A Surface Insulator-to-Conductor Phase Transition in Colossal Magnetoresistive Manganese Perovskites Thin Films
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A Surface Insulator-to-Conductor Phase Transition in Colossal Magnetoresistive Manganese Perovskites Thin Films C.N. Borca, Bo Xu, Takashi Komesu, Hae-Kyung Jeong, S.-H. Liou, P.A. Dowben Department of Physics and Astronomy and the Center for Material Research and Analysis, University of Nebraska, Lincoln, NE 68588-0111, U.S.A. ABSTRACT We have observed a distinct surface phase transition for an important class of colossal magnetoresistive materials, La0.65D0.35MnO3 (with D = Sr, Pb) occurring in a surface layer compositionally different from the bulk. The surface phase transition occurs around 240 K compared to 350 K for the bulk and is fundamentally different. In the bulk, a ferromagnetic metal to paramagnetic ‘bad metal’ occurs, while the lower-temperature surface transition is from an n-type (in case of La0.65Sr0.35MnO3) or a p-type (in case of La0.65Pb0.35MnO3) semiconductor to a semimetal with increasing temperature.
INTRODUCTION Surface phase transitions and surface electronic structure can have a great impact on the electronic transport properties through the surface region. For materials (like manganese perovskites) considered as candidates for spin valves and spin tunnel junctions, the effects may be profound. Phase transitions, largely restricted to the surface, have been observed in ferroelectric [1] and ferromagnetic [2] materials, and across a nonmetal to metal transition [3]. Surface transitions for complex oxides must be also considered. We report on a novel surface phase transition for the manganese perovskite La0.65D0.35MnO3 (with D = Sr, Pb) that occurs at a different temperature than the bulk ferromagnetic phase transition. Tailoring surfaces of complex CMR compounds may lead to improvements in device performances. The current results shed considerable light on the device performance of the CMR perovskites in tunnel magneto-resistive junctions and spin valves in the thin film limit.
EXPERIMENTAL DETAILS The La0.65Sr0.35MnO3 (LSMO) and La0.65Pb0.35MnO3 (LPMO) thin films (1000 Å in thickness) were grown on (100) LaAlO3 substrates by rf sputtering [4]. The films are polycrystalline, highly oriented along the substrate normal and single phase. Detailed studies of the surface composition and electronic structure were performed into an ultra high vacuum (UHV) system, with a base pressure of 2x10-10 torr. To insure surface cleanness, we annealed the samples up to 520ºC for 2 hours before each experiment. A clear four-fold symmetric low energy electron diffraction (LEED) pattern, together with a negligible amount of C (less than 3% of a monolayer) monitored using X-ray photoemission (XPS), were taken as good indicators of a clean surface with a high degree of crystallinity across the entire area of the sample. It has been shown that this thermal treatment completely changes the surface and selvedge structure in both O9.8.1
La0.65Sr0.35MnO3 [5] and La0.65Pb0.35MnO3 [6] thin films. Compare to the well-known bulk structure [7], these annealed surfaces go through a restructuring transition characterized
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