Nutation: separating the spin from its magnetic moment
arxiv(2024)
摘要
For nearly 90 years, precession and relaxation processes have been thought to
dominate magnetization dynamics. Only recently has it been considered that, on
short time scales, an inertia-driven magnetization dynamics should become
relevant, leading to additional nutation of the magnetization vector. Here, we
trigger magnetic nutation via a sudden excitation of a thin Ni80Fe20
(Permalloy) film with an ultrashort optical pulse, that leads to an abrupt
tilting of the effective field acting on the magnetic moments, separating the
dynamics of the magnetization from that of its angular momentum. We investigate
the resulting magnetization dynamics in the inertial regime experimentally by
the time-resolved magneto optical Kerr effect. We find a characteristic
oscillation in the Kerr signal in the range of about 0.1 THz superimposed on
the precessional oscillations with GHz frequencies. By comparison with
atomistic spin dynamics simulations, we demonstrate that this observation
cannot be explained by the well-known Landau-Lifshitz-Gilbert equation of
motion but can be attributed to inertial contributions leading to nutation of
the magnetization vector around its angular momentum. Hence, an optical and
non-resonant excitation of inertial magnetization dynamics can trigger and
control different magnetic processes, ranging from demagnetization via nutation
to precession in a single device. These findings will have profound
implications for the understanding of ultrafast spin dynamics and magnetization
switching.
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