Magnetic polarons, clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La$_{1-x}$A$_x$MnO$_3$ (A $\equiv$ Ca, Sr, $x=0.07\div01)$ single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy $\Delta E_\rho$, its temperature coefficient $\gamma$, the thermopower activation energy $\Delta E_S$, the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of $275\pm25\,$K, which is approximately twice as high as their Curie point $T_C$ and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10-12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to $T_C$ of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.