Speaker : Igor NGOUAGNIA (SPEC, CEA, CNRS, Université Paris-Saclay, France)
Abstract : Due to the extremely nonlinear nature of the magnetization dynamics associated
with the quantization of the excitation spectrum, the strongly out-of-equilibrium
regimes can reveal new properties in magnetic nanostructures. Here, we present a
detailed study of the ferromagnetic resonance (FMR) in the deeply nonlinear regime
of a 700 nm diameter Bismuth-doped yttrium iron garnet (BiYIG) disk. The increase
of the excitation microwave magnetic field leads first to the well-known foldover of
the resonance line but is then quickly followed by unexpected behaviors. We
observe a rapid saturation of the peak amplitude, corresponding to maximum mean
precession angles of the order of only 15°, accompanied with a broadening of the
line towards both lower and higher magnetic field. To understand these results, we
perform extensive micromagnetic simulations. These simulations reproduce the
main characteristics of the measured FMR lines in the deeply nonlinear regime.
They reveal that a dynamic instability is responsible of the observed behavior.
Moreover, this dynamic instability depends on the exact driving conditions,
resulting in a rich temporal dynamics, which can be quasi-periodic or intermittent.
To experimentally access this complex temporal dynamics, we use a new
spectroscopic technique based on the application of a second microwave excitation
field. The obtained frequency modulation spectra reflect the existence of rich low-
frequency temporal variations in the dynamics of the magnetization, as suggested by
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