Laura Maurel

Polar-Graded Multiferroic SrMnO3 Thin Films

Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. Here we explore the existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO3 thin films, whose polar nature was predicted theoretically and recently demonstrated experimentally. By means of aberration-corrected scanning transmission electron microscopy we map the polar rotation of the ferroelectric polarization with atomic resolution, both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.

Epitaxial stabilization of the perovskite phase in (Sr1-xBax)MnO3 thin films

A novel mechanism of ferroelectricity driven by off-centering magnetic Mn(4+) ions was proposed in (Sr1-xBax)MnO3, in its ideal perovskite phase, which yields enormous expectations in the search for strong magnetoelectric materials. Still, the desired perovskite phase has never been stabilized in thin films due to its extremely metastable character. Here, we report on a thorough study of the perovskite phase stabilization of (Sr1-xBax)MnO3 thin films, 0.2 ≤ x ≤ 0.5, grown by pulsed laser deposition onto (001)-oriented perovskite substrates. X-ray diffraction measurements and scanning transmission electron microscopy reveal that, under appropriate deposition conditions, the perovskite phase is fully stabilized over the nonferroelectric hexagonal phase, despite the latter being increasingly favored on increasing Ba-content. Moreover, we have managed to grow epitaxial coherent cube-on-cube (Sr1-xBax)MnO3 films upon strains ranging from 0% to 4%. Our results become a milestone in further studying perovskite (Sr1-xBax)MnO3 thin films and pave the way for tailoring ferroic and magnetoelectric properties either by strain engineering or Ba-doping.

Probing Strain-Induced Phenomena in Low Dimensionality Multiferroic Oxides

1. Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón (INA), Univ. de Zaragoza, Zaragoza, Spain. 2. Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Barcelona, Spain. 3. Zernike Institute for Advanced Materials, Univ. of Groningen, AG Groningen, Netherlands. 4. Department of Materials Science & Metallurgy, Univ. of Cambridge, UK. 5. Instituto de Nanociencia de Aragón (INA), Univ. de Zaragoza, Zaragoza, Spain. 6. Instituto de Ciencia de Materiales de Aragón, Univ. de Zaragoza-CSIC, Zaragoza, Spain. 7. Luxembourg Institute of Science and Technology (LIST), Esch/Alzette, Luxembourg. 8. Ludwig-Maximilians-Universität München, Munich, Germany. 9. Materials Science and Technology Division, Oak Ridge National Laboratory, Tennessee, US. 10. CEMES CNRS, Toulouse, France.

Magnetic anisotropy of epitaxial Co2Fe-Ge Heusler alloy films on MgO (100) substrates

Films of Co2Fe-Ge Heusler alloy with variable Ge concentration deposited on monocrystalline MgO (100) substrates by magnetron co-sputtering are investigated using microstructural, morphological, magnetometric, and magnetic resonance methods. The films were found to grow epitaxially, with island-like or continuous-layer morphology depending the Ge-content. The ferromagnetic resonance data versus out-of-plane and in-plane angle indicate the presence of easy plane and 4-fold in-plane anisotropy. The magnetometry data indicate additional weak 2-fold in-plane anisotropy and pronounced at low fields rotatable anisotropy. The observed magnetic anisotropy properties discussed in correlation with the microstructure and morphology of the films.