  We use tunable dipolar-interactions between the spins of nitrogen-vacancy
(NV) centers in diamond to rotate a diamond crystal. Specifically, we employ
cross-relaxation between the electronic spin of pairs of NV centers in a
trapped diamond to enhance the anisotropic NV paramagnetism and thus to
increase the associated spin torque. Our observations open a path towards the
use of mechanical oscillators to detect paramagnetic defects that lack optical
transitions, to investigation of angular momentum conservation in spin
relaxation processes and to novel means of cooling the motion of mechanical
oscillators.
