Experimental determination of the Weiss temperature of Mn-12-ac and Mn-12-ac-MeOH

We report measurements of the susceptibility in the temperature range from 3.5 to 6.0 K of a series of Mn-12-ac and Mn-12-ac-MeOH samples in the shape of rectangular prisms of length l(c) and square cross section of side l(a). The susceptibility obeys a Curie-Weiss law, chi=C/(T-theta), where theta varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain theta for an infinitely long sample corresponding to intrinsic ordering temperatures T-c approximate to 0.85 K and approximate to 0.74 K for Mn-12-ac and Mn-12-ac-MeOH, respectively. The difference in T-c for two materials that have nearly identical unit cell volumes and lattice constant ratios suggests that, in addition to dipolar interactions, there is a nondipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules.