Spintronics: Electronics with a spin and “the ultimate potential of graphene”

An abstract image to illustrate spintronics

SciTech Europa explores some of the research taking place in the exciting field of spintronics, from that being conducted by the Graphene Flagship to work investigating spin-orbit coupling, Spin current switches, AI, and practical spintronic devices.

Spin electronics, often referred to as ‘spintronics’, is the study of the intrinsic spin of an electron and its associated magnetic moment (a quantity representing the magnetic strength and orientation of the electron). Spintronics uses ‘spin’, meaning the fundamental property of particles, for information processing.

The European Graphene Flagship is known for its strong collaborative ethos and this is now starting to result in the increasingly fast technological development of graphene and related materials (GRMs). Graphene has very good tuneable electronic properties and is therefore an ideal material for the application of spintronics. Out of the six divisions packages implemented by the flagship, involving 20 work packages, the division ‘Enabling Science and Materials’ includes Work Package 2 – ‘Spintronics’.

According to the Graphene Flagship website’s content on Work Package 2: ‘Spintronics is an establishment of the ultimate potential of graphene and two-dimensional materials for spintronic applications. This starts with the engineering of efficient room temperature spin injection and detection, long distance spin transport, spin gating and spin manipulation in graphene devices. Spin transport is studied in large scale polycrystalline graphene samples and optimisation of proximity effects between graphene and magnetic insulators and strong spin-orbit coupling materials is prioritised.

The consortium works towards the demonstration of practical graphene spintronic devices such as coupled nano-oscillators for applications in fields of space communication, high-speed radio links, vehicle radar and inter-chip communication applications.’ Continually, this particular Work Package also focuses on making techniques that create high quality graphene on an industrial scale.

Graphene spintronics – a promising trend

Writing in Nature Communications, Graphene Flagship researchers in the Netherlands (based at the University of Groningen and led by Professor Bart van Wees) have published a new study in which they have created a ‘graphene-based device in which electron spins can be injected and detected at room temperature with high efficiency.’ The team showed that they could greatly improve the efficiency of the injection and detection of spin electrons into graphene by using the insulator boron nitride in between the graphene layer and the ferromagnetic spin injector/detector electrodes. Additionally, it was found that by ‘using a two-atom layer of boron nitride resulted in a very strong spin polarisation of up to 70%, 10 times the normal result.’

The leader of this Graphene Flagship division, Professor Vladimir Falko, has previously stated that: “The encapsulation of graphene in boron nitride and the use of heterostructures of these two materials for new devices, including tunnelling transistors, is a promising trend in graphene research that has previously delivered many interesting results. The reported observation takes graphene spintronics to the qualitatively new level.”

Moreover, Professor Stephan Roche, Spintronics Work Package Deputy Leader, added to this by stating that: “This experimental breakthrough evidences that the Spintronics work package is acting as a pathfinder to bring all the potential of graphene and related materials to future practical applications in spintronics. It is, however, also clear that more efforts and investment are crucially needed to accelerate large scale integration of such type of realisations into the European fab environment for acquiring industrial leadership beyond the recognised European scientific excellence in this field.”

The Graphene flagship – at the forefront of their field

Towards the end of last year, Graphene Flagship researchers Khokhriakov and Cummings et al. released a paper on the Science Advances website, cited by www.graphene-flagship.eu, explaining their recent findings that demonstrate ‘how heterostructures built from graphene and topological insulators have strong, proximity induced spin-orbit coupling which can form the basis of novel information processing technologies. Graphene’s spin-orbit coupling, and high electron mobility make it appealing for long spin coherence length at room temperature.

‘Researchers showed a strong tunability and suppression of the spin signal and spin lifetime in heterostructures formed by graphene and topological insulators. This can lead to new graphene spintronic applications, ranging from novel circuits to new non-volatile memories and information processing technologies. This paper brings us closer to building useful spintronic devices. The innovation and technology roadmap of the Graphene Flagship recognises the potential of graphene and related materials in this area. This work yet again places the Flagship at the forefront of this field, initiated with [the] pioneering contribution of European researchers.’

