Combined experimental and theoretical study of hydrostatic He-gas pressure effects in α−RuCl3

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on various structural and magnetic aspects of the layered honeycomb antiferromagent $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}₃$. Through measurements of the magnetic susceptibility $\ensuremath{\chi}$ performed under almost ideal hydrostatic-pressure conditions by using helium as a pressure-transmitting medium, we find that the phase transition to zigzag-type antiferromagnetic order at ${T}_N$ = 7.3 K can be rapidly suppressed to about 6.1 K at a weak pressure of about 94 MPa. A further suppression of ${T}_N$ with increasing pressure is impeded, however, due to the occurrence of a pressure-induced structural transition at $p\ensuremath{\ge}$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\ensuremath{\chi}$ under varying pressure in the paramagnetic regime ($T>{T}_N$) and before dimerization ($p<$ 100 MPa) reveal a considerable increase of $\ensuremath{\chi}$ with pressure. These experimental observations are consistent with the results of ab initio density functional theory (DFT) calculations on the pressure-dependent structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}₃$ and the corresponding pressure-dependent magnetic model. We find that pressure strengthens the nearest-neighbor Heisenberg $J$ and off-diagonal anisotropic $\mathrm{\ensuremath{\Gamma}}$ coupling and simultaneously weakens the Kitaev $K$ and anisotropic ${\mathrm{\ensuremath{\Gamma}}}^{\ensuremath{'}}$ coupling. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults, reveal that by the application of different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome.