direction of the electron's path and generates the current in the loop which opposes the change in the loop due to applied magnetic field by exerting an induced magnetic field that aligns in the opposite direction of applied magnetic field. This phenomenon results in diamagnetic behavior. Mathematically this opposing tendency of matter in the presence of an external magnetic field is expressed by the negative sign in magnetic susceptibility. Fig. 2.1(a) shows the magnetic susceptibility in diamagnetic materials. Fig. 2.1(b) shows no change in magnetic susceptibility with changing temperature as diamagnetic materials do not have any temperature dependence.

Fig.2.1. Behavior of diamagnetic and paramagnetic materials in the presence of external magnetic field and their temperature dependence.15

2.2.2 Paramagnetic Materials

According to the Langevin model, the origin of the paramagnetism comes from the unpaired non- interacting electrons. Most of the electrons are in pairs with their spin aligning in the opposite direction from each other due to Pauli exclusion principle. These paired electrons have no magnetic moments but orbital motion of these electron pairs can give rise to diamagnetism. In paramagnetic material, the electron that remains unpaired shows non-zero permanent magnetic moment. In the absence of an externally applied magnetic field, these magnetic moments are randomly orientated and show zero net magnetic moment. With an applied magnetic field, magnetic moments within

6 paramagnetic materials align with the external field direction. Fig.2.2 shows the paramagnetism in a material before and after the magnetic is applied.

Fig. 2.2. Magnetic dipole moment configuration of a paramagnetic material before and after the external field is applied.16

After removal of the applied magnetic field, paramagnetic materials do not retain their magnetization. Copper, cobalt, iron, nickel, chromium, gadolinium are some examples of paramagnetic materials. Unlike diamagnetism, paramagnetism is temperature dependent. At significantly high temperature, it becomes difficult for the magnetic moments to align with the 퐶 applied field. This behavior is experimentally proven by Curie’s law χ = where, C is the curie 푇 constant and T is the temperature given in kelvin. Fig. 2.1(c-d) shows paramagnetic behavior where the slope of M vs H is positive, and the susceptibility drops at high temperature.

2.2.3 Ferromagnetic Materials

Unlike diamagnetic and paramagnetic materials, a ferromagnetic (FM) material can retain the magnetic field even after the external magnetic field has been removed. This type of material is easy to magnetize and in a strong magnetic field the magnetization of the material reaches to a certain limit known as saturation magnetization, Ms. The relationship between M and H is non- linear in ferromagnetic material, it rather exhibits a hysteresis loop as shown in Fig. 2.3.

Fig. 2.3. Magnetic hysteresis cycle in ferromagnetic materials. Non-linear response of magnetization (M) in applied field (H).17

With an increase in applied field, magnetization increases to a saturation point where no further increase is possible. This saturation point is an intrinsic property and does not depend on the volume or the size of the material. When the applied field is reversed back to zero a significant amount of magnetization remains in the sample, which are known as remnant magnetization, MR. With further reversal of the magnetic field, magnetization reaches zero to a certain value of the applied field strength denoted as the point of coercivity, HC. The magnetization reaches a saturation point in the opposite direction and repeats the cycle.

Fe, Co, and Ni are examples of ferromagnetic materials at room temperature. The temperature dependence of ferromagnetic materials is given by Curie-Weiss law:

퐶 χ = 푇−푇퐶 where TC is the Curie temperature. Above TC ferromagnetic materials become paramagnetic due to thermal agitation. Ferromagnetic domains consist of small regions of magnetic dipoles that align in parallel to each other in the presence of an external magnetic field. Without the external

8 magnetic field, the magnetization vectors in different domains have different orientations, resulting in net magnetization as zero.

Domains are an integral part of demonstrating ferromagnetism in ferromagnetic materials. The process of occurring domains starts from a quantum mechanical phenomenon known as exchange energy which provides strong driving force in aligning magnetic moments parallel to each other. However, a single domain would try to minimize the total energy. Therefore, formation of multiple domain regions would allow the material to minimize the exchange component of the energy to the total magnetic energy.

2.2.4 Antiferromagnetic Materials

These types of materials consist of two interpenetrating identical sublattices of magnetic ions. Unlike ferromagnets, magnetic moments in antiferromagnetic (AFM) materials are aligned anti parallel to each other. One of the two sets of identical magnetic ions are spontaneously magnetized below the Neel temperature (some critical temperature) and the other set is also spontaneously magnetized but in the opposite direction. Thus, this type of material has no net spontaneous magnetization. Fig. 2.4 shows the antiparallel spin alignment in ferromagnetic materials.

Fig. 2.4. Antiparallel spin alignment in antiferromagnetic materials.18

퐶 Temperature dependence in antiferromagnets is given by Curie-Weiss law: χ = where, TN 푇+푇푁 is the Neel temperature. Above this Neel temperature the magnetic moments in antiferromagnetic materials become randomly orientated therefore paramagnetic.

2.2.5 Ferrimagnetic Materials

Ferrimagnets are similar to ferromagnets in many ways. Like ferromagnets, ferrimagnets have magnetization below a certain temperature, even in the absence of external magnetic fields. Ferrimagnets also have two sublattices of magnetic ions. However, net magnetic moments between these sublattices are nonzero as they are not identical. Similar to ferromagnets, these magnetic moments in two sublattices tend to align antiparallel to each other, since they are not identical, they do not completely cancel out. Ferrimagnetic materials are temperature dependent but sublattices do not vary monotonically with temperature. Hence, accounting for the temperature effect on magnetization in ferromagnets becomes more complicated. Fig. 2.5 shows the spin orientation in ferrimagnets.

