The Effect of Fourfold Anisotropy on the Angular Dependence of the Exchange Bias

The effect of fourfold anisotropy on the angular dependence of exchange bias (ADEB) in the ferromagnetic (FM)/anti-FM bilayers has been investigated. When the external field is absent at the initial magnetization stage, it is found that the bilayers will be in monostable, bistable, and quadristable states, which are controlled by the competition among unidirectional, uniaxial, and fourfold anisotropies of the bilayers. By tuning the magnitude of the fourfold anisotropy, the bilayers will show a transition between these initial states, which induces a remarkable variation of the ADEB. It is confirmed by theoretical analyses that the jump phenomenon is still existent in the ADEB when the fourfold anisotropy is included in the bilayers. In addition, two distinct modes of magnetization, whole-plane and half-plane rotation modes, will be shown in the magnetization reversal processes.