Growth and magnetic properties of single crystal Fe1−xCoxS2 (x=0.35–1)

The pyrite Fe1−xCoxS2 (0.25⩽x⩽0.9) was predicted to be half metal, insensitive to Fe–Co disorder. To verify its half metallicity, we have grown single crystals of Fe1−xCoxS2 (x=0.35, 0.5, 0.75, 0.9, and 1). Crystals of up to a few mm in size by temperature gradient (923–873 K) solution growth from Te melts, which showed high crystalline quality by scanning electron microscopy, x-ray diffraction, and magnetic measurements. A half-metallic magnetization of 1±0.03μB per Co atom was found for x=0.35–0.9. Point contact Andreev reflection measurements, however, showed spin polarization of 47%–61% with the maximum around x=0.5, well below predictions. Ferromagnetic resonance measurements give g∼2.08 with large intrinsic damping, which indicates a reduced half-metallicity. Inductively coupled plasma and energy dispersive x-ray spectroscopy results confirm a sulfur deficiency of ∼1.5%–10% [(Fe,Co)S2−y,0.03⩽y⩽0.2] and small amounts of Te. Electronic structure calculations show the extra electron provided by Co substitution to FeS2 populates a sulfur-derived conduction band. Consequently, S deficiency and/or S site disorder are expected to affect the transport properties. This is consistent with the low polarization measurements and again indicates the importance of crystalline order in obtaining highly spin-polarized materials.