Fang Wang, Xiaohong Xu, Yan Liang, Jing Zhang, and Haishun Wu

1. FeAu/FePt exchange-spring media fabricated by magnetron sputtering and post-annealing Fang Wang, Xiaohong Xu, Yan Liang, Jing Zhang, and Haishun Wu School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China Results Conclusion Introduction Recently, exchange coupled com-posite (ECC) media and exchange spring (ES) media have been proposed to be the most effective method to achieve an areal density beyond 1 Tbit/in.2 in perpendicular recording media. The ECC and ES media are composed of a magnetically hard layer and a magnetically soft layer, which are coupled by exchange interaction through their interface. The hard layer with a high anisotropy can overcome the superparamagnetic limit caused by the decrease of the grain size, but results in an unfavorable increase in coercivity, which can exceed the field of the current write head. Thus, a soft layer is needed to be deposited on the hard layer in order to switch the hard layer at a lower switching field while maintaining a high thermal stability. Soft/hard bilayers consisting of a FeAu layer with different thicknesses and a 20 nm L10-FePt layer have been fabricated by magnetron sputtering and post-annealing. FeAu soft layer not only can promote the ordering degree of FePt layer because of the small lattice mismatch between them and the diffusion of Au atoms into FePt boundaries, but also can reduce the coercivity due to the soft/hard exchange coupling. The results of X-ray photoelectron spectroscopy indicate that a graded interface is formed in the FeAu/FePt bilayer after annealing, which is beneficial to reduce the pinning field. The magnetization reversal in the FeAu/FePt exchange-spring media occurs by the nucleation and propagation of a domain wall from soft layer into hard layer. At 400 oC the FePt film can be trans-formed from soft fcc phase to hard L10 phase by introducing a FeAu soft layer. FIG. 1. Perpendicular hysteresis loops of single FePt, FeAu layers and FeAu(5 nm)/FePt(20 nm) bilayer annealed at 400 oC. The coercivities of the bilayers decrease with the increase of the thickness of soft layer due to the strong exchange coupling between FeAu and FePt layers. FIG. 2. Hc as a function of the soft layer thickness(ts) for FeAu/FePt bilayers annealed at 400 oC and 550 oC. The coercivities of the FeAu/FePt bilayers annealed at 550 oC are smaller than the ones annealed at 400 oC. It is because the interdiffusion at the soft/hard interface is favorable to the formation of the graded interface. The larger value of tG can reduce the pinning field. References Experimental details We choose L10-FePt with an extremely high anisotropy as a hard layer, and select L10-FeAu with a high saturation magnetization as a soft layer. FeAu/FePt bilayers were prepared using magnetron sputtering from separated FeAu and FePt composite targets. The base pressure of the chamber was 8×10-5 Pa and high purity Ar pressure of 0.8 Pa was kept during sputtering. The as-deposited films were then annealed at 400 C and 550 C for an hour in a vacuum better than 2×10-4 Pa. [1] D. Suess, T. Schrefl et al., Appl. Phys. Lett. 87, 012504 (2005). [2] R. H. Victora and X. Shen, IEEE Trans. Magn. 41, 537 (2005). [3] J. P. Wang, W. K. Shen et al., Appl. Phys. Lett. 86, 142504 (2005). [4] D. Suess, Appl. Phys. Lett. 89, 113105 (2006). [5] A. Y. Dobin and H. J. Richter, Appl. Phys. Lett. 89, 062512 (2006). FIG. 3. Compositional depth profiles of FeAu(5 nm)/FePt(20 nm) films: (a) as-deposited and (b) annealed at 550 oC. Acknowledgements This work was supported by NSFC (No. 60776008), NCET-07-0527), FIG. 4. Perpendicular hysteresis loops of the FeAu(t nm)/FePt(20 nm) bilayers annealed at 400 oC [(a)-(d)] and 550 oC [(e)-(h)].