On the origin of itinerant electron behaviour and long-range ferromagnetic order in La_1-xSr_xCoO₃

Abstract

The itinerant electron ferromagnetism in La_1-xSr_xCoO₃ (0.1<or=x<or=0.5) has been reinvestigated. AC magnetic susceptibility studies on these systems have been carried out for the first time. The nature of the susceptibility indicates the freezing of superparamagnetic clusters at low values of x (<0.3) and long-range ferromagnetic order at higher values of x (>or=0.3). Electrical resistivity studies show that itinerant electron behaviour with a positive temperature coefficient of resistance at low temperatures is obtained for x >or=0.2. Metallization and long-range ferromagnetic order thus seems to be associated with different origins. Chemical systematics obtained from the changes in lattice parameters show that the expansion of LaCoO₃ on the substitution of La by Sr stabilizes the paramagnetic states of Co ions which are in predominantly diamagnetic low-spin state in LaCoO₃. The core S=1 spins of trivalent Co seems to be coupled ferromagnetically by the itinerant electron in a manner similar to the Zener double-exchange mechanism.