| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 92 === This work focused on the study regarding the nano oxide layers (NOLs) used in the GMR and TMR multilayers, especially on interlayer coupling and spin transport properties through the nano oxide layers. Two main topics have been discussed in this dissertation. The first topic is the investigation of the interlayer coupling between two ferromagnetic layers (FM) across NOLs. Inserting the NOLs into the pinned layer of a spin-valve structure, various types of interlayer coupling would be observed depending on the oxidation methods and kinds of NOLs. In the case of the naturally oxidized NOLs, a ferromagnetic interlayer coupling revealed through the magnetic channels formed in NOLs. Hence, both of the two ferromagnetic layers exhibited the exchange-biased effect. However, the magnetic channels have been eliminated from the NOLs fabricated by plasma oxidation. Two distinct kinds of interlayer coupling have been observed in the plasma-oxidized NOLs depending on the magnetic characteristics of NOLs. In the case of the plasma-oxidized CoOx- or CoFeOx-NOLs, an isolation of the exchange biasing effect across the NOLs would be observed. On the other hand, a novel biquadratic coupling would be generated between two ferromagnetic layers through the plasma-oxidized NiFeOx NOLs. The mechanism for the biquadratic coupling is due to the plasma-oxidized NiFeOx layer possessing the spin-glass-like characteristic. Therefore, the heterostrucutre comprising of AFM/FM (bottom-pinned CoFe)/ spin-glass-like layer (NiFeOx), in which the spins of spin-glass-like layer repeatedly switch their orientations back and forth parallel to the axis of the CoFe magnetization, forms a spin-compensated plane, namely the artificial AFM layer. According to the theoretical prediction, a uniaxial anisotropy in the FM layer in contact with the spin-compensated AFM layer is induced perpendicular to the direction of the spin axis in the AFM layer and no exchange bias is observed, which can explain the generation of the biquadratic coupling across the plasma-oxidized NiFeOx in this study. Furthermore, the spins in the artificial AFM layer can be manipulated by controlling the temperature or the thickness of the magnetic layers in structure. By controlling the temperature and the cooling field, a fully spin-compensated or spin-uncompensated AFM can be obtained, leading to a fully understanding of exchange bias lying in controlling the spin structure at the interface of AFM. Meanwhile, the biquadratic coupling strength could be optimized by controlling the IrMn thickness due to the variation of exchange path distribution. Finally, we also investigated the long-range coupling between the spin-compensated AFM and ferromagnetic layer across the Cu spacer layer. At a specific temperature range, the oscillatory behavior on biquadratic coupling strength would be observed. The oscillation behavior can be successfully explained by considering the RKKY-like interlayer coupling in this study.
In the second topic, we studied the ion irradiation effect on TMR junctions. A 26 % enhancement of TMR ratio, compared to the non-irradiated sample, can be generated at the Ni ion dose of 1×10^15 ions/cm2. In addition to the enhancement of TMR ratio, a drastic reduction of RA can be also observed. The observed results can be attributed to the impurity-assisted tunneling in the ion-doped barriers. By utilizing the ion irradiation, the properties of TMR junctions can be modulated to meet the requirements for read heads.
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