Revealing Intrinsic Anisotropy of Collective Magnetic Excitations in Twinned Crystals of a Kitaev–Heisenberg Quantum Magnet

Quantum magnets with competing interactions often emerge from delicate balances among microscopic parameters, making it essential to disentangle the intrinsic spin dynamics from extrinsic disorder effects. Here we introduce a multimodal optical approach combining infrared and domain-resolved micro-Raman spectroscopies at high magnetic fields to reconstruct the intrinsic magnetic excitation spectrum of twinned crystals of the Kitaev–Heisenberg antiferromagnet Na3Co2SbO6. Far-infrared spectroscopy reveals multiple field-tunable magnetic excitations, but the intrinsic response is obscured by replica features arising from twin domains. By correlating magneto-infrared and domain-resolved Raman spectra, we isolate the single-domain magnon response and uncover a pronounced 2-fold in-plane magnon anisotropy. This anisotropy far exceeds that expected from the measured in-plane g factor anisotropy and is instead dominated by anisotropic bond-dependent exchange interactions. By unifying high-field, high-resolution, and spatially selective optical probes, our work establishes a broadly applicable framework for revealing intrinsic spin dynamics and constraining the spin Hamiltonian in multidomain quantum magnets.