  Energy-efficient switching of nanoscale magnets requires the application of a
time-varying magnetic field characterized by microwave frequency. At finite
temperatures, even weak thermal fluctuations create perturbations in the
magnetization that can accumulate in time, break the phase locking between the
magnetization and the applied field, and eventually compromise magnetization
switching. It is demonstrated here that the magnetization reversal is mostly
disturbed by unstable perturbations arising in a certain domain of the
configuration space of a nanomagnet. The instabilities can be suppressed and
the probability of magnetization switching enhanced by applying an additional
stimulus such as a weak longitudinal magnetic field that ensures bounded
dynamics of the perturbations. Application of the stabilizing longitudinal
field to a uniaxial nanomagnet makes it possible to reach a desired probability
of magnetization switching even at elevated temperatures. The principle of
suppressing instabilities provides a general approach to enhancing thermal
stability of magnetization dynamics.
