Magnetization reversal in a fine ferromagnetic grain is simulated for the c
ase of an instantaneously applied reversal magnetic field. The Hamiltonian
of the system contains the exchange interaction, uniaxial anisotropy, Zeema
n energy, and dipole-dipole interactions. A cubic grain is discretized into
64 cubic subgrains and the coupled gyromagnetic equations of motion are so
lved without phenomenological damping. A scheme to solve these equations is
developed that utilizes only two variables per sub-cube magnetization and
strictly conserves the absolute magnitude. The initial stage of reversal is
uniform rotation followed by a nonlinear excitation of nonuniform magnetic
oscillations driven by this uniform mode. An excess of the initial Zeeman
energy is transformed into nonlinear spin waves, allowing the average magne
tization to substantially reverse. The process of magnetization reversal in
fine quasi-single-domain grain exhibits general features of Hamiltonian wa
ve systems with nonlinear diffusion. This nonlinear diffusion is forbidden
fur either a strong reversal field and/or a small grain size.