Spin-Dependent Transport in Manganite Trilayer Junctions
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INTRODUCTION Doped mnanganite i6 a good model system for the study of magnetotransport in spin-polarized ferromagnetic conductors. The large spin-polarization of conduction carrier[1-4] results in strongly spin-dependent conductivity, giving rise to large magnetoresistance. High quality epitaxial manganite thin films can be grown using a vriety of methods, including laser ablation, sputtering, and more recently, thermal evaporation-based molecular beam epitaxy (MBE). It forms (at least locally) atomically sharp interfaces with perovskite insulators such as SrTiO 3 , enabling the formation of a trilayer structure with current perpendicular (CPP) transport[5]. This allows for quantitative studies of spin-polarized transport across an artificially controlled interface, in the form of a manganite magnetic trilayer junction (IMITJ). Indeed, several research groups around the world have successfully fabricated MIMITJ structures[5-8]. At low"temperatures. M1MITJ exhibits very large magnetoesistance (MIR). To date. a factor of 10 chaige in junction dc resistance has been observed at 14 K and 100 0e8]. •MIMTJ has also demonstrated strong spin-momnentum transfer (SMIT) effect[r912]. It led to the first experimental demonstration of a spin-current driven magnetic switch, using M•IM•ITJ with naturally occurring manganite particulate from laser ablation[12]. Several key issues remain to be resolved before MIMTJ can be considered a technologically viable device. The MIR of M.MIITJ diminishes too rapidly with increasing temperature. Materials imperfections at the interface, such as oxygen or cation stoichiometry variation, or strain variation could result in increased magnetic disorder at the manganite-barrier interface. Fundamental physics limitations, such as an altered interface magnetic state due to a discontinuous boundary for the electronic states, can also suppress the interface magnetic ordering. It is important to understand which is the leading cause of the large suppression of the operating temperature for M.MITJ. Also important is the understanding and improvement of the uniformity of junction resistance and MR. This apparently involves the improvement of the barrier material. But it is also related to the control and engineering of the magnetic state of the junction, because of the large magnetoresistance in these devices. MATERIALS SYNTHESIS AND DEVICE FABRICATION MM.ITJs are made using in-situ grown nianganite-insulator-nmanganite trilayers. Both bottom and top electrode are epitaxial manganires. in most cases (100) oriented Lao.e7Sru.:sa hlOQ• (LS-MIO) filmis. The barrier material i.a uwsually a layer of (100) epitaxial SrTiO:j (STO;. 20 to 50 A thick. Lat.ser ablttion is Conunonly u.sed for filmn depositioi!5. 7;. Recently. reactive 273 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
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SrTiO 3 barrier
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Figure 1: A schematic view of the LSMO-barrier-LSMO trilayer thin film junction structure. Left: top view of the device; right: side-view of the current-p
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