Half-metallic ferromagnets: materials fundamentals for next-generation spintronics

Semiconductors (such as silicon) underpin so many aspects of modern life, through electronics and data processing for the WWW, telecoms, medicine, transport, etc., that it is hard to overstate their importance. However, silicon chip technology is approaching hard physical limits and alternatives are needed. One radical approach is spintronics, where the both the "spin" and charge of electrons are used for data storage and processing. Spin is a fundamental property of electrons related to magnetism: in a magnetic field, a spin prefers to align in one of two ways, along or against the field. Full utilisation of spin would enable revolutionary new chip designs, which are fast, energy-efficient and fully integrate data storage with logic.

We will study half-metallic ferromagnetic (HMF) materials. HMFs are a class of materials discovered theoretically in the 1980s which combine the properties of a semiconductor and a ferromagnetic metal. Only one of the two electron spin alignments can easily move inside an HMF - they are "100% spin-polarised". They should hence be ideal materials for use in spintronics. However, despite major research efforts to make HMF devices, in most cases HMFs do not outperform ordinary magnetic materials (which are typically 30-40% spin-polarised). There is no clear understanding of why this is the case, which prevents the potential of HMFs being unlocked for advanced spintronics. We propose to solve this outstanding problem with a comprehensive and rigorous study of HMFs in the physical form which is actually used in devices, i.e. in thin-films on an oxide or semiconductor substrate.

We will combine our expertise in four areas: (1) production of high quality thin films of HMFs, (2) characterisation of magnetic thin films down to the atomic level, (3) accurate theoretical description of these materials, and (4) fabrication of HMF spintronic devices. This will enable us to study holistically the most likely culprits for weakened HMF performance, namely temperature, defects and the HMF /substrate interface. Spin-polarisation collapses as an HMF heats up, and this cut-off, for a practical device, must be well above room temperature. We will measure this explicitly and model it with state-of-the-art theory developed recently in Warwick. Residual defects in the thin films can destroy spin polarisation and we will both understand these via atomic-scale imaging / modelling and adjust our thin film growth to minimise them. Finally, there must always be an interface between the HMF and its substrate, which also influences the spin polarisation and functional performance. We will image and model the interfaces, and again adjust our growth to optimise them. Atomic-scale imaging and analysis is possible using cutting-edge aberration-corrected electron microscopes (York and Warwick each have such a microscope, with complementary capabilities). Finally, this fundamental work will be correlated with the functional performance of the HMFs in prototypical spintronic devices. We will be able to fabricate devices, using established designs, and subsequently measure the atomic-scale interfaces and defects on the actual device structure.

This unique combination of capabilities ranging from first-principles theory to device performance will enable the first comprehensive and rigorous study of half-metallicity in real thin film structures. Our goals are to understand in a fundamental way the limitations of HMFs in real structures, to guide future HMF device design, and also develop the highest possible room temperature spin polarisation in HMF thin films. Between York and Warwick, we have growth expertise in three different classes of HMF material (transition metal pnictides, magnetite and Heusler alloys) which will enable us both to produce a generalised understanding of HMFs and find the best materials for ultra-high spin polarisation films.

1. What is the background to this research project?
Silicon technology has been spectacularly successful, supporting an information revolution which has profoundly affected almost every aspect of life in the developed world. However, silicon devices are approaching the ultimate limits of miniaturisation meaning that improvements in processing speed, memory capacity and energy efficiency are slowing down. In a silicon chip, electric charges (electrons) are moved at high speeds. This requires quite a lot of energy and limits the speed at which information can be processed (so, for example, laptops can get very hot and their batteries can run out quickly). But we could instead manipulate the "spin" of electrons to process and store information, which might take as little as a hundredth of the energy. This is called "spintronics", and the development of advanced spintronic devices promises continued progress in information technology (IT), with a huge reduction in its energy demands. We will make, investigate and learn how to optimise a particular family of materials which should be ideal for making new and efficient spintronic devices.

2. Who could benefit from our research?
The long-term beneficiaries of our research will be consumers and the general public, who could benefit from advances in efficient IT, through information processing and storage technology built using materials developed in this project (e.g. more powerful mobile devices with far longer battery life or computers which do not need a lengthy boot-up to start). Environmental benefits would arise particularly from improved energy efficiency in large-scale IT (e.g. data centres supporting WWW services). Our work will help to attract continuing R&D investment to the UK from global companies in the IT hardware sector, and boost the innovation capacity of UK-based companies in the field of spintronics. The project will also benefit public understanding of science via specific engagement activities planned around our use of ultra-powerful microscopes capable of "seeing atoms" inside materials. Finally, the younger researchers working on our project will develop a broad skills base so that they will be extremely well placed to develop careers in or beyond the academic / high-tech commercial sectors.

3. How will we promote these benefits?
To realise the long-term technology benefits of the project, we will need to engage with industrial researchers. We have a substantial network of existing industry collaborators in, for example, the data storage sector, and will invite industrial researchers to our regular progress meetings and to a larger 1-day meeting to be organised in the second year. Our work will also feed into another EPSRC-funded spintronics project in which we are involved, with its own key industrial R&D partner. The younger researchers on our project will take on short industry placements, aiding both commercial engagement and their personal skills portfolios. The project covers an unusually broad range of experimental and theoretical techniques, giving the early-career researchers great opportunities for networking and development of wider technical competences. They will also benefit from training in transferable skills and public engagement, further boosting their employability and career options. The Physics Departments at Warwick and York have active schools outreach programmes, and the project team will participate in specific activities such as the "World of Physics" annual event in York and schools visits to the Microscopy suite run by the Ogden Trust Physics Teacher-Fellow in Warwick.