Experimental determination of the dipolar field in Mn 12 -acetate

Crystals of the molecular magnet ${\text{Mn}}_{12}$-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 to make a direct determination of the dipole field in ${\text{Mn}}_{12}$-ac. We find that the dipolar field of a fully magnetized crystal is $51.5\ifmmode\pm\else\textpm\fi{}8.5\text{ }\text{mT}$, consistent with theoretical expectations.