Low energy excitations in molecular antiferromagnetic rings, Fe10 and Fe12

Abstract 1H-NMR measurements have been performed to study the low energy excitations in molecular antiferromagnetic rings [ Fe ( OCH 3 ) 2 ( O 2 CCH 2 Cl ) ] 10 and [ Fe ( OCH 3 ) 2 ( C 5 H 9 NO 2 ) ( CIO 4 ) ] 12 . The spin–lattice relaxation rates T 1 - 1 above 40 K show the spin fluctuations in the continuous energy levels of the spin states reflecting the classical nature of the spin systems. On the other hand, the temperature dependence of the relaxation rates below 5 K is expressed by an activation-type function with a gap energy, reflecting their discrete energy levels, that is quantum nature of the spin systems. The field dependence of the gap energy in Fe10 is revealed by the T 1 - 1 measurements and is analyzed by the Hamiltonian with the exchange interaction and a single-ion-type anisotropy. The remarkable peak of the relaxation rates is observed around 15 K, which corresponds to the values of exchange interactions both in Fe10 and Fe12.