Potential advances in spintronics through spin-orbit coupling

Outside of the Graphene Flagship, and according to a paper published by Nature Communications, a graduate electrical and computer engineering student from Purdue University, Chaun-Hsun Li and his team, have developed a testing ground for quantum systems where they can turn certain particle interactions on and off. It is believed that this could potentially pave the way for advances in spintronics, especially in accordance to spin transport electronics and the revolutionising of electronic devices.

Compared to conventional electronics, spintronics might be faster and more reliable as a result of its spin being able to be changed quickly, and over less power. Standard electronics encode information by an electron charge, whereas spintronic devices use the intrinsic spin property on an electron. However, one of the main current problems in this field according to Chaun is when spin current decays, because the signal is then lost. He said: “People want to manipulate spin formation so we can use it to encode information, and one way to do this is to use physical mechanisms like spin-orbit coupling (the interaction between a particle’s spin and momentum). However, this can lead to some drawbacks, such as the loss of spin information.”

Despite this, Chaun’s study demonstrated how, by using a mini quantum fluid collider for BEC (Bose-Einstein condensate), spin current decay can be enhanced. The researchers used lasing lasers, and Rubidium-87 atoms within a vacuum chamber were trapped and cooled nearly to absolute zero. As they get colder, the atoms then reached a quantum state, and thus began to overlap with one another and stopped behaving like individuals. The fluid collider was then used to collide two BEC’s with opposite spin to partially penetrate each other, so delivering a spin current.

Chaun has commented on this procedure: “Using this system, researchers can literally turn spin-orbit coupling on and off, which allows them to isolate its effect on spin current decay. This cannot be done with electrons in solid-state materials, which is part of what makes this system so powerful. One important challenge for spintronics and other related quantum technologies is to reduce decay so we can propagate spin information over longer distances, for longer times. With this new knowledge of the role of spin-orbit coupling, this may help people gain new insights to reduce spin decay and potentially also design better spintronic devices.”

As programmes such as the European Graphene Flagship begin to expand their research into spintronics, so does our understanding of the current and potential future of this field. Research studies such as those conducted by Khokhriakov and Cummings et al. and Chaun-Hsun Li, shed light on how spintronics might be faster and more efficient compared to current standard electronic devices. However, spintronic research and development is also expanding outside of the Graphene Flagship.

Spin current switches

Research on spintronics at Tohoku University in Japan by Zhiyong Qui and Dashi Hou et al. has discovered a switch to control the spin current, a mechanism needed for information processing with full spin-based devices. A ‘spin current switch’ is arguably the equivalent of the transistor used in electronics to enable and disable the flow of electricity. The research features the development of a newly-developed layered structure of materials that work as a spin current switch. Using the structure, the researchers were able to control the transmission of spin current at a 500% increase at near room temperature.

The report published on the Tohoku University website explains how they were able to do this: ‘The tri-layer structure sandwiches Cr2O3 between yttrium iron garnet (YIG) and platinum (Pt). The YIG/Pt pair is a standard combination of materials used to investigate the spin current flow – both are insulators in which electrons cannot flow. YIG, a ferrimagnetic electrical insulator, generates spin current in response to RF microwave or temperature gradient and Pt, a paramagnetic metal, detects the spin current as an electric voltage via ISHE. By placing Cr2O3 between the materials, the voltage signal at Pt reflects how much the Cr2O3 layer can transmit the spin current. The researchers investigated the change of the voltage against the temperature and the applied magnetic field.’

The researchers then: ‘Observed a massive reduction in the voltage signal when crossing the temperature at around 300K, at which point Cr2O3 changes its phase from paramagnet to anti-ferromagnet (Neel point). The change of the spin current transmission is a near 500% increase under the application of a magnetic field. This behaviour suggests that the layered structure works as a spin current switch when crossing the Neel point of Cr2O3 or applying a magnetic field. Just as the transistor revolutionised electronics by enabling the scalable development of electronic devices, the discovery of a spin current switch is likely to take spintronics in a new direction. It’s a significant development.’

Paving the way for practical spintronic devices

At NTU (Nanyang Technological University) in Singapore, researchers Elbert Chia, Marco Battiatio and Justin Song et al. have started to pave the way for practical spintronic devices by successfully injecting large spin currents into a semiconductor material. This study exceeded the previous record by several orders of magnitude. Previous research had failed to inject spins into semiconductor materials in a way that was efficient and cheap.

