Antiferromagnetic devices for spintronic memory applications

Almost all modern electronic devices require memory devices for large scale data storage with the ability to write, store and access information. There are strong commercial drives for increased speed of operation, energy efficiency, storage density and robustness of such memories. Most large scale data storage devices, including hard drives, rely on the principle that two different magnetization orientations in a ferromagnet represent the "zeros" and "ones". By applying a magnetic field to a ferromagnet one can reversibly switch the direction of its magnetisation between different stable directions and read out these states / bits from the magnetic fields they produce. This is the basis of ferromagnetic media used from the 19th century to current hard-drives. Today's magnetic memory chips (MRAMs) do not use magnetic fields to manipulate magnetisation with the writing process done by current pulses which can reverse magnetisation directions due to the spin-torque effect. In the conventional version of the effect, switching is achieved by electrically transferring spins from a fixed reference permanent magnet. More recently, it was discovered that the spin torque can be triggered without a reference magnet, by a relativistic effect in which the motion of electrons results in effective internal magnetic fields. Furthermore the magnetisation state is read electrically in such MRAMs. Therefore the sensitivity of ferromagnets to external magnetic fields and the magnetic fields they produce are not utilised. In fact they become problems since data can be can be accidentally wiped by magnetic fields, and can be read by the fields produced making data insecure. Also the fields produced limit how closely data elements can be packed.

Recently we have shown that antiferromagnetic materials can be used to perform all the functions required of a magnetic memory element. Antiferromagnets have the north poles of half of the atomic moments pointing in one direction and the other half in the opposite direction leading to no net magnetisation and no external magnetic field. For antiferromagnets with specific crystal structures we predicted and verified that current pulses produce effective field which can rotate the two types of moments in the same directions. We were able to reverse the moment orientation in antiferromagnets by a current induced torque and to read out the magnetisation state electrically.

Since antiferromagnets do not produce a net magnetic field they do not have all the associated problems discussed above. The dynamics of the magnetisation in antiferromagnets occur on timescales orders of magnitude faster than in ferromagnets, which could lead to much faster and more efficient operations. Finally, the antiferromagnetic state is readily compatible with metal, semiconductor or insulator electronic structures and so their use greatly expands the materials basis for such applications.

This proposal aims to develop a detailed understanding of current induced switching in antiferromagnets though a program of research extensive experimental and theoretical studies and to pave the way to exploitation of this effect in future magnetic memory technologies. We will develop high quality antiferromagnetic materials and smaller and faster devices. We aim to achieve devices in which the antiferromagnetic state has not disordered (single domain behaviour) which will have improved technical parameters and which will be ideal for advancing fundamental understanding. We also aim to demonstrate and study the manipulation of regions of antiferromagnets in which there is a transition between two types of moment orientation (domain walls) using current-induced torques. As well as electrical measurements we will directly study the magnetic order in the antiferromagnetic devices using X-ray imaging techniques and we will carry out extensive theoretical modelling.

This proposal has the potential to impact on several groups of beneficiaries. Below we identify those beneficiaries and the nature of the potential impact.

Impact on industry: The ability to efficiently manipulate the state of antiferromagnets on very short timescales could have a transformative impact on the design of information storage and processing devices, where they could deliver reduced energy consumption, faster processing speed and increased storage density. Many global technology companies (e.g. Hitachi, IBM, Samsung, Seagate, Toshiba) have active programmes to develop magnetic memory technologies. The magnetic memory industry will benefit from the knowledge developed in this project through publication of the results in scientific journals and presentations at conferences. Furthermore, the investigators have links to several such technology companies through previous European consortia and training networks and have a well-established collaboration with the Hitachi Cambridge Laboratory (HCL), who are project partners in this proposal. These links with industry represent a route to take any intellectual property (IP) generated by the project to the next stages of product development and commercialisation. With the aid of the University's Technology Transfer Office, mechanisms are in place at the investigating institution to ensure that intellectual property is protected where appropriate and routes to exploitation with industrial partners are recognised and exploited.

Impact on the wider society: The knowledge generated in this proposal could have long term impact on society in that it will lead to the production of faster, more powerful and more energy efficient devices. Such devices may include computers, laptops, mobile phones etc.
In the shorter term, the project can have societal impact in terms of raising public awareness and knowledge of science, in particular the fields of magnetism, spintronics and materials. This will be achieved through the outreach activities of the investigators including visits and lectures for local schools and societies, and publications and appearances in the mainstream media and social media.

Impact on academia: The proposed investigations of the novel interactions of electrical currents with magnetic moments in antiferromagnets and of the generation of non-equilibrium spin polarisation will be of great interest to the broad spintronics community and will be an experimental test of theoretical predictions made by groups in Kiev, Prague, and Colorado. The understanding of current induced torques to be developed in this proposal will provide a tool for the control of AF moments and will be beneficial to those studying antiferromagnetic phenomena such as exchange bias, including several groups in the UK (e.g. York, Leeds, Oxford, Manchester, and Southampton). Our results in scientific journals and through conference presentations at specialised spintronics and magnetism conferences as well as more cross-disciplinary materials conferences. We will also co-organise a series of meetings during the project with collaborating groups and external experts. The crystalline thin film materials developed within this project will be made available to external academic researchers for studies of, for example, nanoscale or high frequency processes.

Impact on researchers: This project will provide training for early-stage researchers in device fabrication, experimental techniques and data analysis, and will provide opportunities to develop presentation and communications skills at scientific meetings and conferences. These are skills desired by employers in academic institutions and industrial companies. There will also be the opportunity for researchers to gain experience of working in an industrial environment at HCL, where matters related to corporate strategy and product development are more to the fore.