Colossal magnetoresistance of layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2(1-z)}$O$_{7}$ and its description by a “spin–polaron” conduction mechanism

The resistance of a La$_{1.2}$Sr$_{1.8}$Mn$_{2(1-z)}$O$_{7}$ single crystal has been studied in magnetic fields from 0 to 90 kOe. The magnetoresistance at temperature $T$ = 75 K, near which a colossal magnetoresistance maximum is observed, has been successfully described in terms of the “spin–polaron” electric conduction mechanism. This value of the colossal magnetoresistance is due to a three-fold increase in the polaron size. The method of separating contributions of various conduction mechanisms to the magnetoresistance developed for materials with activation type of conduction is generalized to compounds in which a metal–insulator transition is observed. It is found that, at a temperature of 75 K, the contribution of the “orientation” mechanism is maximum ($\approx$ 20%) in a magnetic field of 5 kOe and almost disappears in fields higher than 50 kOe.