  In the present paper, we derive the exact solution of the BMT equation for
the spin rotation of relativistic negatively charged particles passing through
a thin bent crystal in the regime of planar channeling. It is shown that during
the passage through the crystal a particle polarization vector oscillates. If
one takes into account, the energy and angular divergence of the initial
particle beam these oscillations die fast with particle penetration length into
a crystal, and particle spin rotation is determined by the crystal bending
angle.
