New Publication: Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

Congratulations to David Osuna, whose paper, ‘Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core’, was published this month in Physical Review B. David has recently finished his PhD in the CDT and is now a postdoctoral research fellow at University of Exeter, as part of the Electromagnetic and Acoustic Materials Group (EMAG).

David explains the paper’s topic:

“This was a very fruitful collaboration with Dr. Paul Keatley, finally published after about 3 years of hard work!

Generally speaking, we have modeled and ‘filmed’ oscillations of the atomic magnetic spins in microscopic magnets and related them to other dynamics revealed from simulations. Understanding this type of dynamics as a whole is key to design spintronic devices, that may be essential for processing information in quantum computers, for example.

Abstract

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disk have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest frequency, azimuthal modes has previously been reported to lead to their splitting and counterpropagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disk thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is permitted only with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disk, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes, enhancing the spiral nature of the curling mode. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disk. The measured spatiotemporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disk. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetized regions. These results are of importance to the fields of magnonics and spintronics that aim to utilize spin wave emission from highly localized, nanoscale regions of nonuniform magnetization, and their subsequent interaction with modes that may be supported nearby.

Fig. Time sequence of a radial-azimuthal spin wave mode simulated and experimentally imaged in a 2 micrometres diameter, 40 nm thick Permalloy disc with an in-plane RF excitation field at 10.24 GHz. Timestep is approximately 24 ps.

Welcome to our new cohort for 2020!

We have seven new students this year, who have just finished the first week of their projects! Having attended a busy virtual induction week, they were pleased to meet the rest of the CDT face-to-face in a socially-distanced outdoor treasure hunt across the city.

Our new students are:

2020 XM² cohort Theme Project Supervisor 1 Supervisor 2
Kyle Arnold  Radiofrequency and microwave Artificial Magnetic Conductor Surfaces for Conformal Antenna Design Alastair Hibbins  Roy Sambles
George Braid  Visible, Infra-red and Terahertz Laser implosion fusion for clean energy generation David Wright Jacopo Bertolotti
David Bruce  Acoustics and Phononics New metasurfaces for acoustic control (Leaky Waves) Alastair Hibbins  Roy Sambles
Jenner Gudge-Brooke  Radiofrequency and microwave Miniaturised helical antennas for superdirectivity Alastair Hibbins  Roy Sambles
Dan Moore  Acoustics and Phononics Making the world sound better – acoustic metamaterials for manipulating sound in air and underwater Alastair Hibbins Roy Sambles
Tom Moynihan  Radiofrequency and microwave Effect of Material Conductivity and Architecture on Electromagnetic Surface Wave Propagation Alastair Hibbins  Roy Sambles
Lee Worgan  Acoustics and Phononics RF and mm-wave imaging using optical modulation Euan Hendry  Nick Stone

Tom is based in Cambridge working with TWI and David is based in Dorset, undertaking his PhD part-time whilst working at Qinetiq.

New Publication: ‘Microwave Superdirectivity with Dimers of Helical Elements’

Congratulations to third year PGR Pavel Petrov, whose article ‘Microwave Superdirectivity with Dimers of Helical Elements’ was published yesterday in Physical Review Applied.

Pavel summarises his article:

This paper demonstrates experimental realisation of the subwavelength superdirective antenna that reaches close to theoretical maximum of directivity without using complex feeding network. We introduce a novel design of superdirective dimer using helical elements, and describe how parameters of such structures can be optimised using our analytical method. We than show how the geometric parameters of helices and the conductivity of material used may affect directivity, efficiency and gain of the optimised dimer structure. This work is a result of the year and a half of hard work which has been sponsored by DSTL and is a part of SYMETA collaboration.

New Publication: Metasurface bilayer for slow microwave surface waves

Congratulations to fourth year PGR Julia de Pineda Gutiérrez, whose paper ‘Metasurface bilayer for slow microwave surface waves’ was recently published in Physical Review B. Julia’s previous publications include Microwave edge modes on a metasurface with glide symmetry and Hexagonal symmetry metasurfaces for broadband antenna applications. Julia’s thesis is on exploration of beam shaping at microwave frequencies using metasurfaces and metamaterials .

Abstract for ‘Metasurface bilayer for slow microwave surface waves’ below.

Abstract

We present a simple two-layer discontinuous crossed metal-strip array that guides microwaves having very high phase and group indices. The strips are arranged on a square lattice with a two-layer unit cell. The difference in this structure resides in the length of the metal strips, which extend to several unit cells. This work focuses on the isotropic wave dispersion at the lower frequencies. In addition, two of the higher-frequency bands give rise to a very strong negative dispersion, and strong beaming occurs, which can be tailored easily by modifying the relative orientation of the layers.

 

Flann and University of Exeter research project aims to pave way for 5G communications revolution

Microwave engineering company Flann Microwave is teaming up with the University of Exeter as part of ground-breaking research which could help pave the way for the next generation of 5G mobile communications.

Flann is working with PhD researcher Julia De Pineda-Gutiérrez from the Department of Physics and Astronomy on a four-year project which aims to use metamaterials to revolutionise antenna design for point to point radio networks – such as mobile phone networks – with the aim of making these smaller, lighter and cheaper to manufacture and install.

It is work which could overcome one of the key barriers in making the leap from 4G to 5G networks, which would require more regularly placed, highly directional microwave antennas to handle the increased data volumes and speeds involved.

The project is the latest in a series of collaborations over many years between the company, based in Bodmin, Cornwall, and the University of Exeter, a relationship which has grown closer with the establishment at the University of the £12 million EPSRC Centre for Doctoral Training in Metamaterials (XM2).

Professor James Watts, Chief Executive of Flann Microwave, serves on the XM2 Oversight Board and was last year made an Honorary Associate Professor by the University.

He said: “This is incredibly exciting work which has implications nationally and internationally in the development of next generation communications networks, which face a considerable challenge in moving from 4G to 5G, much more so than with the step up from 3G to 4G.

“We’re delighted to be continuing our association with the University of Exeter, which has a growing reputation in the field of metamaterials research. We are excited at an academic level and by the practical and commercial opportunities which we hope will flow from this project and which could one day become mainstream in network development.”

Metamaterials involves materials being treated or engineered to give them special properties not normally found in nature. In the case of the research being carried out by Flann and Ms De Pineda-Gutiérrez, this involves developing surface structures and materials which can be used to manipulate radio waves to form the narrow beams needed for communication between mobile base stations.

As demand grows for higher capacity mobile networks, this technology opens the prospect of subtly incorporating antennas into everyday features and structures, potentially avoiding the visual clutter associated with conventional antenna types.

Professor Alastair Hibbins, Director of the CDT in Metamaterials, said: “We are particularly pleased to play an active role in building exemplary relationships between SMEs and the University of Exeter. Our long-standing collaboration with Flann Microwave strengthens the cluster of innovation and research with industry across the South West of England, and demonstrates mutual benefit for us and the local economy.  Our centre trains around 70 highly skilled PhD researchers, and working with industry gives us the opportunity to explore new areas of research, and the interaction with industry is crucial in moulding the training we provide to our students: ultimately we want them to succeed as scientific leaders in industry, and academia.”

With a 60-strong team based in Bodmin, Cornwall, Flann has grown over six decades to secure a global reputation and market leading position in the design and manufacture of precision microwave communications equipment, for example, to allow internet and mobile data to be carried between mobile phone masts or through satellite links.

Its innovations have been pivotal in the development of mobile telephone networks, from the very inception of the technology through to the huge growth in mobile use over the last two decades and right up to its ongoing research into 5G.

The company serves customers in the telecoms, government, automotive, aerospace, defence and research sectors, exporting more than 80 per cent of production. It has also worked with UK and overseas government agencies to write many of the standards used in the industry.

