Waves are commonly used in communications and imaging. Examples include microwaves that enable operation of cell phones and acoustic waves that enable ultrasound imaging. Typically these waves can freely propagate in any direction away from their sources. Many practical applications of such waves could be improved if the waves were made to propagate in only one specific direction. For example, such unidirectional waves enable communication devices that can simultaneously send and receive signals using the same antenna ? this is called full duplex communications. One goal of this project is to create new magnetic materials, in which microwaves and ultrasound waves propagate in only one direction. Another goal is to make a new type of very compact on-chip microwave and acoustic devices called circulators that are based on these novel materials. Such circulators are the core elements making the full duplex communications possible. In the course of this project, the principal investigators will train a number of undergraduate and graduate students, including those from Historically Black Colleges and Universities, in modern materials engineering, nanofabrication, and microwave measurement techniques, which will enhance the U.S. science and engineering workforce.
This project has two major goals: (i) development of chiral materials and meta-materials supporting strongly non-reciprocal spin waves and magneto-acoustic waves and (ii) demonstration of acoustic signal circulators and reconfigurable microwave circulators, which are based on the non-reciprocal waves. Two approaches to the generation of non-reciprocal waves will be employed. In the first approach, the wave non-reciprocity in magnetic films will be induced either via anti-symmetric exchange interactions by doping interfaces with heavy elements or via spin-flexoelectric interactions by applying an electric field perpendicular to the film surface. In the second approach, novel magnetic metamaterials lacking inversion symmetry will be made from arrays of ferromagnetic nano-elements. In these meta-materials, several types of non-reciprocal spin waves can be excited including spectrally protected modes and topologically protected edge states. By employing materials with strong resonant magneto-elastic coupling, the spin-wave non-reciprocity is transferred to magneto-acoustic waves. Non-reciprocal magnetic meta-materials developed under this program will be used to make and test acoustic wave circulators based on non-reciprocal, topologically protected chiral magneto-acoustic waves propagating along the meta-material edges. A reconfigurable ultra-compact microwave circulator based on spin-flexoelectric interactions in ferrimagnetic thin films will be made and tested. The direction of microwave signal propagation in this device can be rapidly controlled via external bias voltage. This research project will advance our understanding of non-reciprocal waves in magnetic meta-materials and will result in the development of a new class of ultra-compact, on-chip, non-reciprocal signal processing devices.
