Magnetic exchange and charge transfer in mixed-valence manganites and cuprates

Metallic resistivity occurs at cryogenic temperatures in insulators with small carrier trap energies, e.g., superconducting cuprates. A similar metallic regime has been reported for the lanthanide (RE) manganites (RE(1-x)(3+)A(x)(2+))MnO3. To interpret the anomalous resistivity rho as a function of temperature and magnetic field in these compounds, a model constructed from the relation for mobility activated semiconduction and the Brillouin-Weiss theory of ferromagnetism has been developed. the resistivity maximum occurs at the susceptibility peak slightly above the Curie temperature T-C and its magnitude is related to the hopping electron trap energy E(hop) by exp(E(hop)/kT(C)). Where T<T-C, rho is metallic because E(hop) is small due to the collinear polarization of spins. For T greater than or equal to T-C, E(hop) increases to a value similar to 0.1 eV equal to the decrease in stabilization energy of the transfer electrons caused by the transition from spin alignment to disorder. The magnetoresistance sensitivity d rho/dH at T=T-C is controlled by TC through (1/T-C)exp(E(hop)/kT(C)). The relative sensitivity (1/rho d rho/dH, the superconducting cuprates occurs because of the frustration of antiferromagnetism. (C) 1996 American Institute of Physics.