Recent progress in material science has enabled the first experimental studies concerning the static magnetization characterization of samples with tubular geometry, such as rolled-up structures, nanowires and nanotubes. The bending of a flat thin-film to a curved surface introduces a break of the local inversion symmetry and can lead to surprising phenomena which do not unfold in flat geometries. As an example, domain walls in nanotubes show a chiral symmetry breaking regarding their propagation and moreover can propagate so fast that can trigger a Cherenkov-type spin-wave radiation. A further example is that this lack of inversion symmetry leads to an asymmetric dispersion relation for the spin waves regarding the sign of the propagation vector. This is a purely curvature induced effect with its origin in the classical dipole-dipole interaction.
Using finite element micromagnetic simulations with our in-house developed program TetraMag (originally developed by Dr. Riccardo Hertel), we study the effect of the surface curvature on the statics and dynamics of typical spin textures as well as on the spin-wave propagation. The usual spin textures are domain walls, vorticies, Bloch points and Skyrmions formed in ferromagnetic nanowires and nanotubes. Although investigating spin-wave and domain-wall dynamics is still a challenge from experimental point of view, a combined effort of the experimental groups in our devision helps to confirm the results predicted by simulations.
Besides the basic research we develop the GPU accelerated micromagnetic codes used in our studies for the finite discretisation as well as for the finite element methods and all related programs required for the analysis of the simulations.
- Spin textures in curved geometries
- Spin-wave propagation in curved nano-membranes
- GPU accelerated micromagnetic code development
- “Multiplet of Skyrmion States on a Curvilinear Defect: Reconfigurable Skyrmion Lattices”, V. P. Kravchuk, D. D. Sheka, A. Kákay, Oleksii M. Volkov, U. K. Rößler, J. van den Brink, D. Makarov, Y. Gaididei, Physical Review Letters 120, 067201 (2018).
- "Frequency linewidth and decay length of spin waves in curved magnetic membranes", J.A. Otalorá, A. Kákay, J. Lindner, H. Schultheiss, A. Thomas, J. Fassbender, K. Nielsh, Phys. Rev. B 98, 014403 (2018).
- ”Curvature-induced asymmetric spin-wave dispersion”, J.A. Otalorá, M. Yan, H. Schultheiss, R. Hertel, A. Kákay, Phys. Rev. Lett. 117, 227203 (2016).
- ”Asymmetric spin-wave dispersion in ferromagnetic nanotubes induced by surface curvature”, J.A. Otalorá, M. Yan, H. Schultheiss, R. Hertel, A. Kákay, Phys. Rev. B. 95, 184415 (2017).
- “Spin-torque-induced dynamics at fine-split frequencies in nano-oscillators with two stacked vortices”, V. Sluka, A. Kákay, A.M. Deac, D.E. Bürgler, C.M. Schneider, R. Hertel, Nat. Commun. 6:6409, doi: 10.1038/ncomms7409 (2015).
- ”Multiscale and multimodel simulation of Bloch-point dynamics”, C. Andreas, A. Kákay, R. Hertel, Phys. Rev. B., 89 (2014).
- ”Spin-Cherenkov effect and magnonic Mach cones”, M. Yan, A. Kákay, C. Andreas, R. Hertel, Phys. Rev. B. 88, 220412(R) (2013).
- ”Chiral symmetry breaking and pair-creation mediated Walker breakdown in magnetic nanotubes”, M. Yan, C. Andreas, A. Kákay, F. Garcia- Sanchez, R. Hertel, Appl. Phys. Lett. 100, 252401 (2012).
- ”Fast domain wall dynamics in magnetic nanotubes: Suppression of Walker breakdown and Cherenkov-like spin wave emission”, M. Yan, C. Andreas, A. Kákay, F. Garcia-Sanchez, R. Hertel, Appl. Phys. Lett. 99, 122505 (2011).
- ”Speedup of FEM Micromagnetic Simulations With Graphical Processing Units”, A. Kákay, E. Westphal, R. Hertel, IEEE Trans. Magn. 46, 2303 (2010).