Bias-Free Tunable Microwave Metamaterial Based on Co₂₅Fe₇₅ Nanowire Array

We report a magnetic metamaterial based on a lithographically patterned array of Co₂₅Fe₇₅ nanowires, in which the ferromagnetic resonance (FMR) frequency is fully governed by shape-induced demagnetizing fields, eliminating the need for an external magnetic bias. The combination of very high saturation magnetization and low Gilbert damping in Co₂₅Fe₇₅ results in strong, spectrally sharp microwave absorption with the zero-field FMR frequency tunable across the K_u and K bands. We developed a theoretical model that accounts for inter-wire dipolar interactions and shows excellent quantitative agreement with the observed absorption spectra. These results demonstrate that controlled magnetic shape anisotropy in low-damping, high-magnetization Co₂₅Fe₇₅ enables a new class of bias-free, tunable, and low-loss magnetic metamaterials. Such structures provide a practical platform for compact integrated microwave components such as notch and bandpass filters, as well as high-efficiency planar antennas.