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 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.

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

“…These observations are in agreement with what has been found also at the interface of Au-Fe 3 O 4 composites [48,49], Co/Co 3 O 4 nanooctahedra [25], and Fe oxidized cubic NP [50], suggesting that lattice mismatch correlates with changes in magnetic anisotropy. There is also evidence that lattice strain induced by acoustic waves applied to some magnetic structures [51,52] can influence the value of the anisotropy constant and its easy axis.…”

“…Remarkably, although the Néel temperature of Co 3 O 4 is below 40 K, the loop shifts persist up to ∼250 K or above, demonstrating the robustness of the exchange coupling between core and shell and the persistence of EB effects up to almost room temperature. Previous reports on the EB effect in systems with coexisting Co/Co 3 O 4 phases indicated that a minor presence of CoO led to the persistence of EB much above its T N [24][25][26][27][28][29]. Analogously, persistence of EB above T N was also observed in Co/FeF 2 [56] and was explained by retention of short range magnetic ordering above T N .…”

“…This process can be controlled under proper synthesis conditions to * oscar@ffn.ub.es; http://www.ffn.ub.es/oscar † sspsg2@iacs.res.in prevent further oxidation, leading to the formation of core/shell structures with the oxide phase (often an AF or ferrimagnetic material [22]) usually formed at the outer part of the structures. In the case of Co NP, most of the published studies [23] report the formation of the CoO phase on the shell, although in some cases the presence of the Co 3 O 4 has also been evidenced by structural [24,25] or magnetic characterization [26], The possibility of observing EB in Co based nanostructures in contact with Co 3 O 4 has been rather less investigated and the published studies focus on layered structures [27][28][29][30][31]. Synthesis of single phase CoO or Co 3 O 4 NP have been achieved by several authors, that have reported AF magnetic behavior with ordering temperatures reduced compared to the bulk values [32][33][34] and remanence values much higher than those for the bulk due to finite-size effects [26,35].…”

Probing core and shell contributions to exchange bias inCo/Co3O4nanoparticles of controlled size

“…Remarkably, although the Néel temperature of Co 3 O 4 is below 40 K, the loop shifts persist up to ∼250 K or above, demonstrating the robustness of the exchange coupling between core and shell and the persistence of EB effects up to almost room temperature. Previous reports on the EB effect in systems with coexisting Co/Co 3 O 4 phases indicated that a minor presence of CoO led to the persistence of EB much above its T N [24][25][26][27][28][29]. Analogously, persistence of EB above T N was also observed in Co/FeF 2 [56] and was explained by retention of short range magnetic ordering above T N .…”

“…This process can be controlled under proper synthesis conditions to * oscar@ffn.ub.es; http://www.ffn.ub.es/oscar † sspsg2@iacs.res.in prevent further oxidation, leading to the formation of core/shell structures with the oxide phase (often an AF or ferrimagnetic material [22]) usually formed at the outer part of the structures. In the case of Co NP, most of the published studies [23] report the formation of the CoO phase on the shell, although in some cases the presence of the Co 3 O 4 has also been evidenced by structural [24,25] or magnetic characterization [26], The possibility of observing EB in Co based nanostructures in contact with Co 3 O 4 has been rather less investigated and the published studies focus on layered structures [27][28][29][30][31]. Synthesis of single phase CoO or Co 3 O 4 NP have been achieved by several authors, that have reported AF magnetic behavior with ordering temperatures reduced compared to the bulk values [32][33][34] and remanence values much higher than those for the bulk due to finite-size effects [26,35].…”