Hall effect measurements on strained and unstrained thin films of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3

We have studied the Hall effect in thin films of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 deposited on SrTiO3 (STO), NdGaO3 (NGO), and LaSrGaO3 (LSGO) substrates in a temperature range from below (10 K) to above (400 K) the metal-insulator transition, in magnetic fields up to 9 T, and for thicknesses between 7 and 75 nm. The charge-carrier density as calculated from the Hall voltage in a single-band picture shows bulklike values for the thick films, but a significant decrease in thin films (below 20 nm), both for strained thin films (on STO) and unstrained thin films on NGO, although less in the case of LSGO. It is well known, however, that a single-band model is not appropriate for the manganates, in which both electron and hole surfaces occur simultaneously. We therefore analyzed the data in a two-band scenario. We still come to the conclusion that the carrier density in the thin films, both strained and unstrained, is lower than in the thicker bulklike films. We discuss this in terms of charge discontinuities at the various interfaces, which appear to play a significant role.