Experimental determination of the dipolar field in Mn$_{12}$-acetate*

Crystals of the molecular magnet Mn$_{12}$-acetate are known to contain a small fraction of defect (minor species) molecules with a small anisotropy barrier against spin reversal. The lower barrier leads to faster magnetic relaxation and lower coercive field. We exploit the low coercive fields of the minor species, and the location of the minor species tunneling resonances, to make a direct determination of the dipole field in Mn$_{12}$-ac. We find that the dipolar field of a fully magnetized crystal is $51.5 \pm 8.5$ mT, consistent with theoretical expectations.