The major challenges of developing a robust strain coupling between nanocrystals and a strain medium are addressed by employing a simple self-assembly-based adhesion process that relies on the formation of metal-oxide bonds between Ni nanocrystals and various surfaces. X-ray photoelectron spectroscopy indicates that the adhesion mechanism necessitates high oxidation potential surfaces capable of metal-oxide bond formation. The process is demonstrated to be reversible, using a reducing plasma to induce adhesion in previously unadhered NiO/ Ni nanocrystals. Adhesion to the substrate develops interfacial stress in the nanocrystals resulting in a 40 Oe increase in coercivity and a 25 K shift in the blocking temperature.

In this work, we demonstrate an experimental realization of a granular multiferroic composite, where the magnetic state of a nanocrystal array is modified by tuning the interparticle exchange coupling using an applied electric field. Previous theoretical models of a granular multiferroic composite predicted a unique magnetoelectric coupling mechanism, in which the magnetic spins of the ensemble are governed by interparticle exchange. The extent of these exchange interactions can be controlled by varying the local dielectric environment between grains. We specifically utilize the strong dielectric dependence of ferroelectric materials to modify the interparticle coupling of closely spaced magnetic nanoparticles using either a change in temperature or an electric field. This coupling modifies the ensemble magnetic coercivity and thus the superparamagnetic-toferromagnetic phase transition temperature. Through the use of two different ferroelectrics, our results suggest that this magnetoelectric coupling mechanism could be generalized as a new class of multiferroic material, applicable to a broad range of ferroelectric/magnetic nanocrystal composites.