Poster: Crystal Structures of Co1-xFexTiSb compounds

Authors: Götz Schuck, Jorge L. Gavilano, Denis Sheptyakov, Jürg Schefer, H. R. Ott, Z. Fisk



Conference: Jahrestagung der Deutsche Gesellschaft für Kristallographie 2008, March 3-6, Erlangen, Germany , PDF



Abstract:

CoTiSb belongs to the family of half-Heusler compounds [1] which have the general form XYZ, with X and Y transition metals and Z an sp electron metal. These materials are thought to have the MaAgAs type structure, derived from the general Heusler structure (X2Y Z) by leaving every other X site vacant. The latter is thought to lead to unusual electronic properties, including the so-called half-metallic ferromagnetism with a 100% polarized electronic band responsible for the metallic conduction [2]. In addition, the ferromagnetism in this series has been claimed to be associated with a metal to semiconductor transition [3]. All this makes the half metallic ferromagnets of this family good candidates for potential applications involving spin-polarized transport (spintronics) [4]. The CoxFe1-xTiSb samples for this investigation (x = 0, x = 0.015 and x = 0.05) have very different transport properties what is shown in Figure 1 were the normalized dc-resistances R(T)/R(300 K) of Ref. [3] is displayed.

fig1_web.jpg

Figure 1: Normalized dc-resistances R(T)/R(300 K) of Ref. [3]

According to electronic band structure calculations [4], should be a paramagnetic semiconductor with an energy gap of the order of 1 eV. However, the situation is complicated because the results are very sensitive to which elements occupy the X, Y and Z positions (Fig. 2) in the structure [5]. For instance, CoTiSb ought to be a nonmagnetic semimetal, and TiCoSb a ferromagnetic metal. Although this claim obtained some support from measurements on a polycrystalline sample of CoTiSb, it was seriously questioned by recent work on single crystals of CoTiSb [6,7]. In fact none of the above possibilities matched the experimental results. In addition it was found that replacing a small amount (5 %) of Co by Fe strongly reduces the low temperature electrical conductivity, by nearly 2 orders of magnitude. This clearly shows that the sample is close to a metal-semiconductor instability. In addition the properties of CoTiSb are sensitive to details of the annealing of the sample.

fig2_web.jpg

Figure 2

For all three samples (x = 0, x = 0.015 and x = 0.05) neutron single crystal measurements on TriCS (PSI) and X-ray single crystal measurements on BM1A beamline (SNBL/ESRF) each at 100 K has been carried out. Presented are preliminary refinement results on the joint refinement on the neutron- and the X-ray datasets, with a new beta-version of the JANA2006 software [8].

References

  1. M. Terada, K. Endo, Y. Fujita, and R. Kumura, J. Phys. Soc. Jpn. 32, 91 (1972).

  2. R. A. De Groot, F. M. Muller, P.G. Van Engen, and K.H. Buschow, Phys. Rev. Lett., 50, 2024 (1983)

  3. M.A. Kouacou, J. Pierre, and R.V. Skolozdra, J. Phys.: Condens. Matter 7, 7373 (1995).

  4. B.R.K. Nanda and I. Dasgupta, J. Phys. Condens. Matter 17, 5037 (2005).

  5. S. Ishida, T. Masaki, S. Fujii, and S. Asano, Physica B Vol. 239, 163 (1997).

  6. L. Degiorgi, A. V. Sologubenko, H. R. Ott, F. Drymiotis and Z. Fisk, Phys. Rev. B, 65, 41101 (2001)

  7. H.R. Ott, Physica B, 318, 77 (2002).

  8. V. Petricek et al., JANA, Inst. of Physics, AVCR, Praha, Czech. Rep. (2000).

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