Field independent rotational hysteresis loss on exchange coupled polycrystalline Ni–Fe/Mn–Ir bilayers

The behaviors of the magnetic torque curve and the rotational hysteresis loss of the polycrystalline ferromagnetic (F)/antiferromagnetic (AF) bilayer whose AF layer is thinner than the critical thickness are discussed as a function of the applied field. The critical thickness of the AF layer is the threshold beyond which the unidirectional anisotropy appears. At the high field, we found some differences between the calculated torque curves based on the simple model proposed by Meiklejohn and the measured ones for the Ni–Fe/Mn–Ir bilayer: (1) the lack of the sin 2θ component and (2) the constant rotational hysteresis loss, in the experimental results. These differences are explained well by the model in which the two-dimensionally random distribution of the magnetic anisotropy axes of the AF grains is taken into account. We conclude the distribution of the anisotropy axes of the AF grains is an indispensable factor to understand the unidirectional anisotropy of the polycrystalline F/AF bilayers.