Complex oxides are strongly correlated electron systems showing a large variety of phases and functionalities: magnetic order, ferroelectricity, and superconductivity. The different order parameters can be coupled leading, for instance, to magnetoelectric responses or phase coexistence. There is now a great deal of research activity on multilayered heterostructures where different oxides are combined. At the interface between two oxides, charge redistribution and broken symmetries give rise to the appearance of electronic states different from those of the participant oxides in bulk. At interfaces coupling between different orders can be controlled. One of the most celebrated examples of novel behaviour at interfaces is the two-dimensional electron gas arising at the interface between two band-gap insulators: LaAlO3/SrTiO3. With the variety of oxides at hand, and with many of them being strongly correlated, the possibilities for electronic reconstruction at interfaces are endless.
The conduction and magnetic electrons in these oxides come from d-orbitals. The d-orbitals, degenerate in isolated atoms, are split in a solid. Degeneracies are also broken when the translational symmetry is interrupted at a surface or interface. As d-orbitals are anisotropic (with a larger probability density in a particular direction/plane), an orbital selection can have consequences when the interaction between all the degrees of freedom (spin, lattice, orbital) is considered. For instance, the carriers can move more freely in one plane than in the perpendicular direction. These considerations are very relevant for designing oxide-based devices.
Among the many complex oxides, manganites are very much studied due to the strong coupling between the magnetic and transport properties (leading to colossal magnetoresistance and tunneling magnetoresistance) and the variety of magnetic orders that they display. We use model Hamiltonians to study the magnetic and transport properties of manganite multilayers. These multilayers exhibit modifications on their magnetic or transport properties leading to new functionalities.
Recent selected publications
“Signatures of a two-dimensional ferromagnetic electron gas at the La0.7Sr0.3MnO3/SrTiO3 interface arising from orbital reconstruction”. Norbert Marcel Nemes, María José Calderón, Juan Ignacio Beltrán, Flavio Yair Bruno, Javier García-Barriocanal, Zouhair Sefrioui, Carlos León, Mar García-Hernández, Carmen Muñoz, Luis Brey, and Jacobo Santamaría. Advanced Materials. Published online Oct 18th, 2014.
“Magnetoelectric coupling at the interface of BiFeO3/La0.7Sr0.3MnO3 multilayers”. M.J. Calderón, S. Liang, R. Yu, J. Salafranca, S. Dong, S. Yunoki, L. Brey, A. Moreo, and E. Dagotto. Physical Review B 84, 024422 (2011). arXiv:1012.1448. On the shift of the exchange bias produced by the application of an electric field (magnetoelectric coupling) in multilayers involving a ferromagnetic metallic manganite and an antiferromagnetic ferroelectric oxide.
“Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator.” A. Baena, L. Brey, and M.J. Calderón. Physical Review B 83, 064424 (2011). arXiv:1009.4548.
“All-Manganite Tunnel Junctions with Interface-Induced Barrier Magnetism”. Z. Sefrioui, C. Visani, M.J. Calderón, K. March, C. Carrétéro, M. Walls, A. Rivera-Calzada, C. León, R. López Antón, T. R. Charlton, D. Imhoff, L. Brey, M. Bibes, J. Santamaría and A. Barthélémy. Advanced Materials 22, 5029 (2010). A thin antiferromagnetic insulating manganite becomes ferromagnetic (while still insulating) in proximity to a ferromagnetic metallic manganite. This magnetic reconstruction can be used as a barrier with spin filtering capabilities as shown experimentally.