Spin physics in Two-Dimensional Layered Ferromagnets

For the last several decades, the development of currently available electronic devices has relied heavily on the downsizing of the transistor, allowing the technology for the small, powerful computers that are the basis of our modern information society. Moore's Law, effectively describing the growth of the number of transistors per unit area (and computing power), has continued ever since, but the end of that trend - the moment when transistors are as small as atoms, and cannot be shrunk any further - will approaching very rapidly. The characteristic feature length of transistors in latest smart phones is 7 nm, within which we can only fit about 14 lattices of silicon crystals. Electronic devices uses the charge of electrons to manipulate them for data-processing. This fundamental concept needs to be revisited now and radically new computing concepts have to be pursued and examined to sustain the further growth of computation efficiency. The concept of spintronics that creates a "spin"-based electronic technology holds potential to replace the charge-based technology of semiconductors and scientists have begun to examine the spin degree of freedom for new electronics.

At the heart of the development of spintronic technologies are new discoveries and understanding of magnetic materials at the nanoscale. Magnetic materials can store digital information by the direction of their dipoles (arrows pointing from South to North poles). Hard disk drives have a vast number of tiny magnets (magnetic domains to be precise) which act as data storages to secure our digital information reliably and cheaply. The reliability of data storage in magnets has been achieved by enormous efforts of understanding magnetic properties (so-called anisotropies) and reversal switching of the recording media as well as developing the controllability of thin-film multi-layers. Spintronics has taken it further to build more functional and active memory devices where local data-processing by flipping magnetic dipoles is performed. Reversing the dipole at a very low power consumption is a key to develop commercially-viable spintronic devices. To do so, continued efforts of discovering new magnetic materials, together with an understanding of their materials properties, is a valid and effective approach.
In this project, we will study a new class of magnetic materials, the van der Waals 2D layered ferromagnets. They are a magnetic version of graphene, and graphene is a single layer of graphite. A pencil is made out of graphite and the reason that we can write words on a paper with a pencil is because we break a bonding between sheets of graphene while writing and a broken piece of graphite (sheets of graphene) is left over on the paper. Scientists in the UK discovered that it is possible to make a single layer of graphene when we carefully break graphite sheets. And most importantly, graphene shows remarkable electronic properties which do not show up in the form of graphite. After the discovery of graphene, many van der Waals materials have been actively studied at the monolayer limit, forming the active research field of 2D materials. In 2017, the discovery of a magnetic version of graphene was made in two different materials and by two independent research groups, which attract a great deal of interest but yet not much is so far known about these materials. We will on this project study fundamental properties of magnetic 2D layered materials to answer important questions such as "are they different from normal 3D magnets?", "If so, how useful are they for our spintronic technologies?". We have specific workplans to answer these questions as much as possible and also to explore new discoveries with the novel class of nano-materials. Answering these questions allows us to advance the current understanding of ferromagnetism at 2D and spin transport therein, potentially leading to the creation of highly efficient spintronic memories.

Our project will have impact across a broad area of research within EPSRC Physical Sciences and ICT themes such as "Condensed matter: magnetism and magnetic materials", "Spintronics", "Functional Ceramics and Inorganics" and "Microelectronic device technology". Knowledge amassed throughout the project holds potential to impact on academic sector by developing a rapidly growing research area of "spin physics of van der Waals 2D ferromagnets (FMs)". Among different types of stakeholders, academia (both within UK and worldwide) and industry sectors will find benefit from this development. Not only the scientifically and technologically relevant sectors, but also the general public will receive benefit through our activities summarised below. To be more specific and measurable, we set the following impact goals.
(1) Hold sessions dedicated to 2D FMs in major international conferences in spintronics
At the end of the three-year programme, we aim to see sessions of 2D FMs in every major international conference, such as IEEE-Intermag, Magnetism and Magnetic Materials (MMM) and APS March meetings. To achieve this goal, we believe that the following two factors are key: 1 the number of researchers working on the topic has to be significantly increased and 2 the visibility of research work on the 2D FMs has to be rapidly enhanced. We can contribute to these two by achieving milestones set in our research programme where we expect at least one high impact journal publication from each. Clearly, the spintronics community can largely benefit from new knowledge and new materials we share, to explore possible exciting research projects with those. We can then generate more funding opportunities within UK and worldwide and train a new generation of early-career researchers (PhD students and postdocs).
(2) People and sample exchanges
During the course of developing the new research field of 2D FMs, we can make impact on local communities. One of major technical challenges of 2D materials in general is to establish good controllability of material thickness. This will be our first major milestone (M1-1), on which our external collaborator (Prof. Eda) will support us by training the Project co-investigator Safe Khan with an extended visit. Upon the completion of M1-1, we will be able to produce thickness-controlled 2D FMs (such as CrGeTe), which will be of interest from our spintronics community. The spintronics community in UK and worldwide will direct benefit from us sending 2D FM films for their own experiments. For example, our colleagues in Exeter holds a time-resolved optical pump probe technique (EXTREMAG: EP/R008809/1) and it would be interesting to observe real-time evolution/interaction of magnetism at the fs timescale where magnetism also interacts with phonons significantly; Also, UK spintronics which has strong capability of growing high quality magnetic multi-layers can design and grow more complex multi-layer samples on top of the 2D-FMs provided.
(3) A review paper on "spin physics of van der Waals 2D FMs"
To increase the visibility of the importance of van der Waals 2D FMs, we plan to write a review paper on the topic at the end of the project. Our international community in the field of both spintronics and 2D materials will be able to learn key results from our research as well as others. We will include a section of future prospective discussing about potential of existing 2D FMs for technological device applications, which can have a chance to be exposed to research & development scientists from industry for any future commercialisation opportunities.
(4) Outreach for general public
One of the key questions in our programme is "is there any difference between real 3D and 2D magnets?" A successful answer to this question on our research can be used to educate our member of public during any outreach lectures. We have several channels to deliver these outreach events which are summarised in Pathways to Impact.