Investigation on new material for high density recording media with high thermal stability

THEORETICAL AND EXPERIMENTAL STUDY OF EXCHANGE COUPLED MEDIA REN HANBIAO NATIONAL UNIVERSITY OF SINGAPORE 2006 THEORETICAL AND EXPERIMENTAL STUDY OF EXCHANGE COUPLED MEDIA REN HANBIAO (B. ENG. TSINGHUA UNIV.) A THESIS SUBMITTED FOR THE DEGREE OF PH. D OF PHILOSOPHY DATA STORAGE INSTITUTE, A-STAR, SINGAPORE ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT NATIONAL UNIVERSITY OF SINGAPORE 2006 ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my supervisors, A/Prof. Wang Jian Ping and Prof Chong Tow Chong, for offering me the chance to study and my research at DSI, and for their invaluable advice and patient guidance throughout all my work done there. I am truly indebted to Dr. Shan Zhensheng, Dr. Zhou Tiejin, from whom I have gained much theoretic knowledge and invaluable advice. Dr Shan also gave me helpful support during VSM and AGM measurement. I am especially thankful to Dr. Chen Jinshen, Lim Boon Chow, and Pock, who aided me greatly in the trouble-shooting of the sputtering machine. They were always so patient whenever I encountered any problem. My thanks also go to: Mr. Soo Eng Wei, Ms. Pang Siew It, Ms. Chow Shiaw Kee, Mr. Hee Ching Hian, and all other staffs and fellow scholars of the Media and Materials Group from the Data Storage Institute, Sun Chengjun, Jiang Weiwei, Shi Xiao, Zheng Yufang, Hu Jiangfeng, Zhao Yan and Lü Meihua, who were extremely helpful with their assistance and friendship. I particularly appreciate the opportunity to spend four years with my fellow scholars. I also would like to thank the National University of Singapore for its financial support and the Data Storage Institute for supplying me with an excellent research environment. Last, but not least, I am especially grateful to my wife Tan Qiuyan, my daughter, Ren Jiayue and my family for their encouragement, care, and support. i Contents ACKNOWLEDGEMENTS . i Abstract . ix List of Figures and Illustrations xi List of Tables xx Introduction . 1.1 Overview of magnetic recording industry 1.2 Brief overview of magnetic recording 1.3 Thin film media . 1.4 1.5 1.6 1.3.1 Substrate 1.3.2 Underlayer and seedlayer 1.3.3 Intermediate layer . 1.3.4 Magnetic layer 1.3.5 Overcoat and lubricant Key performance indices for current medium 10 1.4.1 Areal density . 11 1.4.2 Signal to noise ratio 12 1.4.3 Thermal stability factor . 14 Issues with and resolutions for further increasing the areal density . 16 1.5.1 Traditional medium structure 17 1.5.2 IBM and DSI proposed medium (AFC and LAC) 17 1.5.3 Fujitsu proposed medium (SFM) 18 1.5.4 Newly proposed medium in this work 18 Objective of this research 19 ii 1.7 Project scope . 20 Literature review 24 2.1 2.2 2.3 2.4 The origin of exchange coupling 24 2.1.1 The concept of exchange interaction 24 2.1.2 Direct exchange interaction 25 2.1.3 Indirect exchange interaction 26 2.1.4 RKKY interaction . 26 2.1.5 Superexchange interaction 27 2.1.6 Double exchange interaction . 29 The interlayer exchange coupling and its effect on magnetic properties 30 2.2.1 Exchange anisotropy discovered by Meiklejohn and Bean 30 2.2.2 RKKY interlayer exchange coupling 35 The measurement of exchange coupling . 35 2.3.1 Magnetization measurement . 36 2.3.2 Torque measurements . 37 2.3.3 Ferromagnetic resonance measurements 38 2.3.4 Neutron diffraction measurements 40 2.3.5 Magnetoresistance measurements . 41 2.3.6 AC-susceptibility measurements 42 2.3.7 Domain observation measurements 43 2.3.8 Brillouin scattering measurements 43 2.3.9 Mössbauer effect measurements . 44 Types of exchange coupled system . 45 2.4.1 Small particles . 45 2.4.2 Coated antiferromagnetic single crystals 46 iii 2.4.3 Thin films 46 2.5 2.6 2.7 2.4.4 Oxide thin film AFMs . 47 2.4.5 Metallic thin film AFMs . 48 2.4.6 Other thin film AFMs 48 2.4.7 Ferrimagnets . 49 Factors that affect exchange coupling and the unidirectional anisotropy . 49 2.5.1 Thickness 49 2.5.2 AFM orientation 53 2.5.3 Interface roughness . 54 2.5.4 Block temperature . 55 2.5.5 Training effect . 56 2.5.6 Perpendicular coupling . 