Physics

Nanotechnology

Pure mathematics

Magnetization

Magnetic anisotropy

Mathematics

Ferromagnetism

Magnetic field

Quantum tunnelling

Magnon

Materials science

Condensed matter physics

Ab initio

Quantum mechanics

Monolayer

Anisotropy

Field (mathematics)

Quantum

Exchange bias

Nature Materials

Layered α-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 α-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. 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.

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