The electronic band structure of cobalt disulfide(100)

The pyrite-type transition metal compound CoS2 is an itinerant electron ferromagnet and is predicted, in ground state band structure calculations to be close to half-metallic. The Curie temperature is about 120 K. Although some minority spin states are present at the Fermi level in bulk band structure, the system is predicted to be relatively highly spin polarized. The measured saturation magnetization is less than 1.0 µB/Co expected of ideal half-metallic CoS2, which is consistent with these calculations. The measured transport spin polarizations are 56 - 64 %, as determined from point-contact Andreev reflection. An accurate determination of the surface structure is essential for understanding electron spectroscopy studies, as well as providing a starting point for modeling the interface properties, essential for modeling any spintronics device applications. The success of this work was made possible by the cleavage of sufficiently large CoS2 (100) single crystals. LEED I(V) analysis, was used to determine surface structure. 1S-terminated model is favored, in which, the atoms in sublayer S and Co relax outward (toward the vacuum) and inward (toward the bulk), along the surface normal by approximately 0.03 and 0.11 Å, respectively. This kind of surface relaxation of best-fit model is also confirmed by surface stability studies. Photon energy dependent and angle dependent photoemission spectra were taken in order to map out the bulk band structure (k⊥) along surface normal and surface band structure (k//) respectively. The agreement between experimental and theory calculation suggests that the CoS 2 bulk band structure near the Fermi level is sensitive to the S-S separation, and the half-metallic gap may be controlled by antibonding sulfur p states, rather than exchange splitting of Co d states. The calculated surface band structure for the (100) surface includes the corrections for the known surface relaxation. The comparison with the experimental k// indicates that the photoemission spectra are indeed dominated by surface weighted bands. The surface state and surface resonance band dispersions in theoretical and experimental band mappings are in general agreement. The spin polarized band structure calculations indicate a true surface state in the minority spin band structure