SPICE Workshop on Nanomagnetism in 3D, April 30th - May 2nd 2024
Olivier Fruchart
Many theoretical and micromagnetic simulation reports over the past 20 years have considered domain walls and their motion in cylindrical magnetic nanowires [1]. A specific aspect in them is the existence of a unique domain wall, the Bloch-point wall (BPW). The BPW consists of a Bloch point surrounded by azimuthal curling of magnetization at the wire periphery, whose velocity is predicted to reach around 1km/s. Experiments have confirmed the existence of this specific domain wall and speed of several hundred m/s under the application of an electric current. In parallel, more elaborate situations are being explored in theory, simulation and experiments, such as simple or core-shell tubes, wires modulated in diameter, or in chemical composition, etc. In all these systems, magnetization states displaying azimuthal curling of magnetization are quite common.
In three-dimensional objects such as nanowires, the application of a (spin-polarized) charge current to achieve spin-transfer effects comes with a sizeable Œrsted field, which directly couples to the variety of above-mentioned magnetization textures displaying azimuthal magnetization. As the strength of the Œrsted field grows linearly with the diameter of the structure (or more precisely, the square root of its cross-sectional area), its impact on magnetization dynamics may dominate over that of spin-transfer effects. In this presentation, I will present two such situations, based on time-resolved (TR) experimental magnetic imaging and micromagnetic simulations. The TR imaging is performed using Scanning Transmission X-ray Microscopy (STXM), revealing their magnetization dynamics with a 50 ps time resolution. The spin texture remains very sharp during the billions-averaged TR series, indicating very reproducible processes.
The first situation considers nanowires made of uniform Permalloy Fe20Ni80, in which we investigate in detail the switching mechanism of circulation (azimuthal curling) of BPWs, a phenomenon previously evidenced but with no hint about its magnetization dynamics [2]. The process consists of the fast shrinking of the BPW, followed by the actual switching mechanism, with a total time around 1 ns or more. Micromagnetic simulations show that we evaluate the phenomenological damping parameter from the former, while the latter involves the nucleation, motion and annihilation of objects such as surface vortices and antivortices, and volumic Bloch points. The threshold current for switching is around 5×10^11 A/m^2, similar to pre-existing knowledge, and a critical slowing-down is evidenced when the critical current is approached.
The second situation considers Permalloy Fe20Ni80 nanowires with micrometers-long segments separated by the insertion of Fe80Ni20 platelets of thickness a few tens of nanometers. Magnetostatic energy induces the occurrence of curling magnetization at the chemical modulations, whose circulation has been shown to be switchable using 1-10 ns electrical current pulses, driven by the associated Œrsted field [3]. The threshold current for switching is around 5×10^11 A/m^2. Above typically 7×10^11 A/m^2 the circulation switching occurs in less than 200 ps. This is similar to the rise time of the current pulse at the sample. This points at an ultrafast swiching mechanism, which we explain by the strength of Œrsted field and dynamic dipolar field in nanowires. When approaching the current threshold the switching time tends again to diverge, consistent with expectations from theory and simulations.
The quantitative comparison with micromagnetic simulations and analytical models is underway, with a view to provide a comprehensive picture of these phenomena, which we believe should be found in other curvilinear systems.
Acknowledgements
This work was partially supported by the French RENATECH network, implemented at the Upstream Technological Platform in Grenoble PTA (ANR-22-PEEL-0015). We also aknowledge the team of the Nanofab platform (CNRS Institut Néel).
References
Magnetic nanowires and nanotubes, M. Stano, O. Fruchart, in Handbook of magnetic materials 27, E. Brück Ed., Elsevier (2018)
Fast domain wall motion governed by topology and OErsted fields in cylindrical magnetic nanowires, M. Schöbitz, A. de Riz, S. Martin, S. Bochmann, C. Thirion, J. Vogel, M. Foerster, L. Aballe, T. O. Menteş, A. Locatelli, F. Genuzio, S. Le Denmat, L. Cagnon, J.-C. Toussaint, D. Gusakova, J. Bachmann, O. Fruchart, Phys. Rev. Lett., 123 (21), 217201 (2019)
Micromagnetics of magnetic chemical modulations in soft-magnetic cylindrical nanowires, L. Álvaro-Gómez, S. Ruiz-Gómez, C. Fernández-González, M. Schöbitz, N. Mille, J. Hurst, D. Tiwari, A. de Riz, I. M. Andersen, J. Bachmann, L. Cagnon, M. Foerster, L. Aballe, R. Belkhou, J. C Toussaint, C. Thirion, A. Masseboeuf, D Gusakova, L. Pérez, O. Fruchart, Phys. Rev. B 106 (2022) 054433.