  Magnetization versus field measurements were performed on single crystals of
[(Pc)2TbIII0.02YIII0.98]^TBA+ and [(Pc)2DyIII0.02YIII0.98]^TBA+ (Pc:
phthalocyaninato, TBA: tetrabutylammonium) at 0.04 K. The [(Pc)2TbIII] complex,
the first lanthanide single-molecule magnet, exhibited clear staircase-like
structures, which are assigned to resonant quantum tunneling between entangled
states of the electron and nuclear spin systems.