Pictured below, Professor James Watts and CDT in Metamaterials PGR Julia De Pineda-Gutierrez.

Flann Microwave are sponsoring a postgraduate researcher (PGR) at the University of Exeter. Pictured are CEO James Watts and the PGR Julia de Pineda-Gutiérrez in a physics lab on Streatham Campus. (Photo by Theo Moye 28/11/17 )

New Publication: The true potential of the Weyl particle – A publication in Science

The quest to understand a variety of intriguing phenomena that may advance progress towards the next generation of computing has taken a quantum leap.  Through the science of metamaterials, quantum physics and the mathematics of topology, a team of scientists , including researchers from University of Exeter, have been exploring the properties of so called ‘ideal’ Weyl systems.  They have constructed an artificial analogue to a conventional material that has been carefully tuned to support ‘massless’ particles at microwave frequencies.

The particles exist in materials that have a singularity in their electromagnetic properties, similar to a 3-dimensional graphene, called the Weyl point. In recent years, scientists have tried to create an ideal system, where these Weyl points can exist at the same energy. The ideal nature of the new design, supporting massless particles, makes them candidates for practical applications in electronics, optoelectronics and, perhaps, quantum computing.

Lauren Barr, a postgraduate researcher from Exeter’s College of Engineering, Mathematics and Physical Sciences  and co-author of the research explained: “The key feature that makes our Weyl metamaterial different to others is the broad frequency range around which we can examine its topological nature – this is what makes it ‘ideal’.

“That means we can more easily explore new ways of controlling waves in a Weyl semi-metal, potentially leading to breakthrough advances in technology for faster and lower-power consumer-electronic devices.”

Professor Alastair Hibbins, Director of Exeter’s EPSRC Centre for Doctoral Training in Metamaterials said: “We extremely pleased to have been involved in such a fantastic collaboration. Exeter provided the state-of-the art experimental kit to undertake the microwave work, but thanks to Prof Shuang Zhang and his team at the University of Birmingham for leading the project so well.  Its been great to see our young researchers engaging in such high-quality science and I’m sure that they will reap the rewards of this result for many years to come!”

Ideal Weyl points and helicoid surface states in artificial photonic crystal structures is published in Science, and is highlighted as a prestigious ‘First Release’ article: http://science.sciencemag.org/content/early/2018/01/10/science.aaq1221.

A  Close up of the metallic saddle-shaped wire the metacrystal is made of.
Photograph of the top of the metacrystal showing how the individual “saddles” are arranged and made using circuit board technology.
C  Four Weyl points are illustrated as singularities at the intersections of the red cones in a plot of the metacrystal’s electromagnetic response; these occur at the same energy, omega, indicated by the pale blue sheet. Reproduced with permission from Science.

>> See all XM2 publications on www.exeter.ac.uk/metamaterials/research/publications/ <<

KTN event: Industrial and Commercial Applications of Metamaterials & MRE2018

On 16 Jan 2018 the third KTN event for Metamaterials took place in London, focussing on Industrial and Commercial Applications of Metamaterials. Pavel Petrov, one of our first years, presented a poster for the first time, and our CDT Director, Alastair Hibbins, was invited to introduce the CDT to the about 150 industry and academic representatives with his talk “Creating new leaders for academia and industry: Exeter’s collaborative approach to doctoral training in functional and meta-materials”.

The talk was well received and provided an excellent opportunity to showcase the breadth of the CDT. We used the KTN event to catch up with existing partners and build links with new companies, e.g. Metaboards and Metasonics, who are currently recruiting – we will try to invite them for talks in our Beyond A PhD series later this year.

The next big event in Materials Science will be the Materials Research Exchange & Investor Showcase 2018 (MRE2018) on 12 and 13 March 2018 in London, with over 1000 expected exhibitors. Staff and PGRs are ecouraged to register now – the delegate rate will be £0 until 8 February 2018 (early bird rate).