Tb magnetic moment behaviour in a Gd single crystal: Nuclear orientation study

Low-temperature nuclear orientation has been used to study the behaviour of the Tb magnetic moment in a Gd + 1 at.% Tb single crystal. The γ-ray anisotropy of several ¹⁶⁰Tb lines was monitored in the temperature range of 5-50 mK and in the external magnetic field (B_ext) range of 0-8.5 T. The field was applied either along the a- or along the c-axis of the Gd single crystal. In the case of B_ext parallel to the a-axis the Tb magnetic moment was found to be almost aligned along B_ext even at very low fields, as expected because the hexagonal basal plane is an easy magnetization plane for the Tb ion in terbium metal. On the other hand, when B_ext is along the c-axis the Tb magnetic moment rotates very slowly towards the c-axis. Even at B_ext = 8.5 T the Tb magnetic moment is not along the direction of B_ext. This is in contrast to the fast saturation of the gadolinium ions in pure Gd. Apparently, the Tb-Gd exchange interaction is in competition with the crystal field acting on the Tb ion in a hexagonal lattice, our data indicates that at B_ext = 8.5 T along the c-axis the angle between the Tb magnetic moment and the applied field is of the order of 30°.