Experimental determination of the Weiss temperature of Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH

TL;DR

This study measures the magnetic susceptibility of Mn12-ac and Mn12-ac-MeOH to determine their Weiss temperatures, revealing the influence of both dipolar and non-dipolar interactions on magnetic ordering.

Contribution

It provides the first systematic determination of Weiss temperatures for Mn12-ac and Mn12-ac-MeOH, highlighting the role of ligand molecules in magnetic interactions.

Findings

Weiss temperatures are approximately 0.85 K and 0.74 K for Mn12-ac and Mn12-ac-MeOH.

Susceptibility follows Curie-Weiss law with aspect ratio-dependent Weiss temperature.

Non-dipolar interactions contribute to the differences in magnetic ordering temperatures.

Abstract

We report measurements of the susceptibility in the temperature range from $3.5$ K 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 $T_c \approx 0.85$ K and $\approx 0.74$ 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 non-dipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules.