Nanoscale analysis of a Co-SrTiO 3 interface in a Magnetic tunnel junction
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Nanoscale analysis of a Co-SrTiO3 interface in a Magnetic tunnel junction J.-L. Maurice1, F. Pailloux1*, D. Imhoff2, J.-P. Contour1, A. Barthélémy1, M. Bowen1, C. Colliex2 and A. Fert1. 1
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Unité Mixte de Physique CNRS/Thales, 91404 Orsay, France Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France
ABSTRACT We use High Resolution Electron Microscopy together with Electron Energy Loss Spectroscopy to analyze the crystallography and the chemical configuration of a Co/SrTiO3 interface in a Co/SrTiO3/La2/3Sr1/3MnO3 magnetic tunnel junction. PACS: 75.47.-m, 75.70.Cn, 68.37.Lp, 79.20.Uv keywords: tunnel magnetoresistance, HRTEM, EELS INTRODUCTION Magnetic tunnel barriers present a conduction that depends on the respective spin polarizations of the two electrodes. Because it is a half metal, the La2/3Sr1/3MnO3 (LSMO) compound is very interesting as a probing electrode for spin-dependent spectroscopy: voltagedependent measurements at low bias should in principle reflect the spin-dependent density of states of the other electrode. Several systems have thus been studied by varying the material of the second electrode (Co, FeNi) and also the nature of the insulator [SrTiO3 (STO) amorphous Al2O3, Ce1-xLaxO(2-x)/2], as the insulator also plays a role as a spin filter [1], [2]. These studies outline the key role played by the interfaces in such devices. Our analytical work on the STO/LSMO interface is published elsewhere [3], [4], [5]. Here, we present the analysis of the Co/STO interface in a Co/STO/LSMO junction. This kind of junction still gives rise to TMR effects at 330K, which is quite close the Tc of the LSMO thin films used as first electrode. The understanding of the behavior of these heterostructures requires the use of characterization methods that can probe one monolayer at the interface. Transmission electron microscopy (TEM) in cross section is particularly suitable in such a case. In this paper, we present an atomic scale characterization by high resolution TEM (HRTEM) and electron energy loss spectroscopy (EELS) in scanning TEM (STEM). More precisely in a Co/STO/LSMO tunnel junction, the atomic configuration at the Co/STO interface determines the polarization of the tunneling electrons that are emitted or accepted by the cobalt electrode. De Teresa et al. [1] have observed inverse magnetoresistance in Co/STO/LSMO tunnel junctions, suggesting that the spin of conduction electrons at the Co/STO interface is antiparallel to the magnetization. They explained that tunneling electrons through an STO barrier are d-band electrons and that the d-band in cobalt or at a cobalt surface [6] happens *Now at Laboratoire de Métallurgie Physique, Université de Poitiers, 86962, Futurscope, France
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to be larger at the Fermi level for minority spins. Recently, Oleinik et al. have performed electronic structure calculations of the fcc Co(100)/STO interface, considering various atomic configurations [7]. These authors concluded that the most stable interface configuration between fcc Co (10
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