Efficient spin voltage/current generation in a ferromagnet/semiconductor lateral spin-valve

Lead Research Organisation: University of York
Department Name: Electronics

Abstract

A spin-polarised electron current has been widely investigated to realise new spintronic device application. For example, spin-transfer torque induced by a spin-polarised electron current offers a fundamental physical mechanism on current-induced magnetisation switching (CIMS) as well as domain-wall motion in a ferromagnetic (FM) nanowire. The spin-transfer torque was predicted by Berger and Slonczewski independently, and has been experimentally demonstrated. By spin-scattering layer insertion and shape modification for a giant magnetoresistive (GMR) nanopillar, a critical current density for switching has been reduced to satisfy a Gbit-scale requirement for a magnetic random access memory (MRAM), a 4-Mbit version of which has been introduced by Freescale (now EverSpin Technologies) in 2006. MRAM is expected to replace a Si-based RAM due to the non-volatility and the better thermal stability. Recently, coherent tunnelling in an Fe/MgO/Fe system has been predicted to achieve over 1000% tunnelling magnetoresistance (TMR) and experimentally observed in epitaxial/highly-oriented Fe(Co)/MgO/Fe(Co) junctions. Such coherent tunnelling has been implemented into a nano-pillar to demonstrate the CIMS with 160% TMR ratio at room temperature. By combining the large TMR ratio with the substantial decrease in critical current density down to 2.5x10^6 A/cm2, the requirement for beyond the Gbit-scale MRAM application is satisfied. Hence, government-initiatives have been applied to develop a commercial Gbit MRAM both in the USA and Japan.Recent development in nanometre-scale fabrication techniques will enable us to expand a vertical GMR nanopillar into a lateral configuration, consisting of ferromagnetic nanowires and a non-magnetic nanowire to bridge over the spin injector and detector, enabling precise control of dimensions. In such a lateral spin-valve configuration, spin-polarised electrons can be injected with an electron charge current (local geometry) and without a charge current (non-local geometry). Using non-local geometry pioneering work has been performed by Jedema et al., successfully demonstrating diffusive spin injection from a ferromagnetic Ni80Fe20 nano-electrode, spin accumulation in a non-magnetic Cu nano-wire and spin detection by another NiFe nano-electrode. They have further extended their study into ballistic spin injection by inserting an AlOx tunnel barrier (insulator, I) at the FM/non-magnet (NM) interfaces. Consequently non-local spin-valve systems have been extensively employed to achieve efficient spin injection by minimising interfacial scattering in both diffusive and ballistic contacts and also to detect both spin Hall and inverse spin Hall effects. This clearly indicates the advantages of the lateral device configurations.In this proposed project, we will employ a lateral spin-valve structure instead of a conventional nano-pillar to demonstrate efficient generation of a spin voltage and current, which is not associated with an electron-charge current and hence minimises the Joule heating. In our proposed devices, both a spin current and a spin-polarised electron-charge current will be used to detect the spin voltage/current generation in non-local and local measurement geometries, respectively by changing the measurement geometries. In the non-local geometry , a spin current can be injected efficiently into a non-magnet through a tunnel barrier and detected as a large spin voltage through a second tunnel barrier. This gives a large spin current through a metallic interface. Our proposed device will therefore act independently as a pure spin-voltage and spin-current source with high efficiency. The evaluation of the pure spin-voltage and current will reveal the fundamental mechanism of spin-current transport (without an electron charge), which will encourage further theoretical studies for better understanding of the spin current and will also lead a new type of device architecture.

Publications

10 25 50
 
Description We have fabricated the abrupt Fe/GaAs(001) interface for the first time and characterised its electrical transport properties.
Exploitation Route Our finding allows us to fabricate a spin-polarised transistor with reproducible operationability.
Sectors Electronics

URL http://www-users.york.ac.uk/~ah566/
 
Description We have published our outcomes in scientific journals and gave more than 10 presentations at international conferences. Especially we reported an abrupt interface between Fe and GaAs for the first time, which allows us to make devices with reproducible transport properties.
First Year Of Impact 2009
Sector Electronics
Impact Types Societal

 
Description Spintronic devices for integrated logic circuits
Amount £856,918 (GBP)
Funding ID EP/M02458X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 04/2015 
End 03/2020
 
Description EPSRC-JST Tohoku 
Organisation Tohoku University
Country Japan 
Sector Academic/University 
PI Contribution This collaborative research was carried out under the first EPSRC-JST Cooperative Research Project scheme.
Collaborator Contribution We exchanged students for 1.5 months during summer for three years.
Impact See the publications.
Start Year 2009