Constriction of Hard Direction Hysteresis Loops in Thin Permalloy Films

Abstract

According to single domain theory, thin Permalloy films with uniaxial anisotropy should have straight hard direction M-H loops without hysteresis. Practically, however, the M-H loops of almost all films are open, when the ac field is applied exactly in the hard direction and the earth's magnetic field is carefully compensated. Moreover, films thicker than about 800 A often show hard direction hysteresis loops which are constricted at the origin. As is well known, magnetization reversal in the hard direction is associated with the splitting up of the film into a great number of long domains parallel to the easy axis. Whether the walls separating these domains are of the Bloch or Néel type depends upon the strength of the applied field. It will be shown that the constricted loops can theoretically be constructed when, in addition to the anisotropy energy and the energy of the magnetization in the applied field, the energy of these walls is considered. Bitter observations of the reversal process confirm the theory, showing that the constriction is actually a result of Neél-Bloch-Néel wall transitions.