  A study of the possible interactions between fermions assuming only
rotational invariance has revealed 15 forms for the potential involving the
fermion spins. We review the experimental constraints on unobserved
macroscopic, spin-dependent interactions between electrons in the range below 1
cm. An existing experiment, using 1 kHz mechanical oscillators as test masses,
has been used to constrain mass-coupled forces in this range. With suitable
modifications, including spin-polarized test masses, this experiment can be
used to explore all 15 possible spin-dependent interactions between electrons
in this range with unprecedented sensitivity. Samples of ferrimagnetic
dysprosium iron garnet have been fabricated in the suitable test mass geometry
and shown to have high spin density with very low intrinsic magnetism.
