Resonant Quantum Tunneling of spin chains in 3D-magnetically ordered CoTAC

CoTAC is a well know molecular spin chain system which orders in an antiferromagnetic canted state at $4.15\ K$. We show that, below $300\ mK$, the dynamics of the magnetization in the $c$-direction are governed by resonant quantum tunneling of the magnetization. This conclusion is based on a number of experimental observations: the temperature independence of the relaxation of the magnetization, speeding up of the relaxation at a well defined magnetic field value ($1025\ Oe$) and the increase of the magnetization each time this field is crossed during a succession of minor loops. The key to understanding this behavior could be the absence of resonance in zero field. We propose a mechanism to describe this behavior using a simple model of domain wall nucleation which explains many of the unusual experimental observations.