Quantum Tunneling of the Magnetization in an Iron Cluster Nanomagnet

We have investigated the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy. Below 400 mK the relaxation rate is temperature independent suggesting that tunneling of the magnetic moment across its anisotropy energy barrier occurs. Using the anisotropy constants derived from EPR data, we can calculate both the crossover temperature $T_c$ and the expected tunneling frequency $1/\tau$. The field dependence of the relaxation shows evidence of resonant tunneling.