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 l_c and square cross section of side l_a. 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 T _c ≈0.85K and ≈0.74 K for Mn₁₂ -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.