High anisotropy copt (l1(0) l1(1) phase) magnetic recording media films

High anisotropy CoPt (L10 & L11 phase) magnetic recording media films Yang Yang (B. Eng., Beijing Institute of Technology, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 Acknowledgements I would like to express my deep and sincere gratitude to all individuals who supported and guided me at every stage throughout my Ph.D. study at National University of Singapore. First and foremost, I would like thank my advisors, Prof. Chow Gan Moog and Prof. Chen Jingsheng, for their generous support and encouragement. I am very grateful to be their student and spend five years under their supervisions at NUS. Prof. Chow Gan Moog is a great teacher. His immense knowledge and logical way of thinking have been of great value to me. I have learned so much from him, not only in conducting research, but also in being an integrated person. His enormous passion for scientific research and his rigorous work ethic have had a remarkable influence on me and hopefully my entire career. I am deeply grateful to Prof. Chen Jingsheng for his valuable guidance and continuous support throughout this work. I have been benefited so much by his enthusiasm, patience, motivation, and immense knowledge and expertise in the field of data storage. I still remember the many times Prof. Chen stayed after work and had long discussions with me about my project and gave me instructions and suggestions. Not only he has taught me all the invaluable knowledge and his experience in magnetism, but also has given me the most priceless wisdoms to be a better thinker. I would also like to thank my senior Pandey Koashal, who helped me since I joined this group and guided me along the way. I express my gratitude for his guidance as well the generous time that he is willing to spend. I wish him every success in his future endeavors. i I wish to thank all the research staff and laboratory technologists in both MSE department and DSI. None of this research could have been accomplished without their support. Zhang Jixuan, Chen Qun, Lim Mui Keow Agnes and Kuan Henche have helped me get started on the TEM, XRD, VSM, AFM and XPS facilities and have constantly provided kind assistance. I also greatly appreciate the kind help from Phyoe Wai Lwin, Cher Kiat Min and Lim Boon Chow for showing me how to operate the sputtering machine and prepare TEM samples as well as the help from Dr. Hu Jiangfeng. The experimental facilities provided by DSI and Argonne National Laboratory (USA) are greatly acknowledged. I would like to express my sincere thanks to Dr. Sun Chengjun for his lecture at the beamline site as well as the good Chinese food. I would like to thank all my group members: Qian Lipeng, Xu Dongbin, Yuan Du, Huang Lisen, Li Huihui, Ho Pin, Zhang Bangmin, Lv Wenlai and Ji Xin, for the good and bad times we have shared during the past years. You have helped me numerous times with my problems and complaints. Thank you for your patience with me. Besides my friends in NUS, other people who know what a difficult time I have gone through in thesis writing are Sean Wong, Makoto Imakawa, Zheng Donghua, Guo Fang, Duan Tao and Zhang Xinqian. Without these people’s love and support, I not think I would have reached this stage of my life. This thesis is meaningless without mentioning my parents. Their emotional support and wisdom have kept me focused on what is really important. I would like to express my deepest gratitude to them for their unconditional love and generous support. They have always been there for me through thick and thin. To them, I dedicate this thesis. ii Table of Contents Acknowledgements . i Table of Contents . iii Summary…. vii List of Tables . ix List of Figures . x Chapter Introduction 1.1 Requirements of magnetic recording media for high areal density . 1.2 Current status of various perpendicular recording media 1.2.1 CoCrPt alloy . 1.2.2 Co/Pt (Pd) multilayers 1.2.3 Rare-earth alloys 1.2.4 L10 CoPt and FePt 1.2.5 L11 CoPt . 1.3 Magnetic recording media for next generation 1.3.1 Heat assisted magnetic recording media 1.3.2 Exchange coupled composite media 10 1.3.3 Patterned media 12 1.4 Structure and physical properties of different phases of CoPt . 13 iii 1.5 Review of L10 and L11 CoPt based magnetic recording media . 