High-field torque magnetometry on Fe-6 and Fe-10 molecular magnets

We have applied torque magnetometry to the study of magnetic anisotropy in six- and ten-membered rings of iron(III) ions. By using magnetic fields up to 23 Tesla, we have partially overriden the intramolecular antiferromagnet ic ordering of the spins (J = 20.0 cm(-1) 9.6 cm(-1) in Fe-6 and Fe-10, res pectively) to obtain magnetic ground states with S = 1, 2, 3, etc.. Torque measurements on microgram single crystals at temperatures down to 0.45 K ha ve shown that spin-flip transitions are accompanied by sharp Variations of magnetic anisotropy. The angular dependence of transition fields has provid ed spectroscopic-quality information about the spin-Hamiltonian parameters in the two compounds. In addition, by analyzing the field dependence of the torque signal we have determined the exchange energies and zero-field spli tting parameters of the total-spin multiplets with S less than or equal to 5 in the Fe-10 sample. We conclude that torque magnetometry represents a hi gh-sensitivity, powerful technique for studying field-induced level crossin g in molecular magnets.