Thermally induced variable-range-hopping crossover and ferromagnetism in the layered cobalt oxide Sr2Y0.5Ca0.5Co2O7
Temperature-dependent transport and magnetic measurements on Sr2Y0.5Ca0.5Co2O7 indicate that ferromagnetism appears along with a crossover between two forms of variable-range-hopping (VRH) conductivity on cooling. Efros-Shklovskii (ES)-type VRH conduction was found below approximately 30 K, transformed from Mott-type VRH at higher temperature. The magnitude of the Coulomb gap and the Mott-ES VRH crossover temperature are ~57 K and ~170 K, respectively. These are unusually large compared to those of nonmagnetic disordered materials. The peculiar electronic state for the ferromagnetic Coulomb gap is probably due to Coulomb correlations among d electrons in the disordered system.