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Controlling the Magnetic State of Nickel Nanocrystal in Granular Multiferroic Composites


Multiferroics are a broad class of materials, which couple multiple ferroic ordering parameters into a single composite—e.g., coupling ferroelectric and ferromagnetic materials. The key feature of multiferroic composites is the ability to control the magnetic (dipole) moments of the composite via an applied electric (magnetic) field. Depending on the coupling mechanisms between the two ferroic materials and the choice of materials, the properties of the multiferroic composite can differ wildly—permitting widespread applications such as field sensors, power convertors, energy storing systems, cooling devices, memory systems, etc. Adding to the plethora of multiferroics, we will be focusing on granular multiferroics (GMF), which substitutes the ferromagnetic material with ferromagnetic nanocrystals, referred to as grains

Although only the size of the ferromagnetic material is being changed, GMF composites present new experiments and insights in controlling magnetism at the nanoscale. We use solution processed methods to synthesize superparamagnetic Ni nanocrystals for all of our GMF composites. We developed fabrication methods for strain-mediated GMF composites by employing the reactive Ni nanocrystal surface to bond to a PMN-PT piezoelectric substrate. The magnetic anisotropy of the strain-mediated GMF composite was shown to be controllable and reversible by applying an electric field induced piezoelectric strain. Because the PMN-PT cut used has a biaxial strain, compressive and tensile, we were able to observe inverse magnetic trends for the blocking temperature of the nanocrystal along the two strain axes. Other fabrication methods we developed, modified the nanocrystal surface with diacid ligands to study magnetic interparticle interactions, known as exchange coupling. Theoretical models of exchange coupled GMF composites predicted a novel magnetoelastic coupling mechanisms that can control the magnetic anisotropy of the ensemble by tuning the dielectric environment of the nanocrystal composite. Experimental work was carried out to fabricate and validate this exchange coupled GMF composite, making it the first demonstration of its kind. This thesis outlines our contributions to the ever-growing field of GMF by establishing methods for fabricating novel granular multiferroic composites and evaluating their unique coupling mechanism and magnetic properties.

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