Magnetic Anisotropy of Fe6 and Fe10 Molecular Rings by Cantilever Torque Magnetometry in High Magnetic Fields

We studied the magnetic anisotropy of 2 mol. magnets, Fe6 and Fe10, which comprise 6- and 10-membered rings of antiferromagnetically coupled Fe (III) ions (Si = 5/2), resp. Spin-flip transitions induced by the applied magnetic field (≤23 T) were investigated by cantilever torque magnetometry on microgram single crystals at very low temp. (down to 0.45 K). From the sharp, steplike variations of magnetic anisotropy at the transition fields, we detd. the singlet-triplet energy gap (Δ1) and the axial zero-field splitting parameter (D1) for the triplet state of Fe6 [Δ1 = 15.28(1) cm-1, D1 = 4.32(3) cm-1] and Fe10 [Δ1 = 4.479(4) cm-1, D1 = 2.24(2) cm-1]. By analyzing the addnl. steps obsd. in the Fe10 sample, we evaluated the ΔS and DS parameters for the total-spin multiplets with S ≤5. On the basis of our findings, we discuss the origin of magnetic anisotropy in Fe(III) rings and the application of torque magnetometry to the study of field-induced level crossing in mol. magnets.