Small-polaron transport inLa0.67Ca0.33MnO3thin films

We present a detailed study of the activated resistivity of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ films up to 600 K under the influence of high magnetic fields. Data in zero field can be explained by small polaron hopping as treated in the Friedman-Holstein theory. Based on the spin orientation of ferromagnetic clusters in a magnetic field, we develop a phenomenological model describing the temperature and field dependence of the resistivity with a minimum of free parameters. We find that the polarons have a magnetic contribution to their activation energy for hopping which depends on the variation of the spin order with increasing temperature and can be modified by applied magnetic fields. 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.