Abstract
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 alpha-RuCl3. Through measurements of the magnetic susceptibility 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 >= 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 chi under varying pressure in the paramagnetic regime (T > T-N) and before dimerization (p < 100 MPa) reveal a considerable increase of 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 alpha-RuCl3 and the corresponding pressure-dependent magnetic model. We find that pressure strengthens the nearest-neighbor Heisenberg J and off-diagonal anisotropic Gamma coupling and simultaneously weakens the Kitaev K and anisotropic Gamma' 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.