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: $K_2{\mathrm{(T)=K}}_2{\text{(0)Î}}_{\text{5/2}}[{\mathcal{L}}^{\text{−1}}\mathrm{(m)]}$ where Î is a normalized hyperbolic Bessel function and ${\mathcal{L}}^{\text{−1}}$ 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 $K_6^6{\mathrm{(T)=K}}_6^6{\text{(0)Î}}_{\text{13/2}}[{\mathcal{L}}^{\text{−1}}\mathrm{(m)]}$ . High‐field facilities of the Naval Research Laboratory and the National Magnet Laboratory were utilized.