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Voltage Control of Chiral Spin Structures

Periodic Reporting for period 2 - V-ChiralSpin (Voltage Control of Chiral Spin Structures)

Reporting period: 2020-11-01 to 2021-10-31

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. Determine and quantify the exact effect of strain transfer that allows for voltage control of chiral spin structures.

3. Explore the possibility to control DW and skyrmion propagation with the aim to create gates usable in logic devices.

Combining thin film deposition techniques (Molecular Beam Epitaxy and Sputtering) with magnetic microscopy (Spin Polarised Low Energy Electron Microscopy and Magneto-Optical Kerr Effect Microscopy) with Micromagnetic Simulations and analytical modelling all three objectives have been met. The conclusions of the action are that (1) domain wall types (Bloch vs Neel) may be modulated by means of tuning anisotropies as well as DMI, which can be easier with applied voltage and (2) novel ferromagnetic domain wall structures may be formed under the influence of imprinted ferroelectric domains.
The work was carried out first for two years at the Lawrence Berkeley National Laborstory (LBNL) in Berkeley, California where a Spin Polarised Low Energy Electron Microscopy (SPLEEM) was used. SPLEEM allows simultaneous sample growth by Molecular Beam Epitaxy (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.

In the third year at the University of Leeds, UK, samples were grown by sputtering and characterised mainly using Magneto-Optical Kerr Effect (MOKE) Microscopy. Multiferroic heterostructure consisting of an in-plane magnetised thin film of CoFeB and a BaTiO3 substrate were investigated.

The outbreak of the COVID-19 pandemic and the associated lock-down measures and restrictions both in California and the UK meant that the planned work had to be adjusted. Experimental work that was not possible due to the lack of access to laboratory equipment was substituted with numerical simulations and analytical modelling that could be performed remotely.

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 possibility to control magnetic DW propagation with a voltage and its use in logic devices was also demonstrated using Micromagnetic Simulations.

Results from Micromagnetic Simulations and analytical modelling have been published in Physical Review Letters [Phys. Rev. Lett. 127, 127203 (2021)], and meet Objective 3.

Two publications (arXiv:2111.06191 & arXiv:2111.15381) have been submitted to Physical Review B and Physical Review Materials and report results from Spin Polarised Low Energy Electron Microscopy and Micromagnetic Simulations to meet Objectives 1 and 2.

One further publication on Micromagnetic Simulations and analytical modelling is being prepared and will report further results to meet Objective 3.

Finally, one publication reporting results from Magneto-Optical Kerr Effect Microscopy is in preparation and will report results to support Objectives 1 and 2.

While the outbreak of the COVID-19 pandemic has led to the cancellation of several conferences, results have been presented at three conferences (IOP Magnetism 2021, Intermag 2021, EPSRC International Network for Spintronics Research Symposium 2021) and are scheduled to be presented in two further talks at the Joint MMM-Intermag Conference 2022.
The project has demonstrated the possibility to control chiral spin structures using applied voltages through the electric field tuning of magnetic anisotropies. Using micromagnetic simulations, reprogrammable pinning and reflection sites for magnetic domain wall motion were demonstrated. The dynamics of current driven magnetic domain walls (by spin orbit torques) are distinct from previous observations. A novel analytical theoretical model accounts for the numerical results.

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 socio-economic impact of the results of this project will be found in this field. The new concepts it has established open up an avenue for the continued miniaturisation and increase in efficiency of microelectronics that enable the continued growth of ICT, with wider social implications for a society increasingly reliant on digital technology but needing to mitigate its carbon footprint.