15 1.5.1 L10 CoPt based magnetic recording media 15 1.5.2 L11 CoPt based magnetic recording media 20 1.6 Research objectives 21 1.7 Thesis outline . 23 Chapter Experimental techniques . 25 2.1 Sputtering techniques . 25 2.2 Rapid thermal processing techniques . 26 2.3 Structure and microstructure characterization . 27 2.3.1 Rutherford Backscattering Spectroscopy (RBS) 27 2.3.2 X-ray diffraction (XRD) . 28 2.3.3 X-ray reflectivity (XRR) 31 2.3.4 Transmission electron microscopy (TEM) . 32 2.3.5 Scanning electron microscope (SEM) 33 2.3.6 X-ray photoelectron spectroscopy (XPS) . 34 2.3.7 X-ray absorption spectroscopy . 34 2.4 Magnetic properties characterization . 39 2.4.1 Vibrating sample magnetometer (VSM) 39 2.4.2 Superconducting Quantum Interference Device (SQUID) 43 2.4.3 Alternating gradient force magnetometer (AGFM) . 44 2.4.4 Magnetic force microscopy (MFM) . 46 iv 2.4.5 Measurement of magnetocrystalline anisotropy constant 46 Chapter Development of L10 phase CoPt on single crystal substrates and oxidized Si substrates 50 3.1 Development of L10 phase CoPt on single crystal substrates 50 3.1.1 Experimental methods 50 3.1.2 Effects of CoPt film thickness on microstructural evolution and magnetization reversal mechanism 51 3.1.3 Chemical ordering and magnetic properties of L10 CoPt–SiO2 nanocomposite 55 3.1.4 Summary 63 3.2 Development of L10 CoPt on oxidized Si substrates . 64 3.2.1 Post-deposition annealing of CoPt with MgO underlayer . 64 3.2.2 Post-deposition annealing of (CoPt/MgO)n multilayer structure77 Chapter Development of L10 CoPt based exchange coupled composite (ECC) media…… 99 4.1 Experimental details . 99 4.2 Results and discussions 100 4.3 Summary 113 Chapter Development of L11 phase CoPt on single crystal substrates and glass substrates… . 114 5.1 Development of L11 phase CoPt on single crystal substrates 114 v 5.1.1 Experimental methods 114 5.1.2 Results and discussions 115 5.1.3 Summary 122 5.2 Development of L11 phase CoPt on glass substrates . 122 5.2.1 The evolution of chemical ordering and magnetic properties by varying CoPt film thickness . 122 5.2.2 Effects of intermediate layer on structure and magnetic properties of L11 CoPt thin films 130 5.2.3 Grain size reduction and ordering improvement of L11 phase CoPt by Cu doping . 137 Chapter Conclusion and future work . 150 6.1 Conclusion . 150 6.2 Future work 152 Bibliography . 154 Appendix…. 168 A Rocking curve measurement of [CoPt (t nm)/MgO (4 nm)]n . 168 B LLG simulation parameters for L10 CoPt based ECC media 169 C XRR profiles of L11 CoPt thin films with Ru, Ir and Pt underlayers 170 D RBS profiles of L11 CoPt thin films with Ru, Ir and Pt underlayers 171 E XPS in-depth profiles of L11 CoPt with and without a nm Pt intermediate layer 172 vi Summary For magnetic recording technology, much effort is currently devoted to the achievement of areal density up to Tb/in2 and beyond. The enhancement of the magnetic recording areal density is governed by the characteristics of magnetic recording media. Media material with large magnetocrystalline anisotropy (Ku) is required to delay the onset of superparamagnetism. L10 and L11 phase CoPt thin films have been considered as potential candidates due to their large Ku values and corrosion resistance, which allow smaller thermally stable grains. This research work focused on obtaining desired structure and magnetic properties of L10 and L11 CoPt media films and investigating the mechanism behind them in order to satisfy the media requirements for high areal density magnetic recording. The first part of this thesis was to develop L10 phase CoPt. Two kinds of fabrication methods were adopted: epitaxial growth on MgO (001) single crystal substrates by in-situ heating and non-epitaxial growth on oxidized Si substrates by post-deposition annealing. Effects of CoPt film thickness and SiOx volume percentage on chemical ordering and magnetic properties of L10 CoPt films were studied. Thicker CoPt exhibited higher chemical ordering but lower coercivity. 10 vol.