Stabilization of surface magnetism by organic-acid adsorption*

Magnetically-disordered layers at the surface of nanoparticles and low dimensionality magnetic oxides significantly reduce the magnetization density with respect to bulk values. However, high crystal quality Fe$_3$O$_4$ nanoparticles capped with non-magnetic organic acid molecules display a surprisingly high magnetization, of unknown origin. Here, we present a real space structural, chemical and magnetic characterization of oleic-acid-caped Fe$_3$O$_4$ nanoparticles with nanometer resolution, demonstrating the presence of a strong magnetic surface layer. In combination with theoretical calculations, we establish the key role of the nanoparticle/organic-acid bond. Magnetization is restored in the surface layer because the bonding with the acid's O atoms partially lifts the surface reconstruction, resulting in surface O-Fe atomic configuration and distances close to the bulk values. Our findings have implications for the optimization of magnetic properties of nanoparticles and thin films.