In this paper, we illustrate an approach to discriminate between\ud epitaxial strain and other factors responsible for the decrease of the\ud metal-insulator transition temperature (T-P) in thin La0.7Sr0.3MnO3\ud films grown by pulsed laser deposition. Using this approach, we have\ud estimated the effect of the biaxial strain on T-P. Ultrathin films,\ud independent of epitaxial strain, do not show any metal-insulator\ud transition over the full temperature range. This finding confirms the\ud existence of an interface dead layer. The strain-independent decrease in\ud T-P, relative to its bulk value, observed for a much wider thickness\ud range (up to about 1000 A) can most likely be attributed to oxygen\ud deficiency. (C) 2004 American Institute of Physics

The resistivity of thin $La_{0.7}A_{0.3}MnO_{3}$ films ($A=Ca, Sr$) is investigated in a wide temperature range. The comparison of the resistivities is made among films grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder. In the low-temperature nearly half-metallic ferromagnetic state the prominent contribution to the resistivity scales as $T^{\alpha}$ with $\alpha \simeq 2.5$ for intermediate strengths of disorder supporting the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. For large values of disorder the low-temperature behavior of the resistivity is well described by the law $T^{3}$ characteristic of anomalous single magnon scattering processes, while in the regime of low disorder the $\alpha$ exponent tends to a value near 2. In the high temperature insulating paramagnetic phase the resistivity shows the activated behavior characteristic of polaronic carriers. Finally in the whole range of temperatures the experimental data are found to be consistent with a phase separation scenario also in films doped with strontium ($A=Sr$).Comment: 5 figure

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed\ud laser deposition. X-ray diffraction confirms that the films are only\ud slightly relaxed and are oriented ``square on square{'' relative to the\ud substrate. The measured Raman spectra closely resemble that observed in\ud bulk LaMnO3, which indicates no relevant distortions of the MnO6\ud octahedra induced by the epitaxial strain. Therefore, no detectable\ud changes in the lattice dynamics occurred in our LaMnO3 strained films\ud relative to the bulk case. Mn-55 nuclear magnetic resonance identifies\ud the presence of localized Mn4+ states. Superconducting quantum\ud interference device magnetization measures T-N=131(3) K and a saturation\ud moment mu=1.09 mu(B)/Mn, revealing a small concentration of Mn4+ and\ud placing our films within the antiferromagnetic insulating phase. (c)\ud 2006 American Institute of Physics.

55 Mn nuclear magnetic resonance and magnetization studies of the series of La0.67Sr0.33MnO3 thin films have been performed at low temperature. Two distinct lines were observed, at 322 MHz and 380 MHz, corresponding to two different phases, the former located at the interface, with localized charges, and the latter corresponding to the film bulk, with itinerant carriers (as it was also found in Ca manganite films). The spin-echo amplitude was measured as a function of a dc magnetic field applied either in the film plane or perpendicular to it. The field dependence of both the main NMR signal intensity and frequency shift is quite consistent with that calculated in a simple single domain model. The best fit to the model shows that magnetization rotation processes play a dominant role when the applied field exceeds the effective anisotropy field. Distinctly different magnetic anisotropies are deduced from the interface NMR signal.

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