Ultrafast Magnetization Dynamics in Metallic Amorphous Ribbons with a Giant Magnetoimpedance Response

We report on the dynamics of laser-induced magnetization in magnetic ribbons using the time-resolved magneto-optical Kerr effect. Damped oscillations representing precessional motion of the magnetization vector are observed in a $\mathrm{Co}$-rich ribbon. The frequency and amplitude of these oscillations depend on the fluence of the pump laser pulse and the external magnetic field. With an anisotropy factor defined by the effective field of the magnetic anisotropy, we demonstrate that the frequency of the ferromagnetic mode activated by the laser pulses is close to the frequency of the giant magnetoimpedance response. These findings provide an opportunity to investigate the magnetoimpedance effect in the gigahertz frequency range using ultrashort laser pulses and are particularly interesting for the development of broadband sensor devices. Furthermore, our results link laser-induced magnetization dynamics with magnetoresistive sensing techniques, thus opening up the supergigahertz domain to magnetoimpedance studies.

Figure 1

Schematic picture of the experimental TR-MOKE geometry. The laser pump beam is focused on the ribbon surface to excite magnetization dynamics. The Kerr rotation θ_K of the reflected probe beam monitors the transient out-of-plane magnetization component with a magnetic field along the z axis. The arrows indicate the direction of the axes and the magnetization M on the air side of the ribbon.

Figure 2

(a) Time-resolved Kerr rotation as a function of the delay time Δt for several fluences of the pump beam (0.6, 1.2, and 2.1 mJ/cm²) with H_z = 3 kOe. The red solid lines are fitted using Eq. (1). The curves are offset vertically. The inset shows the time-resolved Kerr rotation for a pump fluence of 2.1 mJ/cm² on a short timescale below 1 ps, with a decay time τ^* ∼ 0.3 ps. The dashed lines are visual guides. (b) Amplitude B of the nonmagnetic background and (c) frequency f of magnetization precession, obtained at different pump fluences.

Figure 3

(a) Time-resolved Kerr rotation as a function of the delay time Δt for different amplitudes of the applied magnetic field (H_z = 0.95, 1.23, 1.62, 2.15, 2.88, 3.82 kOe) at a pump fluence of 1 mJ/cm². The red solid lines are fitted using Eq. (1). The curves are shifted vertically without rescaling of the plots. (b),(c) Dependence of (b) the amplitude A and (c) the frequency of the FMR mode on the amplitude of the magnetic field H_z. The dashed lines are visual guides.

Figure 4

Frequency of the giant magnetoimpedance signal as a function of resonance field H_M. The inset shows the relative impedance variations ΔZ/Z as a function of the magnetic field H_x in the presence of the ac electric current at a frequency of 1 GHz, as obtained using a vector network analyzer.

Figure 5

Static normalized MOKE magnetization reversal in (a) polar and (b) longitudinal geometries. The arrows indicate the values of the effective anisotropy field. (c) Dependence on effective field of the frequencies obtained in the FMR and GMI experiments. The error on the FMR and GMI data are obtained from a fitting procedure (see Fig. 3) and from determining the resonant fields shown in the inset of Fig. 4, respectively. The red solid line is fitted using the Kittel formula [39]. The background shaded in light red schematically shows the approximately 9% variation in the frequency [see Fig. 2] obtained in our experiments in the range of pump fluences from 0.6 to 2.1 mJ/cm². The frequencies obtained in the TR-MOKE and GMI experiments are in good agreement with the fitted curve.