The constant K(_4) for the hexagonal magnetocrystalline anisotropy of the basal plane has been measured at various temperatures between 77K and 180K for alloys of Terbium and Scandium of compositions. Tb(_x)(^so)(_1-x) where x = 0.695, 0.825 and 0.89 using a torque magnetometer with automatic balancing and recording. Simple methods were devised for setting up the instrument and for analysing torque curves so as to correct for shearing and distortions introduced by rotational hysteresis. Instability in the magnetometer was cured by increasing the speed of response. Three methods of calibratior were compared. The variation of K(_4) with temperature for the Tb/So alloys indicates that the anisobropy is of single ion origin and due to the second order magnetostriction of hexagonal symmetry. Analysis of the results of P.K. Ely ( 1967, Ph.D. thesis, University of Durham, unpublished) leads to the same conclusion for pure Terbium. Estimates of at absolute zero indicate that the hexagonal anisotropy contributes significantly to the driving energy to ferromagnetism in pure Terbium end the Terbium rich alloys0Reported changes in the easy axis of Dysprosium have been investigated. Those below 80K are associated with hysteresis in the movement of the domain walls and are not reproducible. Those above 130K may be explained by the existence of fan-like spin structures but could also be due to the uncertainty associated with determinations of easy axis using a torque magnetometer with an unsaturated specimen
