The Surface Electronic Band Structure CoS 2 (001)
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The Surface Electronic Band Structure of CoS2(001) Ning Wu1, Ya. B. Losovyj1,2, M. Manno3, C. Leighton3, and P.A. Dowben1 1 Dept. of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588-0111 2 Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN55455 ABSTRACT Angle-resolved photoemission was used to study the surface electronic band structure of ferromagnetic CoS2 (below 120K) in high-quality single crystal samples. Strongly dispersing Co t2g bands are identified along the k// direction. Fermi level crossings are identified along this Γ - X line (of the surface Brillouin zone) in higher resolution photoemission spectra, suggesting that the overall polarization may be controlled by the details of the band structure, particularly the surface band structure, rather than by exchange splitting on the Co atoms. INTRODUCTION The pyrite-type transition metal compound CoS2 is an itinerant electron ferromagnet. In ground state band structure calculations [1-7], CoS2 is predicted to be highly spin polarized and at least close to being half-metallic, i.e. a ferromagnet with only one conducting spin channel. However, the measured saturation magnetization (0.74 µB/CoS2 [8]; 0.85 µB/Co [9]; 0.87 µB/CoS2 [10]) falls short of the expected 1 µB/Co for the hypothetical half-metallic CoS2. This means that the minority-spin valence states are present at the Fermi level, and CoS2 is not an ideal half-metallic ferromagnet. In this regard, CoS2 resembles the much touted La0.65Sr0.35MnO3 [11,12], although CoS2 is simpler. CoS2 can be alloyed with the narrow band gap semiconductor FeS2 to become the highly spin polarized ferromagnet Fe1-xCoxS2, which more closely resembles an ideal ground state half-metallic ferromagnet, by “tuning” the Fermi level [4-7,13]. The problem with Fe1-xCoxS2 is that these systems will likely suffer from Co segregation [14], making the surface unstable, as is observed for many other nominally half metallic systems [1517]. With an electron spin polarization of about 56%, as determined from point-contact Andréev reflection [13], CoS2 remains highly spin polarized with a Curie temperature in the range of 116 K to 120 K [10,18], but not half metallic. CoS2 is, nonetheless, a starting point for characterizing the electronic structure of the pyrite-type transition metal alloys. We have studied the bulk band structure of CoS2(001) and discovered that Fermi level crossings are very sensitive to the bonding-antibonding splitting in the sulfur dimer [7], as well as to the choice of the exchange-correlation potential [2-3]. Understanding the surface band structure and interface properties is essential for modeling any spintronics device applications [17]. Due to the fact that the free enthalpy of the surface is generally different from the bulk, a simple truncation of the
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