Spin physics in Two-Dimensional Layered Ferromagnets

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology


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.

Planned Impact

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.


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Description We discovered that an electric field can effectively and efficiently control magnetic states of 2D ferromagnetic systems. For example, we can switch on and off magnetism of this material by applying a few volts in a nano-device made out of this. This has been disseminated in our recent Nature Electronics paper [see Publications section].

We formulate the spin-orbit torque symmetry of major van der Waals magnetic materials based on group theory analysis. Together with summary of exchange interactions and magnetic order temperatures of van der Waals magnets, this has been published in Nature Review Physics in 2022.

We experimentally show the photon-magnon coupling between superconducting resonator and van der Waal magnon modes. This coupling is very powerful to study spin dynamics in van der Waals magnets and a manuscript summarising this study is under preparation.

We discover that the magnetic properties of van der Waal magnet Cr2Ge2Te6 can be significantly controlled by chemical doping. It can increase the magnetic-order temperature as well as rotate the magenetic easy axis by this due to electron doping. A manuscript summarising this study is under preparation.
Exploitation Route The fundamental working principles of our discovery are transferrable to other material systems. We envisage that there will be a good number of follow-up research projects across the world where these techniques/concepts are exploited.
Sectors Electronics

Description This project has created a number of new research directions on which associated group members can use for further funding opportunities. For example, the PI has submitted his EPSRC Open fellowship application based on the findings generated during this project. This project has identified acute needs of attaining a 2D transfer system specialised for this project and we have had a successful bid in EPSRC capital equipment calls (EP/V035630/1). This strongly impacts on the group research capability from which various new research paths using this facility are now envisaged. This project also made significant impact on individual researchers' research track record. For example, with several successful research outputs, the PI has been promoted to a professor recently. We successfully created a new PhD studentship with industrial partner NPL and appointed a talented young researcher. The PI chaired an online, international conference on "2D van der Waals Spin Systems" in SPICE workshops (August 4th - 7th 2020), which attracted over 300 participants. One of outcomes of this meeting is that we decided to publicise all talks, more than 20, on Youtube. This has a wide impact not only within the academic community but also other non-academic domains of society. Similarly, PI delivered a number of invited talks for online seminars, some of which were recorded and publicised on Youtube to the broad audience. The PI delivered a number of outreach lectures at UCL and also online. For example, he delivered a keynote lecture to Global Link Online 2022.
Sector Electronics,Other
Impact Types Cultural

Description UCL/NUS on 2D spin systems 
Organisation National University of Singapore
Country Singapore 
Sector Academic/University 
PI Contribution We share multiple research collaborations on various topics related to this funding with Prof. Goki Eda's group in National University of Singapore (NUS). Our group in UCL mainly provide our expertise in charactering 2D spin materials.
Collaborator Contribution Prof. Goki Eda's group in NUS mainly provides materials and device fabrications.
Impact "Controlling the magnetic anisotropy in Cr2Ge2Te6 by electrostatic gating" I. A. Verzhbitskiy, H. Kurebayashi, H. Cheng, J. Zhou, S. Khan, Y. P. Feng and G. Eda, arXiv:2001.10217; Nature Electron. 3, 460 (2020). "A spin dynamics study in layered van der Waals single crystal, Cr2Ge2Te6" S. Khan, C. W. Zollitsch, D. M. Arroo, H. Cheng, I. Verzhbitskiy, A. Sud, Y. P. Feng, G. Eda and H. Kurebayashi. arXiv:1903.00584; Phys. Rev. B 100, 134437 (2019).
Start Year 2013