57 The theoretical research in exchange coupling . 58 2.6.1 Néel domain model . 60 2.6.2 Early random interface model . 61 2.6.3 AF domain wall models 62 2.6.4 Orthogonal F and AF magnetic lattices 63 2.6.5 Random interface field models . 64 2.6.6 The frozen interface model . 66 2.6.7 Local pinning field variation . 69 Perpendicular exchange coupled system 70 2.7.1 Perpendicular ferromagnetic layer thickness dependence 71 2.7.2 AFM layer thickness dependence . 72 2.7.3 Interface between the FM and AFM layers. . 75 2.7.4 Material of FM layers . 76 iv 2.7.5 Material of AFM layers 76 2.8 Summary . 76 Experimental procedures . 90 3.1 Introduction . 90 3.2 Sputter deposition . 91 3.2.1 Plasma generation and DC diode plasma generation 91 3.2.2 Condensation on the substrate to form the film on the surface . 94 3.2.3 Magnetron DC diode plasma generation 95 3.2.4 DSI home designed UHV sputtering machine 96 3.3 Vibrating sample magnetometer (VSM) . 97 3.4 Alternating gradient field magnetometer (AGFM) . 99 3.5 Hysteresis loop 100 3.6 Thermal stability factor (SF) measurement 102 3.7 X-ray diffraction . 103 3.8 Transmission electron microscopy (TEM) . 105 Analytical model of S-W Particle 110 4.1 Introduction . 110 4.2 Stoner-Wohlfarth Model: Uniform Rotation 111 4.3 The switching field hk . 113 4.4 Hysteresis loop 115 4.5 Energy barrier . 118 4.6 Summary . 121 Theoretical analysis on exchange coupled media 123 5.1 Introduction . 123 5.2 Analytical model . 123 v 5.3 5.4 Results and discussion in the case where no external field is applied 125 5.3.1 Case I: No conditional local minimum energy state exists. 129 5.3.2 Case II: Conditional local minimum energy states exist . 130 Results and discussion in the case with external field applied in the direction of the easy axis . 135 5.5 5.4.1 Hysteresis loop: . 135 5.4.2 Coercivity and switching field calculation and exchange bias . 140 5.4.3 Energy barrier calculation . 141 Results and discussion in the case with the field applied with an angle of the easy axis 142 5.6 5.5.1 Hysteresis loop 142 5.5.2 Switching field 143 Special issue: estimation of J considering different easy axis directions 147 5.6.1 Analytical Model 148 5.6.2 Results and discussion 149 5.6.3 θ = case . 149 5.6.4 θ ≠ case . 153 5.7 Results and discussion in the case with strong exchange coupling 157 5.8 Summary . 158 Effects of Dy-doping on magnetic and reversal properties of recording media 161 6.1 Introduction . 161 6.2 Experimental . 161 6.3 Results and discussion 162 6.3.1 Effect of doping Dy into the Cr underlayer 162 vi 6.3.2 6.4 Effect of doping Dy into the Cr top-layer . 164 Summary . 166 Effects of perpendicular exchange coupling on the magnetic properties of CoCrPtB-CoDy bilayers 168 7.1 Introduction . 168 7.2 Literature review of amorphous rare earth-transition metal alloy 168 7.2.1 Magnetization for homogeneous A (RE) – B (TM) alloys [1,2] 169 7.2.2 The compensate point and magnetization curve change with the temperature . 172 7.2.3 Curie temperature change and its effect on composition 174 7.2.4 Perpendicular anisotropy dependence on the Co content of CoDy amorphous film . 174 7.3 Experimental . 175 7.4 Exchange coupling effect of a CoDy toplayer to a perpendicular CoCrPtB magnetic layer . 178 7.5 The exchange bias field (Hex) and interface domain wall energy (σ) dependence on the magnetic properties (Hc) of the CoDy layer . 187 7.6 Magnetic behaviors of exchange coupled CoDy/CoCrPtB bilayers dependence on the Co content of CoDy layer 188 7.7 Magnetic behaviors of exchange-coupled CoDy/CoCrPtB bilayers’ dependence on the thickness of a CoDy layer 209 7.8 Interface domain wall energy estimation by the different magnetization reversals 224 7.9 Training effect of the magnetization and exchange bias for the CoDy/CoCrPtB coupled bilayers 227 vii 7.10 Summary . 231 Conclusion . 