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제목: 스텝-테라스 표면 형상을 갖는 CoFeB 자성체 박막의 자기 이방성
Co60Fe20B20 (CoFeB) films with step-terrace surface morphology were prepared on (0001) Al2O3 substrates with various mis-cut angles from 0.2° to 4° by using DC Sputtering, and their magnetic anisotropy properties within the plane were studied by using a longitudinal magneto-optic Kerr (MOKE) setup. Step-terrace CoFeB films were successfully grown on the step-terrace (0001) Al2O3 substrates which were prepared by a high-temperature annealing treatment. An atomic force microscopy image showed that the CoFeB film had a step-terrace surface morphology with the step-height of a few nanometers and the step-width of about a few hundreds of nanometers, and the MOKE measurement suggested that the shape of magnetic hysteresis loop had a clear difference with the change of measurement direction within plane. CoFeB films grown on step-terrace substrates had an uniaxial magnetic anisotropy which exhibits an easy axis (or a hard axis) with the period of 180° for a rotation within the plane. The normalized remanent magnetization was well fitted a cosine function with the change of in-plane angle (θ). CoFeB films on the 3° mis-cut terrace substrates had a magnetic easy axis along the longitudinal direction of the step-terrace, and the saturation magnetic field values were about 4.3 Oe for the easy-axis direction and about 48 Oe for the hard-axis direction which is perpendicular to the step- terrace. We also studied the change of magnetic anisotropy energy in the CoFeB films deposited on step-terrace substrates with mis-cut angles from 0.2° to 4°. The anisotropy energy was increased with increasing the mis-cut angle and the largest energy was 2.0×103J/m3 in the CoFeB film on the 4° mis-cut substrate. The anisotropy energies of CoFeB films were varied with the change of CoFeB thickness from 1nm to 50nm. The anisotropy energy was 3.11×103J/m3 in6.6nmCoFeB film which is 3 times larger than the energy in 50 nm film, confirming that the magnetic anisotropy in our samples is mainly due to a shape anisotropy effect related to the step-terrace morphology. This study suggests that the magnetic anisotropy can be controlled by adjusting the mis-cut angle of the substrates or adjusting the thickness of the CoFeB films.
Thesis Advisor: Prof. Joonghoe Dho