In the field of magnetic nanomaterials our research activities focus on systems related to potential applications in spintronics and magnetoelectrics. Simultaneously, we work on the development and fundamental aspects of the methods and the instrumentation which we use in the soft x-ray range. These activities mainly involve experiments at beamline UE46-PGM1 at BESSY II.
In the following we present a few examples of research carried out within the last few years.
For a full list of publications of our institute klick here.
Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces
At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from strong Coulomb interactions at and between transition metal and oxygen ions. Such electronic correlations offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metal–oxygen bond.
MN Grisolia, J Varignon, G Sanchez-Santolino, A Arora, S Valencia, M Varela, R Abrudan, E Weschke, E Schierle, JE Rault, J-P Rue, A Barthélémy, J Santamaria, M Bibes, Nature Physics 12, (
See also the associated highlight report
The dipole moment of the spin density as a local indicator for phase transitions
The intra-atomic magnetic dipole moment - frequently called Tz term - plays an important role in the determination of spin magnetic moments by x-ray absorption spectroscopy for systems with non-spherical spin density distributions. In this work, we present the dipole moment as a sensitive monitor to changes in the electronic structure in the vicinity of a phase transiton. In particular, we studied the dipole moment at the Fe2+ and Fe3+ sites of magnetite as an indicator for the Verwey transition by a combination of x-ray magnetic circular dichroism and density functional theory. Our experimental results prove that there exists a local change in the electronic structure at temperatures above the Verwey transition correlated to the known spin reorientation. Furthermore, it is shown that measurement of the dipole moment is a powerful tool to observe this transition in small magnetite nanoparticles for which it is usually screened by blocking effects in classical magnetometry.
D Schmitz, C Schmitz-Antoniak, A Warland, M Darbandi, S Haldar, S Bhandary, O Eriksson, B Sanyal, H Wende, Sci. Rep. 4 (2014)
For magnetite nanoparticles with average diameters of 6 nm (a) and for a 200 nm thick magnetite film on MgO (b), a change of the intra-atomic magnetic dipole moment is observable with X-Ray Magnetic Circular Dichroism (blue circles) but not with Vibrating Sample Magnetometry (gray lines). As verified with density functional theory, it is due to a local change in the electronic structure in the vicinity of the Verwey transition where the crystal structure changes from monoclinic to cubic with increasing temperature. © 2014 Macmillan Publishers Limited.
Orbital Control of Noncollinear Magnetic Order in Nickel Oxide Heterostructures
We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.
A Frano, E Schierle, MW Haverkort, Y Lu, M Wu, S Blanco-Canosa, U Nwankwo, AV Boris, P Wochner, G Cristiani, HU Habermeier, G Logvenov, V Hinkov, E Benckiser, E Weschke, B Keimer, Phys. Rev. Lett. 111, 106804 (2013)
Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields
Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix.
C Schmitz-Antoniak, D Schmitz, P Borisov, FMF de Groot, S Stienen, A Warland, B Krumme, R Feyerherm, E Dudzik, W Kleemann, H Wende, Nature Communications 4, 2051 (2013)
See also the associated highlight report