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

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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.