Abstract
Besides epitaxial mismatch that can be accommodated by lattice distortions and/or octahedral rotations, ferroelectric-ferromagnetic interfaces are affected by symmetry mismatch and subsequent magnetic ordering. Here, we have investigated La0.67 Sr0.33 MnO3 (LSMO) samples with varying underlying unit cells (uc) of BaTiO3 (BTO) layer on (001) and (110) oriented substrates in order to elucidate the role of symmetry mismatch. Lattice mismatch for 3 uc of BTO and symmetry mismatch for 10 uc of BTO, both associated with local MnO6 octahedral distortions of the (001) LSMO within the first few uc, are revealed by scanning transmission electron microscopy. Interestingly, we find exchange bias along the in-plane [110]/[100] directions only for the (001) oriented samples. Polarized neutron reflectivity measurements confirm the existence of a layer with zero net moment only within (001) oriented samples. First principle density functional calculations show that even though the bulk ground state of LSMO is ferromagnetic, a large lattice constant together with an excess of La can stabilize an antiferromagnetic LaMnO3-type phase at the interface region and explain the experimentally observed exchange bias. Atomic scale tuning of MnO6 octahedra can thus be made possible via symmetry mismatch at heteroepitaxial interfaces. This aspect can act as a vital parameter for structure-driven control of physical properties.
Highlights
In heterostructural interfaces of ABO3 perovskite-type structures, the misfit stress can be accommodated by strain induced via BO6 deformations through Jahn-Teller (J-T) distortions or cation displacement
In the second batch the samples were composed of a (BTO)n/(LSMO)m bilayer unit, with the BTO thickness systematically varying from 3–250 uc (n = 3, 6, 10, 25, 250), and the La0.67 Sr0.33 MnO3 (LSMO) fixed at 39 uc (m = 39)
Besides epitaxial misfits, which are primarily accommodated by octahedral distortions and/or off-stoichiometry at the LSMO-BTO interface, the layers are often subjected to broken symmetry
Summary
In heterostructural interfaces of ABO3 perovskite-type structures, the misfit stress can be accommodated by strain induced via BO6 deformations through Jahn-Teller (J-T) distortions or cation displacement. Besides epitaxial mismatch, the interface will experience the so-called symmetry mismatch Effect of such mismatch on octahedral distortions is usually observed within a few unit cells (uc) from the interface. As the underlying (001) BTO films get thinner, roughly below 10 uc the unit cell symmetry is lowered This uc decrease may affect the octahedral deformation of the TiO6 octahedra, followed by subsequent distortions or canting of the MnO6 octahedra. Additional octahedral distortions at the interface could originate from dissimilar MO6 rotational patterns or symmetry mismatch between the underlying BTO and the coherently grown LSMO layer due to J-T like distortions. Local lattice distortions of the (001) LSMO uc within the first few unit cells can be seen as an effect of the broken symmetry at the interface rather than lattice mismatch which is relevant for 3 uc of BTO. One can expect tailored functionalities which is not necessarily a consequence of strain engineering but guided by near neighbor exchange and symmetry mismatch
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