Kai Litzius
Magnetic skyrmions are topologically stabilized spin configurations that, like domain walls (DWs), can react to external stimuli by collective displacement, which is both physically intriguing and bears promises to realize next generation non-volatile data storage technologies. [1] However, skyrmions in ferromagnets move at an angle with respect to the current direction, which complicates the use of skyrmions in wire devices because the motion component perpendicular to the current can move the skyrmion to a wire edge and thereby annihilate it. [2] Antiferromagnetically coupled systems with compensated angular momentum (such as compensated ferrimagnets and natural antiferromagnets) can reduce this skyrmion Hall effect to zero and could additionally provide high speed dynamics to move spin structures at unprecedented speeds. [3,4] Skyrmions are predicted to move at even higher speeds in these materials, thus making these materials challenging but promising candidates for future spintronic devices.
Besides the compensation of perpendicular motion of skyrmions with respect to the drive, the predictability of their trajectories is also of major importance. Analytical equations of motion describe straight 180° DWs in the one-dimensional (1D) model while rigid, circular bubble domains and skyrmions are predicted to move according to the Thiele equation. [5] However, DWs and skyrmions are often not perfectly straight or circular. Here, we study how strongly deformed DWs and bubble skyrmions move in uncompensated ferrimagnetic Pt/CoGd/W in response to current pulses. We find that all 1D spin textures as well as all fully enclosed spin textures, reach speeds >500 m/s and display identical dynamics. While high speeds are indeed reached, the predicted differences between skyrmion and DW dynamics could not be observed. We attribute this deviation from the commonly used model to significant deformations of the skyrmions during their motion.
[2] W. Jiang et al., Physics Reports 704, 1-49 (2017)
[3] S. Woo et al., Nature Communications 9, 959 (2018)
[4] L. Caretta et al., Nature Nanotechnology 13, 1154 (2018)
[5] F. Büttner, I. Lemesh & G. S. D. Beach, Scientific Reports 8, 4464 (2018)