Magnetic Anisotropy of Terbium and Dysprosium

The enormous magnetic anisotropy of Tb and Dy has been studied in the temperature range from 11°K to near the Néel points. In order to directly determine the large axial anisotropy (108 erg/cm3), a unique torque pendulum was used in which the difference between the specimen torque and that of a fixed-coil dipole was determined as a function of applied field. Torque data on single crystals of Tb and Dy in fields up to 140 kOe yielded values of the uniaxial anisotropy constant K2 of 5.5×108 erg/cm3 at 11°K to 1.7×108 erg/cm3 at 205°K for Tb, and 5×108 erg/cm3 at 22°K to 1.7×108 erg/cm3 at 152°K for Dy. The temperature dependence of the data agrees with that calculated from the single-ion interaction theory: K2(T)=K2(0)Î5/2L−1(m) where Î is a normalized hyperbolic Bessel function and L−1 is the inverse Langevin function of the reduced moment. The sixfold basal-plane anisotropy of Dy and Tb were obtained from an analysis of the angular dependence of the basal plane magnetostriction. Values of the basal-plane anisotropy constant K66 for Tb ranged from 2.4×106 erg/cm3 at 4°K to 2×105 erg/cm3 at 140°K, and for Dy from 7.5×106 erg/cm3 at 4°K to 2×105 erg/cm3 at 120°K. As in the case of K2, the strong monotonic temperature dependence can be adequately represented by the single-ion expression. Here K66(T)=K66(0)Î13/2L−1(m). High-field facilities of the Naval Research Laboratory and the National Magnet Laboratory were utilized.