Magnetic Kitaev interactions are a promising way of realizing quantum spin liquid states. A quantum spin liquid state is a state of matter without long-range magnetic order, but with strong magnetic correlations and quantum entanglement, which would be valuable for possible applications in the field of topological quantum computation. In the Kitaev model, magnetic ions are situated on a honeycomb lattice and Ising-like magnetic interactions named Kitaev interactions couple for two neighboring magnetic moments the spin component parallel to the link between the two neighboring magnetic moments to each other. It is an exactly solvable theoretical model, which harbors a quantum spin liquid state as magnetic ground state. The close proximity of the material α-RuCl3 to the Kitaev model was recently discovered and the IFW played a major role in this discovery. Despite the proximity to the Kitaev model, α-RuCl3 orders magnetically at a temperature TN= 7.5 K. However, it has been shown that this long-range antiferromagnetic order can be suppressed by the application of a magnetic field within the honeycomb plane at the critical field m0Hc ~ 7 T towards a possible field-induced quantum spin liquid state.
In this research project, the Kitaev magnet α-RuCl3 was tuned by the application of hydrostatic pressure and by chemical substitution and intercalation. The aim was to test the stability of the magnetic order and to probe the different magnetic states such as quantum spin liquid states, which compete with this magnetic order and which can be induced by these modifications.
Magnetization measurements under hydrostatic pressure were carried out in a pressure cell developed at the IFW Dresden for a high-accuracy magnetization measurement under hydrostatic pressure up to 6 GPa. By applying hydrostatic pressure to α-RuCl3, wediscovered a pressure-induced structural transition around p = 0.2 GPa towards a non-magnetic phase. In this phase the honeycomb lattice is contracted along one direction and the Ru ions build pairs called dimers with a strong antiferromagnetic interaction within the dimer. As a consequence, the magnetization is strongly reduced and the Kitaev magnetic interaction is not the dominant magnetic interaction anymore in α-RuCl3. This pressure-induced state is named a spin singlet valence bond crystal, and which is in competition with the Kitaev physics in this d-electron honeycomb system. More information can be found in the following publication (in open-access):
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, “Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α-RuCl3.” Phys. Rev. B 97, 241108(R) (2018).
Temperature-pressure phase diagram of α-RuCl3. The solid and open black circles represent the structural transition temperature obtained, respectively, by cooling and by warming the sample.
Another route to tune the magnetic interactions in α-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.
In the case of Ru1-xCrxCl3, we found that the introduction of the magnetic ions Cr3+ destabilizes the magnetic ground state of a-RuCl3 into a spin-glass state for a substitution rate of x ≥ 0.1 Cr ions per formula unit. Furthermore, our studies revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links, approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. More information can be found in the following publication (in open-access):
G. Bastien, M. Roslova, M. H. Haghighi, K. Mehlawat, J. Hunger, A. Isaeva, T. Doert, M. Vojta, B. Büchner and A. U. B. Wolter, “Spin-glass state and reversed magnetic anisotropy induced by Cr doping in the Kitaev magnet α-RuCl3”. Phys. Rev. B 99, 214410 (2019).
(x-T) phase diagram of Ru1−xCrxCl3. The black circles, red diamonds, and blue squares are experimental points from magnetization, speciﬁc-heat and ac magnetic susceptibility (f =0.1 Hz) measurements, respectively. The dashed green line indicates the temperature where dSmag/dT =Cp,mag/T undergoes its maximum
This research was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 796048
Grant Fellow: Dr. Gaël Bastien