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

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Abstract
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 (RE3+1−xA2+x)MnO3. To interpret the anomalous resistivity ρ 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 TC and its magnitude is related to the hopping electron trap energy Ehop by exp(Ehop/kTC). Where T<TC, ρ is metallic because Ehop is small due to the collinear polarization of spins. For T≥TC, Ehop increases to a value ∼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 dρ/dH at T=TC is controlled by TC through (1/TC)exp(Ehop/kTC). The relative sensitivity (1/ρ)dρ/dH, however, is proportional to 1/T2C. These results also reinforce the concept that metallic resistivity in the superconducting cuprates occurs because of the frustration of antiferromagnetism.