Hall magnetometry measurements of the susceptibility of variants of Mn$_{12}$-ac

The temperature dependence of the inverse magnetic susceptibility of both Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH is found to give finite temperature intercepts, indicating a ferromagnetic phase at low temperature [1, 2]. A magnetic field applied transverse to the Ising axis suppresses the Curie temperature, T$_{CW}$, to a T = 0 quantum critical point. However, the decrease of T$_{CW}$ with transverse field, H$_{\bot }$, in Mn$_{12}$-ac does not agree with mean field calculations performed with a spin Hamiltonian that includes dipolar interactions between molecules and H$_{\bot }$ effects. We attribute the pronounced suppression of T$_{CW}$ of Mn$_{12}$-ac to the effect of random fields arising from a distribution of molecular easy axis tilts due to ligand disorder [1]. Mn$_{12}$-ac-MeOH is of interest in this regard because it appears to have no ligand disorder. We discuss these experiments as well as ongoing studies of Mn$_{12}$-toluate, a faster relaxing variant of Mn$_{12}$, which has a lower effective anisotropy barrier that permits the study of the susceptibility in larger H$_{\bot }$ at very low temperature. \\[0pt] [1] Wen et.al PRB \textbf{82}, 014406 (2010) \\[0pt] [2] Li et.al PRB \textbf{82}, 174405 (2010)