On Magnetic Relaxation in Spin Clusters

We summarize some of the significant features of the exotic behavior in 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 connected with what may be termed “resonant transitions” at crossings of Zeeman levels, and are probably due to quantum tunneling, what is lacking is a direct confirmation through controllable parameters that are connected with theory. We discuss the need to match theoretical calculations with experimental conditions. This author and others have presented a theory for the hysteresis steps based upon quantum tunneling. However, the size of the observed steps far exceeds that predicted by theory. The results indicate that a combination of other fields - such as nuclear hyperfine fields, dipole fields, and anisotropy fields due to optical phonons - are important.