Magnetization measurements under hydrostatic pressure were used to investigate the effect of pressure on the Kitaev magnet alpha-RuCl3. The design of the pressure cell for such measurements was optimized during this project. As a result, the resolution of magnetization measurements under pressures up to 4.5 GPa was enhanced both via an enlargement of the maximal sample dimensions and via a reduction of the background signal from the pressure cell.
We discovered a pressure-induced structural phase transition in alpha-RuCl3 under the application of a hydrostatic pressure of 0.2 GPa [1]. The high-pressure phase was further characterized via x-ray diffraction studies under hydrostatic pressure and quantum chemistry calculations in collaboration with the TU Dresden and the theory department of the IFW, respectively. We found that the honeycomb lattice is contracted along one direction and the Ru ions build pairs called dimers with a strong antiferromagnetic interaction within the dimer at this phase transition. As a consequence, the magnetization is strongly reduced and the Kitaev magnetic interaction is not the dominant magnetic interaction anymore in alpha-RuCl3. This pressure-induced state is named a valence bond solid.
Another route to tune the magnetic interactions in alpha-RuCl3 undertaken in this project, is the partial substitution of the magnetic ion Ru3+ by another magnetic ion Cr3+ or by non-magnetic ions Rh3+, Ir3+ and Ru2+. The last example of mixed Ru3+ and Ru2+ ions was obtained via the intercalation of K+ ions between the honeycomb layers.
We found that the introduction of the magnetic ions Cr3+ destabilizes the magnetic ground state of alpha-RuCl3 into a spin-glass state for a substitution rate of x=0.1 Cr ions per formula unit [2]. The chemical substitution of Ru3+ by non-magnetic ions Rh3+ and Ir3+ destabilize also the antiferromagnetic ground state. However, no signature of a spin-glass state has been observed despite its theoretical prediction and the nature of the state induced by chemical substitution remains unclear. Contrary to the chemical substitution of Ru3+ ions by non-magnetic ions Rh3+ and Ir3+, the introduction of Ru2+ by chemical intercalation of K+ ions allows displacement of the magnetic ions via hopping of electrons for temperature higher than 200K . This leads to the formation of a regular pattern of magnetic ions Ru3+ upon slow cooling from 300K. For the composition K0.5RuCl3 our results indicate the magnetic structure changing from a honeycomb structure for alpha-RuCl3 to a triangular-lattice antiferromagnet, which is very valuable since the realizations of triangular-lattice of 4d magnetic ions are rare. The formation of a long-range magnetic order on this triangular lattice was also detected at TN = 2.1K.
[1] G. Bastien, G. Garbarino, R. Yadav, F. J. Martinez-Casado, R. Beltrán Rodríguez, Q. Stahl, M. Kusch, S. P. Limandri, R. Ray, P. Lampen-Kelley, D. G. Mandrus, S. E. Nagler, M. Roslova, A. Isaeva, T. Doert, L. Hozoi, A. U. B. Wolter, B. Büchner, J. Geck, and J. van den Brink, Phys. Rev. B 97, 241108(R) (2018).
[2] G. Bastien, M. Roslova, M. H. Haghighi, K. Mehlawat, J. Hunger, A. Isaeva, T. Doert, M. Vojta, B. Büchner and A. U. B. Wolter, Phys. Rev. B 99, 214410 (2019).
