  Numerical simulations of magnetization reversal of a quantum uniaxial magnet
under a swept magnetic field [Hatomura, \textit{et al}., \textit{Quantum
Stoner-Wohlfarth Model}, Phys. Rev. Lett. \textbf{116}, 037203 (2016)] are
extended. In particular, how the "wave packet" describing the time-evolution of
the system is scattered in the successive avoided level crossings is
investigated from the viewpoint of the distribution of the eigenstate
populations. It is found that the peak of the distribution as a function of the
magnetic field does not depend on spin-size $S$, which indicates that the delay
of magnetization reversal due to the finite sweeping rate is the same in both
the quantum and classical cases. The peculiar synchronized oscillations of all
the spin components result in the beating of the spin-length. Here, dissipative
effects on this beating are studied by making use of the generalized
Lindblad-type master equation. The corresponding experimental situations are
also discussed in order to find conditions for experimental observations.
