Fe-Vacancy-Modified Bi-YIG Ferrites with High Saturation Magnetization and High Dielectric Constant for Miniaturization of C-X Band Circulators

The escalating demand for miniaturized high-frequency circulators in radar and satellite systems necessitates ferrite materials that concurrently possess a high saturation magnetization (4π M s ), a narrow ferromagnetic resonance linewidth ( ΔH ), and a high dielectric constant ( ε r ). In this study, a defect-engineered Bi 1.4 Ca 0.3 Zr 0.3 Y 1.3 Fe 4.6−x In 0.1 O 12−δ garnet ferrite was synthesized, in which synergistic Zr⁴⁺/In³⁺ dual-site doping and controlled Fe-vacancies improved magnetic performance. At optimal Fe-deficiency (x = 0.1), the material achieved a high 4π M s of around 2001 G, enabling C-X band operation. Simultaneously, it attained narrow ferromagnetic resonance linewidth ( ΔH ≈ 112   Oe) through reduced magnetocrystalline anisotropy (| K 1 |) and an enhanced dielectric constant ( ε r ≈ 26.2) via the “rattling” effect that compensated for vacancy-induced polarization loss. A straight-circuit microstrip circulator based on this ferrite ( r 0 = 2.14   mm, r 1 = 7.76   mm) was operated in the bandwidth from 4.23 to 9.85   GHz. It exhibited insertion loss ( S 21 ) better than −0.5   dB and isolation/return loss ( S 11 / S 12 ) below −20   dB. A curved-microstrip redesign further reduced the substrate area to 28.7% of the original while maintaining operation from 4.34 to 9.36   GHz, demonstrating exceptional potential for compact C-X band circulators.