Abstract: 
In 1915, Einstein and de Haas and Barnett demonstrated that changing the magnetization of a magnetic material results in mechanical rotation, and vice versa. At the microscopic level, this effect governs the transfer between electron spin and orbital angular momentum, and lattice degrees of freedom, understanding which is key for molecular magnets, nanomagnetomechanics, spintronics, and ultrafast magnetism. Until now, the timescales of electrontolattice angular momentum transfer remain unclear, since modeling this process on a microscopic level requires addition of an infinite amount of quantum angular momenta. We show that this problem can be solved by reformulating it in terms of the recently discovered angulon quasiparticles, which results in a rotationally invariant quantum manybody theory. In particular, we demonstrate that nonperturbative effects give rise to angular momentum transfer on femtosecond timescales, even if the electronphonon coupling is weak.