However, the new study, which has been published in Nature Physics with the authors subsequently being interviewed by Asian Scientist Magazine, has ‘demonstrated that highly efficient spin injection can be achieved even across a bare metal-to-semiconductor interface. The researchers first fabricated a single two-dimensional layer of the semiconductor material molybdenum disulfide (MoS2) coated with a magnetic layer of cobalt. They then used femtosecond laser pulses to generate spins in the cobalt layer, measuring the resulting spin currents generated in the semiconductor MoS2 layer on the other side. As it was difficult to directly measure the ultrafast spin currents with existing technologies, the researchers made use of the high spin-orbit coupling of MoS2, which ensured that most of the spin currents generated would be converted to terahertz waves that could easily be detected. Using this proxy measure, they estimated that their method resulted in a spin current density in the MoS2 layer of 106-108 A/cm2, exceeding the previous record by 10,000 times.’

In the interview with Asian Scientist Magazine, Chia said: “In real devices, such strong spin currents will not be required, so one can get away with considerably weaker excitations. Laser power a thousand times less than what we used in our experiments would be able to achieve the same kind of spin injection that we see in the spintronics industry today.”
Co-researcher Song added to this by saying that “the ability to inject spin currents into semiconductor materials in a highly efficient manner makes spintronic technology compatible with the mature semiconductor industry, thereby greatly expanding its range of applications. Possibly the most striking aspect is that all this was demonstrated using a simple metal-semiconductor interface, without the complicated and costly structural engineering one sees in other spintronic experiments.”

Spintronics and artificial intelligence

Researchers at Osaka University have recently conducted a study on artificial intelligence (AI) hardware and spintronics. Despite the rapid evolution of AI technologies over recent years, AI hardware still needs to be enhanced in terms of its capabilities and becoming more energy efficient. Although there has been developments in the way energy is consumed, AI applications tend to be computation heavy, with power hungry hardware. In response to these limitations, researchers have developed a way to reduce power consumption of MRAM (magnetoresistive random access memory) and AI devices.
Unlike traditional electronics, spintronics exploits electron spins as a further degree of freedom, resulting in implications in the efficiency of data storage and transfer. Spintronics is a widely researched field in which MRAM technology has been developed using magnetic tunnel junctions (MTJs). MRAM uses the direction of a magnetic pole to store information, so it can retain memory without standby power. Thus, by using these technologies, researchers have tried to reduce the energy consumption of AI devices. The problem for spin-transfer-torque MRAM (STT-MRAM) is that its voltage increases rapidly when its write speed is high, using a great deal of power.

By increasing the magnitude of voltage-controlled magnetic anisotropy and changing the magnetic anisotropy in an MTJ, researchers have found that it is possible to write information that uses less energy than STT-MRAM. The magnetic anisotropy depends on the bias voltage due to Joule heating. According to a report published by Analytics India Magazine, it was also discovered that: ‘the research group … observed microwave amplification by an MTJ using the giant magnetic anisotropy change.

Microwave amplification had been previously attempted using a microwave-frequency magnetic field; however, the microwave power obtained by conventional methods was 0.005, and there was no amplification. Heat-driven engines are hard to realise in nanoscale machines because of efficient heat dissipation. However, in the realm of spintronics, heat has been employed successfully — for example, heat current has been converted into a spin current in a NiFe|Pt bilayer system, and Joule heating has enabled selective writing in magnetic memory array.’

In an interview with Analytics India Magazine, researcher Minori Goto said: “Our study is the first report of microwave amplification using spintronics devices. This research will open the way to developing high-performance microwave devices. Moving forward, we anticipate our technology will be applied to new microwave devices with high sensitivity and high output. This will also contribute to low-power-consumption technology for MRAM and AI hardware.”

The amount of research and development in spintronics over recent years is resulting in it becoming one of the leading alternatives to conventional electronics. The main reason for this is that spintronics offers faster information processing, whilst using less energy, by using an electron spin instead of their flow for the carrying of information. The future ahead of spintronics is long and exciting, and research in and outside of the Graphene Flagship will continue to bring these developments forward.

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