The properties of magnetic nanoparticles tend to be depressed by the unavoidable presence of a magnetically inactive surface layer. However, outstanding magnetic properties with a room-temperature magnetization near the bulk value can be produced by high-temperature synthesis methods involving capping with organic acid. The capping molecules are not magnetic, so the origin of the enhanced magnetization remains elusive. In this work, we present a real-space characterization on the subnanometer scale of the magnetic, chemical, and structural properties of iron-oxide nanoparticles via aberration-corrected scanning transmission electron microscopy. For the first time, electron magnetic chiral dichroism is used to map the magnetization of nanoparticles in real space with subnanometer spatial resolution. We find that the surface of the nanoparticles is magnetically ordered. Combining the results with density functional calculations, we establish how magnetization is restored in the surface layer. The bonding with the acid's O atoms results in O-Fe atomic configuration and distances close to bulk values. We conclude that the nature and number of molecules in the capping layer is an essential ingredient in the fabrication of nanoparticles with optimal magnetic properties.
Electric-field control of magnetism has remained a major challenge which would greatly impact data storage technology. Although progress in this direction has been recently achieved, reversible magnetization switching by an electric field requires the assistance of a bias magnetic field. Here we take advantage of the novel electronic phenomena emerging at interfaces between correlated oxides and demonstrate reversible, voltage-driven magnetization switching without magnetic field. Sandwiching a non-superconducting cuprate between two manganese oxide layers, we find a novel form of magnetoelectric coupling arising from the orbital reconstruction at the interface between interfacial Mn spins and localized states in the CuO 2 planes. This results in a ferromagnetic coupling between the manganite layers that can be controlled by a voltage. Consequently, magnetic tunnel junctions can be electrically toggled between two magnetization states, and the corresponding spin-dependent resistance states, in the absence of a magnetic field.
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