Non-local spin transport across electronic phase transitions

Lead Research Organisation: University of Liverpool
Department Name: Physics

Abstract

The successes of nanoscale magnetism and spintronics (where the spin of the electron is manipulated) have been enabling for materials-by-design magnetism. However, these accomplishments have placed ever more emphasis on precisely controlling the transportation of electron spin. This is nowhere more critically seen than in the case of hard disk drive read heads, where continued reduction in read head size places serious limits to the future use of tunnel magneto-resistance sensors - a major challenge for the ICT industry. Low impedance alternatives are now actively sought, with all-metallic devices returning to the forefront of interest.

Remarkably, despite the ubiquity of spintronic devices like read heads, there remain stark gaps in our understanding of spin transport in metallic systems at the nanoscale. Even in the (relatively) simple ferromagnetic and non-magnetic materials used in magnetoresistive devices, recent results have called into question our understanding at this level. This imposes a number of substantial challenges for their future use. Moving beyond these materials even less is known and, in general, the wider interplay between precise electronic phase and spin transport is only beginning to be probed. The impact of such limitations to current technology is readily seen, with the vast majority of spintronic devices limited to considering only the manipulation of long-range ferromagnetic materials, e.g. in storage applications. Indeed, the possibility of controlling state with spin, beyond ferromagnetic switching, could bring entirely new functionality to spintronic devices, potentially leading to transformative new technologies -- an exciting prospect.

The aim of this proposal is to explore mediating phase transitions using pure spin currents. We will first explore pure spin transport, using a device known as a 'non-local spin valve' as a research platform to incorporate complex magnetic materials. Initially this will involve tailoring spin channel properties to systematically bring to light the role of specific defects in limiting spin transport -- crucial results for enhancing spin signals in metallic devices. We will then move to understand the interplay between electronic phase and spin diffusion, attempting to probe spin transport across a host of fundamental phase transitions, including spin glass freezing, metallic to insulating and ferro- to antiferro-magnetic. By doing so, a wealth of new information on the interaction of spin currents with phase will be revealed. Through a number of spin generation techniques, we will examine the role of the torques from absorbed spin currents in stabilising phases, enhancing critical temperatures, moving phase boundaries and inducing critical fluctuations across a host of these transitions. By using the NLSV for these studies, we will be able to explore such effects in a novel but technologically relevant environment.

Planned Impact

To summarise impact in the themes of economy, society, knowledge and people:

Economy

The fundamental research of the project will be performed in a geometry of key interest to the hard disk drive industry (worth >$30bn in 2014). Major manufacturers (including Seagate and HGST) are already actively investigating its use. New technologies are critical in this industry to overcome future scaling difficulties. The research, therefore, will directly pertain to the issues faced by manufacturers. The results will position the UK as a leader in this form of device, and so economic impact can be significant.

This research is particularly timely, coming at the juncture between a need within the ICT sector to find electrically-controlled, solid state logic devices, and the recent development of a wealth of new methods to interact currents and magnetic order. Advances in nanomagnetic devices would be extremely well received by technology companies, several of which aim to commercialise solid-state spintronic memories. The demonstration of a mechanism such as spin current mediated phase change would open a pathway to wholly different forms of device, beyond traditional ferromagnet-based schemes, an idea of great appeal to this industry.

Society

Ensuring the continued ability to store and process data is a major global societal challenge. With digital information ubiquitous in everyday life, there is a continued search for low-power, energy-efficient devices to enable this unprecedented level of connectivity. Over the long term the knowledge generated from the proposed research towards efficient electrical control of state using spin has the potential therefore for real impact on enhancing quality of life. Research from the proposal will align with the UK government vision to 'foster and promote a clear aspiration' in this field through an 'Internet of Things' vision (Blackett Review, UK Gov Office for Science, 2014), and become a leader in this 'second digital revolution'.

Knowledge

Strategically, spintronics and its potential applications for low power computing and data storage align well with EPSRC Grand Challenge of 'nanoscale design of functional materials' and form a tenet of advanced materials and nanotechnology, which represents one of the UK's '8 great technologies'. Knowledge amassed during this work has the potential to impact multiple scientific areas, including: bio-sensing; novel cancer therapy techniques; data storage and logic; and energy harvesting. The UK has a strong international presence within these fields, and building on our knowledge of technology-driven engineered magnetic materials plays to its strengths. It is conceivable that the milestones of the project would provide considerable new research pathways for the fields of spintronics and nano-magnetism in this regard.

In joining efforts from condensed matter physics and materials science, the proposed research will bring new knowledge to each field, capitalising on the strength of both research partners. This combination of advanced fabrication with sophisticated materials deposition will ensure successful generation of long-term key knowledge in this interdisciplinary field, while fostering collaboration and an exchange of expertise between the UK and US.

People

The proposed research will offer considerable professional development opportunities for all researchers (including post-doctoral and masters level). This will be of benefit to their outlook and greatly enhance their cross-discipline skill set. The high impact of the results and short duration of the project will offer them an excellent opportunity to accelerate their career path. These early-stage researchers will gain experience of top-level technical research, with cutting-edge skills in nanoscaleelectronics and fabrication. This will make them valuable highly-skilled workers for future employers, particularly in the UK industrial research and development sector.

Publications

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Description i) Work at Liverpool and Minnesota has begun to understand the underlying microscopic mechanisms limiting spin transport in low atomic number metals. Most importantly, the dominant mechanisms are very different to those that limit traditional electron transport, with spin particularly sensitive to dilute magnetic impurity scattering events. ii) We have discovered that the spin equivalent to the charge Kondo effect is not only remarkably efficient at relaxing spin accumulation, and so destroying spin currents, it can also cause anomalous suppression of spin signals even at room temperature. This is quite unusual given that the traditional charge Kondo effect is normally considered a low temperature effect. iii) Spin transport in ultrathin metallic films is particularly sensitive to the precise distribution of impurities and growth conditions of the film. In heavy metals, such as Pt, this sensitivity can completely disrupt the usual relationships relating spin and charge transport, meaning precise morphological characterisation is necessary to accurately quantify any spin transport parameters.
Exploitation Route Observation of room temperature Kondo effects could have wide implications for fundamental spin transport research. It suggests that in common materials such as Co and Cu, which are often used as materials for a variety of spintronic studies, the measured spin signals could be being suppressed from their intrinsic values (up to 10's of percent) due to complex chemical interdiffusion between each material. It suggests careful choice of materials combinations are needed to best optimise the output from a device. Furthermore, thin film morphology in high spin orbit coupled materials plays an influential role. Other researchers will hopefully put these results to use in designing their spintronic devices, whether studying spin relaxation explicitly, or using the materials to study a more diverse range of spintronic effects. When designing next-generation low resistance magnetoresistance sensors, maximising spin signals will be key. These results can be used to inform material/morphology/purity choices when constructing sensors or sensor design.
Sectors Digital/Communication/Information Technologies (including Software),Energy