Designing a non-reciprocal metasurface with zero net magnetization for large-aperture microwave applications

The key element of most nonreciprocal devices, such as microwave and optical isolators, circulators, and nonreciprocal phase shifters, is a magnetic material, self-biased or placed in an external magnetic field. However, a magnetized material creates its own demagnetization field, which may be nonuniform and thereby seriously degrade the performance of a nonreciprocal device. Another common problem is the magnetic field outside the magnetic component. There are important applications, such as quantum navigation and sensing, that cannot tolerate even a tiny magnetic field. Using single-phased compensated ferrites can also be problematic because they maintain zero net magnetization only at a certain temperature. Here, we introduce a self-biased composite structure of magnetically hard neodymium cylinders incorporated into a flat, magnetically soft matrix of yttrium iron garnet (YIG). The array of neodymium cylinders provides a uniform magnetic bias for the YIG matrix, while YIG itself is responsible for the Faraday rotation. This structure provides a strong and temperature-independent Faraday rotation while maintaining zero net magnetization within a wide temperature range. The aperture of such a Faraday rotator is virtually unlimited. We demonstrate a highly uniform 45-degree Faraday rotation for an X-band microwave beam.