Anisotropy and orientational dependence of magnetization reversal processes in epitaxial ferromagnetic thin films

Abstract

We have undertaken a detailed study of the macroscopic and microscopic magnetization reversal processes in epitaxial ferromagnetic thin films with varying cubic and uniaxial magnetocrystalline anisotropy strengths. The macroscopic magnetization reversal processes were observed with in-plane magneto-optic Kerr effect (MOKE) vector magnetometry as a function of the relative anisotropy strengths and on the orientation of the applied field with respect to the anisotropy directions. Measurements of the component of magnetization in the plane of the sample and perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in-plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotropy constants, when the ratio is less than one. The ratios obtained from MOKE agree well with those obtained by Brillouin light scattering (BLS). MOKE vector magnetometry reveals loop features that can be associated with either one or two irreversible jumps in the direction of the magnetization, depending sensitively on the anisotropy ratio and the orientation of the applied field. Minimum-energy calculations predict that the reversal process should proceed by a continuous rotation of the magnetization vector with either one or two irreversible jumps between single-domain states, depending on the applied field orientation and the nature of the anisotropy of the film. The calculations provide a good qualitative description of the observed reversal process, although the magnetic microstructure influences the exact values of the switching fields.Sequences of Lorentz microscopy images were made in the vicinity of the two switching fields to reveal the microscopic reversal mechanism. These images provide direct evidence that the reversal proceeds by domain-wall motion between single domain states. In the case where the applied field is oriented away from the easy axis direction, a single domain intermediate state is observed during magnetization reversal, as suggested by the MOKE measurements and predicted by the calculations. In addition, the reversal mechanism with the field applied close to an easy axis reveals that the single jump observed in the MOKE loops actually corresponds to a two-step process on a microscopic scale.