Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO_4

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 Tb^3+ ions in generating the thermal Hall effect of Tb_2Ti_2O_7, we investigated the role of Tm^3+ ions in TmVO_4, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO_4 with a magnitude such that the Hall angle, |κ_xy/κ_xx|, is approximately 1 x 10^-3 at H = 15 T and T = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible κ_xy found in Y_2Ti_2O_7, we observe a negative κ_xy in YVO_4 with a Hall angle of magnitude comparable to that of TmVO_4. This shows that the Tm^3+ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of 30 κ_xy was found to have a positive sign, pointing to the possible importance of impurities in the thermal Hall effect of phonons.