The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb2 to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb2 and FeSi (FeSi1−xGex alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment ≈ 1μB per Fe ion with external magnetic field. PACS. 71.27.+a – 71.30.+h The unusual crossover from a small gap semiconductor at low temperatures to a metallic state with enhanced magnetic fluctuations above room temperature observed in FeSi has long been a subject of great interest [1,2, 3,4,5]. Two different models have been proposed to explain this anomaly. One proposal is that FeSi is a Kondo insulator [2]. The second is that FeSi is a nearly ferromagnetic semiconductor [3,4]. This latter proposal is supported by ab initio electronic structure calculations using the LDA+U method [6] which found that a ferromagnetic metallic state was very close by in energy [3]. Further support for this second model comes from the direct observation of this semiconductor-metal transition as the lattice is expanded by the isoelectronic substitution of Ge for Si [7]. In order to determine critical magnetic field a minimal two-band model with interband interaction was suggested. Even in the mean field approximation the model nicely describes the full phase diagram of the FeSixGe1−x alloy series [7]. Recently, a second Fe-compound, FeSb2, was found to have a similar crossover as FeSi from a small gap semiconductor to a metallic state with strong magnetic fluctuations [8,9]. This raises immediately the question whether ab initio calculations confirm the close analogy between the two Fe compounds. This is a nontrivial question since these electronic structure calculations show strong hybridi zation between the Fe-3d orbitals and the s-p electrons of the close Si or Sb neighbors. This strong hybridization clashes with the assumption of weak hybridization between localized and itinerant electrons that underlies a Kondo insulator description and makes it difficult to relate the Kondo insulator model to the ab initio electronic structure. However in view of the differing compositions and crystal structures of FeSi and FeSb2 it is by no means -3 -2 -1 0 1 2 3 Energy [eV] 0 5 10 15 20 D O S [s ta te s/ eV /c el l] total Fe-d Sb-p -1 -0.5 0 Energy [eV] 0 2 4 6 t 2g