Ultrafast Domain Wall Dynamics in Metallic and Insulating Ferrimagnets INVITED

Abstract

A promising approach is 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 currents across racetrack-like wire devices1,2. 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 texture dynamics 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 size3 (packing density), and precessional dynamics, which limit speed4. By using a broader class of multi-sublattice 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 speed5. By using ferrimagnetic Pt/Gd44Co56/TaOx films with a sizeable Dzyaloshinskii–Moriya interaction (DMI), we realize a current-driven DW motion with a speed of 1.3 km s–1 near the angular momentum compensation temperature and room-temperature-stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature (Figure 1). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. In addition to metallic systems, a broader, ubiquitous class of materials – ferrimagnetic insulating garnets – have been extensively studied for their technologically-desired magnetooptical and spintronic properties. Their low damping and ferrimagnetic dynamics also make them ripe candidates for ultrafast DW motion. Although spin-orbit effects and spin-transport phenomenon from adjacent heavy metal layers have been used to manipulate the magnetisation in this class of materials6, the DMI and chiral spin textures had not been discovered. Here, we discover chiral magnetism that allows for pure spin-current-driven chiral DW motion in iron garnet films and elucidate the origins of the chiral exchange interaction in these films. Moreover, by exploiting reduced angular momentum and low-dissipation in ferrimagnetic insulators, 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 (Figure 2). 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 current-driven spin textures can be realized using materials in which magnetisation and angular momentum are compensated.