% SiOx addition exhibited the highest chemical ordering and coercivity. In order to make it commercially viable, L10 CoPt films were developed on oxidized Si substrates. The effects of annealing conditions and MgO underlayer thickness were investigated. In order to further control the CoPt (001) texture, multilayered structure of (CoPt/oxide)n was adopted. Thin CoPt sublayer was effective in inducing L10 CoPt (001) texture, vii whereas thick MgO was preferred for improved chemical ordering and small grain size. The second part of this thesis was to study the L10 CoPt based exchange coupled composite (ECC) media because it is a possible solution for Tb/in2 recording owing to its advantage in writability when compared with conventional perpendicular media at the same level of thermal stability. The effects of direct interlayer coupling between soft and hard layer was investigated both experimentally and theoretically. Increasing soft layer thickness was effective to significantly reduce the switching field of the hard layer with soft layer thickness below nm. The third part of this thesis was to study L11 phase CoPt thin films. The interfacial effects of different underlayers (Ru, Ir, Pt) were investigated and sharp interface such as CoPt/Ru was found necessary to obtain good ordering. To develop L11 CoPt on glass substrates, Ta seedlayer and Ru underlayer were adopted. For different CoPt film thickness, it was found that 10 nm CoPt had the best chemical ordering and magnetic properties. 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Table A ∆θ50 of CoPt (001) peak for the samples annealed at 600 °C and 700 °C. ∆θ50 of CoPt (001) (°) CoPt layer thickness (nm) 600 °C 700 °C 2.5 7.16 6.11 6.82 5.21 3.8 8.40 6.77 8.73 168 B LLG simulation parameters for L10 CoPt based ECC media Modeled hysteresis loops were obtained by solving the Landau-LifshitzGilbert (LLG) equation with LLG Micromagnetics SimulatorTM. LLG Micromagnetics SimulatorTM is a 3-dimensional simulation tool with integrated graphics that solves the LLG equations by relaxation and integration. LLG equation is differential equation describing the precessional motion of magnetization in a solid and including dipolar, anisotropy, exchange, and Zeeman energies. In this study, hysteresis loops were simulated using a ramped, uniform magnetic field applied perpendicular to the film plane. Magnetic anisotropy and lateral exchange coupling were independently controlled for each layer, and vertical exchange coupling between each layer was also controlled to simulate the coercivity dependent interlayer exchange coupling strength. Simulation volume was X=100 nm, Y=100 nm, Z=10+x nm (x=soft layer thickness, 0, 2, 4, nm). Discretization was Nx=10 sub-elements, Ny=10 sub-elements and Nz=(10+x)/2 sub-elements. Anisotropy type was selected to be uniaxial. The media parameters used are as follows: for hard layer, Ms=821 emu/cc, Ku=1.27×107 erg/cc, A=1 µerg/cm, film thickness 10 nm; whereas for soft layer, Ms=960 emu/cc, Ku=0.98×106 erg/cc, A=1 µerg/cm, film thickness from to nm. The interlayer exchange coupling varied: 0, 0.1×10-6, 0.3×10-6, 0.5×10-6, 1×10-6, 1.5×10-6 and 2×10-6 erg/cm. 169 C XRR profiles of L11 CoPt thin films with Ru, Ir and Pt underlayers To investigate the interfacial condition, the XRR profiles of CoPt/Ru (Pt, Ir) were obtained. The experimental and fitted curves were present in Figure C. Figure C (a)-(c) XRR profile measured (black) Ru/CoPt, Pt/CoPt, Ir/CoPt and fitting (red), respectively; (d)-(f) Electron density versus depth deduced from the fitting for Ru, Pt and Ir underlayer, respectively. 170 D RBS profiles of L11 CoPt thin films with Ru, Ir and Pt underlayers To further confirm the inter-diffusion between CoPt and underlayers, RBS measurement was carried out. The experimental data and simulated curves of RBS spectra of CoPt on different underlayers were shown in Figure D. Figure D RBS spectra of CoPt on (a) Ru underlayer, (b) Ir underlayer and (c) Pt underlayer. 171 E XPS in-depth profiles of L11 CoPt with and without a nm Pt intermediate layer In order to examine the diffusion at the interface, XPS in-depth profiles of the samples with and without a nm Pt buffer layer were shown below. Figure E Depth profile of CoPt film (a) without intermediate layer and (b) with nm Pt intermediate layer. 172 [...]... 1.5 1.5.1 Review of L10 and L11 CoPt based magnetic recording media L10 CoPt based magnetic recording media In spite of the many advantages of L10 CoPt as discussed in Section 1.2.4, some challenges to its utilization as magnetic recording media remain, including 15 decreasing ordering temperature, control of CoPt (001) texture and media noise reduction Research on L10 CoPt has thus been focused on addressing... 