Strain Relaxation of Self-Nanostructured Solution Derived La 0.7 Sr 0.3 MnO 3 Films
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Strain Relaxation of Self-Nanostructured Solution Derived La0.7Sr0.3MnO3 Films P. Abellan1, F. Sandiumenge1, C. Moreno1, M. J. Casanove2, T. Puig1 and X. Obradors1 1 Institut de Ciència de Materials de Barcelona, CSIC, 08193 Bellaterra, Catalonia, Spain 2 CEMES, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France ABSTRACT The morphological and microstructural evolution associated with an exsolution driven selfnanostructuration process of La0.7Sr0.3MnO3 films, is investigated using scanning force microscopy, reciprocal space mapping and transmission electron microscopy. The focus is placed on the misfit strain relaxation mechanism. Surfaces with atomically flat terraces are already developed after 1hour at 1000 ºC while first fingerprints of phase exsolution do not appear until 9-10 hours. X-ray diffraction reciprocal-space mapping reveals that 24 nm thick films remain strained during the whole microstructural evolution, while 12 hour annealed films undergo almost total plastic relaxation of the misfit strain at a thickness of 60 nm. Overall, these results point to a kinetic limitation of dislocation mechanisms. It is argued that chemical relaxation provides a significant contribution to misfit strain relief. INTRODUCTION Manganese perovskites exhibit a rich electronic and magnetic behavior including colossal magnetoresistance, large spin polarization, electronic phase segregation, orbital ordering and charge ordering [1], which have generated great expectations in diverse technological fields. A marked singularity of such materials is that they exhibit strong correlations between several degrees of freedom (lattice, charge, spin, orbital) that are in turn strongly sensitive to external perturbations, among which misfit strain outstands as the most suffered one. Taking advantage of property-microstructure correlations obviously involves a deep understanding of microstructural mechanisms to relief misfit strains. In this context, a main conclusion can be inferred from studies on manganite epitaxial films [2-5], namely, that these phases remain (apparently) strained up to thicknesses that may involve elastic energies even one order of magnitude larger than those required for the formation of a misfit dislocation array assuming a classical surface nucleation dislocation loop mechanism [6]. This anomalous behavior in fact unveils an extremely complex lattice response to misfit strain, presumably involving a subtle interplay between chemical disorder and coupled orbital-charge-lattice effects [7]. We have recently demonstrated that solution derived La0.7Sr0.3MnO3 (LSMO) films submitted to extended thermal annealing periods, undergo an unprecedented spontaneous selfnanostructuration process [8]. The process is thermodynamically driven by the exsolution of (La,Sr)Ox islands and Sr3Mn2O7 type Ruddlesden-Popper (RP) inclusions, which self assemble within a room temperature ferromagnetic (Tc=360 K , Ms=590 emu/cm3) film of high crystalline perfection [9]. The study of this system is interesting because
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