Aw investigation of the magnetocrystalline anisotropy in some of the heavy rare earth metals

Abstract

The magnetocrystalline anisotroples of gadolinium, terbium, dysprosium and holmium have been investigated using a torque method. Measurements were made on single crystal oblate spheroids, the major planes of which contained either the hexagonal axis or the basal plane of the crystals. The torque magnetometer used automatically balanced the torque produced in the specimen by applying a current through a small coil suspended in a galvanometer magnet, the servoraechanism being provided by a light beam and photo cell amplifier system. Fourier analysis of the torque curves by computer provided values for the appropriate anisotropy constants. Torque measurements were made in the temperature range from 55 K to room temperature, and in applied magnetic field strengths up to 12.5 k0e. Because of the comparatively small field strengths available it was not possible to obtain any appreciable movement of the magnetisation from the basal plane in the cases of ferromagnetic Tb, Dy and Ho, and therefore no values could be assigned to the anisotropy constants. In the paramagnetic and antiferromagnetic temperature ranges the torque curves were described completely by one anisotropy constant K(_11), although the presence of strain In the specimen could greatly distort the curve. The anisotropy constant in the basal plane, K(_4), was measured for Gd, Tb and Dy. The easy directions in the basal plane were the a-axls for Gd, and the o-axls for Tb and Dy, although changes of easy direction were observed in Dy at temperatures where the anlsotropy was small. A torque measurement made in the basal plane of holmium just above the critical field showed twelvefold symmetry in the anisotropy energy, but the main energy minimum occurred at the b-axis. Comparison of the temperature variation of K(_4) with the prediction of Zener's theory was satisfactory only in the case of Gd. For torque measurements in the (lOIO) plane for antiferromagnetic Dy, the occurrence of ferromagnetism as a magnetic field larger than the critical value was rotated across the basal plane enabled the temperature variation of the critical field to be determined. The variation was found to be in agreement with other workers. Determinations of Ho showed a change of easy axis from the basal plane to the o-axis in the antiferromagnetic region, and back to the basal plane when the system became ferromagnetic. This behaviour contradicted neutron-diffraction measurements but is confirmed by magnetisation measurements. In the paramagnetic region a variation of K(_1) as H(^2)/(T-0)(^2) was established.