Angular Dependent Magnetization Reversal Behavior And Training Effect In Exchange Bias Systems

In1988, the GMR effect was discovered, which results in great progress in magnetic storage industry and entering of the spintronics era. Afterwards, the exchange bias (EB) effect has attracted much attention for its important application in magneto-electronic devices especially for the spin valves and magnetic tunnel junctions. In this thesis, we performed extensive studied in the EB system from the following three aspects:angular dependent magnetization reversal behavior, time dependent behavior of training effect and in-plane field tuning of training effect. The main results are summarized as follows.1. The angular dependent magnetization reversal behavior in exchange bias systemTypical top-pinning exchange biased Co/FeMn bilayers were prepared in an ultra-high vacuum magnetron sputtering system. In order to observe the reversal behavior, angular dependent hysteresis loops with the signals of Mx and My were recorded by a vector vibration sample magnetometer (VVSM) at different angels with respected to the initial pinning direction (PD). Experimental results exhibit that the magnetization reversal behavior is angular dependent and two critical angles i.e., αc1, αc2,(0°<αc1<αc2<90°) exist which represent the reversal pathway and the reversal asymmetry, respectively. From0°to αc1, around the easy axis, the magnetization reverses in a360°full circle during a whole loop, i.e., two My peaks appear with opposite signs. In contrast, above αc1, the system reverses only in one semicircle, i.e., My always remains positive or negative. The system reverses asymmetrically between0°and αc2and above which the asymmetry factor became zero. Our results also exhibit that the two critical angles depend on the thickness of the AF layer and both of them become smaller when the AF layer is thicker.In order to understand these experimental reversal behaviors clearly, numerical simulations were performed based on a modified coherent rotation Stoner-Wohlfarth (SW) model considering the collinear uniaxial and unidirectional anisotropies. The calculation results unambiguously show that these angular dependent reversal behaviors as well as the appearance of two critical angles are originated from the strength competition between the FM intrinsic anisotropy (KU) and the interfacial exchange anisotropy (KE).2. The time dependent behavior of training effect inexchange bias systemTypical top-pinning exchange biased FeNi/FeMn bilayers were prepared in an ultra-high vacuum magnetron sputtering system. By using a VSM we studied the time dependent behavior of training effect systematically. According to the magnetization direction, a whole hysteresis loop was divided into four different stages, i.e., descending branch, ascending branch, the positive and negative saturation stages, respectively. The time dependent behavior was measured by adding additional waiting time (△tre1) to the corresponding magnetized stage. Our experiment results show that different time dependent behaviors appear at these four stages. On the whole, waiting time in the descending (HC1) and ascending branches (HC2) is shorter than that at the two saturation stages. These experimental results could be understood with thermal activation effect and two-level relaxation model by considering that the AF energy switching barrier depends on the FM magnetization direction. 3. Field tuning of training effect in exchange biased systemTypical top-and bottom-pinning exchange biased FeNi/FeMn bilayers were prepared in an ultra-high vacuum magnetron sputtering system. In this work, to understand the training effect clearly, DC tuning field was applied in±90°to tune the training effect at room temperature. Our results show that the exchange bias training effect could be tuned by a DC field. Moreover, it is very interesting to find that the+90°field tuning effect depends on the magnitude of the tuning field. With small tuning fields, both HE and He enhance after tuning. In contrast, they both decrease under relatively larger fields. In addition, tuning in-90°direction only induced the increase of HE and HC.These results could be understood from the two recovery mechanisms of training effect. As for the two different pinning structures, tuning effect in bottom-pinning structure is much more obvious than that in the top-pinning one. In addition, the angular dependent HE and He curve in bottom-pinning structure is much more asymmetrical than that in the top one. These above results suggest that the interfacial spins in the bottom pinning structure is more unstable and tunable compared with those in the top one.