Isotopic Effect on the Quantum Tunneling of the Magnetization of Molecular Nanomagnets

The molecular cluster [Fe8(tacn)6O2(OH)12]Br8.9H2O, Fe8, characterized by S = 10, with biaxial magnetic anisotropy is an ideal system to investigate quantum effects in the dynamics of the magnetization. Resonant quantum tunneling gives rise to stepped hysteresis cycles and below 0.35 K pure quantum tunneling of the magnetization has been observed. The role of hyperfine, fields in promoting the tunneling as a source of dynamic broadening of the states involved in the tunnel transition is investigated by preparing and characterizing two isotopically enriched samples using Fe-57 and H-2. The relaxation rate in the tunneling regime is proportional to the hyperfine field generated by the nuclei. Also, the intrinsic linewidth of the tunneling resonance scales with the hyperfine field as confirmed by calculations of the super (or transfer) and direct hyperfine interactions. Preliminary results on a novel cluster of formula [Fe4(OCH3)6(dpm)6], Fe4, suited for a more dramatic isotope effect on the tunneling rate are also reported.