Fig.2.5. Magnetic moments of ferrimagnetic materials19

2.3 Exchange Bias

The Exchange Bias (EB) effect in a material is signified by the shift of its isothermal magnetic hysteresis loop along the magnetic field axis. Fig. 2.6 Shows the mechanism of the exchange bias effect. When a material with ferromagnetic (FM) and anti-ferromagnetic (AFM) interfaces (FM/AFM) is cooled in the presence of an applied magnetic field through the Neel temperature 8, 20, (TN) of the AFM layer, an unidirectional magnetic anisotropy or EB is induced in the system. 21 The temperature from which the material is cooled is such that TC > T > TN, where TC is the 10

Curie temperature of the FM layer. In order to achieve EB condition in the system TC must be 22 greater than TN.

Fig. 2.6. Shifted hysteresis loop with exchange bias effect.23

Under the applied temperature condition of TC > T > TN, the AFM layer behaves as paramagnetic state and all the moments are randomly oriented. When cooling in the presence of a magnetic field through the temperature T

Sb) have been reported to exhibit EB properties8, 20 where the Mn moments show both parallel and antiparallel alignments. Although the composite alloy does not manifest EB properties by itself, EB can be induced in the system by manipulating magnetic interactions and stoichiometric variations. EB effect has also been reported in zero field cooling (ZFC).7, 8, 20

Heusler alloy Mn2Ni1+xGa1-x series has also been reported to exhibit exchange bias (EB) properties in martensitic phase at low temperature.21 Martensitic phase transition is a structural phase transition. The mechanism is this transformation is about the transition from face-centered cubic lattice to body-centered tetragonal structure. This transformation happens due to small relative displacement in neighboring atoms.24 The regular Mn sublattice is ferromagnetic (FM). The excess Mn atoms occupying the Ga sites couple antiferromagnetically with Mn atoms on the regular sites.22 The co-existence of FM and AFM materials create magnetic disorder in the system, and thus, inducing spin-glass (SG) phase at low temperature. A SG phase in a magnetic system is described by the local static magnetization which is frozen in different directions. Although every intermetallic system with Mn atoms does not exhibit EB properties, by changing stoichiometric variation EB can be induced in the system. Therefore, for the preliminary experiment on the

Mn2Ni1+xGa1-x series the concentration of Mn was kept unchanged, but Ni and Ga concentrations were gradually changed to understand the Mn-Mn interactions.

Usually exchange bias is characterized by an asymmetric shift of the magnetic hysteresis loop in the opposite of the cooling field direction. A shift in the hysteresis loop along the cooling field direction is termed as positive exchange bias (PEB). PEB is a rare phenomenon which was first 25 observed for FeF2/Fe bilayer thin films. It is attributed to AFM exchange coupling in the interface being strongly dependent on the cooling field.25, 26 This exchange coupling has been observed in several AFM/FM systems.26, 27 For large cooling fields, the magnetic hysteresis loop has been found to be shifted in the positive direction.

2.4 EB in Nanostructured Materials

Materials with well-defined grain size can be prepared from bulk alloys by using the melt spinning technique. The size of the grains in melt-spun ribbons may vary from < 10 nm to several hundred nm. It has been shown that Heusler alloys and related intermetallic systems exhibit enhanced EB properties when they are fabricated in ribbon form.

For example, Pan, H. et al., studied a Mn based system Mn50Ni30Al12 alloy in ribbon form. A large

EB field was reported for this system. The maximum HEB of 5300 Oe with field cooling protocol was observed in the material. In Mn based systems the Mn-Mn atomic interactions depend on the distance between the neighboring atoms. In Mn50Ni30Al12, antisite disorder creates competition between AFM/FM interactions, magnetic clusters, and super spin glass (SSG) in the martensite phase. Therefore, antisite disorder is noted to be responsible for achieving large HEB value for this system.28

Chen, J. et al., studied zero field cooled EB in Mn50Ni41Sn9 ribbon by conducting magnetoresistance measurements (MR) along with magnetic (MvH) measurements. Ribbons in this system has average grain size of 3000 nm. The value of HEB was determined from M-H loop and MR loop with FC and ZFC protocols. Right and left coercivities were determined from M-H M MR loops and MR-H curve which are reported in FC mode as left coercivity H1 = -3088 Oe , H1 = M MR -2809 Oe and right coercivity H2 = 444 Oe, H2 = 587 Oe. Coercivity values are reported in M MR M ZFC mode as left coercivity H1 = -2068 Oe, H1 =-2063 Oe and right coercivity H2 = -456 MR Oe, H2 = -235 Oe. The reported HEB values in FC mode are 1322 Oe (from M-H loop) and 1111 Oe (from MR-H curves), in ZFC mode 1262 Oe (from M-H loop) and 1149 (from MR-H curves).

