arXiv subjects

Akira Lentfert

Publications and source records attributed to Akira Lentfert.

7 recordsLinked to original sources

arXiv2026

Phase-dependent parametric amplification of propagating spin waves in YIG nanostructures enabled by local inhomogeneities

As magnonics evolves towards non-conventional computing, the development of phase-conserving and phase-sensitive amplification mechanisms becomes increasingly important. A particularly promising approach is non-adiabatic parametric amplification. In this work, the influence of local inhomogeneities on the parallel parametric amplification of spin waves in nano-scale Yttrium Iron Garnet waveguides is investigated. Micromagnetic simulations reveal that in larger pump regions, where only adiabatic amplification is expected, scattering centers provide additional linear momentum that enables non-adiabatic amplification of propagating spin waves. Importantly, the coherence of the process remains unaffected by the scattering and the generation of co-propagating spin waves enhance the effective amplification. Our simulations are confirmed by micro-focused Brillouin light scattering spectroscopy experiments, reproducing both the phase-dependent behavior and the characteristic features of the time-resolved dynamics. These findings demonstrate the flexibility of the parametric amplification process and provide a key mechanism for the development of large-scale spin-wave computing circuits.

cond-mat.other↗

arXiv2025

Dynamic interfacial effects in ultrathin ferromagnetic bilayers

We investigate the magnetization dynamics of an ultrathin Co (1.5 nm) /Py (1.5 nm) bilayer system from femtosecond (fs) to nanosecond (ns) timescales. Magnetization dynamics in the fs timescales is characterized as a highly non-equilibrium regime due to an ultrafast reduction of magnetization by laser excitation. On the other hand, the dynamics in the ns timescales is characterized as a close-to-equilibrium regime involving the excitation of coherent magnons. We demonstrate that the interfacial interaction between the Co and Py layers in these two non-equilibrium regimes across the timescales is dynamic and simultaneously influences the magnetization loss in the fs timescales and the magnon dynamics in the ns timescales. On ultrafast (fs) timescales, comparison between time-resolved magneto-optical Kerr effect (TR-MOKE) measurements and temperature-based {\mu}T model simulations reveals that the bilayer exhibits demagnetization dynamics intermediate between those of its individual layers. When driven far from equilibrium by ultrashort laser pulse excitation, the magnetization dynamics of the individual Co and Py layers appear to remain decoupled and evolve independently in the initial stages of the ultrafast response. On the other hand, in the ns regime, the two individual layers of the bilayer precess together at the same frequency in a coupled manner as one effective single layer. Furthermore, by correlating the ultrafast demagnetization to precessional damping we attempt to bridge the two non-equilibrium regimes across fs to ns timescales. These results improve our understanding of magnetization dynamics across timescales in ultrathin exchanged-coupled ferromagnetic bilayers and provide valuable insights for the design of high-frequency and energy efficient spintronic device concepts.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic nutation: transient separation of magnetization from its angular momentum

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.

cond-mat.mes-hall↗

arXiv2023

Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient spintronic devices. Here, we experimentally investigate the coherent spin dynamics after ultrashort laser excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin Ni80Fe20 films. We provide a detailed study of the magnetic field and pump fluence dependence of the coherent precessional dynamics. We show that the coherent precession lifetime increases with the applied external magnetic field which cannot be understood by viscous Gilbert damping of the coherent magnons. Instead, it can be explained by nonlinear magnon interactions and by the change in the fraction of incoherent magnons. This interpretation is in agreement with the observed trends of the coherent magnon amplitude and lifetime as a function of the exciting laser fluence. Our results provide a new insight into the magnetization relaxation processes in ferromagnetic thin films, which is of great importance for further spintronic applications.

cond-mat.mes-hall↗

arXiv2023

Coherent magnetization dynamics in strongly quenched Ni thin films

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the strength of demagnetization affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect (tr-MOKE) technique. The largest coherent oscillation amplitude was observed for strongly quenched systems, showing the conservation of coherency for demagnetizations of up to 90%. Moreover, the phase of the excited spin-waves increases with pump power, indicating a delayed start of the precession during the remagnetization.

cond-mat.mtrl-sci↗

arXiv2022

Parametric generation of spin waves in nano-scaled magnonic conduits

The research feld of magnonics proposes a low-energy wave-logic computation technology based on spin waves to complement the established CMOS technology and provide a basis for emerging unconventional computation architectures. However, magnetic damping is a limiting factor for all-magnonic logic circuits and multi-device networks, ultimately rendering mechanisms to effciently manipulate and amplify spin waves a necessity. In this regard, parallel pumping is a versatile tool since it allows to selectively generate and amplify spin waves. While extensively studied in microscopic systems, nano-scaled systems are lacking investigation to assess the feasibility and potential future use of parallel pumping in magnonics. Here, we investigate a longitudinally magnetized 100 nm-wide magnonic nano-conduit using space and time-resolved micro-focused Brillouin-light-scattering spectroscopy. Employing parallel pumping to generate spin waves, we observe that the non-resonant excitation of dipolar spin waves is favored over the resonant excitation of short wavelength exchange spin waves. In addition, we utilize this technique to access the effective spin-wave relaxation time of an individual nano-conduit, observing a large relaxation time up to (115.0 +- 7.6) ns. Despite the significant decrease of the pumping effciency in the investigated nano-conduit, a reasonably small threshold is found rendering parallel pumping feasible on the nano-scale.

cond-mat.mes-hall↗

arXiv2021

Long-range spin-wave propagation in transversely magnetized nano-scaled conduits

Magnonics attracts increasing attention in the view of novel low-energy computation technologies based on spin waves. Recently, spin-wave propagation in longitudinally magnetized nano-scaled spin-wave conduits was demonstrated, proving the fundamental feasibility of magnonics at the sub-100 nm scale. Transversely magnetized nano-conduits, which are of great interest in this regard as they offer a large group velocity and a potentially chirality-based protected transport of energy, have not yet been investigated due to their complex internal magnetic field distribution. Here, we present a study of propagating spin waves in a transversely magnetized nanoscopic yttrium iron garnet conduit of 50 nm width. Space and time-resolved micro-focused Brillouin-light-scattering spectroscopy is employed to measure the spin-wave group velocity and decay length. A long-range spin-wave propagation is observed with a decay length of up to (8.0+-1.5) μm and a large spin-wave lifetime of up to (44.7+-9.1) ns. The results are supported with micromagnetic simulations, revealing a single-mode dispersion relation in contrast to the common formation of localized edge modes for microscopic systems. Furthermore, a frequency non-reciprocity for counter-propagating spin waves is observed in the simulations and the experiment, caused by the trapezoidal cross-section of the structure. The revealed long-distance spin-wave propagation on the nanoscale is particularly interesting for an application in spin-wave devices, allowing for long-distance transport of information in magnonic circuits, as well as novel low-energy device architectures.

cond-mat.mes-hall↗