The Origin for Training Effects in Exchange Bias Systems: Frustration and Multiple Anisotropy Axes at the Interface

The interaction of an antiferromagnet with a ferromagnet can establish a unidirectional anisotropy, which is referred to as an exchange bias. This paper presents a numerical simulations based on a simple coherent rotation model, which imply that the initial irreversibilities observed between the first and second hysteresis loop are driven by the symmetry of the antiferromagnet. The availability of more than one easy axis in a high symmetry antiferromagnet can give rise to a spin-flop transition during the first field reversal, after which the antiferromagnetic spins are trapped in a metastable configuration. The calculations show a very pronounced training effect for the case of antiferromagnets with two anisotropy axes, while no training is observed for antiferromagnets with only one anisotropy axis. This explains quite naturally why training effects are only observed for exchange bias systems with high symmetry antiferromagnets, while they are absent for antiferromagnets with uniaxial anisotropy.