Thermal Conductivities and Lorenz Functions of Gadolinium, Terbium, and Holmium Single Crystals

The thermal conductivities of gadolinium, terbium, and holmium single crystals have been measured as a function of temperature from 5 to 300°K. The steady-state heat-flow method was used. For each element measurements were taken in the $〈11\overline{2}0〉$ ($a$-axis) and in the $〈0001〉$ ($c$-axis) directions of the hexagonal close-packed crystal structure. Electrical resistivity measurements were made on the same samples to obtain Lorenz functions. Anomalies near magnetic transition temperatures are interpreted in terms of magnetic superzones and spin-disorder scattering. High-temperature anisotropy is interpreted in terms of Fermi-surface anisotropy. Lorenz numbers appreciably larger than $L_0$ suggest that phonons and magnons, in addition to electrons, can carry heat.