Fast Skyrmion Dynamics in Ferrimagnetic Thin Films

Yassine Quessab, New York University

March 4, 2022

The development of efficient artificial intelligence technologies highly depends on the resolution of a memory bottleneck – that is, to achieve high memory bandwidth and low latency. Skyrmion racetrack memories are a promising candidate for a fast encoding of bits. While small skyrmions (~10-200 nm) have been observed at room temperature in ultrathin films, a challenge that has yet to be addressed is to move them at high speeds. It is also important to reduce the skyrmion Hall effect that results in a transverse deflection of the skyrmion towards the edge of the racetrack, which can cause information loss.

Many skyrmion studies have been carried out in ferromagnetic thin films. However, the maximum velocity of skyrmion motion induced by spin-orbit torques (SOTs) did not exceed 100 m s-1 [1,2]. In addition, in these films, the skyrmion dynamics is characterized by a large skyrmion Hall effect with a skyrmion Hall angle of 30º - 50º. In this talk, I will show that nearly compensated ferrimagnetic CoGd thin films can host fast isolated skyrmions with a reduced skyrmion Hall angle.

First, I will discuss the key parameters for the nucleation of skyrmions with high mobility and how for that purpose we engineered and optimized our CoGd thin films with large SOTs, low saturation magnetization and net spin density at room temperature [3,4]. Second, by studying the current-induced domain wall dynamics, I will show that our CoGd racetracks have a very low pinning and density of natural defects making them ideal to study skyrmion motion. Finally, I will present the formation and manipulation of skyrmions induced by nanosecond current pulses and studied by magneto-optical Kerr effect and scanning transmission x-ray microscopy [5]. In our experiments, we realized field free nucleation of skyrmions using short current pulses. In addition, I will show evidence that the nucleation is a thermal process. We also achieved fast skyrmion motion induced by SOTs with velocities as high as 610 m s-1. These are the highest skyrmion velocities reported thus far. We measured a skyrmion Hall angle of less than 3º, much smaller than in ferromagnetic thin films.

One of our findings is that there is a tradeoff between achieving fast and energy-efficient dynamics and nucleating skyrmions with a small diameter. Beyond memory applications, our experimental results could pave the way for fast new computing technologies based on skyrmions using ferrimagnetic materials [6].

References

[1] Litzius, K. et al., Nature Electronics 3, 30 (2020)
[2] Juge, R. et al., Physics Review Applied 12, 044007 (2019)
[3] Quessab, Y. et al., Scientific Reports 10, 7447 (2020)
[4] Quessab, Y. et al., Advanced Science 8, 2100481 (2021)
[5] Quessab, Y. et al., Manuscript under consideration (2022)
[6] Vakili, H et al., Journal of Applied Physics 130, 070908 (2021).