J. Tornos

Phase separation enhanced magneto-electric coupling in La0.7Ca0.3MnO3/BaTiO3 ultra-thin films.

We study the origin of the magnetoelectric coupling in manganite films on ferroelectric substrates. We find large magnetoelectric coupling in La0.7Ca0.3MnO3/BaTiO3 ultra-thin films in experiments based on the converse magnetoelectric effect. The magnetization changes by around 30–40% upon applying electric fields on the order of 1 kV/cm to the BaTiO3 substrate, corresponding to magnetoelectric coupling constants on the order of α = (2–5)·10−7 s/m. Magnetic anisotropy is also affected by the electric field induced strain, resulting in a considerable reduction of coercive fields. We compare the magnetoelectric effect in pre-poled and unpoled BaTiO3 substrates. Polarized neutron reflectometry reveals a two-layer behavior with a depressed magnetic layer of around 30 Å at the interface. Magnetic force microscopy (MFM) shows a granular magnetic structure of the La0.7Ca0.3MnO3. The magnetic granularity of the La0.7Ca0.3MnO3 film and the robust magnetoelastic coupling at the La0.7Ca0.3MnO3/BaTiO3 interface are at the origin of the large magnetoelectric coupling, which is enhanced by phase separation in the manganite.

Competition between Covalent Bonding and Charge Transfer at Complex-Oxide Interfaces

Here we study the electronic properties of cuprate-manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high Tc superconductor YBa2Cu3O7-δ and the colossal magnetoresistance compound (La,Ca)MnO3. A net transfer of electrons from manganite to cuprate with a peculiar nonmonotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales.

Characterization of surface metallic states in SrTiO3 by means of aberration corrected electron microscopy

An unusual conducting surface state can be produced in SrTiO3 substrates by irradiation with Argon ions from a plasma source, at low energy and high doses. The effects of irradiation are analyzed here by atomic force microscopy (AFM) and aberration corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Depth sensitive studies demonstrate the existence of a heavily damaged surface layer and an oxygen vacancy rich layer immediately underneath, both induced during the irradiation process. We find a clear dependence of the Ti oxidation state with the depth, with a very intense Ti(3+) component near the surface. Oxygen vacancies act as n-type doping by releasing electrons into the lattice and producing an insulator-to-metal transition, which explains the unusual metallic behavior of these samples.

Resonant electron tunnelling assisted by charged domain walls in multiferroic tunnel junctions

The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic La0.7Sr0.3MnO3 electrodes separated by an ultrathin ferroelectric BaTiO3 tunnel barrier, where a head-to-head domain wall is constrained. An electron gas stabilized by oxygen vacancies is confined within the domain wall, displaying discrete quantum-well energy levels. These states assist resonant electron tunnelling processes across the barrier, leading to strong quantum oscillations of the electrical conductance.

Effect of Interface-Induced Exchange Fields on Cuprate-Manganite Spin Switches

We examine the anomalous inverse spin switch behavior in La0.7Ca0.3MnO3(LCMO)/YBa2Cu3O7-δ (YBCO)/LCMO trilayers by combined transport studies and polarized neutron reflectometry. Measuring magnetization profiles and magnetoresistance in an in-plane rotating magnetic field, we prove that, contrary to many accepted theoretical scenarios, the relative orientation between the two LCMOs magnetizations is not sufficient to determine the magnetoresistance. Rather the field dependence of magnetoresistance is explained by the interplay between the applied magnetic field and the (exponential tail of the) induced exchange field in YBCO, the latter originating from the electronic reconstruction at the LCMO/YBCO interfaces.

Induced Ti magnetization at La0.7Sr0.3MnO3 and BaTiO3 interfaces

In artificial multiferroics hybrids consisting of ferromagnetic La0.7Sr0.3MnO3 (LSMO) and ferroelectric BaTiO3 epitaxial layers, net Ti moments are found from polarized resonant soft x-ray reflectivity and absorption. The Ti dichroic reflectivity follows the Mn signal during the magnetization reversal, indicating exchange coupling between the Ti and Mn ions. However, the Ti dichroic reflectivity shows stronger temperature dependence than the Mn dichroic signal. Besides a reduced ferromagnetic exchange coupling in the interfacial LSMO layer, this may also be attributed to a weak Ti-Mn exchange coupling that is insufficient to overcome the thermal energy at elevated temperatures.

Resistive switching in manganite/graphene hybrid planar nanostructures

We report on the fabrication and magnetotransport characterization of hybrid graphene-based nanodevices with epitaxial nanopatterned La0.7Sr0.3MnO3 manganite electrodes grown on SrTiO3 (100). The few-layer graphene was deposited onto the predefined manganite nanowires by using a mechanical transfer technique. These nanodevices exhibit resistive switching and hysteretic transport as measured by current-voltage curves. The resistance can be reversibly switched between high and low states, yielding a consistent non-volatile memory response. The effect is discussed in terms of changes in the concentration of oxygen vacancies at the space charge region of the Schottky barriers building at the contacts.

Modified magnetic anisotropy at LaCoO3/La0.7Sr0.3MnO3 interfaces

Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, with a stronger spin orbit interaction, magnetic anisotropy is typically much stronger. In this paper, we show that interfacing La0.7Sr0.3MnO3 (LSMO) with an ultrathin LaCoO3 (LCO) layer drastically modifies the magnetic anisotropy of the manganite, making it independent of the substrate and closer to the magnetic isotropy characterizing its rhombohedral structure. Ferromagnetic resonance measurements evidence a tendency of manganite magnetic moments to point out-of-plane suggesting non collinear magnetic interactions at the interface. These results may be of interest for the design of oxide interfaces with tailored magnetic structures for new oxide devices.

High Resolution Studies of Oxide Multiferroic Interfaces in the Aberration-Corrected STEM

1. GFMC, Dept. de Fisica de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain. 2. University of Tokyo, Tokyo 113-8656, Japan. 3. Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas. Cantoblanco 280409, Madrid. Spain. 4. Instituto de Magnetismo Aplicado, Universidad Complutense de Madrid, 28040 Madrid, Spain. 5. Department of Materials Science & Engineering, National University of Singapore, Singapore 117575.

Study of oxygen distortions in titanate - Manganite interfaces by aberration corrected STEM-EELS

Transition metal oxides constitute a most interesting family of materials thanks to the interplay between structure, electronic and orbital degrees of freedom, which are related to collective phenomena like magnetism, ferroelectricity, superconductivity, electron transfer, etc. Interfaces between complex oxide materials provide a promising scenario for novel physical phenomena to arise. The crystal and electronic structures of these interfaces determine their physical behaviors [1]. In particular, distortions from the perfect cubic perovskite structure, such as deformations in the BO6 oxygen octahedron around the cations and collective tilts and displacements of the oxygen sub-lattice, play an important role in the electronic properties [2]. In order to harness the macroscopic properties of oxide interfaces, techniques capable of real space simultaneous characterization of these systems with atomic resolution are needed. Aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) is a most powerful tool to study the structure, chemistry and electronic properties of these oxide interfaces down to the atomic scale and in real space. The high spatial resolution achieved allows the acquisition of annular bright field (ABF) images, sensitive to light atoms like oxygen, and atomic resolution spectrum images [3].