Neutron diffraction and X-ray magnetic circular dichroism studies of the Co₂SiO₄ magnetic properties

Synthetic Co₂SiO₄ crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50K. We have studied the magnetic properties of Co₂SiO₄ by means of both nonpolarized and polarized neutron diffraction as well as X-ray magnetic circular dichroism. Measurements with non-polarized neutrons were made at 2.5 K whereas polarized neutron diffraction experiments were done at 70 K and 150 K in an external magnetic field of 7 T parallel to the b axis. Symmetry analysis of the Co₂SiO₄ magnetic structure proves that it corresponds to the magnetic (Shubnikov) group Pnma which allows the antiferromagnetic configuration (G_x,C_y,A_z) for the 4a site with inversion symmetry (Co1 position) and (0,C_y,0) for the 4c site with mirror symmetry m (Co2 position). The polarized neutron study of the magnetization induced by an applied field indicates a delocalization of the magnetic moment from Co towards neighbouring O due to the superexchange coupling. This is also in agreement with X-ray magnetic circular dichroism measurements performed at the Co K-edge in the temperature range 7K–150K in an external magnetic field of 6T applied close to the b axis of Co₂SiO₄. An important orbital contribution to the total magnetic moment of Co²⁺ in Co₂SiO₄ predicted by non-polarized neutron diffraction was independently proven by means of the X-ray magnetic circular dichroism and polarized neutron diffraction studies which indicate an orbital to spin magnetic moment ratio μ_L/μ_s ≈ 0.3.