The HEB values were obtained for ZFC and FC measurements from magnetization and magnetoresistance shows satisfactory consistence.29

Han, X. et al., studied changes in the microstructure, magnetic properties, and EB in

(Mn0.7Co0.3)65Sn34 after annealing at 1073 K for 1 h. Maximum HEB value 800 Oe was reported to observe with FC MvH hysteresis loop at temperature 2 K. The average grain size of this system before annealing was reported around 930 nm. After annealing the grain sizes increased up to 8410 nm.30

L. González-Legarreta et al. studies on Ni45.5Mn43.0In11.5 alloy ribbon showed a change in magnetic crystal structure after annealing. The crystal structure for as-spun ribbon has monoclinic structure at 100 K temperature, and at 300 K temperature a pure austenite cubic phase was determined from the XRD peaks. After annealing at 973 K temperature, the XRD measurements shows a monoclinic 10M modulated structure for all the temperatures 150 K, 300 K, 350 K that the measurement was collected. Average grain sizes were observed in between 500 and 2000 nm for as-spun ribbon. The grain size has been expanded in size and has grown in between 1000 and 4000 nm. A clear EB

13 reported to observe at T = 5 K and HEB value was determined as 270 Oe or as-spun ribbon sample.

The HEB value increases up to 1210 Oe for annealed ribbon sample. Coercive field, Hc was determined as 310 Oe for as spun ribbon and 440 Oe for annealed ribbon.31

Chapter 3 Experimental Techniques

The experimental procedures utilized in this thesis project can be categorized into three steps: (i) sample fabrication, (ii) magnetization measurements, and (iii) structural analysis. In this chapter, the experimental methods are briefly discussed.

3.1 Sample Fabrication Two sets of samples were investigated in this study. The first set, were the Mn rich bulk

Mn2Ni1+xGa1-x (0 ≤ x ≤ 0.65) samples, which were fabricated by Tiago Schaeffer. The Samples were arc melted under an argon gas atmosphere using high-purity Mn, Ni, and Ga metals. To obtain homogeneity, the samples were sealed in a partially evacuated (partially filled with inert gas) quartz tube and annealed at 800 oC for 3 days in a high temperature tube furnace. The second sets of samples were Mn2-xFexNi1.6Ga0.4 (x = 0, 0.25, 0.5, 1). These samples are Fe doped derivatives of the Mn2Ni1+xGa1-x (x = 0.6) sample. As discussed in chapter 4, this sample shoed enhanced EB properties in the bulk form. Therefore, this sample was selected for fabricating melt- spun ribbons to explore the effects of nanostructuring on the EB properties of the alloy. TO control the AFM/FM phase concentrations in the materials, Fe was added and three additional samples were prepared. Approximately 9 g of each sample was prepared by arc melting.

The as-melted samples were sent to the University of Nebraska where the ribbons were prepared by melt-spinning technique. The technique involves a molten mixture that is formed by melting an ingot and then injected onto a surface of a rotating copper wheel where it rapidly solidified into ribbons.30 For preparing the ribbons in this study, the arc melted ingots were broken into pieces and loaded into the melt spinner inside quartz tube. The alloys were induction melted at 2700W output power. The molten alloy was quenched onto the cupper wheel rotating at 25m/s. The nozzle diameter of quartz tube used in melt spinning was 0.76 mm and was kept at a distance 5mm above the rotating wheel. The ejection pressure of 200 mbar was used. To prepare for the magnetization measurements, the nanostructured ribbons were annealed at temperature 600oC for 30 minutes. Fig. 3.1 shows visual differences between samples prepared in bulk form and nanostructured ribbons form.

Fig. 3.1. Sample in (a) bulk material form (b) nanostructured ribbons form.

3.2 Magnetization Measurement A Physical property measurement systems (PPMS), manufactured by Quantum Design Inc (model 6000, with magnetic field controller model 6700) was used to measure the magnetic properties of the samples. The PPMS is capable of performing automated measurement of multiple physical properties of a sample such as magnetization, heat capacity, magnetic torque, hall effect, DC resistivity, and other thermal and magnetic properties. For this project, the VSM (vibrating sample magnetometer) option of the PPMS was used. With this option, magnetization measurements as a function of field and temperature can be performed in temperature range of 1.75 K – 400 K and at magnetic fields of up to 9 T. A brief description of the major components of the PPMS system is given below.

The Dewar and the Probe The PPMS Dewar is a multi-jacketed system that primarily contains the liquid-helium bath in which the probe is immersed. To conserve reduce the liquid helium boil-off, the main liquid compartment is surrounded by a vacuum jacket, which is further surrounded by a liquid nitrogen Jacket. The Probe is located inside the Dewar and immersed in liquid-helium bath. It incorporates the temperature-control hardware, the superconducting magnets, the helium-level meter, the gas line, the sample puck connectors, and various electrical connections. Major components of the probe include the sample chamber, impedance assembly, optional magnets, baffled rods, and probe head. Sample chamber is located in between two vacuum tubes. The lower part of the sample

16 chamber is constructed of copper and the base of the sample chamber contains a 12-pin connector that connects it to an installed sample puck. The impedance assembly controls helium flow into the cooling annulus from the Dewar. The cooling annulus refers to the region between the sample chamber and inner vacuum tube. Baffled rods run through the probe and contain electrical connections from the impedance assembly to the magnet. Probe head contains the access port that allows samples to be placed into the sample chamber. Probe head also has two helium-fill ports, and the connection ports for all the gas and electrical lines. Fig. 3.2 shows the sample probe design and cross section.

