Optically Driven Collective Spin Excitations and Magnetization Dynamics in the Néel-type Skyrmion Host ${\mathrm{GaV}}_4{\mathrm{S}}_8$

${\mathrm{GaV}}_4{\mathrm{S}}_8$ is a multiferroic semiconductor hosting magnetic cycloid (Cyc) and Néel-type skyrmion lattice (SkL) phases with a broad region of thermal and magnetic stability. Here, we use time-resolved magneto-optical Kerr spectroscopy to show the coherent generation of collective spin excitations in the Cyc and SkL phases. Our micromagnetic simulations reveal that these are driven by an optically induced modulation of uniaxial anisotropy. Our results shed light on spin dynamics in anisotropic materials hosting skyrmions and pave a new pathway for the optical manipulation of their magnetic order.

Figure 1

(a) Magnetic phase diagram of (001) ${\mathrm{GaV}}_4{\mathrm{S}}_8$ (reproduced from Ref. [23]), (b) pump-induced change in the normalized Kerr rotation angle of the probe pulse below (blue) and above (red) $T_C$, (c) the absolute value of the maximum demagnetization as a function of temperature and normalized to the value at $T=11.4\mathrm{K}$, and (d) the differential reflectivity trace taken at $T=10\mathrm{K}$.

Figure 2

Time derivative of the TR-MOKE signal at $T=12\mathrm{K}$ at various external magnetic fields. The inset shows the temperature dependence of the magnetization for $H_{\text{ext}}=30\mathrm{mT}$ (purple) and 50 mT (red). The arrow marks the peak at the Cyc and SkL interface.

Figure 3

(a) Time derivative of the TR-MOKE signal at $T=10\mathrm{K}$ for different external magnetic fields spanning the Cyc, SkL, and ferromagnetic phases and (b) the Fourier transforms of the 20 mT (blue) and 50 mT (red) traces in (a). The dashed line in (a) marks the end of the first oscillatory period. The green lines in (a) are calculated using Eqs. (2) and (3) with $\beta =0.02$ and $\gamma =0.135$ ($\beta =0.1$ and $\gamma =0.25$) for the 20 mT (50 mT) curve.

Figure 4

(a) The Fourier transforms of the simulated ${\dot{m}}_z$, ${\dot{m}}_x$, and ${\dot{m}}_{z^{\prime }}$ for various values of $\alpha$ spanned by the external magnetic field and the rhombohedral axis and (b) the simulated $m_z(t)$, $m_x(t)$, and $m_{z^{\prime }}(t)$ for $\alpha =54.7°$.