Magnetic Resonance in Single-Crystal Terbium Metal at 100 GHz

The absorption of 100-GHz (3-mm) microwave radiation has been studied as a function of temperature and applied magnetic field in single-crystal terbium metal. For magnetic fields up to 26 kOe, three main features are observed in the absorption spectrum: (a) an absorption onset occurring at low fields and low temperatures, which is shown to be associated with the effects of domain rotation; (b) an absorption line occurring around 19 kOe. This line is essentially temperature-independent, although it greatly intensifies below the Curie temperature $T_C$; (c) a temperature-dependent absorption line which exhibits a dramatic shift as the sample is cooled through $T_C$, even in the presence of fields thought to be sufficiently strong to destroy the antiferromagnetic ordering and induce ferromagnetic alignment above $T_C$. The resonance line-widths are very large (∼5 to 10 kOe). At low temperatures this line is in good agreement with the theory of Cooper and Elliott for ferromagnetic resonance in a material with large twofold magnetic anisotropy. Extrapolation to $T=0\mathrm{}$ yields a value of the twofold-anisotropy constant $K_2$ for terbium of 5.3×${10}^8$ erg/${\mathrm{cm}}^3$ ±7%. This is in excellent agreement with the latest static-torque data of Rhyne and Clark for terbium, substantiating their result as well as reducing the uncertainty involved in their technique by more than a factor of 3. The agreement with theory at temperatures near and above $T_C$ is poorer although still qualitatively correct. Considering the approximations involved in the theoretical expression, the agreement is considered satisfactory throughout the temperature range investigated. The need is indicated for a theoretical calculation to higher order.