Fig. 3.2. PPMS dewar and probe a) PPMS probe b) PPMS sample region with the cross section.32

Model 6000 PPMS Controller It is an integrated user interface that contains all the electronics and the gas-control valves. The front panel of the model 6000 contains a power button, display screen, menu keys, and two status LED’s. The status of the LED’s lights up when an error is logged into the data file during and during remote control of the system. The rear panel has all the connections necessary including

17 necessary PPMS connections, optional Quantum hardware connections, and auxiliary connections that accommodate interfacing with other devices.

The VSM The VSM synchronously detects the induced voltage by oscillating the sample at high frequency near a detection coil. The VSM option for PPMS consists of the following major components: linear motor transport (head), a coilset puck and, electronics equipment. The function of linear motor transport is to introduce vibration in order to oscillate the sample. The coilset puck works on detecting the induced voltage. Electronics equipment contributes to driving the linear motor transport and detecting the voltage response from the pickup coil. A copy of a multi-vu software application for automation. The VSM coil function is not designed to be affected by the large magnetic field. Therefore, VSM can work with PPMS to conduct sensitive measurements along with the high magnetic field that is produced by the PPMS.

3.3 X-ray Diffraction Measurements The crystalline properties of the samples were determined by x-ray diffraction (XRD) measurements. A brief description of the basic principles of XRD and the diffractometer used is given below.

3.3.1 Basic Principles of XRD In 1912 Max Von Laue showed a diffraction pattern from a single crystal of hydrated copper sulphate (CuSO4.5H2O) using x-ray diffraction The Diffraction pattern was observed, due to the periodicity in the lattice structure of that crystal. When using XRD, the atomic periodicity in crystal lattice structure permits the x-ray to project the atomic distribution in the crystal lattice with high precision. The diffraction pattern acquired from this method is an inverse transformation of an ordered atomic structure rather than a direct image. Three-dimensional distribution of atoms in a lattice can be constructed from the diffraction pattern only after transforming back to direct space by using the Fourier transformation method which becomes a very complicated process. Therefore, Bragg’s law was introduced which simplifies the method. A nearly perfect single crystal is placed at a specific angle θ with respect to the x-ray beam. According to Bragg’s law, only discrete

18 wavelengths can be transmitted at this wavelength of λ. So, the single transmitted wavelength is a function of interplanar distances of crystal dhkl and θ which can simply be written as 2dhkl sinθ = nλ. This equation is known as the Bragg’s law equation. Fig. 3.3 shows the path of incident and diffracted beam of the Bragg’s law diagram.

Fig.3.3. Bragg’s law diagram showing the path of incident and diffracted beam.33

The incident and diffracted beam with identical wavelength should form the same angle with respect to the surface of the crystal. Bragg’s equation depends on the interplanar distances of crystal structure which has significant importance in selecting x-ray to study crystal structure in opposition to using visible light sources. Atoms are too small to discern by visible light. Therefore, a suitable option to study the individual atom is x-ray as x-ray has the wavelength that is compatible with the atomic sizes and shortest neighboring interatomic distances. 3.3.2 X-ray Diffractometer The Scintag X1 powder diffractometer was used for the measurements. The main components of the x-ray diffractometer were x-ray source, sample holder, a detector, and a high voltage power supply. The main component of a conventional laboratory source of x-ray is an x-ray tube where x-rays are generated by the impacts of high energy electrons on the metal target of anode which is

19 cooled with water. Electrons generated from the cathode (usually heated tungsten filament) accelerate towards the anode. Throughout the process, a high electrostatic potential of 30 to 60 kV is maintained in between anode and cathode. After hitting the anode, each of these electrons emits x-ray beams with specific characteristic distribution of wavelengths that appear due to excitation of electrons to a higher energy state. In order to be excited from their ground state energy levels to excited state, the energy of incoming electrons must exceed that of the energy difference between two nearest possible energy levels of the target material. Generated x-ray beam exits the x-ray tube through the beryllium (Be) windows as shown in Fig. 3.4.

Fig. 3.4. Schematic diagram of x-ray tube used in laboratories.34

The mechanical assembly that holds x-ray source, sample holder, and detector are known as goniometer. The x-ray tube is stationary but, sample holder and the detector position could be changed manually or can be controlled remotely by the Scintag Pad-X controller window. The sample moves by angel θ and the detector simultaneously moves 2θ position.35 Fig. 3.5 shows the schematic diagram of the diffractometer.

Fig. 3.5. Components of a diffractometer and their positions during the experiment.36

3.3.3 Sample Preparation for XRD For an ideal sample preparation, the particles are expected to have completely random distribution of crystallographic orientation in grains or crystallites with respect to one another.35 Therefore, the sample was grinded by mortar and pestle to achieve a reduced particle size for accuracy in measurements. Then, the powder was mounted on a sample holder.

3.4 Scanning Electron Microscope (SEM) The content of this section is sourced from “Scanning Electron Microscope A to Z” hardware manual. For detailed information, please see ref. 37. SEM is used for observing the topography of specimen surfaces in two-dimensional view. After the specimen is irradiated with an electron beam, secondary electrons are emitted from the specimen. The two-dimensional surface image of the specimen is formed by the detected secondary electrons. Fig. 3.6 shows the major components of the SEM instrument.

