title: Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides.
abstract: Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types. Here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust – above room temperature (Curie temperature TC ≫ 300 K) – environmentally stable- ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2-4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co(3+) to Co(2+) at the CoO/Co3O4 interface, with Co(2+) being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets based on oxidation resistant antiferromagnetic transition metal oxides.
authors.affiliation: Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 48047, Duisburg (Germany).
authors.forename: Zi-An
authors.initials: ZA
authors.lastname: Li
authors.affiliation: Departamento de Física Aplicada, Universidade de Vigo 36310, Vigo (Spain).
authors.forename: N
authors.initials: N
authors.lastname: Fontaíña-Troitiño
authors.affiliation: Ernst Ruska-Centre and Peter Grünberg Institute, Research Centre Jülich, 52425 Jülich, (Germany).
authors.forename: A
authors.initials: A
authors.lastname: Kovács
authors.affiliation: 1] Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 48047, Duisburg (Germany) [2] Departamento de Física Aplicada, Universidade de Vigo 36310, Vigo (Spain).
authors.forename: S
authors.initials: S
authors.lastname: Liébana-Viñas
authors.affiliation: Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 48047, Duisburg (Germany).
authors.forename: M
authors.initials: M
authors.lastname: Spasova
authors.affiliation: Ernst Ruska-Centre and Peter Grünberg Institute, Research Centre Jülich, 52425 Jülich, (Germany).
authors.forename: R E
authors.initials: RE
authors.lastname: Dunin-Borkowski
authors.affiliation: Department of Earth and Environmental Sciences, Section Crystallography, LMU Munich, Theresienstr. 41, 80333 Munich (Germany).
authors.forename: M
authors.initials: M
authors.lastname: Müller
authors.affiliation: Department of Earth and Environmental Sciences, Section Crystallography, LMU Munich, Theresienstr. 41, 80333 Munich (Germany).
authors.forename: D
authors.initials: D
authors.lastname: Doennig
authors.affiliation: 1] Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 48047, Duisburg (Germany) [2] Department of Earth and Environmental Sciences, Section Crystallography, LMU Munich, Theresienstr. 41, 80333 Munich (Germany).
authors.forename: R
authors.initials: R
authors.lastname: Pentcheva
authors.affiliation: Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 48047, Duisburg (Germany).
authors.forename: M
authors.initials: M
authors.lastname: Farle
authors.affiliation: Departamento de Física Aplicada, Universidade de Vigo 36310, Vigo (Spain).
authors.forename: V
authors.initials: V
authors.lastname: Salgueiriño