Néel Ferrimagnets in Large Magnetic Fields

In this paper we analyze the high-field magnetization of Néel ferrimagnets and apply this analysis to our measurements of YbIG and DyIG. We show that in some ferrimagnetic systems, as the temperature is increased at fixed field strength, the magnetization can decrease in one temperature interval, be temperature independent in another, and rise or fall in a third. Conversely, at fixed temperature, as the field is increased, there can be intervals of small and large susceptibility with abrupt discontinuities between them, even in the absence of anisotropy. This behavior is ascribed to the formation of angles between the various sublattice moments and the external field. The angled region can be described by a phase diagram in the H-T plane. There are second-order phase transitions at all boundaries, along which the order parameter (the angles) undergoes critical fluctuations. Two features appear technologically promising for some ferrimagnets: (a) a temperature range over which the moment of the ferrimagnet is temperature independent, with discontinuities in dM/dT at both ends of the range, and (b) cooling by adiabatic magnetization, i.e., cooling by increasing the magnetic field strength. We have measured the magnetization of YbIG and DyIG from 1.4°–296°K in fields up to 130 kOe. At low temperatures, the YbIG magnetization is directly proportional to the magnetic field for 3.5 kOe < H < 130 kOe, while at the same time it is nearly temperature independent. Below a critical field of ∼3.5 kOe, sublattice-moment rotation does not take place, and the magnetization is not linearly related to the magnetic field. The 4.2°K spontaneous magnetization of YbIG is 0.069 μb. The DyIG magnetization, on the other hand, is large and highly anisotropic at low temperatures. At 4.2°K technical saturation is achieved near 40 kOe, but a large volume susceptibility (∼0.4×10−3 emu) persists above 130 kOe.