Coherent angular momentum exchange between magnetic and mechanical degrees of freedom in ferromagnets

Conversion of spin angular momentum into its mechanical counterpart was demonstrated by Einstein and de Haas a long time ago, thereby confirming the connection between magnetic moment and angular momentum. This transfer of angular momentum in the Einstein-de Haas (EdH) effect, however, is driven by thermodynamic considerations and owes its existence to an “incoherent” relaxation to the lowest energy state. As a result, the EdH effect relies on energy dissipation in the system. 

We investigate coherent (linear) interaction between elastic and spin waves in a ferromagnet. Coupled equations of motion for the two fields are obtained using a Lagrangian formulation. Employing magneto-elastic boundary conditions, we study excitation of spin waves by elastic waves injected via an on-board transducer. Finally we demonstrate coherent dissipationless transfer of angular momentum from magnetization to the macroscopic motion of an isolated ferromagnet mediated by elastic waves. This concept can be used to control the orientation of an isolated body in outer space with high precision.