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, χ=C/(T−θ), where θ varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain θ 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.
Lampropoulos, Christos; Li, Shiqi; Bo, Lin; Wen, Bo; Sarachik, M P.; Subedi, P; Kent, A D.; Yeshurun, Y; Millis, A J.; Mukherjee, S; and Christou, G, "Experimental Determination of the Weiss Temperature of Mn12-Ac and Mn12-Ac-MeOH" (2010). Chemistry Faculty Publications. Paper 10.