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Magnetic Interface Structures in Exchange Bias Systems Joseph Magnetic Interface Structures in Exchange Bias Systems Joseph Gompertz Doctor of Philosophy University of York Physics September 2019 Abstract Systematic measurements were performed on sputtered polycrystalline mul- tilayer thin lms containing CoFe (5 nm)/IrMn (10 nm). A number of phenom- ena were described and explained for the rst time. The York Model of Exchange Bias was used to interpret the eect of the interface between the ferromagnetic (F)/antiferromagnetic (AF) layers. An ultra-thin interfacial layer of Mn with a thickness between 0 and 0.6 nm was added between the F and AF layers to alter the interfacial composition. The lms were analysed by X-ray diraction (XRD) and transmission electron micro- scope (TEM) imaging. Their magnetic behaviour was measured using temperature- controlled vibrating sample magnetometry (VSM). A marked irreversibility in the loop shift Hex after setting was observed to- gether with large increases in the coercivity H ( 4) and athermal training ¢H . c £ c1 These latter eects are attributed to spin cluster freezing which also increased Hex by up to 50 %. The cluster freezing was observed to occur over a wide range of temperatures ( 200K) and to below 5 K. This is consistent with diusion of Mn È at the F/AF interface which varied with the thickness of the Mn interfacial layer. An asymmetry of the saturation magnetisation ¢ms between the set and re- versed directions was measured in detail. This was found to be consistent with the F alignment of a single Mn atomic layer at the surface of the IrMn. ¢ms was observed to exhibit a peak at 10K. At present this observation is unexplained. Ç ii Contents Abstract ii Contents iii List of Tables vii List of Figures viii Acknowledgements xiii Author Declaration xiv 1 Introduction 1 1.1 Note on Units and Errors . 5 2 Magnetism in Thin Films 6 2.1 Exchange Interactions . 6 2.1.1 Direct Exchange . 6 2.1.2 Indirect Exchange . 8 2.1.3 Superexchange . 9 2.1.4 Dzyaloshinskii-Moriya Interaction . 9 2.1.5 Ferrimagnetism . 10 2.2 Anisotropy . 10 2.2.1 Magnetocrystalline Anisotropy . 10 2.2.2 Anisotropy and Domains . 13 2.2.3 The Stoner-Wohlfarth Model . 15 2.2.4 AF Anisotropy . 19 2.2.5 Other Anisotropies . 20 2.2.6 Exchange Anisotropy . 22 2.2.7 Temperature Dependence of Anisotropy . 24 2.3 Antiferromagnetic Materials . 26 iii CONTENTS iv 2.3.1 Structure of Antiferromagnetic Materials . 26 2.3.2 Structure of IrMn . 27 2.3.3 Interfacial Exchange Coupling . 30 2.3.4 Characterisation of Exchange Bias Hysteresis Loops . 33 3 Features of Exchange Bias 35 3.1 Early Observations . 35 3.1.1 Exchange Field . 35 3.1.2 Rotational Hysteresis . 36 3.1.3 Enhanced Coercivity . 37 3.2 Dynamic Eects . 38 3.2.1 Thermal Activation . 38 3.2.2 The Training Eect . 40 3.2.3 Reversal Asymmetry . 41 3.3 Structural Eects . 42 3.3.1 Texture . 43 3.3.2 Composition . 44 3.3.3 Interfacial Doping Eects . 46 3.3.4 Eects of Diusion . 48 3.4 Related Phenomena . 53 3.4.1 ~Hset Dependence . 53 3.4.2 Trilayer Experiment . 54 3.4.3 Spin Freezing . 55 3.4.4 ms Oset . 56 4 Models of Exchange Bias 58 4.1 Early Models . 60 4.1.1 Meiklejohn and Bean . 60 4.1.2 Fulcomer and Charap . 63 4.1.3 Stiles and McMichael . 65 4.1.4 Domain State Model . 67 4.2 The York Model of Exchange Bias . 69 4.2.1 Thermal Processes . 69 4.2.2 The York Protocol . 73 4.2.3 Measurement of the Blocking Temperature . 76 4.2.4 Measurement of Antiferromagnetic Anisotropy . 78 4.2.5 Setting Limitations . 82 4.2.6 Grain Volume Dependence of Exchange Bias . 84 CONTENTS v 4.2.7 Magnetic Viscosity . 86 4.2.8 Limitations of the York Model of Exchange Bias . 89 4.3 Proposed Extensions to Exchange Bias Models . 91 5 Characterisation 96 5.1 Sample Preparation . 96 5.1.1 HiTUS System . 96 5.1.2 Sputtering Procedure . 99 5.1.3 Film Thickness . 100 5.2 Structural Characterisation . 102 5.2.1 X-ray Crystallography . 102 5.2.2 Grain Size Analysis . 111 5.3 Magnetic Characterisation . 117 5.3.1 Vibrating Sample Magnetometer . 117 6 High-Temperature Behaviour 122 6.1 Irreversibility of Exchange Bias . 123 6.1.1 Setting and Resetting . 123 6.1.2 Time Dependence of Setting . 