Universal mechanism of spin relaxation in solids

We consider relaxation of a rigid spin cluster in an elastic medium in the presence of the magnetic field. A universal simple expression for spin-phonon matrix elements due to local rotations of the lattice is derived. The equivalence of the lattice frame and the laboratory frame approaches is established. For spin Hamiltonians with strong uniaxial anisotropy, the field dependence of the transition rates due to rotations is analytically calculated and its universality is demonstrated. The role of time-reversal symmetry in spin-phonon transitions has been elucidated. The theory provides a lower bound on the decoherence of any spin-based solid-state qubit.