Indium Preferential Distribution Enables Electronic Engineering of Magnetism in FeSb2–xInxSe4 p-Type High-Tc Ferromagnetic Semiconductors

Single-phase samples of the solid-solution series FeSb2-xInxSe4 (0 <= x <= 0.25) were synthesized using solid-state reaction of the elements to probe the effect of electronic structure engineering on the magnetic behavior of the p-type semiconductor, FeSb2Se4. Powder X-ray diffraction data suggest that all samples are isostructural with FeSb2Se4. Rietveld refinements of the distribution of In atoms at various metal positions indicate a preferential substitution of Sb at the M1(4i) position within the magnetic layer A for In concentration up to x = 0.1. FeSb2-xInxSe4 compositions with higher In content show the distribution of In atoms at all metal positions, except for the M3(2d), which is fully occupied by Fe atoms. Interestingly, the ordering of Fe atoms within the crystal structure of FeSb2-xInxSe4 remains essentially unaffected by the degree of substitution (x values) and is comparable to the distribution of Fe atoms reported in FeSb2Se4. X-ray photoelectron spectroscopy confirms the oxidation states of various metal atoms In(+3), Sb(+3), Fe(+2) in the structure. Electronic transport properties indicate p-type semiconducting behavior for all samples. The electrical conductivity above 300 K first increases with In content, reaches the maximum value for x = 0.1, then decreases with further increase in In content. A reverse trend is observed for the thermopower. All samples show drastically low thermal conductivity with room temperature values ranging from 0.45 Wm(-1) K-1 for x = 0 to 0.27 Wm(-1) K-1 for the sample with x = 0.25. Magnetic susceptibility data suggest ferromagnetic-like behavior from 2-300 K for all samples. The magnitude of the magnetic susceptibility rapidly increases with In content, reaches a maximum for x = 0.1, and marginally decreases with further increase in In concentration. The observed surprising change in the magnetic and electronic behavior of samples with high In content (x > 0.1) is rationalized using the concept of antiferromagnetic scattering of charge carriers at the interfaces between overlapping bound magnetic polarons from adjacent layers A and B.