Strain Accommodation and Relaxation Mechanisms in Epitaxially Grown Manganites
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Strain Accommodation and Relaxation Mechanisms in Epitaxially Grown Manganites 1
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Marie-José Casanove, Pierre Baulès, Christian Roucau, David Magnoux , Jean-François Bobo , 2 2 2 Manuel Bibes , Lluis Balcells , Josep Fontcuberta Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS, B.P. 4347, F-31055 Toulouse cedex 4, France 1 Laboratoire de Physique de la Matière Condensée, INSA, F-31077 Toulouse cedex 4, France 2 Institut de Ciència de Materials de Barcelona, Campus UAB. 08193, Bellaterra, Catalunya, Spain
ABSTRACT Epitaxial growth of La1-x(Sr or Ca)xMnO3 manganites (LSMO or LCMO) has been successfully achieved on SrTiO3 (001) and MgO(001) substrates. We report the influence of the deposition conditions, in particular growth rate, annealing and nature of the substrate, on the state of strain in the manganite layers. Fully strained layers are reported at very slow growth rate on SrTiO3 substrates while misfit dislocations are clearly observed in layers grown on MgO and also in layers grown on SrTiO3, after annealing at high temperature. Besides, evidence is given for a rhombohedral (respectively orthorhombic) distortion relaxation in LSMO (respectively LCMO) layers. The microstructure of the manganite layers is analyzed taking into account their non-isomorphic growth on the cubic substrates. INTRODUCTION The discovery of a colossal magnetoresistance (CMR) in perovskite manganites with general formula A1-xBxMnO3 , where A is a trivalent lanthanide (La, Nd ..) and B a divalent alkaline earth (Sr, Ba, Ca) cations [1-2] recently opened the way to a new generation of tunnel magnetic junctions based on manganite heterostructures. The properties of these devices are strongly dependent on the microstructure of the different layers and on the quality of the different interfaces. Moreover, the values of the Mn-O-Mn bond length and angle in the manganite perovskite layers, likely to be adjusted by epitaxial growth induced strain, largely influence the double exchange interaction. In order to control the different parameters involved, we have grown epitaxial La0.66(Sr or Ca)0.33MnO3 , hereafter referred as LSMO or LCMO layers, on different substrates and with various deposition conditions. The state of strain and the microstructure of these layers have been analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) on plan-view and cross-sectional specimens.
STRUCTURAL AND EXPERIMENTAL DETAILS If we consider the structure of bulk La0.66Sr 0.33MnO3 and La0.66Ca 0.33MnO3, none of them is purely cubic while the unit cells of both the substrates are: LSMO and LCMO unit cells are respectively rhombohedral and orthorhombic. The epitaxial growth on SrTiO3 or MgO is therefore not an isomorphic growth. However, LSMO and LCMO unit cells are usually N9.8.1
described as pseudo-cubic with parameter a = 0.387 nm and angle α = 89.74° for LSMO and with a mean parameter 0.386 nm in LCMO where all the angles are equal to 90°. The SrTiO3 substrate, which has also the perovskite structure, is a real cubic wi
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