Ultrafast control of coherent spin precession in ferromagnetic thin films via thermal spin excitation processes induced by two-pulse laser excitation

We investigate the laser-induced spin precession dynamics in ferromagnetic $\mathrm{N}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}$ thin films by using a time-resolved magneto-optical Kerr effect (TR-MOKE) measurement with two pump pulses. By changing the time interval between the pump pulses, the amplitude of the TR-MOKE signal is strongly modulated according to the phase of the TR-MOKE signal at the time when the second pulse arrives. The change in the oscillation amplitude can be explained with the temporal evolution of the spin precession induced by laser heating, as shown with a numerical simulation. We conclude that the ultrafast control of coherent spin precession dynamics is possible by multiple pump pulses via thermal excitation.

Figure 1

A schematic view of the TR-MOKE measurement system. The components are as follows: a polarization beam splitter (PBS), a barium borate crystal (BBO), a Glan-Thompson prism (GT), an objective lens (OL), a beam splitter (BS), a dichroic mirror (DM), a quarter-wave plate (QWP), a Wollaston prism (WP), a charge-coupled-device (CCD), and two delay stages (DS1 and DS2). Inset: Timing chart of the probe and the two pump pulses.

Figure 2

(a) Delay time dependence of the TR-MOKE signal in the Py thin films. Black solid curves represent the fitting results. Data curves are offset for clarity. (b) Amplitude of the sinusoidal TR-MOKE signal after arrival of the second pump pulse, plotted as a function of the double-pump delay time $\mathrm{\Delta }t$ (cyan circles, left axis). In addition, the probe time dependence of the TR-MOKE signal for single-pulse excitation is plotted for reference (red curve, right axis).

Figure 3

Simple model for the thermally driven spin precession induced by the two-pulse laser excitation.

Figure 4

(a) Time dependence of $M_{\mathrm{z}}$ for single- (red data points) and double- pump excitation (orange to purple) calculated by the LLG equation. (b) Amplitude of the oscillation signal after the second pump pulse, plotted as a function of the double-pump delay $\mathrm{\Delta }t$ (cyan circles, left axis). The temporal intensity profile for the single-pulse excitation is plotted as a function of the probe delay $t$ for reference (red curve, right axis).