Magnetism and magneto-structural effects in transition-metal sulphides

Recent density-functional studies of the structural and electronic properti es of a wide range of transition-metal sulphides (Raybaud P, Kresse G, Hafn er J and Toulhoat H 1997 J. Phys.: Condens. Matter 9 11 085, 11 107) are ex tended to consider the effect of magnetic ordering in sulphides formed by 3 d transition metals. We find that CrS is well described as an itinerant ant iferromagnet and that the magnetic ordering leads to a substantial increase of the equilibrium volume and a reduction in the axial ratio of the NiAs-t ype 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 correla tion effects. Correlation effects are also important for stabilizing I:he h igh-spin AFM type-III state of MnS2 over the low-spin state. The phase tran sitions between non-magnetic (NM) NiAs-type FeS and antiferromagnetic troil ite are well described by spin-density-functional theory, but the formation of a semiconducting gap and the magnitude of the magnetic moments and exch ange splitting can be explained only by postulating correlation effects of intermediate strength. FeS2(pyrite or marcasite) and CoS are predicted to b e 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-d ensity theory, in contrast to experiment.