Novel Antiferromagnetic Materials John Sinclair Doctor of Philosophy University Of York Physics August 2018 Abstract There is intense interest in new antiferromagnetic materials due to the development of antiferromagnetic spintronics. Currently, the material IrMn is used, however, Iridium is one of the scarcest and most expensive elements. In this work, a number of novel, thin film antiferromagnetic materials were produced using sputter deposition and then assessed using magnetic and structural characterisation techniques as well as a temperature dependent resistivity technique developed during the project. Initially work was focussed on the ternary Heusler alloys Ni2MnAl and Ru2MnGe. Both alloys were shown to crystallise in their predicted antiferromagnetic phases, however they had low thermal stability due to limited magnetocrystalline anisotropy. Ru2MnGe was demonstrated to be the superior alloy, due to the anisotropy generated by its (111) sheet antiferromagnetic spin structure allowing the median blocking temperature to be determined for a Ru2MnGe (30 nm) / CoFe (2.5 nm) sample as 푇퐵 = (126 ± 3) K. Subsequent work focussed on the compound MnN. Using similar growth and measurement techniques, MnN thin films were shown to have a median blocking temperature as high as (374 ± 4) K and an effective anisotropy constant up to 6.6 × 106 ergs/cm3. These values indicate MnN may have the potential to be integrated into spintronic devices, if it is possible to reduce the antiferromagnet layer thickness, whilst maintaining the stoichiometry and tetragonal distortion. 1 Contents Abstract........................................................................................................................................ 1 Contents....................................................................................................................................... 2 List of Tables ................................................................................................................................ 6 List of Figures ............................................................................................................................... 7 Acknowledgements ................................................................................................................... 14 Declaration ................................................................................................................................ 15 Chapter 1: Introduction ........................................................................................................ 16 1.1 Exchange Interactions ................................................................................................ 16 1.2 HARFIR Project............................................................................................................ 18 1.3 Requirements for an IrMn Replacement.................................................................... 20 1.4 Potential Novel Antiferromagnetic Materials ............................................................ 21 1.5 Notes on the Thesis .................................................................................................... 21 Chapter 2: Classical Antiferromagnets ................................................................................. 22 2.1 Temperature Dependence of Magnetism .................................................................. 23 2.2 Antiferromagnetic Materials ...................................................................................... 25 2.2.1 Transition Metal Oxide Antiferromagnets .......................................................... 27 2.2.2 Elemental Antiferromagnets .............................................................................. 30 2.2.3 Alloy Antiferromagnets ....................................................................................... 31 2.2.4 Synthetic Antiferromagnets................................................................................ 34 2.3 Imaging of Antiferromagnetic Structure and Domains .............................................. 36 2.4 Exchange and Anisotropy ........................................................................................... 39 2.4.1 Magnetocrystalline Anisotropy .......................................................................... 39 2.4.2 Anisotropy in Polycrystalline AF Films ................................................................ 40 2.4.3 Domain Effects in Epitaxial AF Films ................................................................... 40 2.5 Electrical Conduction in Antiferromagnets ................................................................ 41 2 Contents Chapter 3: Exchange Bias ..................................................................................................... 45 3.1 Origin of Exchange Bias .............................................................................................. 45 3.2 Theories and Models of Exchange Bias ...................................................................... 46 3.3 The York Model of Exchange Bias .............................................................................. 50 3.3.1 Energy Barrier and Reversal ............................................................................... 51 3.3.2 Time Dependence ............................................................................................... 52 3.3.3 Blocking Temperature ........................................................................................ 54 3.3.4 Measurement Protocols ..................................................................................... 55 3.3.5 Calculation of Magnetocrystalline Anisotropy ................................................... 57 3.3.6 Grain Size and Film Thickness Dependence........................................................ 57 3.4 Spintronics .................................................................................................................. 59 3.4.1 GMR Devices ....................................................................................................... 60 3.4.2 TMR Devices ....................................................................................................... 61 3.4.3 MRAM ................................................................................................................. 62 Chapter 4: Material Selection ............................................................................................... 64 4.1 Heusler Alloys ............................................................................................................. 64 4.1.1 Magnetic Ordering and Properties ..................................................................... 65 4.1.2 Screening for Antiferromagnetic Heusler Alloys ................................................ 67 4.1.3 Antiferromagnetic Heusler Alloys ....................................................................... 67 4.2 Manganese Nitride ..................................................................................................... 69 4.3 Characterisation of Antiferromagnetic Order ............................................................ 72 4.3.1 Susceptibility Measurement ............................................................................... 72 4.3.2 Neutron Diffraction............................................................................................. 73 4.3.3 Polarised Neutron Reflectometry ....................................................................... 74 Chapter 5: Development of the Magnetoresistance Apparatus .......................................... 76 5.1 Temperature Control .................................................................................................. 77 5.1.1 Cryostat and Control Instruments ...................................................................... 77 3 Contents 5.1.2 Feedback Control of Temperature ..................................................................... 79 5.2 Magnetic Field Control ............................................................................................... 82 5.3 Electrical Measurement ............................................................................................. 83 5.3.1 Four Point and Van der Pauw Configurations .................................................... 83 5.3.2 Probe Head Design ............................................................................................. 85 5.3.3 Sample Preparation ............................................................................................ 86 5.3.4 Measurement Control ........................................................................................ 87 5.4 Calibration and Initial Measurements ........................................................................ 89 5.4.1 Thin Film Platinum Calibration ........................................................................... 90 5.4.2 Measurement of 푇푁 in Single Crystal Chromium................................................ 91 Chapter 6: Sample Preparation and Characterisation .......................................................... 93 6.1 Thin Film Deposition ................................................................................................... 93 6.1.1 Magnetron Sputtering ........................................................................................ 94 6.1.2 BESTEC 2” DC/RF Magnetron Deposition System .............................................. 96 6.1.3 High Target Utilisation Sputtering (HiTUS)