Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3

The authors report a crossover from easy-plane to easy-axis magnetic anisotropy in monolayer RuCl3, which they attribute to an in-plane distortion of the Cl atoms observed in electron diffraction that modify the non-Kitaev exchange terms. The results are useful for overcoming the challenge of realizing a quantum spin liquid. Layered alpha-RuCl3 is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this state has been reported under a modest in-plane magnetic field, such behaviour is largely inconsistent with theoretical expectations of spin liquid phases emerging only in out-of-plane fields. These predicted field-induced states have been largely out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunnelling spectroscopy, magnetotransport, electron diffraction and ab initio calculations to study the layer-dependent magnons, magnetic anisotropy, structure and exchange coupling in atomically thin samples. Due to picoscale distortions, the sign of the average off-diagonal exchange changes in monolayer alpha-RuCl3, leading to a reversal of spin anisotropy to easy-axis anisotropy, while the Kitaev interaction is concomitantly enhanced. Our work opens the door to the possible exploration of Kitaev physics in the true two-dimensional limit.