Development of Half-Metallic Ferromagnetic Films

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

Project details

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.

Funding agency

EPSRC (Standard Research, EP/K03278X/1, value: GBP 568,816)

Starting date

01/10/2013 (for 4 years).
 

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

Project details

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.

Goals:

  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/cm2.
  4. Estimation of a damping constant of the Co2FeAl0.5Si0.5 film.

Funding agency

EPSRC (Strategic Japanese-UK Cooperative Programme, EP/H026126/1, value: GBP 73,084)

Starting date

05/07/2010 (for 3 years).

Ending date

04/07/2013 (successfully completed).

 

Spectroscopic evaluation of spin-resolved band structures of half-metallic Heusler alloy films by circularly polarised light pumping

Project details

This study aims to develop a new method to measure spin density of states (DOS) in a half-metallic ferromagnetic film as a function of depth by using a capping layer.

Funding agency

Royal Society (Research Grant, value: GBP 15,000)

Starting date

01/11/2010 (for 1 years).

Ending date

31/10/2011 (successfully completed).