Metamagnetism in DyBaCo2O5+x, x≈0.5

The temperature dependence of the paramagnetic susceptibility χ_m(T) taken in 2500 Oe, the resistivity ρ(T), and the thermoelectric power α(T) of DyBaCo₂O_5+x, which has Ba and Dy ordered into alternate (001) planes of an oxygen-deficient perovskite, have revealed a phase segregation in the compositional range 0.3?x<0.5. Orthorhombic DyBaCo₂O_5.51 has, in addition, oxygen vacancies ordered into alternate rows of the DyO_0.51 (001) planes; a cold-pressed polycrystalline sample exhibits a first-order insulator-metal transition at T_IM=320 K, a Curie temperature T_C=300 K, and a broadened metamagnetic transition temperature T_M≈265 K in 2500 Oe. A ferromagnetic M-H hysteresis curve fails to saturate at 5 T, and a minority ferromagnetic phase below T_M has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in the DyBaCo₂O_5.5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power α(T)>0 of DyBaCo₂O_5.51 changing continuously across T_IM is interpreted to manifest a metallic minority phase crossing a percolation threshold; α(T) also provides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above T_IM. A previous model of the RBaCo₂O_5.5 phase is extended to account for the Ising spin configuration below T_C, the magnetic order in the presence of higher-spin octahedral-site Co(III), and the α(T) data.