[Paper Review] A Synthetic Skyrmion Platform with Robust Tunability

Yong Li, Qiyuan Feng+9 more|arXiv (Cornell University)|Jul 29, 2019

Magnetic properties of thin films49 references4 citations

TL;DR

This paper proposes a synthetic skyrmion platform in nanostructured synthetic antiferromagnetic (SAF) multilayers where interfacial Dzyaloshinskii-Moriya interaction (DMI) is replaced by antiferromagnetic interlayer exchange coupling (IEC) for skyrmion nucleation and stabilization. The IEC-driven skyrmions are directly imaged via magnetic force microscopy (MFM), confirmed by magnetometry, magnetoresistance, and micromagnetic simulations, demonstrating robust tunability across temperature (4.5–300 K), device size (400–1200 nm), and lattice designs.

ABSTRACT

Magnetic skyrmions are topologically non-trivial spin structure, and their existence in ferromagnetically coupled multilayers has been reported with disordered arrangement. In these multilayers, the heavy metal spacing layers provide an interfacial Dzyaloshinskii-Moriya interaction (DMI) for stabilizing skyrmions at the expense of interlayer exchanging coupling (IEC). To meet the functional requirement of ordered/designable arrangement, in this work, we proposed and experimentally demonstrated a scenario of skyrmion nucleation using nanostructured synthetic antiferromagnetic (SAF) multilayers. Instead of relying on DMI, the antiferromagnetic IEC in the SAF multilayers fulfills the role of nucleation and stabilization of skyrmions. The IEC induced skyrmions were identified directly imaged with MFM and confirmed by magnetometry and magnetoresistance measurements as well as micromagnetic simulation. Furthermore, the robustness of the proposed skyrmion nucleation scenario was examined against temperature (from 4.5 to 300 K), device size (from 400 to 1200 nm), and different lattice designs. Hence, our results provide a synthetic skyrmion platform meeting the functional needs in magnonic and spintronic applications.

Motivation & Objective

• To develop a functional skyrmion platform with ordered and designable skyrmion arrangements for spintronic and magnonic applications.
• To overcome limitations of DMI-dependent skyrmion systems, which often exhibit disordered arrangements due to DMI's sensitivity and non-uniformity.
• To demonstrate that antiferromagnetic interlayer exchange coupling (IEC) in synthetic antiferromagnetic (SAF) multilayers can effectively nucleate and stabilize skyrmions without relying on DMI.
• To validate the robustness of the IEC-driven skyrmion platform under varying experimental conditions, including temperature, device size, and lattice geometry.

Proposed method

• The study employs nanostructured synthetic antiferromagnetic (SAF) multilayers with alternating ferromagnetic and heavy metal layers to engineer strong antiferromagnetic interlayer exchange coupling (IEC).
• Skyrmion nucleation is driven by IEC rather than interfacial Dzyaloshinskii-Moriya interaction (DMI), enabling tunable and ordered skyrmion formation.
• Magnetic force microscopy (MFM) is used to directly image skyrmion textures in the fabricated devices.
• Magnetometry and magnetoresistance measurements are performed to confirm the presence and topological nature of skyrmions.
• Micromagnetic simulations are conducted to model and validate the observed skyrmion configurations and their stability under varying parameters.
• Systematic variation of device size (400–1200 nm), temperature (4.5–300 K), and lattice design is performed to test robustness.

Experimental results

Research questions

• RQ1 Can interfacial Dzyaloshinskii-Moriya interaction (DMI) be replaced by antiferromagnetic interlayer exchange coupling (IEC) for reliable skyrmion nucleation in synthetic multilayers?
• RQ2 Does the IEC-driven skyrmion platform exhibit robustness against variations in temperature, device size, and lattice geometry?
• RQ3 Can the skyrmion states in the proposed SAF platform be directly imaged and experimentally confirmed using MFM and complementary magnetometric techniques?
• RQ4 To what extent does the IEC-driven mechanism enable ordered and designable skyrmion arrangements compared to DMI-dominated systems?
• RQ5 How do micromagnetic simulations compare with experimental observations in predicting skyrmion stability and configuration in the proposed platform?

Key findings

• Skyrmions were successfully nucleated and stabilized in nanostructured SAF multilayers via antiferromagnetic interlayer exchange coupling (IEC), without reliance on DMI.
• Direct imaging of skyrmions was achieved using magnetic force microscopy (MFM), confirming their existence and spatial arrangement.
• Magnetometry and magnetoresistance measurements provided additional evidence for the topological nature of the observed skyrmion states.
• The IEC-driven skyrmion platform exhibited robust stability across a wide temperature range, from 4.5 K to 300 K.
• The platform maintained skyrmion integrity and tunability across device sizes ranging from 400 nm to 1200 nm and across various lattice designs.
• Micromagnetic simulations corroborated experimental observations, validating the IEC-driven mechanism and predicting stable skyrmion configurations under diverse conditions.