The magnetic behavior of a monodomain nanoparticle was first described by Stoner and Wohlfarth nearly sixty years ago, yet this simple system is frequently invoked in discussions of high-density magnetic recording media, magnetic refrigeration materials, and a host of biomagnetic applications. Here we will examine three areas of research involving magnetic nanoparticles.
The ability to make magnetic nanostructures creates a need for new tools that enable us to visualize their magnetization patterns on the nanoscale. Here self-assembled monolayer arrays of Co and Fe3O4 nanoparticles are characterized using magnetic electron microscopy techniques. Electron holography reveals the domain structure and enables quantitative determination of the in-plane magnetic order parameter. Lorentz electron microscopy movies show the slow collective dynamics of these domains as they switch due to thermal fluctuations.
Novel imaging tools are also needed for nanoparticles that are used in biomedicine, and here we consider a possibility of interest for in vitro studies where gold-coated iron oxide nanoparticles can be imaged optically. Because these 25 nm particles are well below the optical diffraction limit, they can be resolved only through their strong surface plasmon scattering. The chemistry of the coating process is described, and dark field optical microscopy is used to show Brownian motion of individual particles in an aqueous dispersion.
Self-assembled nanoparticle arrays have been proposed as data storage media, but this approach has been plagued with problems due to the need for phase transformation and particle sintering. Here we describe an alternative approach that uses the self-assembled array as a nanomask to make patterned media. The results of nanomasking with argon ion milling and with reactive ion etching are presented. High resolution scanning electron microscopy is used to track changes in the sample during the dry etching process. AFM is used to determine the final aspect ratio of the structures, and to demonstrate the ability to address single nanopillar particles. The implications for single particle per bit patterned media are discussed.