YRLG Workshop: Correlation and Topology in magnetic materials, July 16th - 18th 2024
Anulekha De
Anulekha De1, Julius Schlegel2, Akira Lentfert1, Laura Scheuer1, Benjamin Stadtmüller1, Philipp Pirro1,
Georg von Freymann1,3, Ulrich Nowak2, and Martin Aeschlimann1
1Fachbereich Physik and Landesforschungszentrum OPTIMAS, Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau, 67663 Kaiserslautern, Germany
2 Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany 3Fraunhofer Institute for Industrial Mathematics, ITWM 67663 Kaiserslautern, Germany
The interaction of ultrashort laser pulses with ferromagnets can result in a number of new phenomena such as ultrafast demagnetization, all optical switching, etc. Here, we focus on the optically driven magnetization dynamics in the yet unexplored timescale between ultrafast demagnetization (highly non-equilibrium region) and the precessional motion (close to equilibrium region) of the spin system. In this intermediate time window, the direction of the magnetic moment and angular momentum are transiently separated because of the inertia, which results in additional oscillations, known as nutation, superimposed on the usual precession. Nutation oscillations have higher frequencies but smaller amplitudes and relaxation times as compared to precession. Using ultrafast time-resolved magneto optic Kerr effect (TR-MOKE) experiment, we show that a sudden-incoherent ‘kick’ to the spin system of a ferromagnet by a femtosecond optical pulse does not only trigger the well-known precession (GHz range), but on top, a much faster nutation (sub-THz range). We observe a characteristic nutation frequency of ~ 0.1 THz in Ni80Fe20 (Permalloy) ultrathin films, which have a negligible dependence on magnetic field and film thickness. By comparison with atomistic spin dynamics simulations, we reveal that this experimental observation cannot be explained by the well- known LLG equation, but can be attributed to inertial contributions leading to nutation of the magnetization vector around angular momentum similar to the wobbling motion of a gyroscope. Our results enable us to control the interplay between various magnetic processes - from rapid demagnetization to precession via nutation - within a single experiment, promising exciting avenues for future research and technological advancement.