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Heat Assisted Magnetic Recording Media Based on Exchange Bias

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Heat Assisted Magnetic Recording Media Based on Exchange Bias Heat Assisted Magnetic Recording Media Based on Exchange Bias Kelvin Elphick DOCTOR OF PHILOSOPHY UNIVERSITY OF YORK PHYSICS MAY 2016 Abstract A study of a new paradigm for a heat assisted magnetic recording (HAMR) media based on the use of exchange bias is presented. Exchange bias occurs when an antiferromagnetic (AF) layer such as IrMn is grown in contact with a ferromagnetic (F) layer and the system if field cooled resulting in a hysteresis loop shifted along the field axis. In this concept the temperature dependent anisotropy is provided by IrMn. Therefore the information is stored in the AF layer and the recording layer is actually part of the read/write structure when field cooled. The F layer when magnetised, serves to align the F layer in the direction required to store the information and then provides a read out signal indicating in which direction the AF layer is oriented. Hence in a complex way the “recording layer” is actually part of the read/write head. The key to achieve a structure of this kind is the control of the orientation of the Mn ions of the IrMn such that they are aligned perpendicular to the plane of the film. In this way a perpendicular exchange bias required for information storage in the AF layer has been achieved. Over 300 samples have been prepared and evaluated to determine the optimised structure. A segregated sample CoCrPt-SiO2 was sputtered using a pressed powder target in a HiTUS deposition system. Dual Ru seed layers of 8nm and 12nm were deposited using 3mTorr and 30mTorr process pressure, respectively. The median grain size of 퐷푚 = (6.2 ± 0.2)nm was achieved using these sputtering conditions. High resolution cross section TEM imaging has been used to show that the CoCrPt grains remains segregated by SiO2 after the deposition of IrMn. The key feature of this media is that the recorded information is impossible to be erased by a demagnetising field. In order to achieve this requirement the hysteresis loop has to be completely shifted to a negative field. The highest shift or exchange bias of 퐻푒푥 = (240 ± 5)Oe was measured at room temperature. This was achieved by depositing an ultrathin (0.8nm) Co interlayer above a Pt seed layer. 2 Contents Abstract ........................................................................................................................ 2 List of Figures ............................................................................................................... 6 List of Tables ............................................................................................................... 10 DECLARATION............................................................................................................. 11 Chapter 1 .................................................................................................................... 12 Introduction ............................................................................................................... 12 1.2 Units .............................................................................................................. 16 Chapter 2 .................................................................................................................... 17 Principles of Magnetic Recording .............................................................................. 17 2.1 History of Magnetic Disk Recording ................................................................. 17 2.2 Current & Alternative Magnetic Recording ...................................................... 19 2.2.1 Perpendicular Recording ........................................................................... 19 2.2.2 Coupled Granular Continuous Recording Media ....................................... 22 2.2.3 Exchange Coupled Composite Media ........................................................ 25 2.3 Future Magnetic Recording Media ................................................................... 29 2.3.1 Bit Patterned Media ................................................................................... 29 2.3.2 Microwave Assistant Magnetic Recording ................................................ 32 2.3.3 Heat Assisted Magnetic Recording ............................................................ 34 Chapter 3 .................................................................................................................... 38 Physics of Magnetic Recording Media ....................................................................... 38 3.1 Single Domain Particles .................................................................................... 38 3.2 Stoner-Wohlfarth Theory ................................................................................. 40 3.3 Magnetocrystalline Anisotropy ........................................................................ 43 3.4 Interfacial Anisotropy ....................................................................................... 45 3.5 Demagnetising Field in Recording Media ......................................................... 46 3.6 Exchange Interactions ...................................................................................... 48 3.7 The York Model of Exchange Bias .................................................................... 50 3.8 Limitations of Recording Media ....................................................................... 55 Chapter 4 .................................................................................................................... 61 Experimental Techniques ........................................................................................... 61 3 4.1 Thin Film Growth .............................................................................................. 61 4.1.1 High Target Utilisation Sputtering ............................................................. 61 4.1.2. HiTUS Calibration ...................................................................................... 65 4.2 Magnetic Characterisation ............................................................................... 67 4.2.1 Alternating Gradient Force Magnetometer .............................................. 67 4.2.2 Vibrating Sample Magnetometer (VSM) ................................................... 69 4.2.3 Magnetisation Curves ................................................................................ 72 4.2.4 Thermal Activation Measurements ........................................................... 73 4.3 Structural Characterisation .............................................................................. 75 4.3.1 Transmission Electron Microscopy ............................................................ 75 4.3.2 Cross Sectional Sample Preparation .......................................................... 78 4.3.3 Scanning Electron Microscopy ................................................................... 81 4.3.4 Grain Size Analysis ..................................................................................... 84 4.3.6 X-ray Diffractometer .................................................................................. 87 4.3.6 Reflectivity Scans ....................................................................................... 91 4.4 Errors and Calibration ................................................................................... 93 Chapter 5 .................................................................................................................... 94 Perpendicular Exchange Bias Using [Co/Pt]n ............................................................. 94 5.1 Growth Conditions ........................................................................................... 94 5.2 Role of Seed layers ........................................................................................... 99 5.3. Physical Properties ........................................................................................ 103 5.4 Structural Properties ...................................................................................... 106 5.5 Effect of an AF layer on Co/Pt Multilayers. .................................................... 108 Chapter 6 .................................................................................................................. 117 HAMR Based on Exchange Bias ................................................................................ 117 6.1 Growth Conditions ......................................................................................... 117 6.2 Seed Layers and Structural Effects ................................................................. 126 6.3 Rotational Measurements .............................................................................. 137 6.4 Perpendicular Exchange Bias Structure.......................................................... 139 6.5 Introduce Conventional Recording Layer to Perpendicular Exchange ........... 141 6.6 Structure and Temperature Optimisation ...................................................... 143 Chapter 7 .................................................................................................................. 154 4 7.1 Conclusion ...................................................................................................... 154 7.2 Future Work.................................................................................................... 156 Acronyms.................................................................................................................
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