Light-Tunable Ferromagnetism in Atomically Thin ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ Driven by Femtosecond Laser Pulse

The recent discovery of intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has opened up a new arena for spintronics, raising an opportunity of achieving tunable intrinsic 2D vdW magnetism. Here, we show that the magnetization and the magnetic anisotropy energy (MAE) of few-layered ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT) is strongly modulated by a femtosecond laser pulse. Upon increasing the femtosecond laser excitation intensity, the saturation magnetization increases in an approximately linear way and the coercivity determined by the MAE decreases monotonically, showing unambiguously the effect of the laser pulse on magnetic ordering. This effect observed at room temperature reveals the emergence of light-driven room-temperature (300 K) ferromagnetism in 2D vdW FGT, as its intrinsic Curie temperature $T_C$ is $\sim 200\mathrm{K}$. The light-tunable ferromagnetism is attributed to the changes in the electronic structure due to the optical doping effect. Our findings pave a novel way to optically tune 2D vdW magnetism and enhance the $T_C$ up to room temperature, promoting spintronic applications at or above room temperature.

Figure 1

Crystal structure of FGT and the characteristics of the ferromagnetic transition temperature. (a) Side view (left) and unit cell (right) of FGT monolayer atomic structure. Fe1 and Fe2 represent two inequivalent sites of Fe atoms. (b) Temperature-dependent Hall resistance ($R_{xy}$) of seven-layer-thick vdW FGT as a function of the perpendicular magnetic field. The left panel shows the Hall results from 10 to 215 K, while the right panel shows explicitly the results near $T_C$ for clarity. (c) Diagram of the experimental configuration for static MOKE measurements. Light-induced magnetism is measured via recording rotated polarization angles of the reflected linearly polarized beam of a femtosecond pulsed laser. BS and the WS here represent the beam splitter and Wollaston splitter, respectively. (d) Schematic of the laser-excited DOS in few-layered FGT thin films. The photon energy of 3.1 eV causes electron transitions (vertical blue arrows) from occupied states below the Fermi level $E_F$ to the unoccupied states above $E_F$. The simplified DOS diagram is derived from the calculated DOS of the single-layer FGT in Ref. [34], where the exchange splitting is estimated to be $\sim 1\mathrm{eV}$. See details in the main text.

Figure 2

Excitation fluence-dependent ferromagnetism in atomically thin FGT films induced and detected by the femtosecond pulsed laser in static MOKE measurements. (a)–(c) show, respectively, the static magnetic hysteresis loops of seven-, four-, and two-layer thickness at different excitation intensities of the pulsed laser at room temperature. The photon energy is 3.1 eV, and obvious modifications on both Kerr rotations and coercivities are shown in all the thicknesses.

Figure 3

(a) Extracted values of the saturated Kerr rotations and (b) coercivities of seven-, four-, and two-layer FGT films as a function of the excitation fluence. The dashed lines in (a) are guiding lines, showing the approximately linear response of the maximum Kerr rotations within the measured pump fluence range. The thicker layers of FGT show larger slopes of Kerr rotations but smaller coercivities. The dashed guide lines in (b) exhibit the saturating trend of coercivities above the excitation fluence of $450\mu \mathrm{w}$.