Demonstration of high-frequency oscillation in a Co-based Heusler alloy tunnel junction

Keywords: half-metals, Heusler alloys, MRAM, Gilbert damping and exchange biasGoals: 1. Establishment of a reproducible process to fabricate a magnetic tunnel junction (MTJ), consisting of a MgO barrier sandwiched with epitaxial Co2FeAl0.5Si0.5 film electrodes.2. Demonstration of very large tunnelling magnetoresistance (TMR) ratio at room temperature (RT).3. Demonstration of efficient current-induced magnetisation switching (CIMS) based on a small Gilbert damping constant and large spin polarisation.4. Nanofabrication of a prototype of a high-frequency spin oscillation with the Co2FeAl0.5Si0.5 junction for the first time.Approach: 1. Device fabrication using ultrahigh vacuum (UHV) sputtering/molecular beam epitaxy (MBE) growth and nanofabrication at NIMS combined with interfacial atomic analysis by state-of-the-art scanning transmission electron microscopy (STEM) at York.2. Based on the above feedback process, nanopillar fabrication of high-quality Co2FeAl0.5Si0.5/MgO/Co2FeAl0.5Si0.5 junctions.3. Highly sensitive magnetisation analysis both at NIMS [current in plane tunnelling analysis and temperature-dependent TMR measuremsnts] and York [vibrating sample magnetometer (VSM) and magneto-optical Kerr effect (MOKE)].4. High-frequency operation of the CIMS in a Heusler-based nanopillar and electrical detection by coplanar waveguide.Expected outcome: 1. To reveal a correlation between atomic structures at the Co2FeAl0.5Si0.5/MgO interfaces and ballistic spin-polarised electron tunnelling properties.2. Improvement of the world-record TMR ratio (386% at RT and 832% at 9 K) and its temperature dependence to follow the empirical temperature dependence of magnetisation.3. Decrease of a critical current density below 106 A/cm24. Estimation of a damping constant of the Co2FeAl0.5Si0.5 film.An intensive search for a new ferromagnetic material with 100% spin polarisation at room temperature has been carried out recently for the realisation of a future spin random access memory application. We will employ Co2FeAl0.5Si0.5 Heusler alloy films, which hold the highest spin polarisation, resulting the largest tunnelling magnetoresistance at room temperature to date. Magnetic tunnel junctions with the Heusler films will be epitaxially grown at the NIMS by ultrahigh vacuum sputtering and molecular-beam epitaxy, and will then be characterised at York with the state-of-the-art electron microscopy and magnetometry. By improving the interfacial atomic structures of the films against a MgO tunnel barrier, larger TMR ratios will be demonstrated. High-frequency measurements will also be performed to define their damping constants, which are to be smaller than the conventional ferromagnets. These junctions are expected to take significant advantages in both ~100% spin polarisation and a very small damping constant for the realisation of fast and efficient switching in a spin memory. At the end of this project, we will attempt to fabricate a prototype of a high-frequency oscillator with Heusler alloy films for the first time.

This project utilises the existing world leading expertise both in NIMS and York. In particular, NIMS has held the world record in a TMR ratio of a Heusler-based MTJ and has been competing with the other groups in the world. York has been investing intensively onto materials research and nanotechnology, especially the state-of-the-art STEM in the York JEOL Nanocentre. By combining these expertise, we plan to demonstrate high-frequency operation of a Co-based Heusler alloy nanopillar for the first time. Since such a device shows a very large TMR ratio at RT (almost infinite theoretically) and a small Gilbert damping constant, it is expected to offer lower noise GHz magnetisation switching by a current injection as compared with the CoFeB-based nanopillar, which has recently been investigated widely for future MRAM applications. Hence we can develop an absolutely new device with excellent functionality for future memory applications. York and NIMS have already been collaborating in the fields of Heusler alloy analysis and surface analysis as listed in our previous publications. There is a close relationship between the Principal Investigators and the supporting senior professors. By extending our collaboration, we can fabricate a new material with 100% spin polarisation at RT, leading to a new GHz oscillator. During this project, we will hold an exchange visit every year to minimise the risk and ensure the delivery of milestones. This will also ensure that the collaboration is maintained and hopefully broadened. The progress will be presented annually at international conferences (MMM and Intermag) as well as local conferences (CMMP and JSAP). It will also be published in leading scientific journals. Any successful devices will be patented at the final stage of this project with 50:50 sharing of ownership. The major impact will be the extension of an existing link for two younger academics in the U.K. and Japan. The young scientists working on the project will receive excellent training at two institutions that are at the forefront of work in the field in their respective countries. The facilities used in both countries are world leading. During the exchange of young people special arrangements will be made for them to receive language training such as the Cambridge Certificate at York, as well as cultural awareness events. The value of the work is of basic scientific interest but also of significant industrial importance. In particular NIMS has close links to Toshiba Reseach at Kawasaki and York has links to Toshiba Europe in Cambridge. Should MRAM technology be realised, it will be of similar significance to DRAM today. Hence the technical training and cultural awareness will give the young scientists a major advance in the industries of the future.