Magnetoresistance and properties of half-metallic, ferromagnetic double perovskites

Sr₂FeMoO₆ is a half-metallic ‘ferromagnetic’ double perovskite with a Curie temperature T_c ≈ 390 to 450 K depending on thermal history; it exhibits a low-field magnetoresistance (MR) at room temperature. Device feasibility would be improved by an increase of the saturation moment from an observed 3.2 μ_B/formula unit to a theoretical 4.0 μ_B/formula unit. It has been assumed that the reduced moment is solely due to a random disorder of the Fe-Mo pairs on the B sites of the double-perovskite A₂B′B ′′O₆ structure. Therefore, three strategies to improve B-site order in Sr₂FeMoO₆ were explored.; A peculiar M-H hysteresis loop with low remanence, Mf < 0.5 μ _B/formula unit, low coercivity, H_ci < 200 Oe, and a reduced saturation moment M_s(5 K) < 3.5 μ_B/formula unit was observed for all compositions; it was argued it could be due to the presence of antiphase boundaries. A room-temperature low-field MR in polycrystalline samples is shown to be due to spin-dependent electron transfer across grain boundaries and not across antiphase boundaries. In samples of the system Sr _2−xCa_xFeMoO₆, an unprecedented increase in the paramagnetic inverse molar susceptibility χ_mol −1(T) was observed with increasing applied field H; a Weiss constant &thetas; = &thetas;(H) changed from &thetas; < T_c to &thetas; > T_c with increasing field. In this system, an overlap of Fe 3+/Fe2+ and Mo6+/Mo5+ redox energies occurs and strong hybridization of crystal-field minority-spin π-bonding orbitals on neighboring cations creates a minority-spin π* band. Stabilization of the Fe3+/Fe2+ relative to the Mo6+ /Mo5+ couple by a field H can progressively transfer electron density from the Mo to the Fe atoms with increasing H.; The removal of overlapping Fe3+/Fe2+ and Mo6+/Mo5+ redox energies through substitution of W6+ for Mo5+ in Sr₂FeMoO ₆ introduces Fe2+ with localized configuration t _2gα3e_gα2t _2gβ1. A change from metallic to semiconducting behavior is observed near the magnetic percolation threshold x_m ≈ 0.80 in Sr₂FeMo_1−xW_xO₆; for x ≤ x_m, long-range ferromagnetic order was observed. Spin-glass behavior was observed for 0.80 < x ≤ 0.98 and Sr₂Fe2+W 6+O₆ was an antiferromagnetic insulator with T_N ≈ 39 K.