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

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.

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.