235 viii Chapter Effects of perpendicular exchange coupling Switching field distribution dMr/Mr0 0.8 0.6 348 30 nm 0.4 0.2 0.0 -0.2 10 Switching field (kOe) Figure 7.52 The switching field distribution of the sample with the structure of the glass / Ti (40 nm) / CoCrPtB (35 nm) / CoDy (30 nm) / Ti (4 nm). Figure 7.51 shows the multi-minor loops of the sample with a 30 nm CoDy layer. When a positive saturated field was applied, the magnetization of both the CoDy layer and the CoCrPtB layer was aligned to the positive direction. As the negative field was applied, the magnetization of the CoCrPtB layer reversed its direction first due to the large magnetization of the CoDy layer and lower anisotropy. So the remnant magnetization of the exchange-coupled layer went down as the negative applied field increased its value. As Figure 7.51 indicates, as the negative applied field decreased from -2.5 kOe, -4 kOe to -9 kOe, the remnant magnetization decreased. However, when the applied field went to -10 kOe, the magnetization of the CoDy layer reversed, causing the CoCrPtB layer to reverse its direction when the external field was removed and thus making the remnant magnetization suddenly surge. Figure 7.52 shows the switching field distribution curve of the sample with a 30 nm CoDy layer, confirming that the remnant magnetization’s sudden increase came from the switching of the CoDy layer. 223 Chapter Effects of perpendicular exchange coupling 7.8 Interface domain wall energy estimation by the different magnetization reversals The interface domain wall energy indicates the coupling strength intensity between the two layers. Therefore, accurate interface wall estimation is a critical part of the exchange coupling system. For a bilayer coupled system with a shift in the magnetic hysteresis loop, the interface domain wall is fully formed, and the energy of the domain wall can be calculated using σ = H ex M s t [14-15]. However, for a bilayer coupled system without this shift (Figure 7.19), only a partial interface wall is formed. No expression exists to estimate the interface domain wall energy of such an exchange-coupled system. This study involved the development of a method to estimate the partial interface domain wall energy based on the DCD curve for some special cases of a coupled system. The equation was J = H × ∆M r , where H1 was the remnant coercivity and △Mr was the remnant magnetization delta between the switching. A comparison could be made between the interface wall energy values calculated by the two methods for the hysteresis loop (Figures 7.53 and 7.54). From the minor loop of the CoDy layer shown in Figure 7.53, the interface energy could be estimated as [14-15] σ = 2HexM2t2 = ×16.12 × 0.809 = 2.39 (erg/cm2) (7.21) As shown on the figure 7.53, the magnetization of the sample 343 decreased when a negative external field was applied. However, due to the exchange coupling between the CoDy and CoCrPtB layer, the interface domain wall was formed between two layers. Most part of CoCrPtB layer magnetization was coupled by the CoDy layer. If the external field was removed, the interface domain wall energy would be released and made the magnetization of CoCrPtB reverse back to its original direction. 224 Chapter Effects of perpendicular exchange coupling Therefore in the DCD curve, most of the remnant magnetization kept its origin direction even after applying a very large negative field (around 4.12 kOe). However, when the negative field was large than 4.12kOe, the magnetization of the sample suddenly switched irreversibly to its opposite direction. This was because the Zeeman energy, induced by the external applied field, would overcome the interface wall energy between two layers. Therefore we can estimate that the interface wall energy of the sample is almost equal to Zeeman energy of sample at the critical irreversible switching point. Ewall = σS = ΗΜVcos(θ−α) (7.22) Divided by S on both side, noted V= S×t, we can get σ = ΗΜtcos(θ−α) (7.23) Because the field was applied along the easy axis direction, θ equal to zero. Therefore σ = ΗΜcosα = HMrt (7.24) In Figure 7.54, the switching part of Mrt should be equal to (Mrt1+Mrt2)/2. Therefore we estimate the interface wall energy:” σ = H ∆M 1t1 = 4.12 × 10 × (65.2 + 12.4) × 10 −6 16.87 × 10 −2 = 2.04 (erg/cm2) (7.25) The interface exchange energy value estimated by the DCD curve using Eq. 7.25 was slightly lower than that estimated by the exchange bias using Eq. 7.21. Two factors might account for this discrepancy: (1) the interface wall was not fully formed during the CoCrPtB layer switching stage, and (2) the calculation of this stage did not exclude the magnetization of M2 layers. 