Quantum relaxation and quantum coherence in mesoscopic molecular magnets -art. no. 174411

The effect of a magnetic field B-perpendicular to on the resonant tunneling between different magnetic states of the molecular clusters Fe-8 and Mn-12 has been studied by measuring the magnetic specific heat of these compound s between 0.1 K and 6 K. At zero field, we have shown that tunneling procee ds via excited levels and found which states contribute most to quantum tun neling in Fe-8. As B-perpendicular to increases, the superparamagnetic bloc king temperature decreases, thus indicating that tunneling proceeds through progressively lower lying excited levels. At high enough fields, incoheren t tunneling is observed to proceed through the ground state, at rates faste r than 1 s(-1), For still higher fields, the tunnel splitting Delta (t) of their m =+/- 10 magnetic ground states can be made large compared to pertur bations such as hyperfine and dipolar interactions, Under those conditions, the equilibrium specific heat measured at the lowest temperature gives evi dence for the presence of a Delta (t), thus showing that coherent quantum m echanical tunneling can occur in these "mesoscopic" systems. For Mn-12 the dissipation caused by the hyperfine interaction between the electronic and nuclear Mn spins has been studied in detail.