  We study the Landau-Zener dynamics of a tunneling spin coupled to a torsional
resonator. For strong spin-phonon coupling, when the oscillator frequency is
large compared to the tunnel splitting, the system exhibits multiple
Landau-Zener transitions. Entanglement of spin and mechanical angular momentum
results in abrupt changes of oscillator dynamics which coincide in time with
spin transitions. We show that a large number of spins on a single oscillator
coupled only through the in-phase phonon field behaves as a single large spin,
greatly enhancing the spin-phonon coupling. We compare purely quantum and
semiclassical dynamics of the system and discuss their experimental
realizations. An experiment is proposed in which the field sweep is used to
read out the exact quantum state of the mechanical resonator.
