Resonant quantum tunneling of spin chains in a three-dimensionnal magnetically ordered state

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 $[(C{H}_3{)}_{3}NH]CoC{l}_3\cdot 2H_{2}O$, which orders as a canted 3D-antiferromagnet at $T_C=4.15K$. We present three features that prove unambigously the existence of resonant quantum tunneling below $0.3K$ ($T_C\mathrm{/}12$): Temperature independent relaxation of the magnetization, speeding up of the relaxation at a well defined field ($1025Oe$) 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.