Single-Molecule Magnet Behavior and Spin Structure of an Fe Cartwheel Cluster Revealed by Sub-Kelvin Magnetometry and Mössbauer Spectroscopy: The Final Pieces of the Puzzle.

The spin frustration and other magnetic properties of the "cartwheel" heptanuclear cluster [FeO(OCBu)(Me-dea)(HO)] (Me-deaH = -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. Fe 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 ON coordination are oriented mutually coparallel, while these are antiparallel to the spins on the peripheral Fe(3) sites with O coordination, resulting in an overall = 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 ≈ 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 / = -0.47 K and a fourth-order perturbation; the latter enables thermally activated quantum tunneling through the excited sublevel = ± 3/2, with an activation barrier of / = 1.9 K.