Magnetic anisotropy of Fe6 and Fe10 molecular rings by cantilever torque magnetometry in high magnetic fields

We studied the magnetic anisotropy of two molecular magnets, Fe-6 and Fe-10, which comprise six-and ten-membered rings of antiferromagnetically coupled iron (III) ions (S-i = 5/2), respectively. Spin-flip transitions induced by the applied magnetic field (up to 23 T) were investigated by cantilever torque magnetometry on microgram single crystals at very low temperature (down to 0.45 K). From the sharp, steplike variations of magnetic anisotropy at the transition fields, we determined the singlet-tripler energy gap (Delta(1)) and the axial zero-field splitting parameter (D-1) for the tripler state of Fe-6 [Delta(1) =15.28(1) cm(-1), D-1 = 4.32(3) cm(-1)] and Fe-10 [Delta(1) = 4.479(4) cm(-1), D-1 = 2.24(2) cm(-1)]. By analyzing the additional steps observed in the Fe-10 sample, we evaluated the Delta(S) and D-S parameters for the total-spin multiplets with S up to 5, On the basis of our findings, we discuss the origin of magnetic anisotropy in iron (III) rings and the application of torque magnetometry to the study of field induced level crossing in molecular magnets. [S0163-1829(99)01041-3].