We find that ensembles of iron nanocrystals embedded within a matrix of $\mathrm{Si}{\mathrm{O}}_2$ exhibit both a large Faraday rotation and ultrafast magnetic relaxation. The Verdet constant is $6.3°\pm 0.4°∕\mathrm{cm}∕\mathrm{Oe}$ and a still-unsaturated Faraday rotation of $3.8°\pm 0.2°∕\mu \mathrm{m}$ was observed at a wavelength of $532\mathrm{nm}$ for a specimen containing nanocrystals ranging up to $20\mathrm{nm}$ in diameter. The dynamic response of the nanocrystals to a transient magnetic field produced by a current pulse propagating through a lithographically patterned wire was probed using time-resolved magneto-optical Kerr-effect microscopy. The rise time of the magnetic response to a transient out-of-plane field was observed to be as fast as $26\mathrm{ps}$. Magnetostatic interactions between nanoparticles play a significant role in determining the observed static and dynamic properties of the nanocomposite.

• Received 10 March 2004

DOI:https://doi.org/10.1103/PhysRevB.70.174436