  We study how a magnetic field step triggers the precessional switching of the
magnetization in an isotropic thin film . Using a variational approach, we
estimate analytically the switching frequency and compare it to the results
obtained numerically by direct integration of the equations of motion. We show
that the periodic motions of the three magnetization components can be
described satisfactorily with truncated Fourier expansions, indicating a
relatively high spectral purity of the magnetic response. Our analytical
expressions are simple enough to be physically transparent at first sight, in
contrast to the results of the more elaborate models that treat also
anisotropy.
