  We show that resonant quantum tunneling of the magnetization, until now
observed only in zero-dimensional (0D) cluster systems, occurs in the molecular
Ising spin chain $[(CH_3)_3NH]CoCl_3\cdot 2H_2O$, which orders as a canted
3D-antiferromagnet at $T_C=4.15 K$. We present three features that prove
unambigously the existence of resonant quantum tunneling below $0.3 K$
($T_C/12$): Temperature independent relaxation of the magnetization, speeding
up of the relaxation at a well defined field ($1025 Oe$) and increase of the
magnetization each time this field is crossed during a succession of minor
hysteresis loops. A mechanism is proposed to describe the behavior based on a
simple model of 0D domain wall nucleation.
