In cavity quantum electrodynamics,the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom.We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets.Upon tunneling through a magnetic bilayer,we find that the spin transfer torques per electron incidence can become orders of magnitude larger than h/2,made possible by electron's multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer.Over a broad energy range around the tunneling resonances,the damping-like spin transfer torque per electron tunneling features a universal value of (h/2)tan(θ/2),depending only on the angle θ between the magnetizations.These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.