127 6.1.3 Origin of Magnetic Irreversibility . 130 6.1.4 Origin of Compositional Irreversibility . 133 6.2 Thermal Activation . 137 6.2.1 Blocking Temperature for Films with Manganese-Doped Interfaces . 137 6.2.2 Eect of Interfacial Mn on Maximum Exchange Bias . 139 6.2.3 Eect of Interfacial Mn on the Median Blocking Tempera- ture and Anisotropy . 142 7 Low-Temperature Behaviour 149 7.1 Spin Freezing . 150 7.1.1 Spin Freezing of Exchange Bias . 151 7.1.2 Spin Freezing of Coercivity and Training . 153 7.2 Magnetisation Oset . 156 7.2.1 Temperature Dependence of ms Oset . 156 7.2.2 Eect of Training on ms Oset . 164 7.2.3 Eect of Interfacial Mn on ms Oset . 170 7.2.4 Ferromagnetic-Antiferromagnetic Coupling . 180 8 Conclusions and Further Work 182 CONTENTS vi 8.1 Further Work . 185 List of Acronyms and Abbreviations 187 List of Symbols and Units 189 References 198 List of Publications 207 List of Tables 5.1 The nominal values and modelling parameters used in the t of g. 5.8 showing lm thickness df, density ½f and interfacial roughness σf. 110 6.1 Loop parameters for the loops shown in g. 6.2 given for (A) a lm set in a positive eld and (B) the lm set in the opposite direction. 126 6.2 Summary of H , T and K results for varied thickness of interfa- ex h Bi AF cial Mn. 144 7.1 Summary of the exchange bias, coercivity and training at 5.5 and 300 K showing the eect of spin freezing. 152 7.2 Summary of ¢ms(T ) for lms with dierent values of dMn (i) before and (ii) after training. 173 vii List of Figures 2.1 Schematic of the Bethe-Slater curve. 7 2.2 At 4.5 K the saturation eld of a multilayer oscillates as dCr increases. 8 2.3 Three families of crystal directions labelled in (a) a bcc Fe unit cell with (b) their corresponding magnetisation curves. 11 2.4 The stages of magnetising a F material in the hard direction. 14 2.5 The vector diagram for the Stoner-Wohlfarth model for the magneti- sation curve of a single-domain particle. 15 2.6 Calculated hysteresis loops for a uniaxial particle in the Stoner-Wohlfarth model at an angle to an applied eld. 16 2.7 Calculated hysteresis loops for an assembly of non-interacting and randomly oriented uniaxial particles in the Stoner-Wohlfarth model. 17 2.8 The sublattice magnetisations of a AF material in (a) low eld, (b) a eld sucient to induce spin-opping and (c) a eld sucient to induce metamagnetism. 19 2.9 Hysteresis loop of oxide-coated Co particles at 77 K as presented by Meiklejohn and Bean. 23 2.10 A sheet AF material with sublattices A and B. 26 2.12 The spin structure of IrMn3 according to neutron diraction studies of bulk samples. 29 2.13 Example hysteresis loops showing the eect of AF anisotropy and the eect of the setting process on Hex ...................... 32 2.14 Schematic of an exchange-biased hysteresis loop with Hex, Hc and ¢Hc1 labelled. 33 3.1 Torque curves for exchange-biased Co/CoO nanoparticles for (a) in- creasing and (b) decreasing θH~ ......................... 37 3.2 Thermal activation for a IrMn/CoFe lm set for 5400 s at 500 K in (a) H~ 20kOe and (b) H~ 20kOe. .................... 39 set Æ set Æ¡ viii List of Figures ix 3.3 The rst loop training eect and subsequent thermal activation loops for Co/CoO thin lms. 40 3.4 The asymmetry of reversal shown (a) schematically, (b) at a tempera- ture above TNA showing thermal activation . 41 3.5 The asymmetry of reversal shown before and after training at a mea- surement temperature of at 5.5 K. 42 MAX 3.6 The blocking temperature TB of an exchange bias material increases with (a) increasing integrated density and (b) decreasing FWHM of the {111} IrMn diraction peak. 43 3.7 Hysteresis loops for a lm with (a) weak texture on a Cu seed layer and (b) strong texture on a NiCr seed layer. 44 3.8 The dependence of Hex on the composition of IrMn. 45 3.9 The value of Hex increases and goes through a peak with the addition of an ultra-thin layer of Mn with thickness dMn. 46 3.10 Diusion in a IrMn/Co/Cu thin lm measured using atom-probe to- mography before and after annealing. 50 3.11 The eects of diusion on the hysteresis loops of an IrMn/Co thin lm. 51 3.12 The concentrations of atomic species as function of depth through a IrMn/Co/chCu thin lm. ..
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