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

Abstract

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 (Gx,Cy,Az) for the 4a site with inversion symmetry 1̄ (Co1 position) and (0,Cy,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 μL/μs ≈ 0.3.