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Experimental and computational investigation of structure and magnetism in pyrite Co1−xFexS2: Chemical bonding and half-metallicity

K. Ramesha, Ram Seshadri, Claude Ederer, Tao He, and M. A. Subramanian
Phys. Rev. B 70, 214409 – Published 8 December 2004
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  • INTRODUCTION
  • EXPERIMENT
  • COMPUTATIONAL METHODS
  • RESULTS
  • CONCLUSIONS
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    Bulk samples of the pyrite chalcogenide solid solutions Co1−xFexS2 (0⩽x⩽0.5), have been prepared and their crystal structures and magnetic properties studied by x-ray diffraction and SQUID magnetization measurements. Across the solution series, the distance between sulfur atoms in the persulfide (S22−) unit remains nearly constant. First principles electronic structure calculations using experimental crystal structures as inputs point to the importance of this constant S−S distance, in helping antibonding S−S levels pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half metal, despite being nearly isostructural and isoelectronic. We use our understanding of the Co1−xFexS2 system to make some prescriptions for new half-metallic ferromagnetic.

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    • Received 3 May 2004

    DOI:https://doi.org/10.1103/PhysRevB.70.214409

    ©2004 American Physical Society

    Authors & Affiliations

    K. Ramesha* and Ram Seshadri†

    • Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA

    Claude Ederer‡

    • Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA

    Tao He§ and M. A. Subramanian∥

    • DuPont Central Research and Development Experimental Station, E328/219 Wilmington, Delaware 19880-0328, USA

    • *Electronic address: ramesha@engineering.ucsb.edu
    • †Electronic address: seshadri@mrl.ucsb.edu
    • ‡Electronic address: ederer@mrl.ucsb.edu
    • §Electronic address: tao.he@usa.dupont.com
    • ∥Electronic address: mas.subramanian@usa.dupont.com

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    Vol. 70, Iss. 21 — 1 December 2004

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    • Figure 1
      Figure 1
      MX2 pyrite crystal structure showing MX6 octahedra corner connected through X atoms which, in addition, are bonded (shown by sticks) to X atoms on neighboring octahedra. The coordination of X is 3(M)+1(X). X2 sticks and M atoms (at the centers of the octahedra) form two interpenetrating fcc lattices and a structure related to NaCl.Reuse & Permissions
    • Figure 2
      Figure 2
      (a) Powder x-ray Rietveld refinement of CoS2 (x=0) and Co0.5Fe0.5S2 (x=0.5). Data (circles), the Rietveld fit (fit reliability RBragg<8% for all samples) and the difference profiles are shown for each compound. Vertical lines at the top of the plot indicate expected peak positions. (b) Filled circles: Evolution of the a cell parameter (in Å) with x of the solid solution Co1−xFexS2. Error bars are smaller than the circles. The dashed lines connects published crystal structure (Refs. 29, 30) data on the end members (squares). Open circles: S−S distances as function of x. The dashed line connects published (Ref. 29) data (squares).Reuse & Permissions
    • Figure 3
      Figure 3
      (a) Zero-field cooled (dashed lines) and field-cooled magnetization as a function of temperature of the Co1−xFexS2 samples. (b) Field-cooled MT vs T. (c) Tc onset (circles) and midpoint, corresponding to the maximum value (squares) obtained from the MT vs T plot, as a function of x. The lines are guides to the eye.Reuse & Permissions
    • Figure 4
      Figure 4
      (a) Magnetization at 5K as a function of field. Since none of the samples show appreciable hysteresis, only the positive quadrant is displayed. Data were acquired from 5T through 0T. (b) Closed circles are measured saturation magnetization (5K, 5T) as a function of x. The open squares are the calculated LMTO magnetic moments. The dashed line is the expected spin only value assuming every eg electron contributes 1μB.Reuse & Permissions
    • Figure 5
      Figure 5
      (a) LMTO densities of state of hcp Co metal. (b) Densities of state of CoS2. The origin on the energy axis in (a) and (b), indicated by a vertical line, are the respective Fermi energies. (c) Densities of state of nonmagnetic FeS2 split into two spin directions. The energy axis in (c) has been shifted as described in the text. The upper and lower parts of each panel indicate respectively, majority and minority spin states.Reuse & Permissions
    • Figure 6
      Figure 6
      Evolution of total LMTO densities of state as a function in x in Co1−xFexS2: (a) CoS2 (x=0.00), (b) Co0.75Fe0.25S2 (x=0.25), and (c) Co0.50Fe0.50S2 (x=0.50).Reuse & Permissions
    • Figure 7
      Figure 7
      (a) LMTO COHPs of nonmagnetic CoS2 showing Co−S and S−S interactions. The nonmagnetic COHP has been scaled by a factor of 0.5. (b) LMTO COHPs of the Co−S interactions in CoS2 in the two spin directions. (c) LMTO COHPs of the S−S interactions in CoS2 in the two spin directions. In the definition we employ here, positive COHPs correspond to bonding interactions and negative COHPs to antibonding interactions. This is the opposite to the convention used in the original paper (Ref. 19).Reuse & Permissions
    • Figure 8
      Figure 8
      (a) Total energy and magnetic moment per Co atom in rock-salt CoS as a function of the cubic cell parameter, as obtained from spin-polarized LAPW calculations. (b) Total densities of state in the two spin directions of ferromagnetic CoS, calculated for a rock salt (Fm3¯m) structure with a=4.67Å.Reuse & Permissions
    • Figure 9
      Figure 9
      (a) Summary of the nonmagnetic electronic structure of Co1−xFexS2 for large x values (low eg filling). As x becomes slightly less than 1, the eg levels are filled and descend below EF. The p states remain above EF however. The boxlike shape of the unfilled d states ensures the Stoner criterion is satisfied even for small filling. (b) Even at larger eg filling (smaller x), only d states descend below EF, and EF is pinned to the bottom of the unfilled p states. (c) Schematic nonmagnetic states in a more usual material such as CoS, where unfilled states grow gradually, and the Stoner criterion is satisfied only for large filling.Reuse & Permissions
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