Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO4

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb3+ ions in generating the thermal Hall effect in a family of pyrochlores, we investigated the role of Tm3+ ions in TmVO4, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO4 with a magnitude such that the Hall angle, |κxy/κxx|, is approximately 1×10-3 at H = 15 T and T = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible κxy found in the nonmagnetic pyrochlore analog (where the Tb3+ ions are replaced with Y3+), we observe a negative κxy in YVO4 with a Hall angle of magnitude comparable to that of TmVO4. This shows that the Tm3+ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of x=0.3 in Tm1-xYxVO4, κxy was found to have a positive sign, pointing to the importance of impurities in the thermal Hall effect of phonons.