The Effect of Surface Coating on Magnetic Properties and Local Structure in Coated Cobalt Nanoparticles

Magnetic cobalt nanoparticles have potential applications in magnetic data storage devices. They can be arranged densely enough on a substrate to eclipse current data storage methods. At temperatures below the blocking temperature (Tb), these nanoparticles can retain the direction of their magnetic moment even when an applied magnetic field is removed. The direction of each nanoparticle’s magnetic moment corresponds to one bit of information. One challenge to developing these materials is that the Tb for small cobalt nanoparticles is below 10K. One way to increase the Tb is by increasing the surface anisotropy of the nanoparticle with a surface coating. To study this effect, one sample of cobalt nanoparticles was oxidized, forming a CoO shell while another was coated with silicon dioxide to prevent oxidation and serve as a reference to the oxidized sample. The resulting blocking temperatures and local structures were studied. Transmission electron microscopy confirmed the presence of spherical nanoparticles and was used to determine the size distribution. Synchrotron x-ray pair distribution function (PDF) analysis was used to study the effects of coating on the local structure of nanoparticles. The blocking temperature and oxidation state were determined from MPMS magnetometer data. The PDF of the silica coated nanoparticles shows a hexagonal structure, with different lattice constants than the bulk phase. The PDF of the Co/CoO core shell nanoparticles indicates a distorted cubic structure and a large amount of CoO. The magnetization data reveal a Tb of 5K for the silica coated nanoparticles and no trace of oxidation, in contrast to the CoO nanoparticles, where the Tb was 250K.