Fig. 3.6. Schematic diagram of SEM with the basic components of the instrument.37

Electron Gun A thermionic emission gun (TE gun) with a heated cathode (generally a tungsten wire with diameter of 0.1 mm) filament is used to generate the electron beams that flow toward a metal plate (anode). The Anode maintains a positive voltage of 1- 30 kV. A Wehnelt electrode is placed in between the anode and cathode to apply enough negative voltage so that the electron beam current could be adjusted. The TF gun explained here is most generally used but it requires a high vacuum system because of its higher activity. There are other types of electron guns such as the field emission electron gun (FE gun), Schottky-emission gun (SE gun) are also available.

Condenser Lens and Objective Lens Electron microscopes use magnetic lenses. Two stage lenses, condenser lens and objective lens, are located below the electron gun. The lenses are useful in adjusting the diameter of the electron beam. The aperture is located in between condenser and objective lenses, which is made of thin metal plate and contains a small hole. Main purpose of this aperture is to block some of the electron beam from reaching the objective lens and allowing a small part of the beam to pass through. The objective lens determines the final diameter of the electron beam. Specimen Stage The purpose of the Specimen stage is to restrict the unwanted movement of the specimen and support the moves as required. The movements of this component are restricted to horizontal, vertical, specimen tilting, and rotation. Most SEM instruments are using eucentric specimen stages. The usefulness of using this type of stage is that the focus area does not change even after making changes to the field of view while the specimen is tilted. Secondary Electron Detector This detector is used for the detection of secondary electrons that emits after hitting the specimen. The detector is consisting of a scintillator, a Photomultiplier-tube (PMT), a supplementary electrode. Secondary electrons emitting from the specimen are captured by the high voltage generating from the detector. After hitting the scintillator, these electrons are converted to light. This light is directed to the PMT tube, where light is converted to electrons. These converted electrons are amplified as an electrical signal response and can be captured by the detector.

Image Display and Recording The response from secondary electron detector are amplified and transferred to the connecting display unit for creating the 2D imaging that is observed by the user in the monitor. From the display unit the electron probe scan, brightness variation could be controlled by the user. Therefore, the display unit depends on the number of the secondary electrons appearing on the monitor screen.

Chapter 4

Results and Discussion

The experimental results presented in this chapter include the data for the bulk Mn2Ni1++xGa1-x samples as well as data for the melt-spun ribbons of Mn2-xFexNi1.6Ga0.4. Part of the data for the

Mn2Ni1++xGa1-x system was collected by Tiago Schaeffer. The contents of this chapter are divided into two parts. In the first part, the structural and magnetic properties of Mn2Ni1++xGa1-x is presented and discussed. The second part discusses the experimental results on the partially Fe doped Mn2-xFexNi1.6Ga0.4 ribbons.

4.1 The Structural and magnetic properties of Mn2Ni1+xGa1-x

The main goal of this Thesis project was to explore the magnetic and EB properties of the materials. However, to properly interpret and validate the experimental data it is important to confirm the crystalline properties and phase purities of the samples. Therefore, x-ray diffraction (XRD) along with scanning electron microscopy (SEM) measurements were performed on the

Mn2Ni1++xGa1-x samples the results are briefly discussed in the section below.

4.1.1 Structural properties of Mn2Ni1+xGa1-x

The XRD patterns, measured at room temperature, for selected Mn2Ni1+xGa1-x samples are shown in Fig. 13. The XRD patterns for the compound with x = 0 (parent compound) exhibited the I4/mmm tetragonal structure, which is in agreement with the literature.10, 38 The lattice parameters were a = b = 3.9230(7) Å and c = 6.7049(8) Å. These values are also in close agreement with the literature. The tetragonal structure was the primary phase observed in the samples with x  0.10. However, for x  0.15, a 2nd phase related to the seven-fold modulated orthorhombic structure (space group: Pnnm) was also observed in the samples. The peaks primarily related to this structure are marked by an “*” in Fig. 4.1 As x exceeded 0.15, the percentage of this phase significantly increased. For all samples with x  0.15, the two phases coexisted in the samples. A large peak has been observed near to higher 2θ position or sample x = 0.15. The reason for this sharp peak may be attributed to the fact that the XRD samples were polished flat surfaces and not powder. Since the samples were highly ductile it was not possible to prepare powders. Since the samples were

24 not powder, the lattice planes that were exposed to the incident x-rays were not same for all samples.

Fig. 4.1. Room temperature XRD patterns for selected Mn2Ni1+xGa1-x bulk samples. The peaks marked by “*” are related to the Pnnm structure.

The SEM images for the samples were in agreement with the XRD data. Fig. 4.2 shows the SEM images for the Mn2Ni1+xGa1-x samples with x= 0.00 and x = 0.50. For the x = 0 sample, the image shows one shade of gray, which means only one phase existed in the sample. On the other hand, existence of two different phases (tetragonal and orthorhombic) were clearly observed in the sample with x = 0.5.

Fig. 4.2. SEM images for Mn2Ni1+xGa1-x with x=0.00 (left) and x=0.50 (right).

4.1.2 Magnetic and EB properties of Mn2Ni1+xGa1-x

Magnetization measurements were performed under two major conditions; magnetizations as a function of temperature, M(T) and magnetization as function of applied field, M(H). M(T) measurements were conducted to determine the Curie temperature of the samples. And, the M(H) measurements were conducted to study the exchange bias (EB) behavior in the samples.

