Two types of all-optical magnetization switching mechanisms using femtosecond laser pulses

Using a time-dependent electrical investigation of the all-optical switching in ferrimagnetic and ferromagnetic Hall crosses via the anomalous Hall effect, intriguing insights into the rich physics underlying the all-optical switching are provided. We demonstrate that two different all-optical magnetization switching mechanisms can be distinguished; a “single pulse” switching for ferrimagnetic GdFeCo alloys, and a “two regimes” switching process for both ferrimagnetic TbCo alloys and ferromagnetic Pt/Co multilayers. We show that the latter takes place at two different time scales, and consists of a steplike helicity-independent multiple-domain formation within the first 1 ms followed by a helicity-dependent remagnetization on several tens of milliseconds.

Figure 1

(a) Schematic of the studied 5-$\mu \mathrm{m}\text{−}\mathrm{wide}$ Hall crosses with perpendicular magnetization ($z$ axis); a dc current is injected along the $x$ direction while the anomalous Hall voltage $V_{\text{Hall}}$ is measured along the $y$ direction with a temporal resolution of 1 $\mu \mathrm{s}$. A fs laser beam with a FWHM of 60 $\mu \mathrm{m}$ illuminates the film plane along the $z$ direction, at a fixed position about 40 $\mu \mathrm{m}$ from the center of the Hall cross along $x$ direction. (b) Schematic representation of the different areas of the fs laser beam: “A” where multiple magnetic domains are obtained, “B” where AOS is obtained, and “C” where no change of magnetization is induced.

Figure 2

(a) and (b) Magneto-optical Faraday images of a ${\mathrm{Gd}}_{\text{28}}{\mathrm{Fe}}_{\text{48}}{\mathrm{Co}}_{\text{24}}$ continuous film, with an initial magnetization saturation “up” and “down,” illuminated with two consecutive pulses with three different polarizations: from top to bottom, left circularly polarized pulse ($\sigma -$), right circularly polarized pulse ($\sigma +$), and linearly polarized pulse ($\pi$) with a fluence of 19 ${\mathrm{mJ}/\mathrm{cm}}^2$. Each of the two laser pulses illuminates the same region of the continuous film and induces a total magnetization reversal. The white (dark) contrast in (a) [(b)] corresponds to a reversal to down (to up). (c) Electrical measurement of magnetization reversal of the patterned ${\mathrm{Gd}}_{\text{28}}{\mathrm{Fe}}_{\text{48}}{\mathrm{Co}}_{\text{24}}$ Hall cross under the action of ten consecutive pulses as marked with the blue pulses in the upper row, with initial net magnetization “$M_T$” saturated up with $\sigma -$ polarization and a repetition rate of 5 kHz. The evolution of $V_{\text{Hall}}$ corresponds to the FeCo sublattice magnetization change. The Hall voltage signal is normalized to 1 and $-1$ corresponding to the positive and negative magnetization saturations. Each of the ten laser pulses illuminates the same region of the Hall cross and reverses the magnetization within it. (d) Reversal of the anomalous Hall voltage $V_{\text{Hall}}$ in the patterned ${\mathrm{Gd}}_{\text{28}}{\mathrm{Fe}}_{\text{48}}{\mathrm{Co}}_{\text{24}}$ Hall cross under the action of a single laser pulse with three different polarizations and a fluence of 19 ${\mathrm{mJ}/\mathrm{cm}}^2$. The reversal time of the magnetization is shorter than the temporal resolution of the measurement limited to 1 $\mu \mathrm{s}$. Dashed lines are guides to the eyes.

Figure 3

(a) and (b) Magneto-optical Faraday images of a Pt (4.5 nm)/Co (0.6 nm)/Pt (4.5 nm) continuous film, with initial magnetization saturation up and down, illuminated with two consecutive pulses with three different polarizations and with an energy per pulse of 14 ${\mathrm{mJ}/\mathrm{cm}}^2$. Each of the two laser pulses induces thermal demagnetization of the irradiated area. The dark (white) contrast in (a) [(b)] corresponds to a reversal to down (to up).

Figure 4

(a) and (b) Time evolution of the anomalous Hall voltage of a 5-$\mu \mathrm{m}\text{−}\mathrm{wide}$ Pt (4.5 nm)/Co (0.6 nm)/Pt (4.5 nm) Hall cross initially saturated up, under the action of a 35-fs laser beam with a 5 kHz repetition rate and a fluence of 10 ${\mathrm{mJ}/\mathrm{cm}}^2$. The corresponding number of laser pulses is shown in the upper row. The helicity-dependent switching of the studied Hall cross is governed by a two-step process at two different time scales: (a) helicity-independent demagnetization within the first 1 ms and (b) helicity-dependent reversal measured on a 120 ms time scale. (c) and (d) The same behavior is measured as (a) and (b) with initial saturation of the Hall cross down. The experimental temporal resolution of the experiment is 1 $\mu \mathrm{s}$.

Figure 5

(a) and (b) Time evolution of the anomalous Hall voltage of a 5-$\mu \mathrm{m}\text{−}\mathrm{wide}$ Pt (4.5 nm)/Co (0.6 nm)/Pt (4.5 nm) Hall cross initially saturated up, under the action of a 35-fs laser beam with a 5 kHz repetition rate and a laser fluence of 10 ${\mathrm{mJ}/\mathrm{cm}}^2$, with (blue curve) and without (red curve) adding a small external magnetic field $H_{\text{ex}}$ of 20 mT. The corresponding number of laser pulses is shown in the upper row.

Figure 6

(a) and (b) Magneto-optical Faraday images of a TbCo alloy film, with initial magnetization saturation up and down, illuminated with two consecutive pulses with three different polarizations and with an energy per pulse of 19 ${\mathrm{mJ}/\mathrm{cm}}^2$. Each of the two laser pulses induces thermal demagnetization of the irradiated area. The white (dark) contrast in (a) [(b)] corresponds to a reversal to down (to up).

Figure 7

(a) and (b) Helicity-dependent reversal of the anomalous Hall voltage of a ${\mathrm{Tb}}_{\text{27}}{\mathrm{Co}}_{\text{73}}$ based Hall cross with an initial net magnetization $M_T$ saturated up and down within a 120 ms time scale. The evolution of $V_{\text{Hall}}$ corresponds to the Co sublattice magnetization change. The experiment is performed using a 35-fs laser beam with a 5 kHz repetition rate and a fluence of 19 ${\mathrm{mJ}/\mathrm{cm}}^2$. The corresponding number of laser pulses is shown in the upper row. The inset in (a) [(b)] shows the helicity-independent demagnetization of the Hall cross with an initial saturation magnetization up (down) occurring within the first 1 ms.