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제목: CoPt/FeMn 접합 구조의 보자력과 수직 교환 바이어스
Junction structures of antiferromagnetic FeMn and ferromagnetic [Co/Pt]3 (CoPt), which show perpendicular magnetic anisotropy, have been prepared by a sputtering technique and their temperature dependences of perpendicular exchange bias and coercivity have been studied. First, the perpendicular magnetic anisotropy property in Pt/[Co/Pt]n films with the changes of Co thickness, Pt thickness, the repetition number n have been studied. The magnetic hysteresis loop with a good perpendicular magnetic anisotropy is expected to show a normalized remanent magnetization of nearly 1, a square shape, and a short magnetization tail around a saturation field. The optimal deposition condition for this work was thought to be Co thickness of 0.6 nm, Pt thickness of 2.25 nm and the repetition number of n = 2. In order to investigate which junction structure is better for this work, a FeMn layer was deposited on Pt top CoPt multilayer or on Co top CoPt multilayer. The Pt top junction structure showed a perpendicular magnetic anisotropy regardless of the presence of FeMn, but the Co top junction structure displayed an in-plane magnetic anisotropy after the deposition of FeMn. This result suggests that the existence of Pt spacer between Co layer and FeMn layer is effective to keep the perpendicular magnetic anisotropy of CoPt. In FeMn/CoPt junctions with the FeMn thickness from 5 nm to 20 nm, temperature dependences of perpendicular exchange bias and coercivity have been studied. For this experiment, the samples were heated up to 450 K and then the magnetic field of 5 kOe was applied. After then, the samples were cooled down to 18 K, and upon heating up to 400 K the MOKE hysteresis loops were measured at temperatures with every 10 K increment. At the sample with the 5 nm thick FeMn, there is no perpendicular exchange bias. As the FeMn thickness increased up to 10 nm, the perpendicular exchange bias of CoPt layer at 20 K increased, but a further increment of FeMn thickness induced a decrease of the perpendicular exchange bias. The perpendicular exchange bias was maximally 220 Oe in the sample with the 10 nm thick FeMn, and in all samples it gradually decreased with increasing the temperature. Previously, it was reported that the perpendicular exchange bias increased with the increase of FeMn thickness up to 8 nm. On the other hand, this work demonstrated that the perpendicular exchange bias decreases above the FeMn thickness of 10 nm. The blocking temperature, which the exchange bias disappears, with the FeMn thickness displayed a similar tendency with the magnitude of perpendicular exchange bias with FeMn thickness. In addition, the coercivity of CoPt layer also decreased with increasing the temperature and it showed a decrease tendency with increasing the FeMn thickness. The FeMn/CoPt junctions exhibited a smaller coercivity than the CoPt film without contacting with a FeMn layer. A superparamagnetic blocking temperature, which shows a nearly zero coercivity but a saturation behavior, also decreased with increasing the FeMn thickness. Temperature dependence of coercivity in FeMn/CoPt samples could be fitted by the so-called Kneller’s model. Finally, magnetic domain patterns in the magnetization reversal process were observed by a MOKE microscopy, and a clearly different domain evolution was observed in the CoPt layers coupled with FeMn layer in comparison to the CoPt layer without contacting with a FeMn layer.
Thesis Advisor: Prof. Joonghoe Dho