The M(T) data for Mn2Ni1++xGa1-x (0 ≤ x ≤ 0.55) measured under zero field cooled (ZFC) and field cooled (FC) conditions with constant field of 1000 Oe are shown in Fig. 4.3. Mn2Ni1+xGa1-x (x = 0) does not show any ferromagnetic transition within the data acquisition temperature range. Martensitic transition, signified by a sharp jump in magnetization, was observed at C~ 360 K for the alloy as shown in Fig. 4.3a. The large thermal hysteresis between the ZFC and FCC M(T) data demonstrated the first order nature of the transition. The start of the martensitic transition was also observed near 400 K in the ZFC M(T) data for the alloy with x = 0.2. The complete transition is expected above 400 K. This behavior indicated that TM increased with increasing x in

Mn2Ni1++xGa1-x.

Fig. 4.3. Temperature dependence of the dc magnetization of selected Mn2Ni1+xGa1-x samples measured at 1 kOe. With further increase of x the behavior observed in the M(T) data changed significantly. As shown in Fig. 4.3c, ZFC magnetization of x = 0.35 initially increased with increasing temperature and peaked at 65 K. Above this temperature the magnetization decreased with increasing temperature and demonstrated a ferromagnetic transition with Curie temperature, Tc  334 K, as derived from the (dM/dT) plot of M(T). For x > 0.2, all the samples showed similar behavior and exhibited the ferromagnetic to paramagnetic transition below 400 K. Overall, the Curie temperature decreased with increasing Ni concentration in the system.

Fig. 4.4. Magnetization as a function of Magnetic field measured at temperature 5 K under ZFC and FC conditions for Mn2Ni1+xGa1-x (a) x = 0.0, (b) x = 0.20, (c) x = 0.35, and (d) x = 0.50.

The M(H) data for Mn2Ni1+xGa1-x (0 ≤ x ≤ 0.50) measured at 5 K collected under zero field cooled (ZFC) and field cooled (FC) conditions are shown in Fig. 4.4. EB properties were calculated by analyzing the magnetic hysteresis loops from MvH measurements. For x  0.2, the samples exhibited a symmetric hysteresis loop and did not indicate any exchange bias properties. The ZFC M(H) data for the sample with x = 0.35 showed a double shifted hysteresis loop as shown in Fig. 4.4c. The FC M(H) data for the sample exhibited a regular hysteresis loop with the center shifted along the negative filed axis, clearly demonstrating the EB effect. All the samples with x  0.3 exhibited the EB effect, which is clearly visible in the FC M(H) data for the samples (Fig. 4.4c-

4.4d). The coercive field, HC, and the EB field, HEB, were determined from the FC M(H) data of

Mn2Ni1+xGa1-x. HC increased from 320 Oe (x = 0) to 891 Oe (x = 0.5) while HEB changed consistently from 279 Oe (x = 0.3) to 502 Oe (x = 0.5).

Fig. 4.5. Magnetic field dependence of the dc magnetization for the Mn2Ni1+xGa1-x (x = 0.55, 0.60) samples measured at 5 K under ZFC and FC conditions.

Interestingly, the double-shifted hysteresis loop was no longer observed for the samples with x > 0.50 (Fig. 4.5a). The ZFC M(H) data for the alloy with x = 0.55 exhibited a non-uniform hysteretic behavior and showed a spontaneous EB with HEB  330 Oe while the FC data (Fig. 4.5b) for the sample showed a HEB of 800 Oe. The Mn2Ni1+xGa1-x alloy with x = 0.60 (Fig. 4.5c - 4.5d) exhibited a spontaneous EB of 196 Oe and a significantly large FC EB of 4030 Oe.

4.2 The magnetic and structural properties of Mn2-xFexNi1.4Ga0.6 melt-spun ribbons

The experimental results discussed above showed that Mn2Ni1+xGa1-x exhibited enhanced EB properties for 0.3  x  0.6. As mentioned earlier, melt-spun ribbons with well-defined grain sizes 29 typically exhibit stronger EB properties. Therefore, using the Mn2Ni1.4Ga0.6 as the parent compound melt-spun ribbons for four samples, Mn2Ni1.6Ga0.4, Mn1.75Fe0.25Ni1.4Ga0.6,

Mn1.5Fe0.5Ni1.4Ga0.6, and MnFeNi1.4Ga0.6 were investigated and the results are presented in the remaining part of this chapter.

4.2.1 The magnetic properties of Mn2-xFexNi1.4Ga0.6 as-prepared melt-spun ribbons

The MvH data collected at 10 K under ZFC conditions for the ribbons are shown in Fig. 4.6. As shown in Fig. 4.6a, the Mn2Ni1.4Ga0.6 ribbons exhibited a frustrated M(H) loop and the magnetization did not achieve saturation at 50 kOe and showed a value of 15 emu/g. A dramatic increase of magnetization (nearly 40 emu/g) was observed in the Fe doped ribbons (Fig. 4.6b-d).

Additionally, the frustrated nature observed in Mn2Ni1.4Ga0.6 was not observed in the Fe doped materials, and the magnetization almost achieved saturation at 50 kOe.

To explore the nature of the hysteresis loop in the lower field region, the ZFC M(H) data between

-5 kOe and 5 kOe are shown in Fig. 4.7. The center of the loop for the Mn2Ni1.4Ga0.6 ribbons shifted along the negative field axis demonstrating the EB effect with HEB = 1060 Oe. A totally different yet interesting behavior was observed in the M(H) data for the Mn1.75Fe0.25Ni1.4Ga0.6 ribbons (Fig. 4.7b). A nonsymmetrical double-shifted hysteresis loop was observed for the material. The major part of the loop appeared shifted along the positive field axis with an EB field of +768 Oe. This behavior changed for the Mn1.5Fe0.5Ni1.4Ga0.6 ribbons that exhibited an EB field of +102 Oe. The ZFC M(H) data for MnFeNi1.4Ga0.6 showed no EB effect.

