Figure 1

Concept of the experiments. A circularly polarized femtosecond laser pulse (red) is used to switch the magnetization of a GdFeCo sample. The switching is monitored by a Faraday imaging setup, delivering images where black and white areas correspond respectively to magnetic domains with opposite orientation (perpendicular to the sample plane). To gain information about the fundamental microscopic mechanisms underlying all-optical switching, we have tailored the represented laser parameters: chirp, pulse duration, wavelength, and bandwidth. The sample system used was Gd${}_{26}$Fe${}_{64.7}$Co${}_{9.3}$ (Gd26), with a compensation point of $T_{\mathrm{comp}}=380$ K. The inset shows the $M\left(T\right)$ curve for a typical ferrimagnetic system, as investigated in this paper.Reuse & Permissions

Figure 2

Dependence of the threshold fluence $F_{\min }$ on the temporal duration of laser pulses with central wavelength of 780 nm. The pulse duration was manipulated by changing the GVD from approximately $-27000$ to $27000$ fs${}^2$. Accordingly, the plot is divided into regions of positive and negative chirp by the vertical dotted line. The horizontal dashed line shows the value of $F_{\min }$ for unchirped laser pulses with temporal width of 13 ps, central wavelength of 532 nm, and repetition rate of 5 kHz. Error bars on the $y$ axis are calculated by error propagation including the error given by the uncertainty of the laser spot size determination (10$\%$ of the determined spot size) and by the laser power fluctuations. Error bars on the $x$ axis are given by the fluctuations of the laser autocorrelation.Reuse & Permissions

Figure 3

Dependence of the threshold fluence $F_{\min }$ on the central wavelength of the exciting laser pulse. Central wavelengths and corresponding pulse widths are given in the inset. The spectra of the pulses can be found in the Supplemental Material.[19] Error bars are calculated as in Fig. 2, but assuming an error of 25$\%$ of the determined spot size to account for the larger inhomogeneities of the spots created by the NOPA. The solid line represents the expected wavelength dependency of $F_{\min }$ according to the theoretical calculations in Ref. 12.Reuse & Permissions

Figure 4

Phase diagrams for all-optical magnetization reversal. (a) Results of the theoretical model of Vahaplar and co-workers, adapted from Ref. 2. The phase diagram shows for which combinations of the parameters $\Delta t_{\mathrm{eff}}$ and $T_{el}^{*}$ all-optical switching can be achieved (lower boundary), assuming an effective magnetic field of $H_{\mathrm{eff}}=20$ T. (b) Phase diagram extracted from our measurements. The laser pulse duration ${\tau }_{\mathrm{pulse}}$ is extracted from the autocorrelation traces, while the peak electron temperature $T_{el}^{*}$ is calculated from two-temperature model simulations assuming 40$\%$ absorption, together with experimental laser fluences and durations.Reuse & Permissions

Figure 5

Calculations of the time evolution of electronic and lattice temperatures after excitation with two laser pulses with different time duration ${\tau }_{\mathrm{pulse}}$. The calculations have been performed with the two-temperature model assuming two pulses with ${\tau }_{\mathrm{pulse}}=100$ fs and ${\tau }_{\mathrm{pulse}}=10$ ps, respectively. For both pulses we assumed the same laser fluence of 6.22 mJ/cm${}^2$ and absorption coefficient of 40$\%$. The Gaussian laser pulses are centered at time zero. Green short dashed lines indicate the electronic temperature, red long dashed lines indicate the lattice temperature.Reuse & Permissions

Figure 6

Conceptual principle of spin-flip stimulated Raman scattering (SF-SRS). The frequency component ${\omega }_1$ drives a transition from the initial state $E_1$ into a virtual state. Via stimulated emission due to the presence of the frequency ${\omega }_2$ in the pulse spectrum a spin-flip transition to the final state $E_2$ happens, and a magnon with frequency ${\Omega }_m$ is created. The maximal energy difference $E_1-E_2$ depends on the spectral width of the laser pulse. For example, a Fourier-transform-limited pulse with 25 fs duration has a bandwidth of 78 meV, while the bandwidth of an 11-ps pulse is only $0.28$ meV.Reuse & Permissions