Atomic and magnetic order in the weak ferromagnet CoVSb: is it a half-metallic ferromagnet?

Magnetization and neutron powder diffraction measurements have been carried out on CoVSb. Both the transport and magnetic properties are predicted by band theory to depend upon the nature of the atomic order in the C1b lattice. Calculations in which the cobalt atoms occupy the (4b) site predict half-metallic behaviour and a ground state moment of ∼1 μB per formula unit with 1.19 μB on the vanadium atoms and -0.19 μB on the cobalt atoms. However, calculations in which the Sb or V atoms occupy the (4b) site predict larger moments, ∼0.7 μB, on the cobalt atoms, and either zero or a negative moment on the vanadium atoms. In contrast, the magnetization measured in fields of up to 5.5 T is small, amounting to a ferromagnetic moment per formula unit of 0.17 μB at 2 K. Both the field dependence of the magnetization in the ordered phase below Tc∼48 K and the large effective moment obtained from the Curie-Weiss susceptibility above Tc are consistent with weak itinerant ferromagnetism. Confirmation of the small ground state moment and hence the absence of any significant moment on the vanadium atoms was obtained from neutron powder diffraction measurements, which also enabled the crystallographic structure to be determined as C1b with the cobalt atoms occupying the (4b) sites. The results show that CoVSb is not a half-metallic ferromagnet as predicted by band theory