Ultrafast Magnetization Reversal Dynamics on a Micrometer-Scale Thin Film Element Studied by Time Domain Imaging

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Ultrafast Magnetization Reversal Dynamics on A Micrometer-Scale Thin Film Element Studied by Time Domain Imaging

B.C. Choi, G. Ballentine, M. Belov, W.K. Hiebert, and M.R. Freeman Department of Physics, University of Alberta, Edmonton, Alberta T6G 2J1, Canada ABSTRACT Picosecond time scale magnetization reversal dynamics in a 15nm thick Ni80Fe20 microstructure (10µm×2µm) is studied using time-resolved scanning Kerr microscopy. The time domain images reveal a striking change in the magnetization reversal mode, associated with the dramatic reduction in switching time when the magnetization vector is pulsed by a longitudinal switching field while a steady transverse biasing field is applied to the sample. According to the time domain imaging results, the abrupt change of the switching time is due to the change in the magnetization reversal mode; i.e., the nucleation dominant reversal process is replaced by domain wall motion if transverse biasing field is applied. Furthermore, magnetization oscillations subsequent to reversal are observed at two distinct resonance frequencies, which sensitively depend on the biasing field strength. The high frequency resonance at f=2 GHz is caused by damped precession of the magnetization vector, whereas another mode at f≈0.8 GHz is observed to arise from domain wall oscillation.

INTRODUCTION From both fundamental and applications points of view, the switching time required for the magnetization reversal is of utmost significance. While magnetization reversal in micro- and nano-sized magnets has been actively studied recently [1-5], a clear view of the reversal dynamics has not been achieved experimentally. In the case of quasi-static magnetic configurations, magnetic force, Bitter pattern, and other microscopies provide highly detailed results [6,7]. These techniques, however, do not offer enough temporal resolution to observe a real time switching process. For the investigation of reversal dynamics it has been demonstrated that time-resolved scanning Kerr microscopy is a powerful tool [8-12], allowing direct insight into the spatiotemporal evolution of the switching process.

EXPERIMENTAL DETAILS We present ps time scale stroboscopic scanning Kerr microscopy experiments on a 15 nm thick polycrystalline Ni80Fe20 structure (10µm×2µm), patterned by electron beam lithography [13]. During the film growth, an in-plane magnetic field was applied to induce a uniaxial anisotropy. The measured coercivity Hc along the easy axis of the patterned structure is 1.3 kA/m, and the hard axis saturation field of 3.8 kA/m corresponds to the anisotropy field. The Ni80Fe20 structures are made on a 20 µm wide and 300 nm thick gold transmission line that carries a fast current pulse. The current creates an in-plane switching field (Hs) of 24 kA/m along the long axis of the sample with 0.5 ns rise time, 1.5 ns fall time and 10 ns duration [13,14]. The current pulses P4.9.1

are synchronously triggered by a mode-locked Ti-sapphire fs laser (λ=800 nm, 0.8 MHz repetition rate). Magnetization measurements are