果冻传媒

In recent years the concept of chirality has entered the spintronics community, which refers to the preferred rotational sense of the magnetization in spin textures, such as domain walls, skyrmions or spin spirals.

Dzyaloshinskii-Moriya Interaction (DMI), as the key origin of chiral magnetism, is widely studied in the fields of magnetism and spintronics. Short-range chiral magnetic interaction, i.e. interfacial DMI (IF-DMI), plays a key role in spin orbit torque (SOT) driven domain wall movement and an important origin of the formation of chiral structures such as chiral magnetic domain walls and skyrmions.

On the otherhand, long-range chiral magnetic interaction, i.e. interlayer DMI (IL-DMI), is crucial in assisting SOT to achieve field-free perpendicular magnetic anisotropy (PMA) film switching and constructing three-dimensional (3D) topological magnetic structure such as Hopfion. The study of short- and long-range chiral exchange interactions in spintronics devices is particularly important at this time.

Chiral magnetic exchange

For his PhD research, sought to improve the current research on chiral magnetic exchange interactions of both IF-DMI and IL-DMI, and provide a new reference direction for the new application of spintronics.

For IF-DMI, Li and his colleagues proposed a method for coherent domain wall motion (DWM), which is driven by chirality switching (CS) and an ultralow SOT current.

The CS, as the driving force of DWM, is achieved by the sign change of IF-DMI which is further induced by a ferroelectric switching voltage. The SOT is used to break the symmetry when the magnetic moment is rotated to the Bloch direction.

He numerically investigated the underlying principle and the effect of key parameters on the DWM through micromagnetic simulations. Under the CS mechanism, a fast (~100 m/s), ultralow-energy (~5 attoJoule), and precisely discretized DWM was achieved.

Based on the existing simulation results, Li conducted experimental research on DMI modulation. He used PMNPT to generate strain on the upper Pt/Co/Pt film under an applied voltage to explore the changes in its IF-DMI. The research results prove that under the tensile or compressive strain of PMNPT, the IF-DMI presents a butterfly shape similar to the strain changing curve. This provides a reference direction for subsequent strain control of IF-DMI and CS.

Synthetic ferrimagnet

For IL-DMI, Li first studied the IL-DMI in synthetic ferrimagnet (SFi) structure. He exploited interlayer chiral exchange bias fields in SFi to address both the sign and magnitude of the IL-DMI.

IL-DMI behaved as an effective field with chirality and unidirectionality in the switching process. Then, he studied the influence of IL-DMI on the SOT switching process through transient and steady-state detection methods.

In addition to field-free switching, he found that the combination of IL-DMI and SOT can give rise to multi-resistance states, which provides a possible direction for the future design of neuromorphic devices based on SOT.

Step forwards

This work is a step towards characterizing and understanding the IL-DMI for spintronic applications.

However, even so, the origin of the in-plane symmetry breaking of IL-DMI in our sample is still unclear.

Next, Li studied the origin of the in-plane symmetry breaking of IL-DMI. Based on laboratory conditions, he constructed three different Hall bar devices with oblique sputtering, wedge sputtering and normal sputtering method and we detected the IL-DMI effective fields.

This provides a reference study for the origin of in-plane symmetry breaking of IL-DMI.

Title of PhD thesis: . Supervisors: Reinoud Lavrijsen, Xiaoyang Lin, and Bert Koopmans.