Magnetism and magneto-structural effects in transition-metal sulphides

Abstract

Recent density-functional studies of the structural and electronic properties of a wide range of transition-metal sulphides (Raybaud P, Kresse G, Hafner J and Toulhoat H 1997 J. Phys.: Condens. Matter 9 11 085, 11 107) are extended to consider the effect of magnetic ordering in sulphides formed by 3d transition metals. We find that CrS is well described as an itinerant antiferromagnet and that the magnetic ordering leads to a substantial increase of the equilibrium volume and a reduction in the axial ratio of the NiAs-type lattice. MnS(NaCl structure) is correctly described as a high-spin type-II antiferromagnet (AFM) with a very large magneto-volume effect, but the semiconducting gap is underestimated - probably due to the neglect of correlation effects. Correlation effects are also important for stabilizing the high-spin AFM type-III state of MnS2 over the low-spin state. The phase transitions between non-magnetic (NM) NiAs-type FeS and antiferromagnetic troilite are well described by spin-density-functional theory, but the formation of a semiconducting gap and the magnitude of the magnetic moments and exchange splitting can be explained only by postulating correlation effects of intermediate strength. FeS2(pyrite or marcasite) and CoS are predicted to be non-magnetic, while cubic CoS2 is well characterized as an itinerant weak ferromagnet. NiS and NiS2 are predicted to be non-magnetic by local spin-density theory, in contrast to experiment.