Local magnetic order in manganite thin films studied by 1/f noise measurements

The low-frequency 1/f resistance fluctuations of epitaxial La2/3Ca1/3MnO3 t hin films on different substrates have been studied as a function of temper ature and applied magnetic field. A film with a high Curie temperature T-C= 250 K grown on a substrate with perfect lattice match (NdGaO3) shows a low overall noise amplitude, comparable to that of a conventional metal. In con trast to previous work, no increase of the 1/f resistance noise in the ferr omagnetic regime of the La2/3Ca1/3MnO3 film could be observed. The magnitud e of the normalized resistance fluctuations decreases with decreasing tempe rature except for a sharp peak at T-C this peak is completely suppressed by an applied magnetic held of about 2 T. In contrast, the noise level in a f ilm with low Curie temperature (T-C = 110 K) grown on a substrate (SrTiO3) with large lattice mismatch (1.5%) is several orders of magnitude larger an d shows a qualitatively different temperature dependence, with a huge noise increase below T-C. These results are discussed in terms of a strong coupl ing of electric resistance and magnetic ordering, as well as within a perco lative picture of the ferromagnetic transition.