225 Chapter Effects of perpendicular exchange coupling Mt (memu/cm2) 1.0 0.5 343 0.0 -0.5 -1.0 -20 -10 10 20 Applied field (kOe) Figure 7.53 The exchange coupling strength J estimated by the minor loop 0.50 Mrt (memu/cm ) 0.25 0.00 -0.25 343 -0.50 -5.0k -2.5k 0.0 2.5k 5.0k Applied field (Oe) Figure 7.54 The exchange coupling strength estimation by the DCD curve. 226 Chapter Effects of perpendicular exchange coupling 7.9 Training effect of the magnetization and exchange bias for the CoDy/CoCrPtB coupled bilayers 0.12 Moment (memu) 0.06 20 40 HL 10 30 50 HR M2 0.00 -0.06 -0.12 -20k Sample 310, film structure: Ti (40nm)/CoCrPtB (35nm)/ Co79.8Dy20.2 (20nm)/Ti (4nm) -10k 10k 20k Applied field (Oe) Figure 7.55 The hysteresis loop dependence on the field cycle number for sample 310. The number of hysteresis loop goes from to 50. A training effect in the hysteresis loop has often been observed for exchange-coupled systems. The origin of the training effect phenomenon is still unclear, even after much research. [20-22] The training effect is usually described by the variation of switching fields HL (coercivity of the left side of the hysteresis loop) and HR (coercivity of the right side of the hysteresis loop) for the descending and ascending hysteresis curves with the number of field cycles. Nėel [23] believed that the mechanism of the training effect was analogous to the mechanism of the training effect in an FM body. He explained the hysteresis loop shrinking of both sides (HL increase and HR decrease) using the analogy of “tilting” observed in ferromagnets with high negative coupled domains. Nėel also thought that the hysteresis loop shifted along the pinning direction and shrank in the presence of heavy AF domain interaction. However, L. Wee et al.[20] 227 Chapter Effects of perpendicular exchange coupling believed that the training effect was related to the magnetization process activated by thermal energy overcoming the energy barrier in each stable state under the external applied field. Wee et al. found that field cycling appeared to affect coercivity for only one branch (HL), suggesting that thermal activation in the ferromagnets dominated the field training response. The results also implied that the activation rate was larger in a reversed branch loop where the field magnitude was larger than that of the forward branch. In their experiments, the researchers observed the training effect with loop shift opposite to the pinning direction and shrinking of the hysteresis loop. Repeated cycles brought the populations of the reversed and unreversed antiferromagnetic into equilibrium. The hysteresis loop was independent of the field cycle number. In this study, the training effect of the hysteresis loop was also analyzed. The hysteresis loop dependence on the field cycle number is shown in Figure 7.55. The training effect of this system was different from all training effects discussed previously because no hysteresis loop shrinking occurred. Figure 7.56 shows the coercivity dependence on the field cycle number. It suggests the coercivity was almost independent of the field cycle number. The relationship between exchange bias field and the number of field cycles is shown in Figure 7.57. The exchange bias first increased with the field cycle number. Then it slowly saturated as the reversal loops number increased further. This behavior of the training effect was also different from any previously reported. In other research, the exchange bias decreased with subsequent field reversals, due to the reconfiguration of the AFM domain spin. The left and right fields’ switching of CoDy layer dependence on the loop number is shown in Figure 7.58. Both increased as the loop number increased, just like the Hex dependence. The variation of the exchange bias might be