Broadband and Wide-Angle Nonreciprocal Electromagnetic Transport via Gradient Permeability-Near-Zero Metastructures.

Breaking time-reversal symmetry to achieve broadband and wide-angle nonreciprocity is critical in electromagnetic waves manipulation. While Epsilon-Near-Zero materials have revolutionized the control of Transverse Magnetic modes, realizing comparable broadband nonreciprocity for Transverse Electric (TE) modes remains a formidable challenge because the permeability of the magneto-optical materials is isotropic. Here, we propose a novel strategy utilizing gradient Permeability-Near-Zero (PNZ) subwavelength magnetic metastructures based on yttrium iron garnet thin films. By engineering the magnetic permeability tensor near the zero-index regime, we excite robust TE-polarized leaky modes (analogous to magnetic Berreman modes) in deep sub-wavelength films (<λ/100, λ is wavelength). This mechanism unlocks a continuous nonreciprocal window, achieving a maximum nonreciprocity of 0.93 (approx. 12.4 dB) across the entire 35-40 GHz band. Unlike resonance-dependent metastructures, our design exhibits intrinsic robustness, maintaining high performance over an exceptionally wide incidence angle range (> 40°) and thickness tolerances (-28% to +24%). This work fills the critical gap in magnetic light-matter interactions, offering a scalable and robust material platform for next-generation directional electromagnetic energy transport, emission control, and signal isolation.