Magnetothermopower, magnetic entropy change, and magnetoresistance in ferromagnetic cobaltites: R0.7Sr0.3CoO3 (R = La and Gd)

Abstract

We compare the impact effect of external magnetic fields on the magnetic entropy (Sm), electrical resistivity, and thermopower in two metallic oxides R0.7Sr0.3CoO3 that become ferromagnetic below TC = 240 K and 117 K for R = La3+(4f0) and Gd3+(4f7), respectively. While Gd0.7Sr0.3CoO3 shows unusually high values of negative magnetothermopower (≈−35.5%), magnetic entropy change (ΔSm ≈ −12 J/kg K), and negative magnetoresistance (≈−3.7%) at 10 K for a field change of 5 T, these effects are absent in the La sample at 10 K. We suggest that the large ΔSm arises from the field-induced suppression of spin fluctuations associated with localized 4f7 moments of Gd3+ ions, but magnetic interaction among Co ions dictates magnetoresistance and magnetothermopower. Magnetization data indicate that while Co3+ ions adopt intermediate spin state (S = 1) in the La compound, major fraction of them transforms into low spin state (S = 0) in the Gd sample. Hence, the Gd sample is microscopically heterogeneous with a mixture of ferromagnetic metallic and paramagnetic metallic clusters. Spin dependent scattering at the surface of ferromagnetic clusters may be the cause for the coexistence of magnetothermopower and magnetoresistance in this compound. A close correlation between the magnetoresistance and the magnetothermopower is reported.