Crystallographic and magnetic properties in (001) FePt films with graded structure

Autor: Shih-Hsien Liu, 劉士賢
Rok vydání: 2015
Druh dokumentu: 學位論文 ; thesis
Popis: 103
Due to the demand in ever increasing magnetic recording density, media with very small bit size while maintaining thermal stability and writability is necessary. Bit-patterned media (BPM) is one of advanced nano-technologies that are currently considered to satisfy the addressed requests. Magnetic materials with large magnetocrystalline anisotropy (K_u) are required to allow the thermal stability of those individual bits, e.g., L10-FePt (001)-oriented films (K_u=7×〖10〗^7 erg/cm3). L10-FePt films with highly (001) texture can be obtained by rapid thermal annealing (RTA). However, RTA will also cause a dewetting of (001) textured FePt continuous films. The dewetting is an undesirable phenomenon for further applications. In addition, high Ku in L10-FePt films usually causes an unfavorable increase in coercivity, which may exceed the writing field of a magnetic head. In this study, atomically flat surface of (001) graded FePt films with reduced Hc were proposed. Firstly, the single-layered (001) FePt continuous films with atomically flat surface roughness were forming by controlling RTA processing parameters. The dependences of (001) texture on heating rate, surface roughness, and magnetic properties were systematically investigated. Films prepared at 40 K/s have a surface roughness of 0.31 nm and an enhanced perpendicularly magnetic anisotropy. The surface morphology, (001) orientation and magnetic properties of the L10 FePt are strongly correlated with the evolution of in-plane residual tensile stress. Secondly, an anisotropy-graded film, which exhibits a good combination of reduced Hc and near-atomic flatness on glass substrate was prepared for potential use as BPM. Soft CoPt film was deposited on the hard layer at different growth temperatures (Ts : 400 oC - 700 oC). The graded structure can be controlled by inter-diffusion of the hard and soft layers during the deposition of the soft layer. The graded the structure was proven, for the first time, via grazing incidence anomalous X-ray diffraction technology. In comparison to the single-layered FePt, Hc of the optimal-graded film is significantly reduced by a factor of 5 from 14.1 kOe to 3.1 kOe at Ts = 600 oC, while the surface roughness is lower than 0.3 nm. The CoPt/FePt graded films were systematically studied regarding to the depth-resolved structure, magnetic properties, and soft layer thickness. In the third part, the crystallographic structure and magnetic properties of the X/FePt (X= CoPt, FePt, and FePd) graded films prepared at 600 oC were discussed. Compared to the single-layered FePt films, the FePt/FePt graded films exhibit the maximum reduction (~82 %) of Hc 2.5 kOe, while FePd/FePt graded films showed an Hc of 2.7 kOe. The high saturation magnetization of the Fe-based alloys was considered as a key factor for improving performance of the graded films.
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