Thermally induced variable-range-hopping crossover and ferromagnetism in the layered cobalt oxide Sr2Y0.5Ca0.5Co2O7 - art. no. 064401

Temperature-dependent transport and magnetic measurements on Sr2Y0.5Ca0.5Co 2O7 indicate that ferromagnetism appears along with a crossover between two forms of variable-range-hopping (VRH) conductivity on cooling. Efros-Shklo vskii (ES)-type VRH conduction was found below approximately 30 K, transfor med from Mott-type VRH at higher temperature. The magnitude of the Coulomb gap and the Mott-ES VRH crossover temperature ore similar to 57 K and simil ar to 170 K, respectively. These are unusually large compared to those of n onmagnetic disordered materials. The peculiar electronic state for the ferr omagnetic Coulomb gap is probably due to Coulomb correlations among d elect rons in the disordered system.