Magnetooptical Kerr effect measurements of ultrafast spin dynamics in cobalt nanodots

We present our magnetooptical Kerr effect (MOKE) studies on picosecond spin dynamics in closely spaced rectangular Co dots with sizes ranging from 2×6 μm² down to 100 × 300 nm² under in-plane, picosecond, magnetic pulse excitation. A low-temperature-grown GaAs photoconductive switch, excited by femtosecond laser pulses, was used to produce ∼25-ps-long magnetic transients along the surface of the coplanar-waveguide centerline, which contained arrays of patterned Co dots. The resulting spin dynamics in the dots was detected by a time-synchronized train of femtosecond optical probe pulses employing the time-resolved MOKE. Our experimental technique allowed us to measure the initial picosecond dynamics of spins in Co nanodots, followed by damped oscillations. We ascribe the observed results to the small-angle coherent spin precession and show that it depends on the size of magnetic dots.