We report phenomena manifesting nonequilibrium superconductivity induced by spin-polarized quasiparticles in perovskite ferromagnet-insulator-superconductor ͑F-I-S͒ heterostructures. Measurements of the critical current J c , using a pulsed current technique to minimize Joule heating, reveal a monotonic increase with increasing insulator thickness; and for F-IS with thin insulating barriers, a slight increase in J c is observed under small injection currents I m from the ferromagnet, followed by a strong suppression of J c under large I m. In contrast, no effect of injection on J c can be detected in the N-IS control sample ͑N: nonmagnetic metal͒. The spin diffusion time and length in cuprate superconductors are also estimated and compared with our experimental results. ͓S0163-1829͑99͒05638-6͔

Studies of La 0.7 Ca 0.3 MnO 3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T → T C , T C being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T T C ), the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn-Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La 0.5 Ca 0.5 CoO 3 . Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn-Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn-Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites.

The effects of spin-polarized quasiparticle transport in superconducting YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor ͑F-I-S͒ heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor ͑N-I-S͒. Systematic studies of the critical current density (J c ) as a function of the injection current density (J in j ), temperature (T), and the thickness ͑d͒ of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of J c and a rapidly increasing characteristic transport length near the superconducting transition temperature T c only in the F-I-S samples. The temperature dependence of the efficiency (ϵ⌬J c /J in j ; ⌬J c : the suppression of critical current due to finite J in j ͒ in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift * in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO 2 planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity.

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We report an unprecedentedly large spontaneous Hall effect in ferromagnetic La 1Ϫx Ca x CoO 3 (0.2рx р0.5) epitaxial films. The effect exceeds existing theoretical predictions for the value of the spontaneous Hall resistivity xy by several orders of magnitude. The Hall effect is the strongest for xϭ0.2, which is at a doping level nearest to the ferromagnetic percolation threshold in La 1Ϫx Ca x CoO 3 . We suggest that the coexistence of high-and low-spin configurations in the perovskite cobaltites, together with the magnetic percolation behavior, may be responsible for the giant Hall effect.