Torque and Rotational Hysteresis in fcc Single-Crystal Cobalt Films

Abstract

The Stoner-Wohlfarth coherent rotation model is extended to the case of biaxial anisotropy. The rotational hysteresis Wr as a function of reduced field h is zero for all values of h except 0.25≤h≤1, and the maximum value of Wr/K=2.38 at h=0.25. The rotational hysteresis integral W equals 1.54. Measurements on fcc single-crystal Co films (−K=4−6×105 ergs/cm3) prepared by epitaxial evaporation onto the (100) face of single-crystal MgO slabs held at 400°C yield results in wide disagreement with the theory. While the expected angular symmetry of the torque is observed, the model does not correctly predict the magnitude of the torque nor the fields at which specific types of angular dependence will be observed. The rotational hysteresis begins in every case before the predicted onset and continues beyond the expected cutoff. The maximum value of Wr/K and W are found to vary from 1.0 to 1.6 and from 2.9 to 1.7, respectively. It is concluded that the magnetization reversal process in single-crystal Co films is not coherent and that, as in the case of polycrystalline films, noncoherent rotations or wall motions must be investigated. This situation in single-element, single-crystal films calls for a reappraisal of the importance of compositional inhomogeneities and local crystalline anisotropy in polycrystalline alloy films.