due to the configuration of 228 Chapter Effects of perpendicular exchange coupling the spins of ferromagnetic CoDy layers at the interface. This was verified by the CoDy layer change with the magnetization training effect. In the first several reversal loops, some Co and Dy spins did not have anti-parallel configuration in the interface. Total magnetization reached its maximum when the CoCrPtB magnetization reversed. Total magnetization reversal of the coupled system’s dependence on the field cycle number is shown in Figure 7.59. The magnetization first decreased with the field cycle number. Then its value saturated as the field cycle number increased further. These results indicated that the configuration of the CoDy layer varied after the field cycle training. This configuration improved the interface coupling energy between the two layers because more CoDy and CoCrPtB layers coupled and switched together. Thus, the exchange bias of the coupled system increased after field cycle training. 2.90 Hc (kOe) 2.85 2.80 Hc training effect 2.75 2.70 10 20 30 40 50 number of field cycle Figure 7.56 The training effect of the coercivity of CoDy layer of minor hysteresis loop for sample 310 229 Chapter Effects of perpendicular exchange coupling Exchange bias (Hex) 11.2 11.1 Training effect 11.0 10.9 10 20 30 40 50 number of field cycle Figure 7.57 The training effect of the exchange bias field of CoDy layer of minor hysteresis loop for sample 310. Coercivity (kOe) 14.0 Left Hc 13.8 Right Hc 8.4 8.2 8.0 10 20 30 40 50 number of field cycle Figure 7.58 The training effect of Hc of the hysteresis loop of CoDy layer: the left and right switching field of CoDy layer of minor hysteresis loop for sample 310. 230 Chapter Effects of perpendicular exchange coupling Magnetic moment (memu) 0.094 Magnetization training effect 0.092 0.090 0.088 10 20 30 Number of cycles 40 50 Figure 7.59 The training effect of the magnetization of the sample 310. 7.10 Summary This chapter reviewed the exchange coupling between Co atoms and Dy atoms, and magnetic properties of an amorphous CoDy layer. Then the experiment of depositing a CoDy layer on a magnetic CoCrPtB layer with a perpendicular easy axis was investigated. It was found that the magnetic properties (Hc, thermal stability factor SF) of these exchange-coupled bilayers improved when the Co content of the CoDy layer was in the range of 75%-81%. The magnetic switching behaviors of the CoCrPtB layer were found to be changed greatly by exchange coupling from the CoDy layer. By tuning the Co content and thickness of the CoDy layer and the shape of hysteresis loop, the DCD curve and the switching field distribution underwent a systematic transition. The hysteresis loop and DCD curve of the exchange-coupled Ti (40 nm) /CoCrPtB (35 nm) /CoDy (30 nm)/Ti (4 nm) layer showed an unusual shape, which was explained by the large anisotropy of the CoDy layer. A new method based on the DCD curve to estimate the exchange-coupled system without a minor loop was developed. The 231 Chapter Effects of perpendicular exchange coupling estimated value was reliable compared to the value estimated by the minor loop. The training effect of exchange bias of the exchange-coupled bilayers CoCrPtB/CoDy layer was discussed. The magnetization training effect of exchange-coupled bilayers CoCrPtB/CoDy layer was due to the change of the configuration of the CoDy layer after the field cycle training. This varied configuration improved the interface coupling strength between the two coupled layers; therefore, the exchange bias of the coupled system increased after field cycle training. 232 Chapter Effects of perpendicular exchange coupling References: 1. R. Hasegawa, J. Appl. Phys. 46, 5263 (1975). 2. Z. S. Shan, D. J. Sellmyer, S. S. Jaswal, Y. J. Wang and J. X. Shen, Phys. Rev. B 16 10446 (1990). 3. P. Hansen, S. Klahn, C. Clausen, G. Much and K. Witter, J. Appl. Phys. 69, 3194 (1991). 4. H. Wan, A. Tsoukatos and G. C. Hadjipanayis, J. of Magn. Magn. Mater. 125 157 (1993). 5. P. Hansen, C. Clausen, G. Much, M. Rosenkranz, and K. Witter, J Appl. Phys. 66, 756 (1989). 