Transition metal oxides with unpaired electrons in their d orbitals display a rich variety of magnetic behaviors. Nanoparticles and nanocrystals (with increased surface-to-volume ratio) have an important percentage of atoms at the surface, with a consequent lower coordination that results in dramatically different surface magnetic properties, emerging from inversion symmetry breaking and hybridization of electronic states. Polyhedron-shaped nanoparticles have the surface atoms located at specific positions due to edges and facets of the crystalline lattice of the material and accordingly their magnetism varies with the particular local structure and electronic perturbations.
Antiferromagnetic materials at the nanoscale
Modern spintronic devices such as magnetic tunnel junctions and high-density memories take advantage of antiferromagnetic materials at the nanoscale; for example, to modify the switching behavior of adjacent ferromagnets via the exchange bias effect, to adjust their coupling for magneto-logic devices, or to tailor their interactions in dense nonvolatile storage of information. Consequently, the understanding and tuning of spin structures at antiferromagnetic surfaces and interfaces is highly desirable not only for these advancing technologies but also for enabling new insights into the fundamental understanding of the related magnetic effects.
Nanoscale heterostructures composed of magnetically active materials show fascinating properties that can be greatly relevant in spintronic devices such as magnetic tunnel junctions and high-density memories. In the case of transition metal oxides that behave differently at the interface established in the heterostructure, they have been demonstrated to offer very interesting coupling effects, for example, to obtain magnetoelectric multiferroic hybrid materials exploiting strain or an exchange bias effect. The exchange bias effect itself has a significant importance in giant magnetoresistance devices where the antiferro-ferromagnetic (or ferrimagnetic) interaction is used to control the reversal of the ferro- or ferrimagnetic part.
CoO@Fe3O4 core-shell octahedron-shaped nanoparticles (Chemistry of Materials (2014), 26, 5566-5575).
Addressing therefore fundamental questions about the nature of the magnetism at surfaces and interfaces of nanoparticulate solids can provide promising new materials according to the wide variety of electrical and magnetic properties they can display.