Half metal oxides: In search for 100% spin polarised materials

Lead Research Organisation: University of York
Department Name: Physics

Abstract

Spintronics is a rapidly developing field that utilises the electron's spin in addition to its charge to create new devices combining logic, data storage and sensor applications. The huge potential of spintronics has stimulated a wide range of research from spin transport, spin injection/accumulation and spin manipulation to device fabrication such as spin valves and magnetic tunnel junctions. One of the main challenges in the spintronics field is to find/create highly spin polarised materials that are compatible (lattice match, conductivity match, thermodynamically stable, high Curie temperature (Tc), etc.) with CMOS technology. In this proposal magnetite (Fe3O4) is proposed as the optimum highly spin polarised material for spintronics and by understanding the material at the atomic level seeks to solve the challenges in its implementation.
Conventional 3d ferromagnetic metals and their alloys are only 30-40% spin polarised at the Fermi level, thus material systems with better spin polarisation are essential for the next generation of spintronic devices. The existence of 100% spin polarised materials at the Fermi level has been predicted by density functional theory (DFT). Such 100% spin polarised materials, also termed half-metals, have one of the spin channels metallic while the other spin channel is insulating. A rather large number of materials including oxides (Fe3O4, CrO2, manganites), pnictides, chalcogenides, and Heusler alloys have been predicted to be half-metallic. Among these materials magnetite (Tc=855 K) is of special interest since: (i) it has a Tc in excess of 500K, the threshold temperature for device applications; (ii) it has an excellent lattice match with MgO and MgAl2O4, the two most important oxides for spintronic applications; (iii) it can form atomically sharp interfaces with relevant semiconductors (SC) such as GaAs, GaN and SiC; and (iv) its layered structure will allow interface atomic engineering at magnetite/oxide and, magnetite/SC heterojunctions. It is worth noting that no other SP materials have these properties. For example, CrO2 has Tc below 500K and Heusler/SC junctions are not abrupt due to the high annealing temperature required for Heuslers to fully order into a L21 structure that is half-metallic.
In order to incorporate magnetite in device structure, growth of thin films of magnetite and heterostructures of magnetite with suitable oxides, metals and semiconductors (SC) is required.
The two main challenges to overcome for successful application of Fe3O4 are:
1) growth of thin films with control of stoichiometry and structural defects; it is well known that defects such as antiphase domain boundaries (APBs) can completely determine the functionality of magnetite films, thus controlling the APBs nature and density is highly important;
2) engineering the interfaces between magnetite/oxide barriers and magnetite/SC; spin transport across interfaces critically depends on the interfaces' atomic structure.
These are the two crucial steps to understand the structural basis of spin-related phenomena in the magnetite films as well as some of technologically important magnetite/oxide and magnetite/SC interfaces. This knowledge would provide a path and guide for the engineering of spintronic devices based on Fe3O4.
In order to achieve this goal, the direct correlation of the films' functionality and their atomic structure, in this application I propose a joint experimental and theoretical study on the growth of half-metal magnetite oxide films and the atomic and electronic structure of the film, APBs and magnetite/oxide and magnetite/SC interfaces which are of interest for spintronic devices. Film growth will be done by Molecular Beam Epitaxy, spin polarised calculations will be performed by DFT, and High Resolution Transmission Electron Microscopy, High Angle Annular Dark Field Imaging and Electron Energy Loss Spectroscopy will be used to fully characterise these systems on atomic scale.

Planned Impact

Spintronics is one of the most promising fields in nanoscience, and as such has been identified as a top priority for nanoscience and nano-materials by the European Commission. The results from this project will be relevant to the spintronics materials and devices research community in the UK and internationally, as well as the data storage industry. Half-metals have the potential to address one of the main problems in the field of spintronics which is the need for 100% spin injectors and electrodes, which are the key elements in spintronic devices. Full realisation of spintronics will be of great importance for future computational, data storage and sensing devices. In addition to the speed of these devices they will bring new functionality such as integrated logic and data storage. Equally important is the fact that spintronic devices can be operated with significantly lower power than conventional electronic devices. Thus the challenge of heat dissipation as a result of continuous scaling can be resolved.

The electron microscopy capability, and in particular the combination of the microscopy with theoretical modelling and thin film growth which we have available, has enabled us to establish a world lead in this field at the present time. Hence the work is of the highest impact locally, nationally and internationally. Fundamental understanding of how the structural defects (APBs and interfaces) affect the functionality of materials with spinel structure will have an impact on a range of spinel materials besides magnetite, for example: multiferroics (CoFe2O4), ferrites (MgFe2O4), magnetic semiconductors (Fe(Zn)O4, Co3O4) tunnel barrier oxides (MgAl2O4), maghemite (Fe2O3). Therefore magnetite can be used as a model system for spinel structured materials and the knowledge obtained within this project that correlates structure/defects and functional properties could be transferable to a number of spinel materials, and thus enable tuning and tailoring of the properties of these materials. Hence work such as that proposed here will make a significant impact in the academic field by identifying the physical structures that exist in key materials and enabling those involved in materials development and particularly thin film growth, to direct their efforts towards the resolution of key problems.

In addition the knowledge of Fe-oxides with structural and chemical control on an atomic scale is of tremendous importance for a number of technologies in which Fe-oxides are materials of choice. For example: a) Fe-oxide semiconductors (both in spinel and corundum phase) are good choices for direct water splitting under solar radiation due to their band gap, stability in solution and availability; and b) Fe-oxide nanoparticles are the basis of ferrofluids with a range of applications from the automobile industry to medical applications.

The work also fits well with the EPSRC priorities in the research field of Spintronics, in particular: 'the study of spintronic materials and the physical phenomena underpinning this, including research into spin polarisation' as indicated in the EPSRC's 'Our portfolio'.
Because of the specialist capability at York, particularly in the field of TEM, magnetic measurements and also first principles materials modelling, and the fact that we have a critical mass of experimental facilities and academics involved in the field, this work will make an impact on the academic field worldwide.

Publications

10 25 50
 
Description We have demonstrated how to obtain desired properties of halfmetalic Fe3O4 in thin film forms.
By employing atomic level characterisation and modelling we identified and fully understand the nature of the
growth film defects that destroy the desired electronic properties of the thin films Fe3O4. Additionally we have used the knowledge of structural defects in magnetite thin films to magnetite nanoparticles. Magnetite nanoparticles have potential of being used for hyperthermia cancer treatments.
Exploitation Route Our findings can be utilised in electronic and IT industry for creating spin electrodes for novel spin-electron devices.
Sectors Digital/Communication/Information Technologies (including Software),Education,Electronics,Pharmaceuticals and Medical Biotechnology

 
Description Research Pump Priming: High Energy resolution spectroscopy of functional materials
Amount £50,000 (GBP)
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start  
 
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 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 'Atomic study of Fe3O4/SrTiO3 Interface' talk prsented on onternational conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Magnetite heterostructures work presented on largest microscopy conference in the world. The talk initiate discussion and initiate further collaborations.
Year(s) Of Engagement Activity 2015
 
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 'Fe3O4 thin films with bulk like magnetic and magnetotransport behaviour', presented on Annual Conference on Magnetism and Magnetic Materials, Denver CO, USA Nov. 2013 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Work on magnetite thin film presented on largest magnetism conference in the world. Initiate fruitful discussion and ideas for further experiments.
Year(s) Of Engagement Activity 2014
 
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 '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