Phase separation and anisotropy in the electrical properties of weakly doped lanthanum manganites

Variations in the thermopower, electrical resistivity, magnetoresistance, thermal expansion coefficients, and their anisotropy with temperature were detected near room temperature in single crystals of weakly doped lanthanum manganites La(1 - x)A(x)MnO(3) (A = Ca, Sr; x = 0.07-0.125) with orthorhombic structure. The results obtained are discussed in terms of a model of phase separation related to polaron anisotropy. Due to a gain in exchange and elastic energies in the lattice, small-radius magnetic polarons can merge to form polarons of a larger size, which would contain now not one but rather a few electrons (equal in number to the polarons in the cluster). As a result, short-range order in a cluster and phase separation set in at a temperature T-ps approximate to 250-300 K, which is approximately equal to the Curie temperature T-C of conducting manganites with x approximate to 0.2-0.3. (c) 2005 Pleiades Publishing, Inc.