Magnetically assembled iron oxide nanoparticle coatings and their integration with pseudo-spin-valve thin films

Abstract

The ability to prepare ordered crystalline structures by the assembly of magnetic nanoparticles is of great value for the design and fabrication of nanoparticle-based spintronics devices with novel structures and enhanced performances. In this work, nanoparticle coatings with both structural order and magnetic alignment were assembled onto flat substrates using monodisperse iron oxide nanoparticles by spin coating and heat treatment. The binary solvent mixture used as the carrier solvent for the nanoparticles enables solution-processed spin coating. The out-of-plane magnetic field applied during heat treatment promotes nanoparticle assembly. The magnetically assembled nanoparticle coating exhibits larger saturation magnetization and higher coercivity compared with its randomly aggregated counterpart, due to the easy-axis alignment and close packing of the nanoparticles. By tuning the experimental parameters, nanoparticle coatings with different morphologies were produced, in which locally ordered grains were formed due to anisotropic dipolar interactions. The potential of the nanoparticle coating for spintronic applications is demonstrated by integrating it with pseudo-spin-valve thin films, forming a nanoparticle-coated multilayer thin film structure. This composite system shows a magnetization switching behavior and spin-dependent magnetoresistance (MR) change. By decreasing the distance between the nanoparticle coating and the multilayer thin films, nanoparticle-thin film coupling and interlayer coupling were modulated, resulting in enhanced MR ratios. The presented results and proposed coupling mechanism provide insights into designing nanoparticle-based spintronic devices with enhanced performances and improved properties.