Excitation and damping of spin excitations in ferromagnetic thin films

Abstract

We present the results of optical pump-probe measurements made on Ni 81Fe 19 films of thickness d=50, 500 and 5000 Å. We show that the rise time of the pulsed field within the sample may be determined, and that when d is sufficiently small, the value of the Gilbert damping parameter may be obtained from the decay of the magneto-optical signal. The pulsed field was found to rise more slowly when applied perpendicular to the plane of the sample and as the thickness of the sample was increased. By changing the orientation of the pulsed field relative to the static magnetisation we were able to alter the ellipticity of the trajectory of the precessing magnetisation and observe variations in the decay of the precession. We discuss these effects in terms of eddy current shielding of the rising field, eddy current damping of the motion of the magnetisation, and propagation of spin waves from the point at which the sample response is probed. For the thinnest sample studied ( d=50 Å) we found the value of the Gilbert damping parameter to be strongly field dependent, so that, contrary to our expectations, the calculated Ferromagnetic Resonance (FMR) line width increased as the field was reduced. Under certain circumstances a second mode of higher frequency was observed in the thickest film. We believe this to be a magnetostatic surface mode that has not previously been observed by means of the optical pump-probe technique.