Anion substitution effect on the crystal structure and properties of high-coercive Fe0.25TaX2 (X = S, Se)

Using the solid-phase method, a series of layered compounds Fe0.25TaS2-ySey has been synthesized and studied using X-ray diffraction, magnetization, electrical resistivity and magnetoresistance measurements to reveal the evolution of the crystal structure and properties with the replacement of sulfur by selenium. The crystal structure of Fe0.25TaS2-ySey consists of chains along the c axis, in which tantalum and iron atoms alternate in trigonalprismatic and trigonal-antiprismatic coordination, respectively. The replacement of sulfur by selenium is accompanied by an anisotropic expansion of the crystal and a relative elongation of the structure in the direction perpendicular to the plane of the layers. All the Fe0.25TaS2-ySey compounds exhibit ferromagnetic behavior with a huge coercive field (Plc - 40-60 kOe at low temperatures) which decreases exponentially with increasing temperature. It is assumed that an almost twofold decrease in the value of TC (from 120 K to - 60 K) when sulfur is replaced by selenium occurs mainly due to a decrease in the polarization of the 5d electrons of tantalum, through which an indirect exchange interaction between the 3d electrons of Fe occurs. The change of the lowtemperature (2 K) coercive field with the Se for S substitution is found to correlate with the concentration dependence of the Curie temperature, which is indicative of the Ising spin state of Fe ions in these compounds.