Figure: (top-left) Schematic of the charge-transfer phenomena. Mn3+ is present at both interfaces of La2NiMnO6 because of the polar discontinuity with the substrate. The additional electron (in yellow) is captured by LaNiO3 and results in the appearance of Ni2+. (bottom-left) Evolution of the ferromagnetic Curie temperature in interface-engineered (green circles) and bare heterostructures (red squares). (right) X-ray absorption spectroscopy collected at ALBA of selected heterostructures highlights the pivotal role of Ni3+ in displacing the excess electrons from La2NiMnO6 to LaNiO3.
Cerdanyola del Vallès (Spain), 23rd of August 2022. Transition metal oxides and their atomically sharp interfaces are at the forefront of materials science research because they hold the promise to enable the development of a completely new generation of technological devices potentially starting the rise of the oxide electronics era.
In these systems, many fascinating properties are driven by charge-transfer phenomena. These effects involve a redistribution of the electronic charge at the interface between different oxides often resulting in a change of the transition metal valence state. Such a modification allows engineering functionalities absent in the bulk materials (for instance the emergence of a two-dimensional electron liquid, interfacial ferromagnetism and exchange bias) and this is why mastering the deterministic control of the charge transfer would be beneficial.
In a recent publication, scientists from University of Zurich (UZH), University of Bern (UniBe), the Swiss Federal Laboratories for Materials Science and Technology (EMPA), the Swiss Light Source (SLS) and the ALBA Synchrotron, demonstrated that a polar ferromagnetic insulator grown on a non-polar substrate experiences an interfacial charge transfer effect due to a polar discontinuity between the film and the substrate. This electron redistribution is detrimental for the magnetism in films that are 5 unit cells (2 nm) or thinner. Finally, an appropriate top-interface design allows for remotely manipulating the charge configuration of the buried interface and restore the ferromagnetic properties.
La2NiMnO6 as a model system to study charge-transfer phenomena
Double-perovskite insulating ferromagnetic La2NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are an interesting system to investigate because of the scarcity in nature of ferromagnetic insulators. In particular, the researchers found that an oxygen-vacancy-assisted electronic reconstruction takes place when the LNMO films are grown on substrates having different polarity (SrTiO3 and LSAT) than LNMO itself. No interfacial electronic reconstruction is observed when the substrates have the same polarity of LNMO.
The additional electrons present at these electron-doped double perovskite interfaces are mainly incorporated in the Mn states shifting the valence of this element from 4+ to 3+. This new electronic configuration of the Mn is detrimental for Mn and Ni rock salt ordering and, as a result, the magnetic properties of ultrathin LNMO films are affected beyond dimensionality effects.
Top-layer engineering using LaNiO3
To restore the correct electronic configuration of the heterostructure, the researchers decided to introduce a top electron-acceptor layer to redistribute the electron excess directly in a top layer. The choice fell on LaNiO3 (LNO) because it is a paramagnetic material characterized by the presence of a Ni3+ ion. Because of the strong electronegativity difference between Ni3+ and Mn3+, an electronic charge transfer between these two ions (and the two distinct LNMO and LNO layers) can occur, as further verified by density functional theory calculations.
X-ray absorption spectra (XAS) of the LNO/LNMO//STO samples were performed at the BOREAS beamline of the ALBA Synchrotron to verify this hypothesis. XAS was used to characterize, at room temperature, the oxidation state of both Ni and Mn in the integral multilayer. The valence state resulted to be extremely different between the bare LNMO heterostructures and the capped films, with the capping depleting all the Mn3+ in favour of Ni2+.
These findings were crucial to understand the degree of charge-transfer engineering when employing the top electron acceptor layer. Such a top-interface engineering approach to manipulate the electronic state of a buried interface is another step forward for reaching a systematic and exact control of the desired functionalities in complex oxide heterostructures.
With the collaboration of Fundación Española para la Ciencia y la Tecnología. The ALBA Synchrotron is part of the of the Unidades de Cultura Científica y de la Innovación (UCC+i) of the FECYT and has received support through the FCT-21-17088 project.
