Energy barrier and effective thermal reversal volume in columnar grains

A method to determine the minimum energy path between two stable states in a magnetic system is used to investigate magnetic reversal of an irregularly shaped columnar grain. The method describes the magnetization reversal process initiated by thermal excitations in zero applied field or fields below the intrinsic switching field. In addition to the energy barrier, an effective volume V-eff is calculated as a function of material exchange interaction, uniaxial crystalline anisotropy, and particle length. The minimum energy barrier paths follow that of intrinsic switching where with decreasing exchange the reversal modes go from uniform rotation to end nucleation and subsequent expansion to reversal by vortex excitation. The effective reversal volume decreases with decreasing exchange and increasing crystalline anisotropy. Decreasing the particle length also decreases the effective volume once the length becomes less than the excitation length. A very simple analytic wall model nicely indicates these trends. Analysis of measured thermal reversal fields for CrO2 and Fe explain the inferred effective volumes in these particles. (C) 2003 Elsevier B.V. All rights reserved.