Dependence of the spontaneous appearance of reverse domains on anisotropy in polycrystal ferrite materials

The theory of generation of reverse domains in poly-crystal magnetic material with an allowance for the effect of magnetic material with an allowance for the effect of magnetic permeability caused by anisotropy is presented. Specific domains are discussed that generate at the boundaries of grains and nonmagnetic inclusions. The energetic state of domains is analyzed, and the expression for the critical sizes of grains and inclusions is given where the reverse magnetization domains start to appear. From this expression it follows that at the critical radii are proportional to the ratio where is the overall constant of anisotropy, Msis the saturation magnetization and σwis the domain-wall energy density. In this case the expression for the critical radii coincides with the one obtained earlier by Néel and Goodenough. At the expression for critical radii is proportional to and is practically independent of Ms. The physical sense of the result obtained is discussed. The expression obtained is used to explain the effect of cobalt additions on the static properties of nickel-zinc ferrites.