Controlling Acoustic Metamaterials with Magnetic Resonances: The Best of Both Worlds

The world around us is full of devices, ranging from smartphones to airplanes. Moreover, our civilization is defined to a great degree by the functionalities that those devices can deliver. However, when constructing and indeed even conceiving a device, engineers operate within constraints set by properties of materials available, either in nature or via industrial processes. These material properties together with the laws of physics then restrict functionalities that the device may have. Radically new dynamical properties and advanced functionalities can be created by tailor-tuning the spectra of wave excitations in structured media - so-called metamaterials. Recently demonstrated and proposed practical applications of such artificial materials include e.g. optical fibres (manipulating light), lasers (manipulating electrons), and noise absorption and heat steering (manipulating acoustic waves).

The properties of 'surface acoustic waves' (SAWs) have been investigated for over one hundred years, but it was the invention of electro-acoustic "interdigital" transducers in 1965 that enabled surface acoustic wave devices to be developed for a wide and diverse range of functions, including analogue signal processing in mobile phones and sensing. Recently, the field of metamaterials research has expanded to acoustic waves, promising a method to control and manipulate propagation of surface acoustic waves. These so called acoustic (or phononic) metamaterials could both extend the functionality of existing devices and underpin totally new device concepts. However, to date there have been very few suggested ways of designing acoustic metamaterials that can be dynamically reconfigured and tuned, limiting their use in applications. Integration with magnetic materials, well known for their ability to store information e.g. in magnetic hard disk drives, offers an exciting route for achieving non-volatile tuning of acoustic metamaterials.

Our project aims to develop a new class of magneto-acoustic metamaterials in which the role of their building blocks ("meta-atoms") is played by magneto-acoustic resonators. Such metamaterials will add exquisite magnetic field tunability to structures aimed to control the propagation of surface acoustic waves, opening intriguing opportunities both in fundamental science and technology. Technologically, the memory phenomenon inherent to magnetism will enable significant energy savings in non-volatile magneto-acoustic data and signal processing devices. For instance, they would be instantly bootable and could be more easily integrated with the existing magnetic data storage devices. From the point of view of fundamental science, the magneto-acoustic metamaterials developed in our project will serve as an excellent test bed for studying the physics of wave propagation in non-uniform and non-stationary media.

The collaborative research programme will be conducted jointly by the Department of Materials Science and Engineering at the University of Sheffield and the College of Engineering, Mathematics and Physical Sciences at the University of Exeter. The Sheffield team will contribute to the project their internationally leading expertise in magnetostrictive and multiferroic materials and nanotechnology, while the Exeter team will contribute their world leading expertise in dynamical characterization and theoretical modelling of acoustic and magnetic metamaterials and devices. By joining their forces together, the two teams will ensure that UK will remain at the forefront of the acoustics and magnetism research and technology, in particular opening the new interdisciplinary field of magneto-acoustic metamaterials.

On the time scales of the project duration, our research will have direct and obvious impact within its own interdisciplinary topic at the interface between Physical Acoustics and Nanomagnetism, including their Physics, Materials Science and Engineering sectors. The impacts of the proposed research within the broader acoustics and magnetism communities will develop on the time scales of years, by way of supplying important information on the key underpinning physics. This is likely to include experts in applied mathematics, many of whom work in the field of acoustics. The research will reach out to other fields of study of metamaterials and waves of other nature, such as photonics, plasmonics, etc, on somewhat longer time scales, perhaps also a few years. The impact will be achieved via propagation of theoretical concepts adoptable within the said research fields through their publication in multidisciplinary journals and presentation at multidisciplinary scientific meetings and academic seminars. Subject to successful delivery of the main targets proposed here, the interdisciplinary field of magneto-acoustics can experience a renaissance on a longer time scale of 3-6 years, leading to the emergence of completely novel interdisciplinary paradigms and associated technologies, the impact of which is difficult to predict at this point.

Technologically, the proposal is centred on a new device paradigm potentially able to make a step change in the hardware underpinning information and communication technologies (ICT). ICT will therefore be the main vehicle through which our research will impact society and the economy, both in the UK and internationally. Indeed, the recent advances in ICT, in particular including the proliferation of the use of internet and mobile network devices, have changed working practices, hobbies, healthcare, and the overall quality of life, penetrating all layers of the society. In this context, our research will benefit wider ICT research from the outset, also including via competition and synergy with the established ICT paradigms.

Our project will contribute to training of the personnel needed to drive the growth of physics, materials science and the ICT sector within the UK. The postdoctoral researchers at Exeter and Sheffield will gain experience of the use and development of advanced fabrication and measurement techniques, of the development and application of theoretical models implemented using the state of the art computational tools, and of working within a distributed team where effective communication is required to deliver ambitious goals. These young researchers will develop a comprehensive capability for the conduct of independent research that will prepare them for permanent academic positions or employment within industry.

Finally, the project will contribute to strengthening and development of the critical mass of the acoustics and magnetics technology research base in the UK. In longer term, when related commercial applications are realised, this will prove important in the decision making with regard to the location of the key research and manufacturing businesses (ranging from start-ups to branches of the established transnational industrial giants, such as Intel or Seagate) to be set up, hopefully within the UK or perhaps in the EU.