Current-Induced Domain Wall Motion in a Compensated Ferrimagnet

Saima A. Siddiqui, Jiahao Han, Joseph T. Finley, Caroline A. Ross, and Luqiao Liu

Phys. Rev. Lett. 121, 057701 – Published 30 July 2018

Abstract

Owing to the difficulty in detecting and manipulating the magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. By employing heavy-metal–rare-earth–transition-metal alloy bilayers, we experimentally study current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum at the angular momentum compensation point. With experiment and modeling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy-metal–ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.

Received 27 March 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.057701

Physics Subject Headings (PhySH)

Domain walls Spintronics

Ferrimagnets

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Saima A. Siddiqui¹, Jiahao Han¹, Joseph T. Finley¹, Caroline A. Ross², and Luqiao Liu¹

¹Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
²Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

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Vol. 121, Iss. 5 — 3 August 2018

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Images

Figure 1
(a) Saturation magnetization and coercive fields of $Pt / {Co}_{1 - x} {Tb}_{x}$ thin films from vibrating sample magnetometry. (b) Schematics of the device geometry and electrical setup for Hall resistance measurement. (c) Anomalous Hall resistance of $4 - μ m$ -wide $Pt / {Co}_{1 - x} {Tb}_{x}$ Hall bars. The coercive fields of the patterned structures differ from that of the continuous films due to domain nucleation and pinning processes at the wire edges.
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Figure 2
(a) Electrical setup for the domain wall motion measurement. The yellow and green regions represent $↓$ and $↑$ domains in the MOKE microscope image of $4 - μ m$ -wide ${Co}_{0.69} {Tb}_{0.31}$ wire. (b) Domain wall motion in ${Co}_{0.69} {Tb}_{0.31}$ wire with consecutive current pulses. Blue and red dotted lines show the initial positions of $↓ ↑$ and $↑ ↓$ domain walls in the top MOKE image, respectively. (c), Domain wall velocity as a function of current density for $Pt / {Co}_{1 - x} {Tb}_{x}$ at $x = 0.17$ , 0.21, 0.26, 0.31, 0.33, 0.38, and 0.41 (from bottom to top panel). The error bars reflect standard deviations from multiple measurements. (d) Domain wall mobility extracted from the dotted lines in (c) for $Pt / {Co}_{1 - x} {Tb}_{x}$ . (e) The calculated current-induced domain wall velocity for a series of ferrimagnetic samples with different net angular momenta, $S_{eff}$ .
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Figure 3
Domain wall velocity as a function of current density at different longitudinal fields along the length of the $Pt / {Co}_{0.79} {Tb}_{0.21}$ sample for (a) $↓ ↑$ and (b) $↑ ↓$ domain walls. Schematic illustration of the domain wall texture for $↓ ↑$ and $↑ ↓$ Néel domain walls in (c) magnetically Co-dominant and (d) magnetically Tb-dominant samples. Blue and green arrows represent the magnetizations from Co and Tb sublattices, respectively. The chirality of the $↓ ↑$ and $↑ ↓$ domain walls remains the same as the composition changes from the Co-dominant to the Tb-dominant side. The effective fields from Slonczewski-like torque ( $H_{SL}$ ) on the Co and Tb sublattices are shown by yellow and brown arrows, respectively. It can be seen that $H_{SL}$ on the two sublattices works constructively to move domain walls. The domain wall motion direction remains the same in both samples. The black $↓$ and $↑$ arrows show the domain orientation as detected in the MOKE measurement. The length of the blue and green arrows below the domain wall region reflects the influence of external field $H_{x}$ on domain wall chirality. Domain wall velocity as a function of the in-plane field for samples of the (e) $Pt / {Co}_{0.79} {Tb}_{0.21}$ , (f) $Pt / {Co}_{0.67} {Tb}_{0.33}$ , and (g) $Pt / {Co}_{0.59} {Tb}_{0.41}$ wires, respectively. Red squares (negative current) and red circles (positive current) represent $↓ ↑$ domain walls and blue triangles (negative current) and blue stars (positive current) represent $↑ ↓$ domain walls, respectively. Red solid lines and blue dashed lines are the linear fit of the experimental data for $↓ ↑$ and $↑ ↓$ domain walls. There is a sign reversal in the slopes of the red and blue lines among (e), (f), and (g).
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