Magnetic and electronic properties of La1-x Srx Co O3 single crystals across the percolation metal-insulator transition

Lightly doped La1-x Srx Co O3 single crystals (i.e., x≤0.15) have been intensively studied primarily due to the occurrence of spin-state transitions and magnetoelectronic phase separation. We report here the electronic transport and magnetic properties of more heavily doped crystals (0.15<x<0.30) with compositions clustered around the percolation metal-insulator transition (MIT). While the magnetic ordering temperature and saturation magnetization increase gradually as x is increased above 0.17, the resistivity and coercivity show drastic decreases in the range 0.17<x<0.20, due to the percolation and coalescence of the metallic ferromagnetic (FM) clusters that form in a semiconducting non-FM matrix at low doping. The doping dependence of the coercivity can be qualitatively understood in terms of the thermal stability of isolated FM clusters and the formation of domains in percolated networks. The magnetoresistance shows a smooth crossover from the intergranular giant magnetoresistance (GMR)-type behavior that is dominant at low x (due to spin-dependent transport between FM clusters) to the conventional negative magnetoresistance occurring in the vicinity of the ordering temperature at high x. These two mechanisms coexist in the range 0.17<x<0.20, providing further evidence of the spatial coexistence of insulating and metallic phases. The data suggest that although magnetoelectronic phase separation certainly occurs, it may be active over a smaller doping range than in polycrystals. At a composition between x=0.20 and 0.30 the signatures of phase coexistence in the magnetotransport disappear. Interestingly, this is coincident with a transition in the high-temperature (above the Curie point) transport from semiconducting-like to metallic-like. The largest "colossal magnetoresistance-type" negative magnetoresistance effects on the metallic side of the MIT are ∼20% in La1-x Srx Co O3, smaller than the equivalent manganites, due to the absence of close competition with a strongly insulating phase. The incompatibility of some of the relevant spin states and crystal symmetries with a static, coherent, Jahn-Teller distortion, and the absence of long-range antiferromagnetic order in the La1-x Srx Co O3 phase diagram are likely key factors in this regard.