David Dunmire, Danilo Romero, and Angela Hightwalker
Optical Technology Division, Physics Laboratory
Presented are the results of our investigation of magnetic cobalt nanoparticles (approximately equal to 10 nm), materials that are of current interest for applications from information storage to drug delivery. Low-frequency Raman spectra of cobalt nanoparticles have been obtained. A triple-grating Dilor XY Raman spectrometer with Ar+ and He-Ne, excitation sources are used in conjunction with liquid nitrogen cooled CCD detection. With extremely high Rayleigh rejection capabilities, the ability to obtain low-frequency vibrational spectra (from 10 cm-1 to 400 cm-1) is demonstrated with reported spectra. Optical characterization is supported by other methods such as TEM, AFM and SQUID magnetometry. A cryo-magnet coupled to a triple grating spectrometer enables the measurements at a range of sample temperatures (4 K to 300 K) and magnetic field strengths (0 T to 8 T). Cobalt nanoparticles have a complicated phase diagram which reveals three unique crystal structures resulting in distinctive and tunable magnetic properties and a self-assembly capability well suited for building SERS-active substrates. Coating of the cobalt particles with a monolayer of gold is also being pursued to further enhance the Raman scattering signal while maintaining the unique magnetic properties of the particles.