Experimental limits allow for fundamental forces of nature millions of times stronger than gravity at distance ranges below 1 mm. Physics beyond the Standard Model predicts a variety of phenomena in this range, including signatures of extra spacetime dimensions, scalars motivated by string theory, violations of Lorentz symmetry, and dark energy and dark matter. The last includes the axion, a light pseudoscalar motivated by the strong CP problem of QCD, which could couple to spin. In this talk I will discuss an experiment sensitive to fifth forces that couple to both mass and spin. It uses planar, 1 kHz mechanical oscillators as test masses with a stiff conducting shield in between them to suppress backgrounds. It is being modified to search for spin-dependent forces by augmenting the test masses with a polarized material which exhibits orbital compensation of the magnetism associated with the electron spins, substantially reducing the magnetic backgrounds. Another experiment uses NMR techniques to probe the interaction between a dense, non-magnetic source and a sample of polarized helium-3 atoms. It is expected to further improve sensitivity to axion-mediated forces by several orders of magnitude, comparable to limits inferred from searches for the neutron electric dipole moment.
