Spin Glass Behaviour and Spin-Dependent Scattering in La_0.7Ca_0.3Mn_0.9Cr_0.1O_x Perovskites

The magnetic, electrical and thermal transport properties of the perovskite La_0.7Ca_0.3Mn_0.9Cr_0.1O_x have been investigated by measuring dc magnetization, ac susceptibility, the magnetoresistance and thermal conductivity in the temperature range of 5--300K. The spin glass behaviour with a spin freezing temperature of 70K has been well confirmed for this compound, which demonstrates the coexistence and competition between ferromagnetic and antiferromagnetic clusters by the introduction of Cr. Colossal magnetoresistance has been observed over the temperature range investigated. The introduction of Cr causes the ``double-bump'' feature in electrical resistivity ρ(T). Anomalies on the susceptibility and the thermal conductivity associated with the double-bumps in ρ(T) are observed simultaneously. The imaginary part of ac susceptibility shows a sharp peak at the temperature of insulating--metallic transition where the first resistivity bump was observed, but it is a deep-set valley near the temperature where the second bump in ρ(T) emerges. The thermal conductivity shows an increase below the temperature of the insulating--metallic transition, but the phonon scattering is enhanced accompanying the appearance of the second peak of double-bumps in ρ(T). We relate those observed in magnetic and transport properties of La_0.7Ca_0.3Mn_0.9Cr_0.1O_x to the spin-dependent scattering. The results reveal that the spin--phonon interaction may be of more significance than the electron (charge)--phonon interaction in the mixed perovskite system.