Spin dynamics and level crossing in nanoscale molecular magnet V15 cluster studied by 1H-NMR

Abstract

Abstract 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 K 6 [V 15 As 6 O 42 (H 2 O)]8H 2 O (in short, V15). The temperature ( T ) and magnetic field ( H ) dependences of 1 H-NMR spectrum, nuclear spin–lattice relaxation rates (1/ T 1 ) and spin–spin relaxation rates (1/ T 2 ) provide evidence for a gradual slowing of fluctuations of the V 4+ spins with increasing the magnetic field. The T and H dependence of 1/ T 1 is 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 T 1 data 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/ T 1 due to peculiarities of the S = 1/2 triangle configuration in V15. An enhancement of 1/ T 1 was observed at a critical field H c ∼ 2.7 T where the ground state changes from S = 1/2 to 3/2. The T -dependence of 1/ T 1 at 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 V 4+ spins is found to be very slow, i.e., less than the NMR frequency at the level crossing.