Microstructural Studies of L10-FePt Thin Films with High Coercivity Fabricated at Low Deposition Temperatures
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Microstructural Studies of L10-FePt Thin Films with High Coercivity Fabricated at Low Deposition Temperatures Z.L. ZHAO, J. DING, Y. LI, G.M. CHOW, J.S. CHEN, and J.P. WANG The influence of ultrathin nonmagnetic Ag layers on the formation of the ordered fct-L10 PtFe phase and their magnetic properties have been studied, when the thin FePt films were deposited on MgO (100) single-crystal substrates. Epitaxial growth of the FePt (001) films was observed at the deposition temperature of 400 °C. With ultrathin Ag intermediate layers deposited between FePt layers, the surface morphology changed from the interconnection network to isolated-island character. The perpendicular coercivity of the FePt film dramatically increased from 6.5 to 32.5 kOe. The formation mechanism of the isolated island morphology of FePt thin films is discussed. I. INTRODUCTION
THE fct ordered L10 FePt alloy has a uniaxial magnetocrystalline anisotropy of 7.0 3 106 J/m3 and a saturation magnetization of 1150 G at 298 K.[1] Because of its high anisotropy and large saturation magnetization, the ordered L10 FePt is considered as an attractive candidate for magnetic recording media. However, an as-deposited FePt thin film has the soft-magnetic disordered fcc structure as its major phase, which possesses a very low anisotropy of K1 5 6 3 103 J/m3.[2] A heat-treatment temperature above 1000 °C is necessary to obtain the ordered L10 phase in bulk FePt alloys. Though the transformation temperature would be dramatically decreased in FePt thin films because of their nanosized grains and the large mobility of the atoms in thin films, a heat-treatment temperature of 600 °C or higher is generally required to form the hard-magnetic ordered L10 phase.[3] The high annealing temperature may damage the substrate and result in large surface roughness and grain growth. Thus, it is important to develop methods to form the ordered L10 FePt films at a relatively low temperature for industrial applications. Several strategies, such as element doping and[4–8] ion bombardment,[9,10] have been used to fabricate L10 FePt films at relatively low temperatures. However, there are some inevitable drawbacks: the magnetization of the FePt alloy is inevitably reduced, and the easy axis cannot be controlled in the direction normal to the film. Additional layers, such as Ag,[6] Au,[12] and Cu,[13] were reported to be effective in reducing the ordering temperature of L10 FePt alloys. A nonmagnetic top layer could enhance the coercivity of FePt films by promoting the disorder/order transformation of FePt thin films. However, the Z.L. ZHAO, formerly with the Data Storage Institute, Singapore 117608, and the Department of Material Sciences & Engineering, National University of Singapore, Singapore 119260, is with Heraeus Singapore Pte Ltd., Singapore. J. DING and Y. LI, Associate Professors, and G.M. CHOW, Professor, are with the Department of Material Sciences & Engineering, National University of Singapore, Singapore 119260. Contact e-mail: [email protected] J.S. CHEN is with the Data Storage Ins
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