  Strongly out-of-equilibrium regimes in magnetic nanostructures exhibit novel
properties, linked to the nonlinear nature of magnetization dynamics, which are
of great fundamental and practical interest. Here, we demonstrate that
field-driven ferromagnetic resonance can occur with substantial spatial
coherency at unprecedented large angle of magnetization precessions, which is
normally prevented by the onset of spin-wave instabilities and magnetization
turbulent dynamics. Our results show that this limitation can be overcome in
nanomagnets, where the geometric confinement drastically reduces the density of
spin-wave modes. The obtained deeply nonlinear ferromagnetic resonance regime
is probed by a new spectroscopic technique based on the application of a second
excitation field. This enables to resonantly drive slow coherent magnetization
nutations around the large angle periodic trajectory. Our experimental findings
are well accounted for by an analytical model derived for systems with uniaxial
symmetry. They also provide new means for controlling highly nonlinear
magnetization dynamics in nanostructures, which open interesting applicative
opportunities in the context of magnetic nanotechnologies.
