Single-Molecule Magnet Behavior and Spin Structure of an FeIII₇ 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 [Fe III 7 O 3 (O 2 C t Bu) 9 (Me-dea) 3 (H 2 O) 3 ] (Me-deaH 2 = 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. 57 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 O 5 N coordination are oriented mutually coparallel, while these are antiparallel to the spins on the peripheral Fe(3) sites with O 6 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 z = ± 3/2, with an activation barrier of U / k B = 1.9 K.