Domain wall pinning in amorphous CoZrNb films

Abstract

A series of CoZrNb amorphous films, with thickness t ranging from 200 to 630 nm, has been studied by using several techniques: Kerr imaging, dc and audio frequency hysteresis loops, field dependence of the permeability, torque balance, broadband frequency susceptometer (1 kHz≤f≤500 MHz). Only samples with t≤500 nm have simple domain structures typical of an in-plane anisotropy (with a small dispersion of the easy axis). Although domain walls (DW) are visible, they do not contribute to the initial permeability in the observed frequency range (even in the favorable case where the magnetic field is parallel to the easy axis), but they do play a role on the irreversible magnetization processes as shown by the existence of a critical field Hcr, typical of the onset of irreversible DW motion. These observations are consistant with a model which assumes that the DW are pinned at the film surfaces: the calculations give a negligible DW bulging contribution to the initial permeability, and predict the occurence of a DW unpinning field, thickness dependent as observed.