Transport Properties of Thulium Single Crystals

Single crystals of thulium have been grown by the strain-anneal technique. The electrical resistivity, thermal conductivity, and Seebeck coefficients were measured along the $〈11\overline{2}0〉$ ($a$-axis), $〈10\overline{1}0〉$ ($b$-axis), and $〈0001〉$ ($c$-axis) crystallographic directions as a function of temperature from 5 to 300°K. All measurements indicated a magnetic transition near 57°K, in good agreement with a Néel temperature of 56°K as determined in earlier work by the Oak Ridge National Laboratory neutron diffraction group. The Seebeck coefficient measured along the $c$ axis exhibited a second anomaly at 32°K, in agreement with low-field ac susceptibility measurements; but this anomaly did not appear in the neutron-diffraction measurements. The anomalies observed in the electrical resistivity and thermal conductivity at the Néel temperature are interpreted in terms of magnetic superzone planes. All transport properties exhibited a significant anisotropy between the $a$- or $b$-axis and $c$-axis directions. The high-temperature anisotropy of the electrical resistivity and the thermal conductivity is interpreted in terms of the anisotropy of the Fermi surface.