Ensembles of iron nanocrystals up to 25 nm in diameter embedded in SiO(2) were found to exhibit an ultrafast magnetic response to a transient out-of-plane magnetic field. The response time varies as a function of in-plane bias magnetic field with the fastest rise times, as short as 26 ps, observed for both zero and high bias fields (140 kA/m). Analytical modeling and micromagnetic simulations confirm that magnetostatic interactions between nanoparticles play an important role in the dynamic response.

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We find that ensembles of iron nanocrystals embedded within a matrix of SiO 2 exhibit both a large Faraday rotation and ultrafast magnetic relaxation. The Verdet constant is 6.3°± 0.4°/ cm/ Oe and a still-unsaturated Faraday rotation of 3.8°± 0.2°/ m was observed at a wavelength of 532 nm for a specimen containing nanocrystals ranging up to 20 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 ps. Magnetostatic interactions between nanoparticles play a significant role in determining the observed static and dynamic properties of the nanocomposite.

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Oriented ferromagnetic Fe-Pt alloy nanoparticles produced in Al 2 O 3 by ion-beam synthesisThe properties of FePt nanoparticles formed by the implantation of FeϩPt into c-axis-oriented Al 2 O 3 single crystals followed by thermal annealing are shown to be strongly dependent on the annealing environment. Annealing in a reducing environment ͑flowing Arϩ4% H 2 , or ultrahigh vacuum͒ gives rise to ferromagnetic FePt nanoparticles with the L1 0 structure and very high magnetic coercivity ͑greater than 20 kOe͒. FePt alloy formation does not occur during annealing in an oxidizing environment. Instead, the implanted Pt precipitates out forming oriented Pt nanoparticles and the implanted Fe redistributes with ϳ40% segregating to the surface where it forms epitaxial ␣-Fe 2 O 3 precipitates at the surface; the remainder of the implanted Fe remains in the bulk, most likely in solid solution in the matrix. Results obtained by sequential annealing of FeϩPt implanted samples in reducing ͑oxidizing͒ environments followed by annealing in an oxidizing ͑reducing͒ environment suggest that equilibrium, rather than kinetic, effects are responsible for the observed microstructures.