Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides
by Zi-An Li, N Fontaíña-Troitiño, A Kovács, S Liébana-Viñas, M Spasova, R E Dunin-Borkowski, M Müller, D Doennig, R Pentcheva, M Farle and V Salgueiriño
Polar oxide interfaces are an important focus of solid state 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 therefore important to extend the understanding of electronic reconstruction phenomena to a broader class of materials and structure types.
In the present study, researchers from the University of Duisburg-Essen, the Universidade de Vigo, the LMU Munich and the Ernst Ruska-Centre report on a robust above room temperature (Curie temperature Tc ≫ 300 K) environmentally stable-ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 24 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals based on high-resolution transmission electron microscopy and quantitative magnetometry measurements. Density functional theory calculations with an on-site Coulomb repulsion parameter have been used to identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ 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.
Zi-An Li, N Fontaíña-Troitiño, A Kovács, S Liébana-Viñas, M Spasova, R E Dunin-Borkowski, M Müller, D Doennig, R Pentcheva, M Farle and V Salgueiriño: Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides, Nature Scientifc Reports 5 (2014) 7997.