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

Lead Research Organisation: University of Warwick
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.

Publications

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Burrows CW (2014) Interaction of Mn with GaAs and InSb: incorporation, surface reconstruction and nano-cluster formation. in Journal of physics. Condensed matter : an Institute of Physics journal

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Nedelkoski Z (2016) Controlling the half-metallicity of Heusler/Si(1 1 1) interfaces by a monolayer of Si-Co-Si. in Journal of physics. Condensed matter : an Institute of Physics journal

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Maskery I (2016) Bulk crystal growth and surface preparation of NiSb, MnSb, and NiMnSb in Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

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Glover SE (2018) Magnetic and structural depth profiles of Heusler alloy CoFeAlSi epitaxial films on Si(1 1 1). in Journal of physics. Condensed matter : an Institute of Physics journal

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Burrows C (2018) Hybrid Heteroepitaxial Growth Mode in physica status solidi (a)

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Burrows C (2019) Epitaxial growth and surface reconstruction of CrSb(0001) in Results in Physics

 
Description We are worked on 3 types of advanced magnetic material to combine with semiconductor materials. Structures including our materials will enable new types of efficient spin-electronic devices to be made. We discovered a combination of materials which has outstanding properties for supporting semiconductor spintronics and in collaboration with a group in Japan demonstrated an efficient spintronic device.
Exploitation Route We have worked with Toshiba (in Cambridge) to make electronic devices with our advanced materials which could provide low-power, efficient data storage and processing for next-generation computer devices.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology

URL http://www.warwick.ac.uk/halfmetals
 
Description Our spintronic materials have been investigated by industry partner for use in devices. Furthermore our theoretical methods have been used by a new industry partner to investigate energy recovery materials. A substantial team of early career researchers in York and Warwick, including 3 UK PhD students and 4 overseas PhD students, have graduated from the project into further science and technology jobs, including industry collaborations and at prestigious research institutes.
First Year Of Impact 2016
Sector Digital/Communication/Information Technologies (including Software),Energy
Impact Types Societal

 
Description PhD studentship - Centre for Scientific Computing
Amount £16,000 (GBP)
Organisation University of Warwick 
Sector Academic/University
Country United Kingdom
Start 11/2013 
End 11/2015
 
Description PhD studentship CO
Amount £53,730 (GBP)
Organisation Government of Nigeria 
Sector Public
Country Nigeria
Start 02/2013 
End 02/2016
 
Description Epitaxy, interface control and surface modification of novel spin-functional materials 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution The project is a joint 3.5 year PhD studentship with Diamond Light Source Ltd. The PhD student, Philip Mousley, began work on the project in October 2013. We will study materials grown in the joint York-Warwick project as well as materials grown in Warwick's single crystal growth facility. We will be able to exploit the world-leading facilities of Diamond Light Source to analyse half-metallic and topological insulator surfaces.
Collaborator Contribution The facilities of beamline I07 and the Surface Village at Diamond Light Source.
Impact Obtained beam time at the SPring-8 synchrotron radiation facility in Japan to run experiments on novel spintronic structures.
Start Year 2013
 
Description ILL internship / collaboration 
Organisation Institut Laue–Langevin
Country France 
Sector Academic/University 
PI Contribution PhD student internship at Institut Laue-Langevin for 3 months, plus research collaboration on polarized neutron reflectivity.
Collaborator Contribution Training and data analysis.
Impact Several papers and important contribution to PhD thesis; advances in PNR analysis including work with author of major software package used worldwide.
Start Year 2015
 
Description Toshiba sample growth contract 
Organisation Toshiba Research Europe Ltd
Country United Kingdom 
Sector Private 
PI Contribution Sample growth of novel spintronic heterostructures for Toshiba Research Europe Ltd.
Collaborator Contribution Magneto-transport measurements.
Impact Multi-disciplinary: electrical engineering & physics.
Start Year 2013