Magnetic chains of Fe clusters in {Fe3 YO2} butterfly molecules

Abstract

Clusters in molecules with transition metals M and rare earths Ln can show single molecule magnet (SMM) behaviour. While M provides most of the cluster magnetization, Ln metals generate an enhanced magnetic anisotropy via the intracluster M-Ln interactions. The use of Y as a non-magnetic substitution for Ln in isostructural compounds provides a way to explore possible interactions between the transition metals. The >butterfly> molecule Fe3Y(μ3−O)2(CCl3COO)8(H2O)(THF)3 (in brief {Fe3YO2}) includes three close Fe3+ ions which build a robust Fe3 cluster with a strong intracluster antiferromagnetic exchange J = −50 K and a total spin S=5/2. We present heat capacity and DC susceptibility measurements below 2 K, which show the cluster anisotropy and intercluster interactions. However, no phase transition to a long-range magnetically ordered phase is observed down to 20 mK. The intercluster interaction is analysed in the framework of the one-dimensional Blume-Capel model with an antiferromagnetic chain exchange constant J '≈ −0.04 K, and a uniaxial anisotropy with parameter D ≈ −0.56 K. This is proposed to be associated to chains of Fe3 clusters oriented along the shortest intercluster distances displayed by the crystal structure of {Fe3YO2}. AC susceptibility measurements reveal magnetic relaxation below 5 K with quantum tunnelling of the magnetization below 0.2 K, and a thermally activated process for higher temperature.