Ferrimagnetism of the magnetoelectric compound Cu₂OSeO ₃ probed by ⁷⁷Se NMR

We present a thorough ⁷⁷Se nuclear-magnetic-resonance (NMR) study of a single crystal of the magnetoelectric compound Cu ₂OSeO₃. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high- T paramagnetic phase to a low- T ferrimagnetic state with 3/4 of the Cu²⁺ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction in the crystal symmetry from its high- T cubic P 2₁3 space group. These results are in agreement with high-resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos [Phys. Rev. B 78, 094416 (2008)]10.1103/PhysRevB.78.094416. We also develop a mean-field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic (J_afm ≃68 K) and one ferromagnetic (J _fm ≃-50 K) nearest-neighbor exchange interaction.