The effect of particle interaction on the coercive force of ferromagnetic micropowders

Abstract

Previous theoretical work by Stoner & Wohlfarth (1948) on the magnetic properties of single domain ferromagnetic particles is extended to take some account of the interaction between the particles. Earlier theoretical considerations of the interaction problem are critically discussed (§2), and the experimental results for micropowder materials under different degrees of compression are reviewed (§3). A general relation for the magnetic interaction energy of two prolate spheroidal particles is derived (§4). The magnetic properties of assemblies of parallel particles (two particles, linear chains, two- and three-dimensional assemblies) are discussed, and relations are derived for the coercivity change due to the interaction (§5). Results are similarly derived for two non-parallel particles and for ordered assemblies of such particles (§§6 and 7). The two principal effects of interaction which have been covered are an interaction-dependence of the effective shape anisotropy coefficient, and a mutual magnetization change due to an interaction dependent rotation of the magnetization vectors. Both these effects are in general present and as a result the coercive force H_c depends on the interaction, being given by the general relation H_c (p) = H_c(0) – I₀(Ap+Bp^5/3+...), where p is the packing factor and I₀ the saturation magnetization. The coefficients, particularly A, may depend critically on the orientations and the geometrical arrangement of the particles. This formula is contrasted with those obtained previously, based on an 'effective field’ treatment.