Quantum relaxation and quantum coherence in mesoscopic molecular magnets

The effect of a magnetic field B_⊥ on the resonant tunneling between different magnetic states of the molecular clusters Fe₈ and Mn₁₂ has been studied by measuring the magnetic specific heat of these compounds between 0.1 K and 6 K. At zero field, we have shown that tunneling proceeds via excited levels and found which states contribute most to quantum tunneling in Fe₈. As B_⊥ increases, the superparamagnetic blocking temperature decreases, thus indicating that tunneling proceeds through progressively lower lying excited levels. At high enough fields, incoherent tunneling is observed to proceed through the ground state, at rates faster than 1 s^-1. For still higher fields, the tunnel splitting Δₜ of their m=±10 magnetic ground states can be made large compared to perturbations such as hyperfine and dipolar interactions. Under those conditions, the equilibrium specific heat measured at the lowest temperature gives evidence for the presence of a Δₜ, thus showing that coherent quantum mechanical tunneling can occur in these ``mesoscopic'' systems. For Mn₁₂ the dissipation caused by the hyperfine interaction between the electronic and nuclear Mn spins has been studied in detail.