Atomic-resolution studies of epitaxial strain release mechanisms inLa1.85Sr0.15CuO4

In this paper we present an atomic-resolution electron microscopy study of superlattices (SLs) where the colossal\udmagnetoresistant manganite La_(0.67)Ca_(0.33)MnO_(3) (LCMO) and the high critical temperature superconducting\udcuprate La_(1.85)Sr_(0.15)CuO_(4 (LSCO) are combined. Although good quality epitaxial growth can be achieved, both the choice of substrate and the relatively large lattice mismatch between these materials (around 2%) have a significant impact on the system properties [Phys. C 468, 991 (2008); Nature (London) 394, 453 (1998)]. Our samples, grown by pulsed laser deposition, are epitaxial and exhibit high structural quality. By means of cutting-edge electron microscopy and spectroscopy techniques we still find that the epitaxial strain is accommodated by a combination of defects, such as interface steps and antiphase boundaries in the manganite. These defects result in inhomogeneous strain fields through the samples. Also, some chemical unhomogeneities are detected, up to\udthe point that novel phases nucleate. For example, at the LCMO/LSCO interface the ABO3-type manganite adopts a tetragonal LSCO-like structure forming localized layers that locally resemble the composition of La_(2/3)Ca_(4/3)MnO_(4). Structural distortions are detected in the cuprate as well, which may extend over lateral distances of several unit cells. Finally, we also analyze the influence of the substrate-induced strain by examining superlattices grown on two different substrates: (LaAlO_(3))_(0.3)(Sr_(2)AlTaO_(6))_(0.7) (LSAT) and LaSrAlO_(4) (LSAO). We observe that SLs grown on LSAT, which are nonsuperconducting, present reduced values of the c axis compared to superlattices grown on LSAO (which are fully superconducting). This finding points to the fact that the proper distance between copper planes in LSCO is essential in obtaining superconductivity in cuprates