Magneto-optical Kerr microscopy investigation of magnetization reversal in Co2FeSi Heusler alloy thin films

Abstract

Magneto-optical Kerr effect microscopy investigation of magnetocrystalline anisotropy and the magnetization reversal (MR) process in Co2FeSi (CFS) Heusler alloy thin films reveals the following: Regardless of the strength of the anti-site atomic disorder, all CFS films of fixed thickness (t = 50 nm) exhibit variations in the remanent magnetization (Mr) and coercive field (Hc) with the “in-plane” magnetic field (H) angle, φH, that are characteristic of the “in-plane” uniaxial anisotropy (UA) with an easy axis along φH = 0°. The observed variations with the field-angle are well described by the two-phase pinning (TP) model. Exceptions to this rule are the films with t ≥ 25 nm, having a maximum B2 atomic order, which show completely different angular variations in Mr and Hc. In such exceptional cases, we find that the TP model reproduces Mr(φH) and Hc(φH) only when it takes into account two mutually exclusive UAs, UA1 and UA2, with easy axes perpendicular to one another and UA1 ≫ UA2. When H points along the easy axis (φH = 0°), MR in all CFS films proceeds through the nucleation of reverse domains and their subsequent growth by domain wall movement as H increases. Atomic disorder has essentially no effect on the MR process, but the domain wall pinning at defects/imperfections affects the magnitudes of Hc and the UA field, Hk. At φH ≃ 45°, MR takes place through the formation of ripple domains. As H increases, the reverse domains grow at the expense of ripple domains, and a single domain configuration is established at high fields. When φH = 90°, MR in all films involves nucleation and subsequent growth of reverse domains by the field-induced movement of the 180° domain walls.