ABSTRACT MAGNETISM and ASSOCIATED EXCHANGE BIAS EFFECTS in Mn2ni1+Xga1-X HEUSLER ALLOYS and SELECTED Fe DOPED DERIVATIVES By
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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 by Sutapa Biswas Miami University Oxford, Ohio 2020 Advisor: Prof. Mahmud Khan Reader: Prof. Herbert Jaeger Reader: Prof. Steve Alexander ©2020 Sutapa Biswas This thesis titled MAGNETISM AND ASSOCIATED EXCHANGE BIAS EFFECTS IN Mn2Ni1+xGa1-x HEUSLER ALLOYS AND SELECTED Fe DOPED DERIVATIVES by 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 iii 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 iv 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 v 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 vi 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. ix 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. 1 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