1.2.5 L11 CoPt Although L10 phase CoPt is a potential candidate for high density magnetic recording media, its ordering temperature is higher than 600 °C Media materials with comparable Ku but lower ordering temperature are therefore needed L11 CoPt with large magnetocrystalline anisotropy (~107 erg/cm3) and relatively low fabrication 8 temperature (300-400 °C) becomes another media candidate for high density... dependent magnetic properties of recording media material In HAMR, the recording media is heated during the writing process close to its Curie temperature This reduces the magnetic anisotropy and thus requires only a very small field to enable magnetization reversal The media is then quickly cooled back to its initial stage to store the data HAMR allows the use of large Ku magnetic materials as recording media. .. been exerted both in improving current media to compete thermal instability, and in designing alternative methods for data storage Heat assisted magnetic recording media, exchange coupled composite media, and patterned media are considered to be the most effective methods for the next generation 1.3.1 Heat assisted magnetic recording media Heat assisted magnetic recording (HAMR) was proposed to improve... Section 1.2 Section 1.3 describes the magnetic recording media for next generation General properties of CoPt thin films will be presented in Section 1.4 Current research on L10 and L11 CoPt films will be reviewed in Section 1.5 At the end of this chapter, research objectives and thesis outline are presented 1.1 Requirements of magnetic recording media for high areal density Many factors contribute to... composite media However, there have been relatively few experimental studies of L10 CoPt based ECC media It is known that the magnetic anisotropy constant of CoPt films depends on the deposition temperature High temperature deposition leads to high Ku L10 phase CoPt, whereas room temperature deposition leads to low Ku fcc phase CoPt Therefore, it is an ideal homocomposite system to investigate exchange coupling... information the magnetic media can hold, which can be quantified by areal density.7, 8 To achieve high areal density magnetic recording, it is necessary to have high signal-to-noise ratio (SNR) to recover data reliably because the higher SNR, the more easily the data can be detected In conventional recording media, the SNR is determined as SNR=10×log(N), where N is the number of grains per recording bit... application 1.5.1.1 Decreasing ordering temperature The earliest studies of CoPt thin films focused on the growth of continuous polycrystalline films and their magnetic properties As-deposited CoPt films show chemically disordered fcc phase and are magnetically soft To obtain L10 phase CoPt films, in-situ heating or post-deposition annealing higher than 600 °C is required, which is not practical for hard disk... perpendicular media. 32-34 However, to fabricate tilted disk media is a big technical challenge, which prevents it from being industrially realized Nonetheless, the tilted media has inspired the development of exchange coupled composite (ECC) media. 35-37 ECC media consists of magnetically isolated grains which have two regions with different magnetic properties One is magnetically hard and the other is magnetically... media, it is quite promising for ultra -high density recording Experimental studies on ECC media were reported based on Co/Pd multilayers35 and CoCrPt-SiO2 alloy media3 8, but the magnetic anisotropy constant Ku were not very high Girt et al.39 examined the Co74Pt22Ni4 based ECC media and experimental evidence of domain wall assisted switching was observed in composite media However, there have been relatively . 1.2.4 L1 0 CoPt and FePt 8 1.2.5 L1 1 CoPt 8 1.3 Magnetic recording media for next generation 9 1.3.1 Heat assisted magnetic recording media 9 1.3.2 Exchange coupled composite media 10 1.3.3. Patterned media 12 1.4 Structure and physical properties of different phases of CoPt 13 iv 1.5 Review of L1 0 and L1 1 CoPt based magnetic recording media 15 1.5.1 L1 0 CoPt based magnetic recording. High anisotropy CoPt (L1 0 & L1 1 phase) magnetic recording media films Yang Yang (B. Eng., Beijing Institute