Experimental Determination of the Weiss Temperature of Mn12-Ac and Mn12-Ac-MeOH

We report measurements of the susceptibility in the temperature range from 3.5 to 6.0 K of a series of Mn12-ac and Mn12-ac-MeOH samples in the shape of rectangular prisms of length lc and square cross section of side la. The susceptibility obeys a Curie-Weiss law, =C / T− , where varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain for an infinitely long sample corresponding to intrinsic ordering temperatures Tc 0.85 K and 0.74 K for Mn12-ac and Mn12-ac-MeOH, respectively. The difference in Tc 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.