Field dependence of the elastic constant c66in the basal-plane ferromagnet terbium

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Abstract
The dependence of c66 in ferromagnetic Tb on a magnetic field applied along a hard axis in the basal plane has been studied experimentally. It has been predicted that c66 should vanish at the second-order transition from a phase where the magnetisation is parallel to the field to a phase which has a finite moment perpendicular to the field. Although it has not been possible to detect the ultrasonic waves in the close neighbourhood of the transition, c66 is found to be substantially reduced at fields slightly higher than the critical value. The observed behavior of c66 can be described quite accurately, at all temperatures below the Curie point, by a random-phase theory without the use of any free parameters.