Angular Dependence Of The Exchange Bias And The Jump Phenomenon In Ferromagnetic/Antiferromagnetic Systems

The exchange bias between ferromagnetic/antiferromagnetic systems has received much attention in recent years for its technological importance in magnetoresistive sensors and spin valves. Recently, it has been confirmed by experiments that a jump phenomenon will be emerged in the angular dependence of the exchange bias. Additionally, it is found that the ferromagnetic/antiferromagnetic systems will exhibt the full-plane and half-plane rotation modes in the magnetization reversal processes. However, the mechanism for these new features was not interpreted in these works. In this thesis, a new method was proposed to analyze the magnetization processes of the systems. The magnetization reversal processes of the ferromagnetic/ antiferromagnetic systems have been investigated in detail. The jump phenomenon together with the full-plane and half-plane rotation modes have been explained by this method.According to the energy spectrum of the systems, it is found that the ferromagnetic/antiferromagnetic systems will be in monostable, bistable, tristable and quadristable states when the applied field is absent at the initial magnetization state. Furthermore, the orientations of ferromagnetic magnetization for the energy minima and energy maxima in the initial magnetization state are defined as the intrinsic easy axes and intrinsic hard axes, respectively. Based on the positions of the intrinsic easy and hard axes, the whole angular range of the magnetization can be divided into several angular regions. The magnetization processes are analyzed when the external field is applied in every angular region. It is found that when the ferromagnetic magnetization crosses the whole of the intrinsic hard axes during the magnetization cycles, the systems will exhibit a full-plane rotation mode during the magnetization reversal processes. Otherwise, the magnetization reversal will show a half-plane rotation mode when the ferromagnetic magnetization crosses partial intrinsic hard axes or it does not cross any intrinsic hard axis in the magnetization cycles. In addition, when the external field is applied along the intrinsic easy and hard axes, it is found that one of the switching fields at the descending or ascending branch of the hysteresis loop makes an abrupt change, while the other switching field keeps continuity, and consequently the exchange bias field and the coercivity will show the jump phenomenon in the angular dependence of the exchange bias. Based on this method, the effect of various anisotropies on the magnetization processes for ferromagnetic/antiferromagnetic systems has been investigated. The dependence of the exchange bias field and the coercivity on the orientation of the applied field has also been investigated in the thesis. Some main results are generalized as follows:Firstly, only the uniaxial anisotropy is considered in the ferromagnetic/antiferromagnetic systems. By tuning the magnitude and the orientation of the exchange anisotropy, the initial magnetization state of the systems can be divided into monostable state and bistable state, which determine the angular dependence of exchange bias immediately. When the external field is applied along the orientations of the intrinsic easy and hard axes, the exchange bias field and the coercivity will show a jump phenomenon obviously. The numerical calculations indicate that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jump. At the jumping points of the intrinsic easy axes, the coercivity reaches the maximum; at the jumping points of the intrinsic hard axes, the exchange bias field will reach the maximum, at the meantime the coercivity vanishes itself suddenly. The half-plane and full-plane rotation modes will be shown in the magnetization reversal processes, the orientations of the intrinsic easy axes and hard axes are just the critical magnetization angle of these two rotation modes.Secondly, the effect of the external stress on the exchange bias has been investigated.'Our results demonstrate that both the magnitude and orientation of the external stress will affect the angular dependence of the exchange bias significantly by making a transition between monostable state and bistable state in the systems. The jump phenomenon is also existent in the angular dependence of the exchange bias. The external stress is a viable way to control and tune the exchange bias of the ferromagnetic/antiferromagnetic systems. The effect of the applied stress on the exchange bias will be more remarkable when the ferromagnetic materials have a larger magnetostriction. Thirdly, both the uniaxial and cubic anisotropies are considered in the ferromagnetic/antiferromagnetic systems. By tuning the relative magnitude and orientation of the cubic anisotropy, the systems will be in monostable, bistable, tristable and quadristable states. It displaces a complex angular dependence of exchange bias in the systems, which manifests itself by a visible oscillation in the curves. The jump phenomenon is also existent in the angular dependence of the exchange bias. Additionally, the half-plane rotation and full-plane rotation modes are also shown in the magnetization reversal processes. The numerical calculations indicate that the coercivity of the system was enhanced significantly by reinforcing the cubic anisotropy, and the times of the jumps are increased in the curves when the systems make the transition from monostable state to quadristable state.Lastly, the effect of domain wall in the antiferromagnet on the angular dependence of the exchange bias has been investigated based on the planar domain wall model. It is shown that the jump phenomenon is also existent in the angular dependence of the exchange bias. Additionally, the half-plane rotation and full-plane rotation modes are also displaced in the magnetization reversal processes. When the energy of the antiferromagnetic domain wall is larger, the changing range of pinning angle is narrower, which indicates that the better pinning effect of the antiferromagnetic layer is exerted on the ferromagnetic layer. When the energy of the domain wall tends to infinity, the planar domain wall model coincides with the M-B model. On the contrary, if the energy of the antiferromagnetic domain wall is weaker, the changing range of pinning angle is wider, which means a weaker pinning effect exists in the systems. When reducing the energy of the domain wall without limit, the antiferromagnetic magnetization will rotate synchronously with the ferromagnetic magnetization. Therefore, the magnetizing behavior of the exchange-biased ferromagnetic/antiferromagnetic systems will be same to the single ferromagnetic layer.This work was supported by the National Natural Science Foundation of China (Grant No.10762001), the Program for New Century Excellent Talents in University of China (Grant No.2005-0272), the key project of Chinese ministry of education (grant No.2006024).and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.200801260003).