Monodomain magnetic nanoparticles act in many ways like giant spins. They differ from bulk magnets because they can move, and they differ from atomic magnetic moments because they can be imaged and tracked individually. These particles are an excellent model system for investigations of phase transitions on the nanoscale, and in addition they are relevant to magnetic recording media and applications in biomedicine. Following a brief introduction to the physics of magnetic nanoparticles, examples from each of these areas will be described.
For nanoscale magnetic imaging, monodisperse, surfactant-coated nanoparticles are first self-assembled into ordered monolayer arrays using Langmuir layer techniques. While the behavior of non-interacting particles has been understood for many years, the nature of collective behavior in nanoparticle assemblies has been controversial. Here we use Lorentz micrscopy to show that structural disorder disrupts the domains and leads to spin glass-like dynamics. We demonstrate with electron holography the existence of dipolar ferromagnetism, with long-range magnetic order that is stable over time, almost up to the bulk Curie temperature.
Similar nanoparticle arrays are also used as nanomasks for thin films, to make patterned data storage media. Self-assembly can create regular arrays with features smaller than those achievable by lithographic methods. However, the particles in the arrays are not crystallographically oriented, and it is a tremendous challenge to create regular multiphase nanostructures by chemical methods. Here we combine the advantages of thin film growth techniques and self-assembled nanoparticle arrays to prepare and pattern oriented multilayer thin films. High resolution scanning electron microscopy is used to image the samples at different processing stages, and to demonstrate pattern transfer from the arrays into the underlying film.
Magnetic field gradients can guide the motion of single nanoparticles, but imaging them in biocompatible media is a challenge because they are smaller than a wavelength of light. Here we describe how magnetic particles are coated with gold, so that scattering at their plasmon resonance can be used to track their positions using dark field optical microscopy. We demonstrate magnetically-controlled motion within living cells.