Source-linked AI summary
Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory
Xichao Zhang, G. P. Zhao, Hans Fangohr, J. Ping Liu, W. X. Xia, J. Xia, F. J. Morvan
TL;DR
Skyrmion-based racetrack memory requires reliable bit spacing and a way to prevent skyrmions from clogging at the racetrack end. The paper numerically investigates skyrmion repulsions and shows that a notch-tip can enable bits beyond the reading element to exit at low current density.
Problem
The study addresses how skyrmion-skyrmion and skyrmion-edge repulsions affect reliable bit spacing and clogging in skyrmion-based racetrack memory.
Method
The paper uses numerical simulations incorporating applied and magnetostatic self-interaction fields to investigate skyrmion behavior in racetracks.
Results
A notch-tip enables skyrmionic bits beyond the reading element to exit the racetrack at low current density (~ 10^10 A/m2).
Takeaways & Limitations
The results identify notch-tips as a practical way to avoid skyrmion clogging in skyrmion-based racetrack memory.
Takeaways & Limitations
The simulations indicate that skyrmionic bits are not stable on the racetrack in the examined condition.
Abstract
from arXiv · showhide
Magnetic skyrmions are promising for building next-generation magnetic memories and spintronic devices due to their stability, small size and the extremely low currents needed to move them. In particular, skyrmion-based racetrack memory is attractive for information technology, where skyrmions are used to store information as data bits instead of traditional domain walls. Here we numerically demonstrate the impacts of skyrmion-skyrmion and skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory. The reliable and practicable spacing between consecutive skyrmionic bits on the racetrack as well as the ability to adjust it are investigated. Clogging of skyrmionic bits is found at the end of the racetrack, leading to the reduction of skyrmion size. Further, we demonstrate an effective and simple method to avoid the clogging of skyrmionic bits, which ensures the elimination of skyrmionic bits beyond the reading element. Our results give guidance for the design and development of future skyrmion-based racetrack memory.
Results
Skyrmion repulsions establish reliable bit spacings and depend on material and racetrack parameters. At the track end, edge and skyrmion interactions cause clogging and compression, which can be avoided using a notch-tip elimination method without increasing current density.
- Skyrmion-skyrmion repulsion: For di = 30 nm, d increases from 30 nm to 52 nm within 1 ns and reaches 58 nm at 10 ns, while rs grows from 5 nm to ~ 11 nm.The corresponding repulsion force falls from ~ 10-7 N/m to ~ 10-10 N/m within 0.5 ns, then to ~ 10-12 N/m at 10 ns.
- Parameter dependence: Increasing PMA from 0.7 MJ/m3 to 0.9 MJ/m3 decreases de from 64 nm to 52 nm and relaxed rs from 14 nm to 8 nm.Both de and relaxed rs decrease approximately linearly with increasing PMA; above a critical PMA or applied field, skyrmions become unstable.
- Clogging mitigation: A notch-tip eliminates moving skyrmionic bits when they touch it, preventing chain compression so all skyrmions move coherently without increasing current density.The bit is attracted by magnetization tilts, evolves into a domain wall pair at the notch edge, and is cleared by the currents.
Discussion
Skyrmion–skyrmion and skyrmion–edge repulsions determine reliable bit spacing, density, and motion in skyrmion-based racetrack memory. A notch-tip enables bits beyond the reading element to exit easily, avoiding clogging, compression, and congestion.
- Bit spacing: A reliable initial spacing prevents mutual repulsion from affecting skyrmionic bits and thereby protects storage density, processing speed, and data robustness.The spacing must be sufficiently large to avoid bit interference and write/read errors.
- Parameter tuning: Both equilibrium distance de and relaxed skyrmion size rs decrease approximately linearly with increasing PMA and applied field, while also changing with racetrack width.Increasing track width generally increases de and rs; within the stable range, narrower tracks offer higher-density fabrication possibilities.
- Repulsion effects: ~ 60 nm is the spacing threshold above which the skyrmionic bit chain is not influenced by repulsions and can move as a coherent unit when Fss ≈ 10-12 N/m.Below this spacing, skyrmion–skyrmion repulsion can influence the chain motion.
- Clogging: The skyrmionic bit chain clogs at the end of a normal racetrack and skyrmion size reduces because of skyrmion–edge and skyrmion–skyrmion repulsions.The clogging occurs at the racetrack end and produces compression of the skyrmionic bits.
- Notch-tip solution: A notch-tip lets skyrmionic bits beyond the reading element exit at low current density (~ 1010 A/m2), effectively avoiding clogging, compression, and congestion.The notch-tip’s more in-plane magnetization facilitates reversal of the skyrmion boundary and core during exit.
Methods
The study uses OOMMF-based micromagnetic simulations to model skyrmion dynamics in cobalt racetracks, with LLG equations extended for CPP spin-polarized currents. Simulations incorporate exchange, anisotropy, applied, magnetostatic, and DMI energies using specified material parameters and discretizations.
- Simulation setup: Micromagnetic simulations use OOMMF with a DMI extension to model skyrmion motion in cobalt racetracks.The racetracks are 0.4-nm thick, 30 ~ 100 nm wide, and 200 ~ 800 nm long.
- Magnetization dynamics: Magnetization dynamics without spin-transfer torque are governed by the Landau-Lifshitz-Gilbert ordinary differential equation.The effective field derives from exchange, anisotropy, applied, magnetostatic, and DMI contributions.
- Parameters and discretization: The simulations use MS = 580 kA/m, A = 15 pJ/m, D = 3 mJ/m2, K = 0.8 MJ/m3, and α = 0.3.Normal racetracks use 1 × 1 × 0.4 nm3 discretization, whereas notched racetracks use 0.5 × 0.5 × 0.4 nm3 for numerical accuracy.
Acknowledegments
The acknowledgments thank external collaborators for suggestions and discussions, identify author contributions, report funding sources, and declare no competing financial interests.
- Acknowledgments: The authors thank collaborators from CNRS/Thales, Université Paris Sud, Shinshu University, and CAS for suggestions and discussions.The discussions concerned CPP and DMI.
- Acknowledgments: The work was supported by NSFC grants, the Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan, and SSRIP of SICNU.The listed NSFC grants are No. 11074179 and No. 10747007; the Sichuan program is No. 12TD008.
- Author contributions: G.P.Z., H.F., and X.C.Z. conceived and coordinated the project, while X.C.Z. performed micromagnetic simulations and analyzed the data.These contributions are explicitly assigned in the author-contribution statement.
- Author contributions: X.C.Z., G.P.Z., H.F., J.P.L., and W.X.X. interpreted the results, and X.C.Z., G.P.Z., H.F., J.X., and F.J.M. prepared the manuscript.All authors commented on the manuscript.
- Competing financial interests: The authors declare no competing financial interests.This statement appears under the paper’s competing-financial-interests disclosure.
How to cite this article
The article is cited as Zhang, X. C. et al., “Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory,” published in Scientific Reports in 2015.
- The authors are cited as Zhang, X. C. et al.
- The article title is “Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory.”
- The citation is Sci. Rep. 5, 7643, with DOI 10.1038/srep07643, published in 2015.