  Exchange forces on the movable dot ("shuttle") in a magnetic shuttle device
depend on the parity of the number of shuttling electrons. The performance of
such a device can therefore be tuned by changing the strength $U$ of Coulomb
correlations to block or unblock parity fluctuations. We show that by
increasing $U$ the spintro-mechanics of the device crosses over, at $U=U_c(T)$,
from a mechanically stable regime to a regime of spin-induced shuttle
instabilities. This is due to enhanced spin-dependent mechanical forces as
parity fluctuations are reduced by a Coulomb blockade of tunneling and
demonstrates that single-electron manipulation of single-spin controlled
nano-mechanics is possible.
