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Spin Currents

Final Report Summary - SPINICUR (Spin Currents)

Spintronics offers the potential for logic operations that are faster and consume much lower power when compared to conventional semiconductors. Passive spintronic devices are already the basis for a multi-billion dollar industry producing read heads for hard discs and storage cells in MRAM. Alternatives to semiconductor RAM and logic are being actively sought with spintronics offering exciting possibilities such as: the Spin Hall Effect, spin pumping and the spin Seebeck effect to name a few – all of these effects are based on spin currents.

Pure spin currents are a consequence of a charge current flowing across the interface between a ferromagnet and a normal metal. The charge current is spin polarised as it derives from a ferromagnet whereas currents in normal metals are not polarised. At the interface therefore there will be more carriers of one spin creating an accumulation. Since there is a concentration of spins at the interface there will be diffusion of that spin away from the interface and importantly, diffusion of the opposite spin towards the interface. Equal numbers of spins will travel in opposite directions so that there is no net flow of charge and hence no energy losses due to Joule heating. Under these conditions the spin current is known as a pure spin current and may be exploited to reduce the energy costs of future electronics.

These pure spin currents and their fundamental understanding is the scientific objective of SPINICUR, an FP7-PEOPLE-2012-ITN: 316657 that ran from 2012 to 2016. SpinIcur (from spin currents) is a training network of European experts dedicated to providing state-of-the-art education and training for early stage and experienced researchers. SpinIcur trained 15 fellows in the Physics and Material Science of spin currents using cutting edge techniques from world leading laboratories in academia, research institutes and industry.

The training was delivered using secondments, lectures and hands-on intensive topical courses. These instruments, developed within Spinicur, worked well and have served as a model for use in the future. Fellows were guided by a Director of Training from IBM Zurich embedding business and industrial appreciation into their PhD programmes. The complementary skills courses were extensive, ranging from courses to stimulate creativity, protecting intellectual property to starting your own business. Network meetings were very well attended and provided fellows with the opportunity to present their results, discuss their understanding, and field questions from their peers and the PIs. Working together was encouraged by planning secondments at the meetings driven by the fellows’ needs. The SpinIcur conference, held at the end of the students’ second year, was a unique example of training. Complete control of the conference was given to the fellows including the budget. The ERs formed the conference committee and the ESRs were the session chairmen. They were given assistance, guidance and help, but only where requested. The conference was a resounding success and the costs were under budget.

SpinIcur fellows organized or participated in many different outreach activities, ranging from Open Days for School Pupils to European Research Nights. These events – reaching more than 700 pupils and members of the public - are organised by the host institution and involve the fellows presenting lectures, discussing posters and making demonstrations, tours round the laboratory all designed to engage the public with physics and in particular, SpinIcur research topics.

SpinIcur fellows delivered a total of 24 talks (including three invited) and 19 posters at international conferences, by any standard this is an exceptional number because the distribution was not evenly spread – where fellows engaged fully they delivered about three times more than the average for students outwith networks like SpinIcur. This did a great deal to help advertise the SpinIcur collaboration and has enhanced the career prospects of the fellows.

The network has been at the forefront of research in spintronics and magnetic devices. Amongst the main results emerging from this project, we can find the following:
• Measurements in freestanding single-crystalline magnetic structures fabricated by ion bombardment, where nanostenciled magnetic nanostrips show changes limited in lateral extension by focussed ion beam milling (published in Applied Physics Letters).
• A collaboration in current induced switching in antiferromagnets detected by anisotropic magnetoresistance measurements, demonstrating the possibility for fast, electrical manipulation of antiferromagnets (published in Science).
• Spin pumping due to magnetopolaritons in ferromagnetic/normal bilayers as detected by inverse spin Hall voltages, a demonstration of the spin-optical coupling (Physical Review B).
• Measurements of anisotropic magneto-thermopower, which magnetic field dependence shows a significant contribution down to 1 Oe fields but with different symmetry and angular dependence to the spin Seebeck effect (submitted to Physical Review B).
• The development of yttrium iron garnet (YIG) films with extremely low damping and grown via industry-friendly RF sputtering. The magnetic properties of these films are strongly influenced by Gd diffusion after annealing, a process that affects all epitaxial YIG films (to be submitted to Nature Materials). Spin wave propagations exceedes 10 μm.
• The observation via improved magnetic imaging techniques and simulation of Bloch and Néel walls in wires of 11 to 75 nm in width for high density information storage and the development of electrical manipulation in magnetic memories (under preparation).
• The development of lateral spin valves with improved sample growth resulting in spin diffusion lengths up to 100% larger that ever published in similar systems with no downturn at low T, as well as the separation of Joule and Peltier effects (under preparation).
• The fabrication of tunable spin transfer torque oscillators without breakdown and the observation of nuclear spin polarisation in Fe/GaAs systems. The slow decay of the nuclear spin polarisation provides a memory mechanism (under preparation).

The fellows of SpinIcur have produced 20 publications so far with a further 10 that are in preparation. Of these 3 are in very high impact journals.

In terms of providing the platform from which our fellows could achieve their potential, SpinIcur has succeeded, we can already say that the training has enhanced their career prospects because it has gone beyond that of students who do not work within such networks.

The industrial partners of Spinicur are testimony that the research objectives are relevant to future technology. Some of the results will lead to an accelerated uptake by technology, for example, better tunnel junctions and the reduction of the energy cost of switching are in the right direction to produce more economic and sustainable sensors or MRAM. Currently, the carbon footprint of the internet exceeds that of air travel, and new industrial facilities generate such vast amounts of heat that need to be built close to the artic circle in order to benefit from natural cooling (e.g. Facebook’s servers). Our research in the Spinicur network has provided a strong contribution towards the foundations of a new technology with lower power dissipation and more environmentally friendly –while providing a platform that enhances the performance of current devices.

We have also sought to increase the dissemination of spin current technologies to a wider scientific audience as well as the general public. Members of the network have made contributions to the general media through accessible articles that highlight the issues and suggested solutions above. We have also participated in science festivals, delivered talks in second grade schools and other learning institutions in different environments that we hope will bring a new generation of bright stars to the field, without distinction of gender or socio-economic background.

Through the concentration of effort on fundamental understanding and training, there were no results deemed necessary for protection and therefore the results were and are being published in refereed journals and on open access websites. Recruitment was carried out in accordance with the principles of gender equality and awareness of gender issues was raised as part of the complimentary skills training on ethical’s and impact in science. Where possible, courses were delivered by women, for example the course on presenting your ideas in a business or industrial setting was given by a woman reinforcing the notion that women are successful leaders. Our outreach activity was broad with many small events such as tours and demonstrations for prospective undergraduates to seven major events at host institutions as outreach More importantly, the achievement of the primary objective of Spinicur - the training of 15 fellows in much-needed skills - is the most significant impact and contribution to sustainable European industry that Spinicur has had.