Small-polaron transport in La0.67Ca0.33MnO3 thin films

We present a detailed study of the activated resistivity of La0.6Ca0.33MnO3 films up to 600 K under the influence of high magnetic fields. Data in zer o field can be explained by small polaron hopping as treated in the Friedma n-Holstein theory. Based on the spin orientation of ferromagnetic clusters in a magnetic field, we develop a phenomenological model describing the tem perature and field dependence of the resistivity with a minimum of free par ameters. We find that the polarons have a magnetic contribution to their ac tivation energy for hopping which depends on the variation of the spin orde r with increasing temperature and can be modified by applied magnetic field s. The average magnetic clusters contain 4-6 ions. Hall measurements show a temperature dependent electronlike linear low field slope predicted by the theory of the Hall mobility of small polarons. [S0163-1829(98)04646-3].