Experimental observation of quantum relaxation and quantum coherence in molecular magnetic clusters

By applying a transverse magnetic field B-perpendicular to to the uniaxial molecular magnetic clusters Fe, and Mn,,, the quantum mechanical tunnel spl itting Delta (t) of their S-z = +/- 10 magnetic ground states can be made l arge compared with perturbations such as hyperfine and dipolar interactions . We have measured the magnetic specific hears between T = 0.1 and 6 K at d ifferent B-perpendicular to (0-10 T). Both the field dependence of the equi librium-to-non-equilibrium transition and the energy difference of the tunn el-split levels have been studied. At high enough fields, which depend on t he magnitude of the magnetic anisotropy, the spin-lattice relaxation time b ecomes independent of temperature. Moreover, the equilibrium specific heat measured at the lowest temperature shows evidence for the presence of coher ent quantum mechanical tunneling in these 'mesoscopic' systems. For Mn,, th e decoherence caused by the hyperfine interaction has been studied. (C) 200 1 Elsevier Science Ltd. All rights reserved.