Anomalous change in the size of a spin polaron in the paramagnetic temperature range in La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

In this work, we continued the study of the magnetic and electrical properties of the double perovskite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$, which has a colossal magnetoresistance in excess of 1200 near the Curie temperature. It is shown that the colossal magnetoresistance observed in La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ is well described on the basis of the “orientational” and “spin-polaron” conduction mechanisms. It was found in the work that in the absence of a magnetic field, the linear size of the spin polaron decreases with increasing temperature in the ferromagnetic region, and upon the transition of manganite to the paramagnetic state, the linear size begins to increase, reaching a maximum at 180 K. At temperatures exceeding 180 K, an anomalous temperature change the size of the spin polaron disappears. In the absence of a magnetic field, the detected peak on the temperature curve of the change in the size of the spin polaron is maximal; with the inclusion of the magnetic field, the peak height decreases. Mechanisms are proposed to explain this anomalous temperature behavior of the spin polaron size.