6. H. Jouve, J. P. Rebouillat, and R. Meyer, AIP Conf. Proc. 29, 97 (1976). 7. K. H. J. Buschow, J. Appl. Phys. 51, 2795 (1980). 8. P. C. M. Gubbens, A. M. van der Kraan, and K. H. J. Buschow, Phys. Status Solidi. A 64, 657 (1981). 9. M. Takahashi, A. Yoshihara, T. Shimamori, T. Wakiyama, T. Miyazaki, K. Hayashi, and S. Yamaguchi, J. Magn. Magn. Mat. 75, 252 (1988). 10. X. Yang and T. Miyazaki, J. Appl. Phys. 64, 5489 (1988). 11. R. C. Taylor and A. Gangulee.J. Appl. Phys. 47, 4666 (1976). 12. G. E. Reberts, W. L. Wilson. Jr., and H. C. Bourne, Jr., IEEE Tran. Magn. Mag-13, 1535 (1977). 13. .C. J. Sun, G. M. Chow, J. P. Wang, E. W. Soo and J. H. Je, J. Appl. Phys. 93, (2003) 14. A. P. Malozemoff, Phys. Rev. B 35 (1987) 3679. 15. A. P. Malozemoff, Phys. Rev. B 37 (1988) 7673. 233 Chapter 16. Effects of perpendicular exchange coupling M. Ali, C. H. Marrows, M. Al-Jawad and B. J. Hickey, A. Misra, U. Nowak and K. D. Usadel. Phys. Rev. B. 68, 214420 (2003) 17. S. Mangin, G. Marchal, and B. Barbara, Phys. Rev. Lett. 82, 4336 (1999) 18. F. Canet, S. Mangin, C. Bellouard, M. Piécuch, and A. Schuhl, JAP 89, 6916 (2001) 19. F. C. S. da Silva and J. P. Nibarger, Phys. Rev. B. 68, 012414 (2003) 20. L. Wee and R. L. Stamps, L. Malkinski, Z. Celinski and D. Skrzypek, Phys. Rev. B, 69, 134425 (2004) 21. K. Zhang, T. Zhao and H. Fujiwara, J Appl. Phys. 6902 (2002). 22. Hideo Fujiwara, Kunliang Zhang, Tadashi Tai, Tong Zhao, J. Magn. Magn. Mater. 235, 319 (2001) 23. L. Néel, Ann. Phys. (N. Y.), 2, 61, (1967) 234 Chapter Conclusion Conclusion My PhD study focuses on utilizing the exchange coupling effect to improve the thermal stability of the magnetic recording medium. Systematic experimental and theoretical investigations of exchange-coupled thin film media have been conducted throughout the project. The major accomplishments of this research are: 1. A theoretical model has been developed for the exchange-coupled bilayer system based on the Stoner-Wolhfarth particle. The anisotropy, magnetization, thickness of both layers, exchange coupling between the two layers, and external applied field were considered in the theoretical model. Hysteresis loop, switching field and energy barrier of the exchange coupled bilayers system was calculated. The analytical results is well fit to the experimental results shown in chapter 7. 2. Based on the calculation results, The magnetic properties (Hc, energy barrier) of the exchange-coupled medium was found to be improved with the exchange coupling introduced by the coupling layer. The improvement depends on the magnetic parameters (anisotropy, magnetization, and thickness) of the coupling layer and the introduced exchange coupling strength. 3. The calculation results showed that thermal energy and exchange coupling strength are critical for magnetization anti-parallel configuration in the remnant state for the AFC medium. For long-time measurement (VSM) at room temperature, the required J decreases by 80%. 4. The exchange bias estimation formula was modified for the medium with the easy axis distribution: Hex = a J/M2t2. a is the revised factor constant that 235 Chapter Conclusion depends on the easy axis distribution. For a medium with randomly distributed easy axes, a = 0.38. For a medium with higher orientation, if ORMr =1.58, a = 0.45; if ORMr = 3.14, a = 0.49. 5. Doping Dy in both the underlayer and toplayer improved the Hc and SF of the magnetic layer. The improvement may be due to the antiferromagnetic coupling between the magnetic layer and interface that contained the Co and Dy atoms. 6. The antiferromagnetic coupling between Co and Dy atoms was utilized to improve the magnetic properties of recording medium A CoDy layer was deposited on a perpendicular CoCrPtB magnetic layer to introduce the perpendicular exchange coupling between CoDy and CoCrPtB layers. The improvement of magnetic properties (Hc, thermal stability) was achieved under certain conditions. 