Experimental determination of the Weiss temperature of several variants of Mn12-ac

We report measurements of the susceptibility in the temperature range from 3.5 K to 6.0 K of a series of Mn12-ac and Mn12-ac-MeOH samples in the shape of rectangular prisms. The susceptibility obeys a Curie-Weiss Law, where the temperature intercept varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain the Curie-Weiss intercept for an infinitely long sample corresponding to intrinsic ordering temperatures Tc $\sim $ 0.85 K and $\sim $ 0.74 K for Mn12-ac and Mn12-ac-MeOH, respectively [1]. The difference in Tc for the two materials suggests an additional non-dipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules. A similar comparison will also be reported for Mn12-toluate.