  We have studied the relaxation of a spin-polarized gas in a magnetic field,
in the presence of short-range spin-dependent interactions. As a main result we
have established a link between the specific properties of the interaction and
the dependence of the spin-relaxation rate on the magnitude of the holding
magnetic field. This allows us to formulate a new, extremely sensitive method
to study (pseudo-) magnetic properties at the sub-millimeter scale, which are
difficult to access by other means. The method has been used as a probe for
nucleon-nucleon axion-like P,T violating interactions which yields a
two-order-of-magnitude improved constraint on the coupling strength ($g_s g_p$)
as a function of the force range ($\lambda$): $g_s g_p \lambda^2 < 3 \times
10^{-27}$ m$^2$.
