Elastic Moduli of Amorphous Rare-Earth Alloys

Abstract

Crystalline electric field (CEF) effects have been observed using ultrasonic techniques in a variety of magnetic and paramagnetic solids. Dramatic changes in the velocity of sound have been reported in rare earth compounds which undergo structural transitions¹ and in magnetically ordered rare earth compounds with large magnetostrictions². In both cases results are interpretable as arising from the large magnetoelastic coupling, i.e., coupling of the strain field of the sound wave with the 4f electron charge distribution. Large magnetoelastic effects are not confined to crystalline materials. We have shown that the velocity of sound in bulk amorphous rare earth iron alloys is very sensitive to the application of a magnetic field. Here we wish to concentrate on one feature of those measurements, namely the minimum which occurs in the sound velocity as a function of magnetic field. The physical importance of this minimum is its relation to the CEF at the rare earth sites. We will discuss the temperature dependence of this minimum in the light of the random anisotropy model originally introduced to describe the magnetic properties of these alloys. By extending the random anisotropy model to include magnetoelastic coupling, we are able to account for much of the magnetic field dependence of the sound velocity. In particular the model predicts the minimum to occur at twice the local anisotropy energy divided by the value of the local moment. If this description is correct, we then have a measure of the local anisotropy energy for these alloys. It further follows that we have the possibility of determining the temperature dependence of this important physical quantity by observing the field