The magnetic anisotropy of the cyclic octanuclear Fe(III) cluster [Cs subsetFe(8)[N(CH(2)CH(2)O)(3)](8)]Cl was investigated. Based on a spin Hamiltonian formalism and the consequent use of all symmetries, the magnetic anisotropy could be calculated exactly to first order, i.e., in the strong exchange limit. Experimentally, the magnetic anisotropy was investigated by magnetic susceptibility and high-field torque magnetometry of single crystals. The field and angle dependence of the torque at 1.7 K could be accurately reproduced by the calculations with one single parameter set, providing accurate results for the coupling constant and single-ion zero-field-splitting. These magnetic parameters are compared to those of several related hexanuclear ferric wheels and are discussed with respect to magneto-structural correlations for both coupling constant and single-ion anisotropy.

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In our work [1] on the molecular ferric wheel NaFe 6 , we observed butterfly hysteresis of the magnetic torque at the first level-crossing field, basically due to a phonon bottleneck. We used this phenomenon as a tool to investigate the processes changing the spin state at the level crossing, providing new insight into these issues. Beyond the ones given in [1], our work is based on a host of earlier works, which were not cited. Butterflylike behavior was found in, e.g., [2]; magnetocaloric cooling was observed and discussed in many materials, e.g., antiferromagnets [3], ferrimagnets [4], superconductors [5], and small metal complexes [2,6,7]; the phonon bottleneck and mechanisms of spin relaxation were observed and discussed in many works, e.g., [8][9][10].

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At low temperatures, the magnetization of the molecular ferric wheel NaFe6 exhibits a step at a critical field B(c) due to a field-induced level crossing. By means of high-field torque magnetometry we observed a hysteretic behavior at the level crossing with a characteristic butterfly shape which is analyzed in terms of a dissipative two-level model. Several unusual features were found. The nonzero bias field of the level crossing suggests the possibility of cooling by adiabatic magnetization.