ON MAGNETIC-RELAXATION IN SPIN CLUSTERS

We summarize some of the significant features of the exotic behavior i n the relaxation of molecular spin clusters and comment on aspects of some theories intended to account for observations. We point out that while the peaks in the relaxation rate and the hysteresis steps are co nnected with what may be termed ''resonant transitions'' at crossings of Zeeman levels, and are probably due to quantum tunneling, what is l acking is a direct confirmation through controllable parameters that a re connected with theory. We discuss the need to match theoretical cal culations with experimental conditions. This author and others have pr esented a theory for the hysteresis steps based upon quantum tunneling . However, the size of the observed steps far exceeds that predicted b y theory. The results indicate that a combination of other fields - su ch as nuclear hyperfine fields, dipole fields, and anisotropy fields d ue to optical phonons - are important.