7. Co content alternated the exchange coupling strength and magnetization of the CoDy layer, thus changing the magnetic properties of the exchange-coupled CoCrPtB/CoDy bilayers. The shape of hysteresis loops, DCD curves, and switching field distribution curves of exchange-coupled CoCrPtB/CoDy bilayers underwent a systematic transition as Co content varied. Two-step and three-step magnetization switching behaviors were observed in the hysteresis loop. The switching field was observed to change from an even distribution to an uneven distribution with a peak. The peak shape and position could be tuned by the exchange coupling between CoDy and CoCrPtB layers. As the Co content decreased, the peak position shifted to the larger value direction, indicating an increase of the switching field. However, the peak amplitude 236 Chapter Conclusion lowered and the peak width broadened, indicating that the exchange coupling weakened. 8. The magnetization switching behaviors of CoCrPtB/CoDy bilayers were highly dependent on the CoDy thickness. As the CoDy layer increased from to 30 nm, the increasing exchange coupling strength and anisotropy of the CoDy layer, the hysteresis loop shape first became flatten; then the two-step magnetization switching and three-step magnetization switching occurred, and finally the hysteresis loop became an unusual shape. The transition process of hysteresis loop of the exchange coupling system is well consistent with the theoretical study in the chapter 5. 9. A method used the DCD curve to estimate the exchange coupling strength for the exchange-coupled bilayers without having minor hysteresis loop was developed. The estimated value was reliable compared to the value estimated by the minor loop. 10. The exchange bias training effect was observed, and for the first time the magnetization training effect was found for the exchange-coupled bilayer system. The training effect was due to the varying configuration of the CoDy layer after the field cycle training. This varied configuration improved the interface coupling strength between the two coupled layers; therefore, the exchange bias of the coupled system increased after field cycle training. In summary, for the exchange-coupled medium, the magnetic properties, exchange bias dependence on the magnetic parameters of the two layers and the exchange coupling were studied. The switching field, coercivity, hysteresis loop, and energy barrier of the exchange-coupled bilayers were calculated. Thermal energy and 237 Chapter Conclusion exchange coupling strength were critical for magnetization anti-parallel configuration in the remnant state. External field direction effect on the exchange bias estimation was first clarified. A correction factor was addressed to account for the easy axis distribution. In the experimental research, the magnetic properties of the CoCrPtB medium were observed to be improved by an exchange-coupled CoDy top layer. The improvement of magnetic properties and shape transformations of the magnetic hysteresis loops, DCD curves with varying Co content, and CoDy layer thickness were investigated. Based on this theoretical and experimental study, the exchange coupling greatly affected the magnetic loops, the DCD curves, and the switching field of the exchange-coupled medium. This study also showed that using exchange coupling energy to improve thermal stability was an effective way to extend the thermal limitation of the current medium. This research work extended the fundamental understanding of perpendicular exchange-coupled medium and, for the first time, successfully demonstrated the usage of exchange coupling energy to improve thermal stability of a CoCrPtB perpendicular recording medium. 238 [...]