Direction of Easy Magnetization in Gadolinium

Abstract

The direction of easy magnetization in Gd is studied as a function of temperature and pressure. The pertinent experimental results have already been given by Robinson, Milstein, and Jayaraman. A Gd sample was used as the core of a small transformer; a constant input voltage was supplied to this transformer. The secondary voltage was monitored as a function of temperature at several constant pressures. The secondary voltage of the transformer, being proportional to the permeability of the sample, is a sensitive indicator of changes in magnetic structure in the Gd sample. Typically, the secondary voltages behave as follows as the temperature of the transformer is lowered: A sharp rise in the secondary voltage occurs at the Curie temperature, followed by another sharp rise at a temperature well below the Curie point. The magnitude of the secondary voltage at this latter transition is from two to three orders of magnitude greater than at the transition from the paramagnetic to the ferromagnetic state. This transition is interpreted as a magnetic transition in which the direction of easy magnetization in Gd deviates from the $c$ axis. The angle of deviation is discussed in terms of the (small) anisotropy constants resulting from the weak crystalline field in Gd. In terms of the output voltage, one is able to obtain the relative deviations of the direction of easy magnetization from the $c$ axis. The temperatures, as a function of pressure, at which the maximum deviations occur can also be obtained. As the temperature is lowered further, the direction of easy magnetization starts to approach the $c$ axis again. Comparisons are made with other measurements of this angle of deviation.