Materials Science Research Lecture
Abstract:
Magnetic skyrmions are topological spin textures, stabilized due to Dzyaloshinskii-Moriya interactions, that exhibit quasi-particle like behavior and consequently can be directed with low electric currents. The controlled manipulation of magnetic skyrmions at room temperature in thin films is envisioned to enable skyrmion-based low-power information technologies, and consequently has engaged the interest of the scientific community in recent years [1]. Using trilayered heterostructures we have demonstrated how diverging electric charge currents combined with the spin Hall effect in a heavy metal layer can be used to generate and manipulate magnetic Néel skyrmions in an adjacent ferromagnetic layer [2,3]. The process of skyrmion formation, visualized using magneto-optical Kerr effect microscopy, appears stunningly similar to droplet formation in surface-tension driven fluid flow. Under application of homogeneous currents, the motion of magnetic skyrmions is experimentally shown to exhibit transverse motion relative to the current direction, i.e., the skyrmion Hall effect [4]. This effect arises due to the non-trivial topological charge of the skyrmions and is the analogue of the ordinary Hall effect for electrical charges in the presence of a magnetic field. With increasing current density, the skyrmion Hall angle first increases monotonically, which can be linked to the influence of pinning by defects, and then saturates, indicating the flow regime for motion has been reached. From an applications perspective, minimizing the skyrmion size is equally important to controlling the creation and motion of skyrmions. To this end, inversion asymmetric [Pt/FM/X]N multilayers have been investigated, where FM is a ferro- or ferrimagnet and X a transition or rare earth metal, which allows for various competing interactions to be tuned. The skyrmion size varies depending on the choice of metal X, the trilayer repetition number N, and the magnetic field. Finally, artificially ferrimagnetic heterostructures are explored, as they are expected to allow for faster skyrmion velocities without the hinderance of the skyrmion Hall effect.
[1] W. Jiang, et al., Physics Reports 704, 1 (2017).
[2] W. Jiang, et al., Science 349, 283 (2015).
[3] O. Heinonen, et al., Phys. Rev. B 93, 094407 (2016).
[4] W. Jiang, et al., Nature Phys. 13, 162 (2017).
More about the Speaker:
Dr. Suzanne te Velthuis, physicist in the Neutron and X-ray Scattering group of the Materials Science Division at Argonne National Laboratory, has been studying the physics of magnetic thin films and nanostructures throughout her career, focusing on using techniques such as neutron scattering, nuclear magnetic resonance, x-ray magnetic circular dichroism and magneto-optical Kerr microscopy. Dr. te Velthuis started at Argonne in 1999 as a postdoctoral fellow, after obtaining her doctorate degree from Delft University of Technology, which was preceded by a Masters degree in Applied Physics from the Eindhoven University of Technology, both in the Netherlands. She was instrument scientist of the POSY1 polarized neutron reflectometer at IPNS from 2001 until the shutdown of that facility in 2009 and is a world recognized expert in polarized neutron scattering, and reflectivity in particular, which is the most commonly used neutron scattering technique for studying magnetic films. With her work she has contributed to the understanding of exchange bias and exchange coupling, competition of various interactions at the interfaces of complex oxide heterostructures, such as superconducting spin switches, magnetic tunnel junctions, and digitally ordered manganites. In recent years she has additionally made advances in the field of magnetic skyrmions.
She is a Fellow of the American Physical Society (2014) and Neutron Scattering Society of America (2018). Her recent professional activities include serving as Chair of the American Physical Society's Topical Group on Magnetism and its Applications (GMAG), as General Chair for the 2019 Joint Intermag/MMM Conference, and serving as co-Scientific Director of the annual National School on Neutron and X-ray scattering, hosted jointly by Argonne and Oak Ridge National Laboratories (2008-2017).