... coupling This novel structure showed an improved thermal stability and will be one of candidates for future extremely high areal density recording media x List of Figures and Illustrations Figure 1.1 The rate of evolution of rigid disk technology represented as a graph of areal density versus production year [4] Figure 1.2 Comparison of HDD and DRAM recording areal density improvement over the years [4] Figure... grain size distribution, and chemical isolation to break exchange and keep the media noise within acceptable bounds [27] 1.4.3 Thermal stability factor To maintain both linear resolution and media SNR requirements, the grain size must be scaled down and eventually impose a limit on the achievable areal density because of the onset of thermal instabilities Especially for the high density medium, the...Abstract Thermal instability due to the superparamagnetism is one of key concerns for future magnetic recording media Indirect exchange coupling through an interlayer has been successfully used in recently proposed and commercialized antiferromagnetically coupled medium (AFC) to solve the thermal stability issue for longitudinal recording media This study proposed and investigated for the first... Hc, SF on the Dy content in CrDy underlayer Figure 6.2 XRD diffraction spectra of the films with different Dy content in CrDy underlayer Figure 6.3 Dependence of Hc, SF on the Dy content in CrDy toplayer Figure 6.4 XRD diffraction spectra of the films with different Dy content of CrDy toplayer Figure 6.5 Hysteresis loop for the medium with CrDy toplayer (10% Dy) Figure 7.1 The Co concentration dependence... information storage in hard disk is magnetic recording My study focuses on the magnetic materials used for magnetic recording Brief magnetic recording and disk structure are explained in the subsequent paragraphs Figure 1.1 The rate of evolution of rigid disk technology represented as a graph of areal density versus production year [4] 2 Chapter 1 Introduction Figure 1.2 Comparison of HDD and DRAM recording. .. exchangecouple with a magnetic layer (recording layer), which was not only meaningful for longitudinal media but also important for future perpendicular media and/or heat assisted magnetic recording media A theoretical model for the proposed exchangecoupled bilayers structure was built up for the first time Magnetization switching behaviors of such medium structure were calculated based on this model... replacements With the development of magnetic recording, another recording mode was proposed: tilted recording [6-10] In the titled recording mode, the magnetic layer is magnetized tiltled to the film plane It has the advantage of lower requirement for the write head, higher orientation, and better thermal stability compared to the other two recording modes The magnetic medium that stores information is very... high permeability, high saturation magnetization, and low coercivity Even though high saturation materials are used, the thickness of the SUL ranges between 100 and 400 nm This thickness is much larger than that of layers in current longitudinal recording media and poses a considerable challenge to deposition tools In addition, surface roughness of the SUL, which tends to increase with layer thickness,... with different temperatures xvi Figure 7.8 Hc and thermal stability factor (SF) dependence on the Co content of CoDy layer Figure 7.9 Hc and thermal stability factor (SF) dependence on the Co content of CoDy layer Figure 7.10 Switching field (Hs) dependence on the Co content of CoDy layer Figure 7.11 The saturated magnetization dependence on the Co content of CoDy layers Figure 7.12 XRD spectrum of... Compositional variation of the compensation temperature for amorphous RE-TM alloys with RE = Gd, Tb, Dy, Ho for TM = Co Figure 7.5 Compositional variation of the Curie temperature for amorphous RE-Co alloys with RE = Gd, Tb, Dy, Ho, Here Dy-Co (circles) Figure 7.6 The anisotropy of CoDy film dependence on Co content of CoDy Figure 7.7 The magnetic properties of samples deposited with different temperatures . exchange- couple with a magnetic layer (recording layer), which was not only meaningful for longitudinal media but also important for future perpendicular media and/or heat assisted magnetic recording media. . improved thermal stability and will be one of candidates for future extremely high areal density recording media. xi List of Figures and Illustrations Figure 1.1 The rate of evolution of rigid. viii 7.10 Summary 231 8 Conclusion 235 ix Abstract Thermal instability due to the superparamagnetism is one of key concerns for future magnetic recording media. Indirect exchange coupling