Quantum tunneling of magnetization in Mn(12)Bz clusters: Evidences of spinparity effect

We present a comparison of the field dependence of the relaxation times obs erved for Mn(12)Bz, (Bz=benzoate) spin cluster, and its derivative containi ng one extra electron, r-Mn(12)Bz. At low temperatures Mn(12)Bz has a total spin S=10, while for the reduced sample r-Mn(12)Bz the spin of the ground state is S=19/2. Theory predicts that half integer spin should not present quantum tunneling, the so-called spin parity effect. We show evidence from experiments that quantum tunneling is partially inhibited in the reduced sp ecies. Low temperature magnetization hysteresis loops of the reduced variet y do not show well-defined steps. In addition, the ac susceptibility as a f unction of the field also does not show a clear resonance at the expected l evel crossing fields, as seen in the nonreduced sample. (C) 2000 American I nstitute of Physics. [S0021-8979(00)17608-X].