Gyrotropic metamaterials with tailored magnetization

Abstract: Magnetic materials are crucial in nonreciprocal electromagnetic devices, such as isolators, circulators, and nonreciprocal phase shifters. However, their use is often limited by the need for a uniform bias magnetic field and nonuniform demagnetizing fields, resulting in the restricted aperture of free-space devices, poor temperature stability, and incompatibility with magnetic field-sensitive applications. Alternative methods have been developed to achieve nonreciprocity using active, nonlinear, and time-varying metamaterials, each with its own advantages and limitations. Here, we present a new approach based on self-biased gyrotropic metamaterials composed of magnetically hard magnets (specifically NdFeB) embedded in a magnetically soft ferrite matrix. In this configuration, the NdFeB magnets provide the magnetic bias for the ferrite matrix, which produces a nonreciprocal response. This gyrotropic metamaterial can exhibit zero net magnetization while producing strong and uniform Faraday rotation over a broad temperature range. Without bias and demagnetizing fields, the aperture of this Faraday rotator can be virtually unlimited. Using this method, we demonstrate uniform 45-degree Faraday rotation and effective isolation across the microwave X-band.