Single-Molecule Magnet Behavior and Spin Structure of an Fe^III₇ Cartwheel Cluster Revealed by Sub-Kelvin Magnetometry...

The spin frustration and other magnetic properties of the "cartwheel" heptanuclear cluster [Fe^III₇O₃(O₂CBu)₉(Me-dea)₃(H₂O)₃] (Me-deaH₂ = N-methyldiethanolamine) have been previously investigated; we present here a Mössbauer spectroscopic study and sub-Kelvin magnetization and ac susceptibility measurements which enable a complete magnetic picture of this frustrated cluster. ⁵⁷Fe Mössbauer spectra above 150 K showed three doublets in a 1:3:3 ratio, which could be assigned by their respective quadrupole splittings to the central Fe(1) and the peripheral Fe(2) and Fe(3). The field dependence of the corresponding magnetic sextets at 3 K showed that the spins on the central Fe(1) and the three peripheral Fe(2) sites with O₅N coordination are oriented mutually coparallel, while these are antiparallel to the spins on the peripheral Fe(3) sites with O₆ coordination, resulting in an overall S = 5/2 ground state. This provides experimental confirmation of the previously proposed spin ground state structure. Upon cooling to sub-Kelvin temperatures, a crossover to spin blocking with T_B ≈ 0.21 K could be observed. This single-molecule magnet behavior had been expected but had not been observable with a conventional SQUID. The anisotropy barrier, of 3-fold symmetry, can be described in terms of the parameter D/k_B = -0.47 K and a fourth-order perturbation; the latter enables thermally activated quantum tunneling through the excited sublevel m = ± 3/2, with an activation barrier of U/k_B = 1.9 K.