| Summary: | 碩士 === 國立彰化師範大學 === 光電科技研究所 === 104 === Antiferromagnets (AFM) have attracted considerable research interest in the field of magnetism. AFM layers can exert various effects such as coercivity(Hc) enhancement, exchange bias field, and magnetization switching on adjacent ferromagnetic layers. The work of this thesis focuses on the study of induced perpendicular magnetic anisotropy (PMA) in AFM/ferromagnetic (FM) bilayers and FM/AFM/FM trilayers systems. In the first part of this thesis, we report an experimental investigation of the structural effects of the AFM ultrathin film on the induced PMA in epitaxially grown Mn/2 ML Co/14 ML Ni films. Magneto-optical Kerr effect (MOKE) and X-ray Photoemission Electron microscopy (X-PEEM) show an establishment of PMA in FM Co/Ni films while monolayer Mn film was deposited. Further increasing Mn layer thickness (tMn) leads to presence of in-plane magnetic anisotropy (IMA). The PMA is established again when the tMn is larger than 6 ML, occurred concurrently with a structural transition of the Mn film from the contracted (c)-face-centered-tetragonal (fct) (c/a=0.97) to expanded (e)-fct (c/a=1.05). Temperature-dependent hysteresis-loops and magnetic domain imaging give further evidence that the e-fct Mn film induces PMA for the adjacent Co/Ni film, whereas the c-fct Mn film contributes IMA. Our results clearly indicate that the interlayer distance of the AFM thin film is a crucial parameter for the induced PMA in the AFM/FM bilayers.
In the second part, the magnetic anisotropy and interlayer coupling of 12 ML Ni/Co/Mn/5 ML Co/Cu(100) magnetic thin films were in situ investigated by using MOKE and X-PEEM. The magnetic-hysteresis loops show that both 12 ML Ni/1 ML Co/Cu(100) and 0 ~ 6.5 ML Mn/5 ML Co/Cu(100) behave in-plane magnetic anisotropy. As combining above two films into 12 ML Ni/1 ML Co/Mn/5 ML Co/Cu(100), the magnetic anisotropy of top Ni/Co ferromagnetic film is found to be changed from in-plane to perpendicular direction when the thickness of Mn film is greater than about 4 ML, occurred concurrently with an establishment of antiferromagnetic ordering of the Mn film. Meanwhile, the X-PEEM images and magnetic-hysteresis loops show that the interlayer coupling between top Ni/Co and bottom Co layer is changed from parallel, antiparallel, to non-collinear. Our results suggest that the antiferromagnet-induced PMA could surpass the long-range interlayer coupling as a dominated mechanism in FM/AFM/FM exchange-coupled systems.
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