Effects of partial surface anisotropy on a fine magnetic particle

Abstract

Surface effects on fine magnetic particles become very pronounced because of the large surface-to-volume ratio. Experiments have shown that surface modification of fine γ-Fe2O3 particles can enhance the coercivity [see, for example, A. E. Berkowitz et al., IEEE Trans. Magn. 24, 2871 (1988) and Z. Lin, J. Magn. Magn. Mater. 116, 147 (1992)]. Using micromagnetic simulations, we have demonstrated that surface anisotropy with the proper sign contributes to the increase of coercivity [K. Zhang and D. R. Fredkin, J. Appl. Phys. 79, 5762 (1996)]. The situation where only part of the particle surface has anisotropy has not yet been investigated. To investigate this effect, we studied a single domain γ-Fe2O3 fine particle (with a length of 3000 Å and an aspect ratio of 4.6:1) using micromagnetics and the finite element method. Two regions of the particle surface have been modified by the surface anisotropy: (1) the region centered around the equator, and (2) the region around one end of the particle. In both cases the area of the modified region is about 20% of the total surface area. The surface anisotropy energy, axially symmetric about the surface normal, is added to the total magnetic energy of the particle. Hysteresis loops were obtained by minimizing the total energy at a given applied field. Numerical results show that the addition of positive surface anisotropy in case (1) enhances the coercivity while in case (2) the coercivity decreases. In both cases the surface magnetization tends to be tangential to the surface and the magnetization reversal is incoherent and starts from the surface. Further, in case (2) the incoherence of the magnetization is more prominent than in case (1), which explains the different effects of surface modification in regions (1) and (2) on coercivity.