# Ultrafast Domain Wall Dynamics in Metallic and Insulating Ferrimagnets - Livestream

A promising approach to encode bits of information for next-generation memory and logic is by using solitons, such as chiral domain walls (DW) or topological skyrmions, which can be translated by currentsacross racetrack-like wire devices. One technological and scientific challenge is to stabilize small spin textures and to move them efficiently with high velocities, which is critical for dense, fast memory. However, despite over a decade of research on ferromagnetic materials, current-driven spin texturedynamics faced a “speed limit” of a few hundred m/s, and room-temperature-stable magnetic skyrmions were an order of magnitude too large to be useful in any competitive technologies. These problems were rooted in two fundamental characteristics of ferromagnets: large stray fields, which limit spin texture size (packing density), and precessional dynamics, which limit speed. By using a broader class of multisublattice magnetic materials, namely compensated metallic and insulating ferrimagnets, fundamental limits plaguing ferromagnets can be overcome.

Here, we engineer compensated chiral ferrimagnets with reduced magnetisation and angular momentum, realizing order-of-magnitude improvements in both bit size and speed. In metallic, ferrimagnetic Pt/Gd44Co56/TaOx films with a sizeable Dzyaloshinskii - Moriya interaction (DMI), we realize currentdriven DW motion of 1.3 km s-1 near the angular momentum compensation temperature and roomtemperature- stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature. Moreover, by exploiting reduced angular momentum and low-dissipation in ferrimagnetic insulating garnets, we drive DWs to their relativistic limit using pure spin currents from the spin Hall effect of Pt, achieving record velocities in excess of 4300 m/s, within ~10% of the relativistic limit. We observe key signatures of relativistic, motion including Lorentz contraction and velocity saturation. The experimental results are well-explained through analytical and atomistic modeling. More broadly, these observations provide critical insight into the fundamental limits of the dynamics of magnetic solitons and establish a readily-accessible experimental framework to study relativistic solitonic physics. Technologically, this work shows that high-speed, high-density spintronic devices based on currentdriven spin textures can be realized using materials in which magnetization and angular momentum are compensated.

Speaker: Lucas Caretta, UC Berkeley

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## Tuesday, 09/28/21

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