Time-dependent rotatable magnetic anisotropy in polycrystalline exchange-bias systems: Dependence on grain-size distribution

摘要

Angular-resolved measurements of the exchange-bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange-bias layer systems. No simple analytical model is as yet available which considers time-dependent effects such as enhanced coercivity in magnetic easy and hard axis configurations arising from the grain-size distribution of the antiferromagnet. In this work, we expand an existing model class describing polycrystalline exchange-bias systems by a rotatable magnetic anisotropy taking into account the relaxation time of thermally unstable grains. Our calculations show that coercivity mediated by the rotatable magnetic anisotropy can be distinguished from coercivity arising from ferromagnetic anisotropy by the shape of the angular dependence. Additionally, we performed angular-resolved magnetization curve measurements using vectorial magneto-optic Kerr magnetometry. Fitting the proposed model to the experimental data shows excellent agreement and reveals the ferromagnetic anisotropy and properties connected to the grain-size distribution of the antiferromagnet. Therefore, a distinction between the different influences on coercivity and magnetic anisotropy becomes available.