Fig. 4.6. Magnetization field dependence of the dc magnetization obtained at 10 K under ZFC condition for the as-spun ribbons samples.

Fig. 4.7. Magnetic field dependence of the dc magnetization at the lower field region obtained at 10 K under ZFC condition for the as-spun ribbons samples.

The FC M(H) data for the ribbons collected at 10 K are shown in Fig. 4.8. Before measuring the magnetization, the samples were cooled down from 300 K to 10 K in presences of a 50 kOe field. When the temperature stabilized at 10 K, the M(H) loops were measured. For clear visualization of the hysteresis loop near H = 0, data between -5 kOe and 5 kOe are shown in Fig. 4.8. For

Mn2Ni1.60Ga0.4 the center of the loop largely shifted along the negative field axis demonstrating the EB effect with HEB = 712 Oe (Fig. 4.8a). As the Fe concentration increased, the coercive field,

HC, and HEB, for the ribbons decreased (Fig. 4.8b-4.8c). For MnFeNi1.4Ga0.6 FC and ZFC M(H) loops were nearly identical and showed no EB effect.

The EB properties obtained from the ZFC and FC M(H) data are listed in d Mn1.5Fe0.5Ni1.4Ga0.6 showed smaller but noticeable EB effect and HEB values are listed in Tables 4.1 and 4.2, respectively. A closer look at the M(H) data discussed above, showed that the

Mn1.75Fe0.25Ni1.4Ga0.6 sample showed a non-typical behavior when compared to the other ribbons studied. Therefore, the Mn1.75Fe0.25Ni1.4Ga0.6 ribbons were subjected to further investigation.

Fig. 4.8. Magnetic field dependence of the dc magnetization obtained at 10 K under FC condition for the as-spun ribbons samples.

Table 4.1 HEB, Hc, and Ms values obtained at 10 K obtained under ZFC conditions for the as-prepared ribbons.

Samples HEB (Oe) ZFC Hc (Oe) ZFC Ms (emu/g) ZFC Mn2Ni1.6Ga0.4 -1060 4020 15 Mn1.75Fe0.25Ni1.4Ga0.6 +768 double shifted 45 Mn1.5Fe0.5Ni1.4Ga0.6 +102 738 38 MnFeNi1.4Ga0.6 0 210 33

Table 4.2 HEB, Hc, and Ms values obtained at 10 K obtained under FC conditions for the as- prepared ribbons.

Samples HEB (Oe) FC Hc (Oe) FC Ms (emu/g) FC Mn2Ni1.6Ga0.4 -712 2630 15 Mn1.75Fe0.25Ni1.4Ga0.6 -424 1200 45 Mn1.5Fe0.5Ni1.4Ga0.6 -295 1078 38 MnFeNi1.4Ga0.6 0 276 33

The M(T) data was colleted for the Mn1.75Fe0.25Ni1.4Ga0.6 ribbons, as shown in Fig. 4.9. The data was collected for two different magnetic fields. Fig 4.9a shows the data collected at small field of 50 Oe. As shown in the figure, a large thermomagnetic irreversibility was observed below 290 K in the M(T) data. Such irreversibility is generally observed in magnetic materials where competing AFM and FM interactions coexists. Also, the ferromagnetic transition observed in the FC M(T) data was fairly broad, which was also indicative of the presence of competing magnetic interactions. Additionally, the ZFC M(T) data showed negative magnetization below 290 K, which was also indicative of a competing magnetic interactions that resulted in frustrated magnetic behavior in the material.

The thermomagnetic irreversibility was also observed in the M(T) data obtained at 1 kOe (Fig. 4.9b). However, the magnitude of the irreversibility was significantly smaller and the ZFC magnetization becomes positive at 35 K, a temperature much lower than 290 K as was in the case for the M(T) data obtained at 50 Oe. The Curie temperature for Mn1.75Fe0.25Ni1.4Ga0.6 was evaluated to be, Tc  250 K.

Fig. 4.9. Temperature dependence of magnetization for Mn1.75Fe0.25Ni1.4Ga0.6 as-spun ribbon measured at magnetic fields of (a) H = 50 Oe (b) H = 1 kOe.

4.2.2 The magnetic properties of the annealed ribbons

To explore the effect of annealing on the magnetic properties, the Mn1.75Fe0.25Ni1.4Ga0.6 ribbons were annealed at 6000C and the magnetic properties were measured. The M(H) data obtained at 10 K between -2 kOe and 2 kOe under ZFC and FC protocols are shown in Fig. 4.10. The saturation magnetization obtained from the M(H) data was, MS= 36.74 emu/g, which was smaller than the saturation value found before annealing (45 emu/g). The ZFC M(H) data for the annealed sample was significantly different than the ZFC M(H) data for the as-prepared sample (Fig. 4.7b).

Fig. 4.10. Magnetization vs applied field for annealed Mn1.75Fe0.25Ni1.4Ga0.6 ribbons measured at 10 K under a) FC b) ZFC protocol.

