Formation of Reversed Domains in Plate-Shaped Ferrite Particles

Abstract

The coercive force of fine-particle magnets consisting of hard ferrite particles in a nonmagnetic matrix is always considerably less than 2K/IS. A coherent rotation model is not adequate to describe this behavior. It is suggested that reversed domains come into existence at places where strong stray fields are present. Plate-shaped particles are considered with a large ratio of diameter to thickness with the easy axis perpendicular to the surface. At the edge the magnetization is forced to incline due to the large horizontal component of the stray field. The magnetization of the platelets is described by three parameters. Numerical minimization of the total energy, which is carried out for BaFe12O19 and SmCo5, shows that at a certain field, Hn, an antiparallel oriented domain comes into existence. Hn is positive (parallel to the initial magnetization direction) for particle thicknesses d>1.5 μ for BaFe12O19 and for d>1.8 μ for SmCo5. For d<345 Å (BaFe12O19) coherent rotation becomes energetically more favorable. This model shows that formation of reversed domains in a wide range of plate-shaped particle thicknesses determines the coercive force.