Magnetization reversal in cobalt-surface-coated iron-oxide particles

The reduced critical field h_n for magnetization reversal by curling of Co-surface-coated iron-oxide particles has been investigated in terms of the atomic layer model (ALM). The most simple case is a rectangular Co distribution (particle composed of a γ-Fe₂O₃ core and a shell of CoFe₂O₄). In this case, the results agree very well with the curves calculated by A. Aharoni (J. Appl. Phys., vol.92, p.2576-7, 1987) in the frame of micromagnetism. Furthermore, the influence of Co diffusion into the particle core is taken into account. It turns out that h_n first increases considerably with proceeding diffusion. Depending on the anisotropy magnitude, the further alteration may be either positive or negative. This behavior offers the possibility to explain the different temperature dependences of the coercivity before and after annealing