Magnetic properties of the magnetoelectric compound Cu₂OSeO ₃: Magnetization and ⁷⁷Se NMR study

We present magnetization and ⁷⁷Se Nuclear Magnetic Resonance (NMR) measurements in single crystals of the magneteoelectric compound Cu ₂OSeO₃. The temperature and field dependence of the magnetization suggest a ferrimagnetic ordering at T_c ≃ 60 K in a 3up-1down configuration. The easy axis of the magnetization is along the [100] crystallographic direction. The ⁷⁷Se NMR line shape data collected at 14.09 T are consistent with the symmetries imposed by the cubic P2₁3 space group in the temperature range 20-290 K and confirm that the magnetic transition is not accompanied by any lowering of the crystal symmetry as has recently been proposed by Bos et al. [Phys. Rev. B 78 094416 (2008)].