The hysteresis loop observed in the ZFC M(H) data was relatively more symmetric when compared to that of the as-prepared sample. The FC M(H) data for the annealed sample (Fig. 10b) showed EB effect with HC = 400 Oe and HEB = -175 Oe. These values are also much smaller than the values determined for the as-spun ribbons (Table 4.2).

The M(T) data obtained at a constant magnetic field of 1 kOe under ZFC conditions for

Mn1.75Fe0.25Ni1.4Ga0.6 is shown Fig. 4.11. With increasing temperature magnetization increased and reached a peak at around 70 K then decreased with increasing temperature. The Curie temperature for the sample was determined to be 300 K, which is higher than the Curie temperature determined from the sample before annealing.

Fig. 4.11. Magnetization as a function of temperature for annealed Mn1.75Fe0.25Ni1.4Ga0.6 ribbons measured at an applied magnetic field of 1 kOe. The data presented and discussed above showed that the magnetic properties of the ribbons change significantly due to annealing, which generally causes a change in the microstructure of ribbons.

To determine the effect of annealing on the microstructure of the Mn1.75Fe0.25Ni1.4Ga0.6 ribbons SEM micrographs were collected for the as-prepared and annealed samples. The results are discussed below.

4.2.3 The crystalline properties of Mn1.75Fe0.25Ni1.4Ga0.6 ribbons

XRD patterns, measured at room temperature, for the as-spun Mn1.75Fe0.25Ni1.4Ga0.6 ribbons is shown in Fig. 4.12. The patterns are comparable to the data for the bulk samples (Fig. 4.1), showing that the crystalline structure for the ribbons are comparable to the bulk samples.

Fig. 4.12. Room temperature XRD pattern for as-spun nanostructured ribbon sample

Mn1.75Fe0.25Ni1.4Ga0.6.

Fig. 4.13 shows the SEM image for as-spun Mn1.75Fe0.25Ni1.4Ga0.6. A blue line marked in the figure represents a size of 2 μm (2000 nm). By using this line as reference, the average grain sizes in the ribbon could be estimated. The sizes ranged from 500 nm to 2000 nm. As was shown in Fig. 4.2, no such grain formation was observed for the bulk samples. According to the reported values for nanostructured ribbons, these grain sizes are acceptable to be identified as nanostructured grains.30, 31

Fig. 4.13. SEM image for Mn1.75Fe0.25Ni1.4Ga0.6 before annealing. The line (in blue) shows about 4 grains in the nanostructure formation.

Fig. 4.14 shows the SEM images for annealed Mn1.75Fe0.25Ni1.4Ga0.6 (annealed at 600⁰C for 30 minutes) ribbons. From the image of the free surface area (on the left), an arbitrary line has been chosen (colored in blue) in order to determine the grain size. The SEM image revealed a much uniform distribution of nearly equally sized grains in the annealed ribbons. The average size of majority of the grains was estimated to be approximately 1000 nm or 1µm. A few smaller sized grains were also observed. The effect of annealing was clear in Fig. 4.12 and the observation is consistent with literature.30, 31 A minor grain growth is observed from the SEM imaged of the wheel surface.

Fig. 4.14. SEM images for annealed sample Mn1.75Fe0.25Ni1.4Ga0.6. Free surface showing the grains formation (on the left), wheel surface (on the right).

The differences observed in the magnetic properties of the as-prepared and annealed

Mn1.75Fe0.25Ni1.4Ga0.6 ribbons may be attributed to their microstructures as shown in Fig. 4.12 and Fig. 4.13. The magnetic exchange interactions between the grains are strongly dependent on their sizes. Due to the random distribution of grains with different sizes in the as-prepared sample, unsymmetrical ZFC hysteresis loop was observed for them (Fig. 4.7b). The loop became more symmetrical or the annealed sample that exhibited much uniformly (similar sized) distributed grains.

The experimental results presented and discussed in this chapter showed that although the bulk and ribbon samples exhibited similar structural properties, their microstructures and associated magnetic properties were significantly different, particularly in case of the as-prepared ribbons. The results also showed that preparing ribbons followed by annealing might be an efficient way to control the magnetic and EB properties of the materials.

Chapter 5

Conclusion

The magnetic properties and exchange bias properties of the Mn based Heusler alloy system

Mn2Ni1+xGa1-x and a Fe doped derivative Mn2-xFexNi1.6Ga0.4 system in form of bulk and ribbons, respectively, were studied in this Thesis. For selected values of x all samples showed EB properties. The magnetization measurements obtained at 10 K for the ribbon samples exhibited some interesting properties including large EB effect except for the MnFeNi1.4Ga0.6 sample that showed no EB effect. The ribbons for Mn1.75Fe0.25Ni1.4Ga0.6 were studied extensively for this thesis as the material showed interesting properties such as asymmetric double shifted loop for ZFC condition, large HEB value, coercive field, and high magnetic saturation. The Curie temperature was determined from the M(T) data. A high Curie temperature of 300 K was observed for annealed

Mn1.75Fe0.25Ni1.4Ga0.6 ribbons, which was higher than the TC of 250 K for the as-spun ribbon. While the structural properties of the bulk and ribbon samples were comparable, the microstructures of the ribbons were significantly different from those of the bulk samples. This difference in microstructure was well reflected in the magnetic properties of the samples. In future work, better experimental results could be achieved by changing the annealing temperature and time.

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