  Relaxation of the Larmor magnetic moment by spin-exchange collisions has been
shown to diminish for high alkali densities, resulting from the linear part of
the collisional interaction. In contrast, we demonstrate both experimentally
and theoretically the elimination of spin-exchange relaxation of high magnetic
moments (birefringence) in alkali vapor. This elimination originates from the
nonlinear part of the spin-exchange interaction, as a scattering process of the
Larmor magnetic moment. We find counter-intuitively that the threshold magnetic
field is the same as in the Larmor case, despite the fact that the precession
frequency is twice as large.
