  Current-induced control of magnetization in ferromagnets using spin-orbit
torque (SOT) has drawn attention as a new mechanism for fast and energy
efficient magnetic memory devices. Energy-efficient spintronic devices require
a spin-current source with a large SOT efficiency (${\xi}$) and electrical
conductivity (${\sigma}$), and an efficient spin injection across a transparent
interface. Herein, we use single crystals of the van der Waals (vdW)
topological semimetal WTe$_2$ and vdW ferromagnet Fe$_3$GeTe$_2$ to satisfy the
requirements in their all-vdW-heterostructure with an atomically sharp
interface. The results exhibit values of ${\xi}{\approx}4.6$ and
${\sigma}{\approx}2.25{\times}10^5 {\Omega}^{-1} m^{-1}$ for WTe$_2$. Moreover,
we obtain the significantly reduced switching current density of
$3.90{\times}10^6 A/cm^2$ at 150 K, which is an order of magnitude smaller than
those of conventional heavy-metal/ ferromagnet thin films. These findings
highlight that engineering vdW-type topological materials and magnets offers a
promising route to energy-efficient magnetization control in SOT-based
spintronics.
