Heterogeneous nested composite substrates significantly enhance circulator performance over single-ferrite substrates. This study establishes two distinct interface engineering strategies, diffusion-suppressed co-firing and mechanically interlocking brazing, for the fabrication of robust NiCuZn ferrite (NCZF)/(Mg,Ca)TiO₃ (MCT) joints, overcoming the limitations inherent to conventional adhesive bonding. The addition of low-melting-point ZnO–B₂O₃–SiO₂ (ZBS) tailored the sintering behavior and dielectric properties of MCT, enabling co-firing with NCZF at 1050 °C. Thermal shrinkage adjustment effectively suppresses ion interdiffusion driven by compressive stresses from radial extrusion of the outer ring contraction, and the width of the transition region is just 29 μm. Subsequently, brazed substrates were fabricated on the basis of the wettability of the La₂O₃–CaO–ZnO–B₂O₃–SiO₂ (LCZBS) glass. The ceramic boundaries undergo localized dissolution by the erosive interaction of molten glass, whereas the width of the brazing seam increases with brazing temperature (T_b), with the narrowest transition region of 30 μm. Ti⁴⁺ ions from MCT diffused through the glass network, forming a ZnO–TiO₂ enrichment interfacial layer at the NCZF boundary. Moreover, the Mg₂TiO₄ whiskers grow into the solder region through in situ reactions at the MCT interface, forming a mechanically interlocked architecture. This structure serves as the primary contributor to the superior shear strength of brazed substrates relative to co-fired substrates. By elucidating the distinct interfacial regulatory mechanisms in co-firing and brazing, this study establishes a foundation for precision interface design in high-reliability composite substrates, supporting the development of high-performance circulators for microwave applications.

The application of NiCuZn ferrites (NCZFs) in high-power communication systems is constrained by their nonlinear excitation. To reduce nonlinear effects, it is essential for ferrite materials to possess a relatively high spin-wave linewidth (ΔH_k). Doping with ions such as cobalt and rare-earth (RE) ions with fast relaxation has proven effective in increasing ΔH_k of ferrites. However, the regulatory mechanism of doping NCZFs with RE ions with larger ionic radii remains unclear. In this study, Ho³⁺-substituted NCZFs were synthesized via a solid-state reaction route. The spatial distribution and substitution amount of the Ho³⁺ ions were carefully investigated via elemental and phase composition analysis, revealing the limited solid solubility of the Ho³⁺ ions in NCZFs. Some of the Ho³⁺ ions enter the lattice and occupy the octahedral sites, accelerating relaxation and increasing ΔH_k to a maximum value of 2.63 kA·m⁻¹. Insoluble Ho³⁺ ions combine with Fe³⁺ ions to form a HoFeO₃ heterogeneous phase with Fe³⁺ ions at the grain boundaries, leading to iron deficiency within NCZF crystals and significantly reducing the dielectric loss tangent at microwave frequencies. These results reveal the great potential of Ho³⁺-substituted NCZFs for high-power, low-loss microwave applications.