Study On Spin Glass Type Exchange Bias Effect

The study of Spintronics mainly focuses on the spin degree freedom of electron and its application in devices. There are many subfields in Spintronics. Because of its great importance in fundamental research and device application, exchange bias is an important subfield in Spintronics. In this thesis, exchange bias, especially spin glass type exchange bias, is extensively studied.Significant exchange bias (EB) training effect has been observed in sputter deposited FeAu/FeNi bilayers wherein the exchange field (HE) exhibits special sign-changeable temperature dependence. Very interestingly, albeit the absent multiple easy axes in the FeAu spin glass (SG) layer, an abrupt drop of HE between the first and the second magnetic cycles still exists, followed by a gradual change in subsequent cycles. The training behavior cannot be described by the empirical n-1/2-law due to the asymmetric magnetization reversal processes. We propose a modified Binek’s model to include the asymmetric changes of the pinning SG spins at the descending and the ascending branches. This new model can well describe the training effect in FeAu/FeNi bilayers.We also report experimental and theoretical studies on the cooling field (HFC) and temperature (T) dependent exchange bias (EB) in spin glass (SG)/ferromagnet (FM) bilayers. When the iron constituent in the co-sputtered FexAu1-x SG alloys is varied from x= 8% to 14%, with elevating T the exchange bias field (HE) is initially negative and decreases in magnitude, then changes its sign to a maximum positive value, and finally vanishes at the EB blocking temperature (TB). Meantime, the T dependent coercivity (HC) exhibits a unimodal distribution, and the peak position is concurrent with the maximum positive HE. On the other hand, HFC decreases (increases) the strength of HE in the negative (positive) region, resulting in the entire He～T curve moved leftwards and upwards, while HFC affects He weakly. Using a short-range SG vector model, the HE behaviors influenced by T and HFC are reproduced qualitatively. It is proposed that randomly FM and antiferromagnet (AFM) exchange interactions as well as noncollinear spin orientations in the SG phase are indispensable to these abnormal EB phenomena. Significantly, the results of the impact of HFC on EB imply that the sign-changeable HE against T may be an intrinsic property in the SG based systems.