Field-induced transitions in the Kitaev material α-RuCl3 probed by thermal expansion and magnetostriction

High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of alpha-RuCl3 in magnetic fields applied parallel to the Ru-Ru bonds. The length changes were measured in the direction perpendicular to the honeycomb planes. Our data show clear thermodynamic characteristics for the field-induced phase transition at the critical field mu H-0(c1) = 7.8(2) T where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional transition or crossover around mu H-0(c2) approximate to 11 T. The extracted Gruneisen ratio shows typical hallmarks for quantum criticality near H-c1 but also displays anomalous behavior above H-c1. We compare our experimental data with spin-wave calculations employing a minimal Kitaev-Heisenberg model in the semiclassical limit. Most of the salient features are in agreement with each other, however, the peculiar features in the region above H-c1 cannot be accounted for in our semiclassical modeling and hence suggest a genuine quantum nature. We construct a phase diagram for alpha-RuCl3 in a magnetic field along the Ru-Ru bonds, displaying a zigzag ordered state below H-c1, a quantum paramagnetic regime between H-c1 and H-c2 and a semiclassical partially polarized state above H-c2.