Abstract
We report measurements of the electrical transport properties of films of La 0.7Ca 0.3MnO 3 deposited over a range of thicknesses on lanthanum aluminate and strontium titanate substrates. We find that the dependence of the resistance on temperature and magnetic field is very different in films with a thickness less than 10 nm which show a large mismatch-induced strain. We show that the hysteresis in the resistance of such films on changing the magnetic field is a consequence of logarithmic relaxation of the resistive state. We conclude these results, as well as similar data in the literature can be explained on the basis of a superparamagnetic state induced in ultra-thin films by local structural disorder. Since in the ultra-thin films a very high colossal magnetoresistance (CMR)>1000% is maintained over a temperature range of over 200 K, films showing these effects may be more appropriate for applications than are thicker films in which the CMR effect is confined to a narrow temperature range near the Curie temperature.
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