Controlling laser-induced magnetization reversal dynamics in a rare-earth iron garnet across the magnetization compensation point

In this work we explore the ultrafast magnetization dynamics induced by femtosecond laser pulses in a doped film of gadolinium iron garnet over a broad temperature range including the magnetization compensation point $T_M$. By exciting the phonon-assisted ${}^{6}S\to {}^{4}G$ and ${}^{6}S\to {}^{4}P$ electronic $d-d$ transitions simultaneously by one- and two-photon absorption processes, we find out that the transfer of heat energy from the lattice to the spin has, at a temperature slightly below $T_M$, a large influence on the magnetization dynamics. In particular, we show that the speed and the amplitude of the magnetization dynamics can be strongly increased when increasing either the external magnetic field or the laser energy density. The obtained results are explained by a magnetization reversal process across $T_M$. Furthermore, we find that the dynamics has unusual characteristics which can be understood by considering the weak spin-phonon coupling in magnetic garnets. These results open new perspectives for controlling the magnetic state of magnetic dielectrics using an ultrashort optically induced heat pulse.

Figure 1

Static magneto-optical properties of the garnet film as a function of temperature. (a, b) Normalized polar Kerr hysteresis loops measured over a broad temperature range above (a) and below (b) the compensation temperature $T_M$. (c, d) Temperature dependence of the coercive field $H_C$ and the polar Kerr rotation ${\mathrm{\Theta }}_K$ measured at the photon energy 3.1 eV.

Figure 2

Laser-induced magnetization dynamics in the garnet film. (a, b) $\mathrm{\Delta }{\mathrm{\Theta }}_K$ and $\mathrm{\Delta }R/R$ induced by an absorbed energy density of $0.89\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$ for $H_{\mathrm{ext}}=\pm 0.75\mathrm{T}$ and at a temperature of 240 K. (c) The optical absorption spectra of the sample at 300 K, including the laser pulse spectrum and the $d-d$ transition associated to the peak at 1.38 eV. Insets: (a) Sketch of the TR-MOKE configuration and (b) $\mathrm{\Delta }R/R$ measured at two different time delays of 0 and 20 ps as a function of the pump energy density. The solid lines are the fits to the data using $\mathrm{\Delta }R/R=b{\left(E_{\mathrm{pump}}\right)}^2$.

Figure 3

Temperature dependence of laser-induced magnetization dynamics. The absolute value of the amplitude $\mathrm{\Delta }{\mathrm{\Theta }}_K$ ($t=1.6\mathrm{ns}$) as a function of the temperature. The inset shows $\mathrm{\Delta }{\mathrm{\Theta }}_K$ measured at selected temperatures below $T_M$. All measurements are obtained for a pump energy density $E_{\mathrm{pump}}=1.34\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$ and $H_{\mathrm{ext}}=0.5\mathrm{T}$.

Figure 4

Pump energy density dependence of the magnetization dynamics. (a) $\mathrm{\Delta }{\mathrm{\Theta }}_K$ measured at different pump energy densities. The solid lines are the fits using Eq. (1). (b–d) Pump energy density dependence of (b) the amplitude $\mathrm{\Delta }{\mathrm{\Theta }}_K$ ($t=1.6\mathrm{ns}$) and (c, d) the elapses time $t_s$ and characteristic time $\tau$ of the magnetization reversal dynamics. The inset shows $\mathrm{\Delta }{\mathrm{\Theta }}_K$ in logarithmic scale. The solid lines in the inset and (b–d) are guides to the eyes. All measurements are obtained at the temperature 240 K and for $H_{\mathrm{ext}}=1\mathrm{T}$.

Figure 5

Magnetic field dependence of laser-induced magnetization dynamics. (a, b) Variation of the elapsed time $t_s$ and the characteristic time $\tau$ of the magnetization reversal dynamics as a function of $H_{\mathrm{ext}}$. (c) Field dependence of the amplitude $\mathrm{\Delta }{\mathrm{\Theta }}_K$ ($t=1.6\mathrm{ns}$). All measurements are obtained at the temperature 240 K and for $E_{\mathrm{pump}}=1.07\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$.