In-situ picosecond time-resolved probing of magnetization dynamics in polycrystalline ferromagnetic thin films
Date
2009-08-31T20:11:58Z
Authors
Rudge, Jonathan
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Abstract
Magnetization dynamics in polycrystalline Permalloy thin films were studied in-situ using a time-resolved magneto-optic Kerr effect microscope (TR-MOKE). The films, in thicknesses from 9 to 22 nm, were thermally evaporated in a high-vacuum (<10⁻⁸ mbar) environment. Two important dynamic parameters of the magnetization, the precessional frequency and effective damping constant α eff, are obtained from the picosecond time-resolved evolution of the magnetization after a magnetic field pulse excitation. For all film thicknesses investigated, the magnetization carried out precessional motion at a frequency of ~2 GHz. The effective damping constant α eff is extracted from the precessional decay time τ. The decay time is obtained by fitting the experimental time trace of the magnetization to a damped sine function of the form M(t)=Mo e -t/τ sin(ωt-φ), where ω is the angular frequency of the precession mode and φ is the initial phase of the precession. For the thinnest film investigated, α eff reaches the value of 0.32, considerably higher than any previously reported values. The physical origin of the increased magnetic damping is discussed in terms of the surface roughness induced extrinsic damping in magnetic thin films, but the experimentally found thickness-dependence of α eff, however, does not agree with the prediction. The discrepancy is attributed to the percolation of Permalloy islands into connected clusters occurring at the thickness of ~18 nm.
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magnetism, dynamic, time-resolved, Kerr effect