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

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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.