  Scaling magnets down to where quantum size effects become prominent triggers
quantum tunneling of magnetization (QTM), profoundly influencing magnetization
dynamics. Measuring magnetization switching in an Fe atomic chain under a
carefully tuned transverse magnetic field, we observe a non-monotonic variation
of magnetization lifetimes around a level crossing, known as the diabolic point
(DP). Near DPs, local environment effects causing QTM are efficiently
suppressed, enhancing lifetimes by three orders of magnitude. Adjusting
interatomic interactions further facilitates multiple DPs. Our study provides a
deeper understanding of quantum dynamics near DPs and enhances our ability to
engineer a quantum magnet.
