Angular momentum in spin-phonon processes

Although a century has passed since the celebrated Einstein - de Haas experiments, the microscopic mechanisms leading to the conversion of the spin angular momentum to the mechanical angular momentum in elastic solids has not been fully understood. In this paper D. A. Garanin and E. M. Chudnovsky explore the concept of the phonon spin recently suggested by L. Zhang and Q. Niu, Physical Review Letters 112, 085503 (2014). They show how it enters the problem of the relaxation of the atomic spin in the elastic body and explicitly demonstrate the conservation of the total angular momentum in spin-phonon processes at the microscopic level.