Miniaturized High-Harmonic-Suppressed Circulators with Integrated Design Enabled by Cu2+/Ti4+ Co-Doped NiCuZn Ferrites (High-εr) and LTCC Technology

This work presents a miniaturization strategy for ferrite circulators by enhancing the dielectric constant ε r of NiCuZn ferrites and LTCC technology. Highly polarizable Cu 2+ and Ti 4+ substitutions for Ni 2+ and Fe 3+ increase dielectric constant, while non-magnetic Ti 4+ reduces magnetocrystalline anisotropy and narrows ferromagnetic resonance linewidth ΔH , with Cu 2+ concurrently lowering sintering temperatures. In the Ni 0.42-x/2 Cu 0.18+x Zn 0.4 Ti x/2 Fe 1.98-x O 4-δ system, the x = 0.1 composition (sintered at 975 °C) achieves minimal ΔH (82 Oe), ε r ≈ 14.6, and 4π M s ≈ 4219 G, whereas x = 0.6 (sintered at 925 °C) balances ε r ≈ 20.8 with magnetic performance ( ΔH ≈167 Oe, 4π M s ≈ 2117 G), confirming LTCC compatibility. A heterostructured substrate fabricated by co-firing Ni 0.12 Cu 0.78 Zn 0.4 Ti 0.3 Fe 1.38 O 4-δ ferrite (x = 0.6) with a microwave dielectric (50 wt.% SrCu 0.95 Mn 0.05 Si 4 O 10 + 50 wt.% Sr 3 V 2 O 8 ) exhibits controlled interfacial diffusion. HFSS simulations demonstrate that the integrated circulator-filter achieves significant miniaturization with >30 dB harmonic suppression (14-24 GHz) while maintaining in-band performance (| S 21 | < 1 dB, | S 11 | and | S 12 | < -20 dB at 7-12 GHz). The validates high- ε r NiCuZn ferrites and LTCC composite substrates for compact multifunctional microwave components.