Thickness and angular dependencies of exchange bias in ferromagnetic/antiferromagnetic bilayers

Abstract

A generalized Meiklejohn–Bean model in which the exchange coupling includes bilinear (direct exchange) and biquadratic (spin–flop) terms is proposed to study the exchange bias in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers. For the strong antiferromagnetic anisotropy, an explicit formula for the exchange bias is derived and used to discuss the thickness and angular dependencies, and a lot of recent experimental results can thus be illustrated. The results show that both the critical thickness of the antiferromagnetic layer displaying the exchange bias and the saturation value of the exchange bias depend on the exchange coupling constants, the angle between the FM easy axis and AFM easy axis, as well as the orientation of an applied field. The bilinear and biquadratic terms will provide competitive contributions to the exchange bias, while the angular variation of the applied field can tune the exchange bias.