Shape Demagnetizing Effects in Permalloy Films

Abstract

An experimental and theoretical study was made of the shape-dependent demagnetizing fields in thick (∼1 μ) uniaxial Permalloy films. Measured values of the wall motion coercive field Hc and of the rotational anisotropy field Hk change by factors of 20 and 3, respectively, as the lateral sample dimensions vary from 20:1 through 1:20. The values of Hc agree with a theory in which magnetization switching occurs when the total internal field (applied field plus demagnetizing field) exceeds an intrinsic coercive field which is independent of sample shape. In films for which the demagnetizing effects are significant this leads to a well-defined range of effective switching fields and to highly skewed hysteresis loops. The total internal field also gives agreement with the measured values of Hk, reflecting the fact that the remanent state of a hard direction hysteresis cycle is multidomain, with a consequent reduction of the easy axis demagnetization. The theoretical analysis approximates the rectangular film by a very flat ellipsoid; the good agreement with the experiments indicates that stray fields from corners and edges can be ignored when studying magnetic films.