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Voltage Control of Chiral Spin Structures

Periodic Reporting for period 1 - V-ChiralSpin (Voltage Control of Chiral Spin Structures)

Reporting period: 2018-11-01 to 2020-10-31

The goal of V-CHIRALSPIN is to use voltages to manipulate, create, and delete chiral spin structures, i.e. chiral domain wall (DW) spin textures and skyrmions, thus providing the conceptual basis necessary to build a new generation of high density memory technologies with low power consumption. Chiral spin structures allow for fast current-driven spin dynamics and large densities in solid-state devices. The project aims to develop voltage control of chiral spin structures as a powerful strategy towards energy-efficient memory and logic applications.
Combining the fields of electric field control of magnetism and chiral spin structures would enable voltage control of chiral spin structures and thus spintronic devices with reduced power consumption and added functionality by reducing or even eliminating the need for electric currents or magnetic fields. Using voltages to create and delete chiral spin structures or to affect their dynamics offers an energy efficient approach superior to manipulation with currents.
In light of this, the research project aims to develop strain-coupled multiferroic heterostructures to demonstrate voltage control of chiral spin structures (i.e. manipulation, creation, and deletion), determine and quantify the coupling effects, and explore potential device concepts. The project merges two fields of nanomagnetism, functioning as a starting point for a novel research direction. The project is to be explored in detail through three key objectives:
1. Establish suitable multiferroic heterostructures and demonstrate the writing, deleting and tuning of chiral DW spin textures and skyrmions using voltages.
2. Explore the possibility to control DW and skyrmion propagation with the aim to create gates usable in logic devices.
3. Determine and quantify the exact effect of strain transfer that allows for voltage control of chiral spin structures.
So far, two main techniques have been used: Spin Polarised Low Energy Electron Microscopy (SPLEEM), and Micromagnetic Simulations. The project is thus being investigated using both experimental and computational approaches.
SPLEEM allows simultaneous sample growth by MBE, structural characterisation, and high resolution magnetic imaging. It was used to deposit perpendicular magnetised multilayers on ferroelectric BaTiO3 and piezoelectric PMN-PT substrate to obtain multiferroic heterostructures. The magnetisation in these heterostructure was imaged in the same instrument. Deposition on BaTiO3 yielded a suitable multiferroic heterostructure, but the application of an electric field in the SPLEEM turned out to be unachievable, due to shorting in the experimental setup. The outbreak of the COVID-19 pandemic and the associated lock-down measures in California hampered further investigations with SPLEEM.

Micromagnetic Simulations were successfully used to demonstrate the tuning of chiral DW spin textures using voltages. The application of a voltage was simulated as a tuneable magnetic anisotropy. This corresponds to the effect observed in multiferroic heterostructures with piezoelectric substrates. There, interfacial strain transfer and inverse magnetostriction yield a magnetic anisotropy when an electric field is applied. Such a voltage tuneable anisotropy is also obtained in systems exhibiting a so-called voltage controlled magnetic anisotropy (VCMA), which is the direct effect of a voltage on the magnetic anisotropy of a thin film. The figure shows that similar to the effect of a Dzyaloshinskii–Moriya Interaction (DMI), a uniaxial anisotropy can tune between magnetic Bloch and Néel domain walls. The possibility to control magnetic DW propagation with a voltage and its use in logic devices was also demonstrated using Micromagnetic Simulations.

Using SPLEEM and Micromagnetic Simulations, a large part of the first two key objectives of the proposal have been met. The analysis of results is underway, and two publications are being prepared.
The project has demonstrated the possibility to control chiral spin structures using applied voltages. It was also shown that this control can be used as gates in logic devices. These key results were obtained using Micromagnetic Simulations.

By the end of the project, these results are expected to be implemented and demonstrated in an experimental system as well. This will make use of the deposition and characterisation techniques available at the University of Leeds. It is furthermore expected that the results obtained for chiral DW spin textures will be extended to include skyrmion systems. Finally, the third objective, to determine and quantify the exact effect of strain transfer that allows for voltage control of chiral spin structures, will be met before the end of the project.

Information and Communications Technologies (ICT) - enabling new consumer products and driving revolutions in everything from healthcare to cars - are accompanied by an ever-increasing electrical power consumption (associating ICT to a significant carbon footprint), as the demand for ICT grows faster than its energy efficiency. The results of this projects, and the new concepts they establish, open up an avenue for the continued miniaturisation and increase in efficiency of microelectronics that enable the continued growth of ICT.
Tuning between magnetic Bloch and Néel domain walls with DMI or anisotropy.