Engineering Magnetic Transitions in Fe1–xSnxBi2Se4 n-Type Ferromagnetic Semiconductors through Chemical Manipulation of Spatial Separation between Magnetic Centers

Understanding the nature and origin of high-temperature ferromagnetic-like ordering in complex semiconducting transition metal selenides, such as FeBi2Se4, is extremely important and could shed light on novel approaches to develop high-T-c ferromagnetism in traditional semiconductors. Here, we report on the effect of partial substitution of Fe by Sn on the distribution of magnetic centers within the Fe1-xSnxBi2Se4 crystal lattice and its impact on the electronic and magnetic properties. Several compositions of the Fe1-xSnxBi2Se4 (0.1 <= x <= 0.5) solid solution were synthesized by combining high-purity elements in the respective stoichiometric ratios. Powder X-ray diffraction suggests that the synthesized phases are isostructural with the parent compound, FeBi2Se4, despite the large difference in the ionic radii of Fe2+ and Sn2+ in octahedral coordination. Single-crystal X-ray diffraction reveals increased ordering in the distribution of Fe2+, Sn2+, and Bi3+ atoms in various metal sites within the crystal structure, with full atomic ordering reached for the Fe0.5Sn0.5Bi2Se4 (x = 0.5) composition. High-temperature direct current (dc) magnetic susceptibility measurements reveal that all Fe1-xSnxBi2Se4 samples remain ferromagnetic over a wide temperature range and the Curie transition temperature, T-c, decreases from similar to 450 K for compositions with x <= 0.15 to 325 K for compositions with high Sn content. The observed drop in T-c is ascribed to an increased separation between the magnetic centers for compositions with x > 0.15. Hall effect measurements confirm the n-type semiconducting nature of all compounds. Interestingly, the carrier density of Fe1-xSnxBi2Se4 samples gradually decreases with decreasing temperature down to 150 K, below which a remarkable transition from semiconducting to metallic behavior is observed for compositions with x >= 0.25.