  Spin-orbit torques (SOTs), which rely on spin current generation from charge
current in a nonmagnetic material, promise an energy-efficient scheme for
manipulating magnetization in magnetic devices. A critical topic for spintronic
devices using SOTs is to enhance the charge to spin conversion efficiency.
Besides, the current-induced spin polarization is usually limited to in-plane,
whereas out-of-plane spin polarization could be favored for efficient
perpendicular magnetization switching. Recent advances in utilizing two
important classes of van der Waals materials$-$topological insulators and
transition-metal dichalcogenides$-$as spin sources to generate SOT shed light
on addressing these challenges. Topological insulators such as bismuth selenide
have shown a giant SOT efficiency, which is larger than those from
three-dimensional heavy metals by at least one order of magnitude.
Transition-metal dichalcogenides such as tungsten telluride have shown a
current-induced out-of-plane spin polarization, which is allowed by the reduced
symmetry. In this review, we use symmetry arguments to predict and analyze SOTs
in van der Waal material-based heterostructures. We summarize the recent
progress of SOT studies based on topological insulators and transition-metal
dichalcogenides and show how these results are in line with the symmetry
arguments. At last, we identify unsolved issues in the current studies and
suggest three potential research directions in this field.
