Introduction

In Chapter 12 we described the mean-field theory of magnetization for ferromagnetic materials, which consist of only one type of magnetic species. The atoms (or ions) comprising a simple ferromagnet are coupled via exchange interactions to their neighbors, and the sign of the exchange integral ℐ is positive everywhere. In this chapter we develop the mean-field model of magnetization for amorphous ferrimagnetic materials, of which the rare earth–transition metal (RE–TM) alloys are the media of choice for thermomagnetic recording applications. Ferrimagnets are composed of at least two types of magnetic species; while the exchange integral for some pairs of ions is positive, there are other pairs for which the integral is negative. This leads to the formation of two or more subnetworks of magnetic species. When the ferrimagnet has a uniform temperature, each of its subnetworks will be uniformly magnetized, but the direction of magnetization will vary among the subnetworks. In simple ferrimagnets consisting of only two subnetworks, the magnetization directions of the two are antiparallel.

In thin-film form, amorphous RE–TM alloys exhibit perpendicular magnetic anisotropy, which makes them particularly useful for polar Kerr (or Faraday) effect readout. Being ferrimagnetic, they possess a compensation point temperature, Tcomp, at which the net moment of the material becomes zero. Tcomp can be brought to the vicinity of the ambient temperature by proper choice of composition. This feature preserves uniform magnetic alignment in the perpendicular direction by preventing the magnetization from breaking up into domains.