Exploring spin-orbit interactions induced magnetization dynamics towards device applications

September 14, 2023

Zilu Wang defended his PhD thesis at the department of Applied Physics and Science Education on September 14th.

In the history of science, significant technological advancements usually arise from the convergence of different subbranches of disciplines. For example, the telephone was born from the intersection of acoustics and electricity. By exploring both the magnetic and electronic properties of materials, spintronics has enabled numerous high-performance device applications. It is a crucial technology for overcoming the power consumption bottleneck of electronic devices in the "post-Moore era". In his PhD thesis, Zilu Wang undertook an in-depth exploration of high-performance spintronics devices based on spin-orbitronics.

For his PhD research, Zilu Wang investigates the magnetization dynamics driven by spin-orbit interaction, particularly within the context of synthetic antiferromagnets (SAFs). The findings shed light on the next generation of high-performance spin-orbitronics based memory and oscillator devices.

Spin-orbit torque

First, Wang explored a basic phenomenon in spin orbitronics, namely spin-orbit torque (SOT) effect. Conventionally, to promote the performance of SOT devices, most efforts have been devoted to enhance the damping-like component of SOT.

Recently, some studies noticed that the other component of SOT, namely field-like torque, also plays a crucial role in the nanosecond-timescale SOT dynamics. However, there is not yet an effective way to tune its relative amplitude. Wang experimentally investigated the tunning of the field-like spin-orbit torque in W/CoFeB/MgO trilayers by tuning the interfacial spin accumulation. The results shows a new path to further improve the performance of SOT-based ultrafast magnetic devices.

Novel materials

Next, Wang explored novel materials for spintronics devices. As the process node moves on, conventional ferromagnet-based spintronic devices meet the superparamagnetic size limit. Under such circumstance, perpendicular synthetic antiferromagnets (p-SAFs) with high scalability, strong immunity to external field and ultrafast dynamics, become a competitive choice for the next generation spintronics memory and logic devices.

However, to efficiently operate their magnetic order by current-induced SOTs, an unfavored high external magnetic field is conventionally required to break the symmetry. Wang and his collaborators achieved the field-free SOT switching of a p-SAF through the introduction of interlayer Dzyaloshinskii–Moriya interaction (DMI).

He observed the existence of interlayer DMI in our SAF sample by an azimuthal angular dependent anomalous Hall measurement. Deterministic field-free switching is accomplished in such a sample and depicted by macro-spin and micromagnetic simulations. Results from the thesis provide a new strategy for SAF based high performance SOT devices.

Single-domain model

Based on the study of the ultrafast dynamics of p-SAF driven by SOT, Wang designed and simulated a single-domain model of a spin Hall nano-oscillator (SHNO) device. The proposed device is compatible with the magnetic tunnel junction (MTJ) device structure, allowing for higher scalability and readability of the proposed oscillator device.

The observed oscillation exhibits a significant perpendicular component, resulting in an electric output signal with a high signal-to-noise ratio when utilizing a perpendicular reference layer in the MTJ.

Wang’s research demonstrates the feasibility of constructing a high-performance oscillator based on SOT devices.

Title of PhD thesis: Exploring spin-orbit interactions induced magnetization dynamics towards device applications. Supervisors: Bert Koopmans, Weisheng Zhao (external), and Reinoud Lavrijssen.