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

Lead Research Organisation: University of York
Department Name: Physics

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description We have achieved atomic level understanding of what controls the functional properties of predicted 100% spin polarised materials such as Heusler alloys and magnetite.
We have demonstrate that atomically sharp interface between Heusler alloy and semiconductors e.g. Si and Ge is feasible. We currently demonstrated proof of principke planar spin valve devices using CoFeAlSi-Ge system.
Exploitation Route Utilizing our finding of optimal structure for spin injectors in Ge.
Sectors Digital/Communication/Information Technologies (including Software),Education

 
Description Our findings are currently utilised into fabrication of plan spin injection devices. These devices are first step into realisation of spin transistor at room temperature.
First Year Of Impact 2016
Sector Digital/Communication/Information Technologies (including Software),Electronics
Impact Types Societal

 
Description International Exchange Grant
Amount £11,900 (GBP)
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2017 
End 03/2019
 
Description JSPS Core-to-Core Programme
Amount £856,000 (GBP)
Funding ID EP/M02458X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2015 
End 03/2020
 
Description Research Studentship
Amount £60,000 (GBP)
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start 10/2016 
End 09/2019
 
Description Strategic PhD studenship
Amount £60,000 (GBP)
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start 09/2015 
End 09/2018
 
Description Strategic PhD studentship
Amount £60,000 (GBP)
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start 01/2014 
End 01/2017
 
Description Strategic PhD studentship
Amount £60,000 (GBP)
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start 01/2014 
End 01/2017
 
Description Cadiz 
Organisation University of Cadiz
Country Spain 
Sector Academic/University 
PI Contribution Materials growth, experimental data collection, structure determination
Collaborator Contribution Develop computer algorithms and scripts for image analysis
Impact We have jointly published several publications in peer reviewed journals.
Start Year 2013
 
Description Halfmetal Semiconductor heterostructure 
Organisation Osaka University
Department Center for Spintronics Research Network
Country Japan 
Sector Academic/University 
PI Contribution We have shown that atomically, chemically and magnetically sharp interfaces between full Heusler halfmetalic alloys and Ge are feasible. Based on experimentally derived atomistic models we have shown that spin polarisation at Co2FeAlSi/Ge can be preserved, which is the condition for efficient spin injection into Ge. Also we have shown that by suitable control of atomic structure at Heulser/Si interfaces spin polarisation can be also preserved at this interface.
Collaborator Contribution Prof. Hamaya group from Osaka are developing spin injection devices in Si and Ge using full Heusler alloys as a spin injector electrode. They do the device fabrication and perform device functionality measurments i.e. non-local spin injection ito both Si and Ge. Jointly we are aiming to fully describe what effect good and efficient spin injection into both Ge and Si.
Impact We have published 4 joint publication and one is submitted. 1. The antiphase boundary in half-metallic Heusler alloy Co2Fe (Al, Si): atomic structure, spin polarization reversal, and domain wall effects 2. Controlling the half-metallicity of Heusler/Si (1 1 1) interfaces by a monolayer of Si-Co-Si 3.The role of chemical structure on the magnetic and electronic properties of Co2FeAl0. 5Si0. 5/Si (111) interface 4. Realisation of magnetically and atomically abrupt half-metal/semiconductor interface: Co2FeSi0. 5Al0. 5/Ge (111)
Start Year 2015
 
Description Nagoya 
Organisation Nagoya University
Country Japan 
Sector Academic/University 
PI Contribution Material growth, data analysis
Collaborator Contribution Experimental infrastructure, e.g. electron microscopy
Impact Joint research publications, and exchange of knowledge and experiance
Start Year 2013
 
Description TIT 
Organisation Tokyo Institute of Technology
Country Japan 
Sector Academic/University 
PI Contribution Structural analysis and mode;l;ing of half-metal oxides
Collaborator Contribution Pulsed Laser Deposition of half-metal oxides
Impact Joint research publications. Transfer of knowledge and expertise between the groups.
Start Year 2013
 
Description UWM 
Organisation University of Wisconsin-Milwaukee
Country United States 
Sector Academic/University 
PI Contribution Materials growth and analysis with focus on atomistic models of the interfaces and defects commonly met in halfmetals which are crucial when those materials are used in devices.
Collaborator Contribution Materials growth and Electronic and total energy calculations on half-metals and topological insulators.
Impact Exchange of knowledge and expertise, and joint research publications.
Start Year 2013
 
Description Warwick halfmetal colloboration 
Organisation University of Warwick
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution We provide the research group in Warwick with half metal specimens (Heuslers and magnetite). Also we exchange expertise and knowledge on theoretical calculations and carried out joint experiments.
Collaborator Contribution Warwick has provide us with MnSb and NiMnSb specimens, also they took lead on polarised neutron reflection experiments.
Impact joint publications Joint experiments
Start Year 2013
 
Description 'Atomic Structure and Magnetic Properties of Co2FeAl0.5Si0.5 Thin Films on Ge(111) as a Function of Annealing Temperature' talk presented on international confgerence 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk presented on Magnetism and Magnetic Materials, Pittsburgh, USA, November 2017
Year(s) Of Engagement Activity 2017
 
Description 'Atomic and electronic structure study of a Co2FeAl0.5Si0.5 half-metal thin film on Si (111)' 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk presented on Microscopy and Microanalysis, Columbus, USA, July 2016
Year(s) Of Engagement Activity 2016
 
Description 'Fe3O4 thin films with bulk like magnetic and magnetotransport behaviour' 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk presented Annual Conference on Magnetism and Magnetic Materials, Denver CO, Nov. 2013
Year(s) Of Engagement Activity 2013
 
Description 'Origin of reduced magnetization and domain formation in small magnetite nanoparticles' 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk presented on Magnetism and Magnetic Materials, Pittsburgh, USA, November 2017
Year(s) Of Engagement Activity 2017
 
Description 'Structural and spectroscopic characterisation of heterostructures for semiconductor spintronics applications 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This talk was presented on the International Microscopy Conference, Sydney, Australia, September 2018
Year(s) Of Engagement Activity 2018
 
Description 'The Role of Antiphase Boundaries on Magnetic Domains Formation in Fe3O4 Thin Films', talk for MMM 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This talk was presented on Magnetism and Magnetic Materials, Pittsburgh, USA, November 2017
Year(s) Of Engagement Activity 2017
 
Description Co2FeSiAl/Si(111) heterointerface: magnetic and atomic structure' talk presented on international conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This work was presented on Magnetism and Magnetic Materials, San Diego, USA, January 2016
Year(s) Of Engagement Activity 2016