  We propose a mechanism of torque generation by injection of an orbital
current, which we call $\textit{orbital torque}$. In a magnetic bilayer
consisting of a nonmagnet (NM) and a ferromagnet (FM), we consider a situation
where the spin-orbit coupling (SOC) is present only in the FM. Although the SOC
is absent in the NM, the orbital Hall effect can arise in the NM. When the
resulting orbital Hall current is injected to the FM, the SOC of the FM
converts the orbital angular momentum into spin, which exerts torque to the
magnetization of the FM. Remarkably, even for small SOC strength comparable to
that of $3d$ FMs, the orbital torque can be comparable to the spin torque
induced by the spin Hall effect of the NM with strong SOC. This provides a way
to experimentally probe the OHE and opens a new venue to achieving spin-torque
devices based on light elements that exhibit gigantic orbital response.
Experimental implications are discussed.
