We have carried out nuclear magnetic resonance (NMR) on protons in the magnetic field range H = 0.23-8.2 T below 4.2 K in order to investigate the spin dynamics in the molecular magnet K6[V15As6O42(H2O)]8H2O (in short, V15). The temperature (T) and magnetic field (H) dependences of1H-NMR spectrum, nuclear spin-lattice relaxation rates (1/T1) and spin-spin relaxation rates (1/T2) provide evidence for a gradual slowing of fluctuations of the V4+spins with increasing the magnetic field. The T and H dependence of 1/T1is well explained by the model in terms of spin phonon interaction in the magnetic field region where the ground state of the cluster is S = 3/2. On the other hand, the T1data can not be reproduced by the model in the low magnetic field region where the ground state is formed by two S = 1/2 doubly degenerate states, suggesting the existence of the another contribution to 1/T1due to peculiarities of the S = 1/2 triangle configuration in V15. An enhancement of 1/T1was observed at a critical field Hc∼ 2.7 T where the ground state changes from S = 1/2 to 3/2. The T-dependence of 1/T1at the level crossing field suggests the existence of an energy gap between the levels (level anti-crossing) in the V15 cluster. The fluctuating frequency of the V4+spins is found to be very slow, i.e., less than the NMR frequency at the level crossing. © 2005 Elsevier Ltd. All rights reserved.
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