Non-adiabatic Landau-Zener transitions in low-spin molecular magnet V-15

The V-15 polyoxovanadate molecule is made of 15 spins: with antiferromagnet ic couplings. It belongs to the class of molecules with very large Hilbert space dimension (2(15) in V-15, 10(8) in Mn-12-AC). It is a low spin/large molecule with spin S = 1/2 Contrary to large spins/large molecules of the M n-12-AC type, V-15 has no energy barrier against spin rotation. Magnetizati on measurements have been performed and despite the absence of a barrier, m agnetic hysteresis is observed over a timescale of several seconds. This ne w phenomenon characterized by a "butterfly" hysteresis loop is due to the e ffect of the environment on the quantum rotation of the entangled 15 spins of the molecule, in which the phonon density of states is not at its equili brium (phonon bottleneck). (C) 2000 Elsevier Science B.V. All rights reserv ed.