Nanowire-Embedded Bias-Free Gyrotropic Metamaterials

Nonreciprocal magneto-optical devices such as isolators and circulators typically rely on ferrite materials biased by external magnetic fields, which limits scalability and compatibility with magnetic-field-sensitive applications. We recently introduced a prototype gyrotropic metamaterial composed of magnetically hard NdFeB magnets embedded in a magnetically soft ferrite matrix, achieving strong and uniform 45-degree Faraday rotation across the microwave X-band without external bias and enabling configurations with zero net magnetization. Building on this concept, in this study, we demonstrate a nanostructured implementation using nanoporous anodic aluminum oxide (AAO) membranes infiltrated with Co and Ni nanowires (NWs). This approach eliminates rare-earth elements while preserving bias-free operation. Through numerical simulations and experimental measurements, we show that single-material NW arrays exhibit significant Faraday rotation within a high-Q microwave cavity, and stacking Co and Ni NW arrays magnetized in opposite directions achieves near-zero net magnetization with additive nonreciprocal effects. Our results confirm that nanowire-embedded metasurfaces can extend the performance of bulk structures, offering a scalable pathway for bias-free nonreciprocal devices. These findings open new opportunities for compact, magnetization-free metasurfaces in communication, sensing, and quantum technologies.