Bonding Mechanisms of BTO-NZF Laminated Cofired Bodies Featuring High-Quality Heterojunctions

The shrinkage rate of Ni_0.6Zn_0.4Fe_1.8O₄ (NZF) ferrite was controlled to enable cofiring with BaTiO₃ (BTO) at 1250 °C, forming BTO-NZF laminated structures. Postcofiring microstructural analysis revealed a well-bonded interface with minimal elemental interdiffusion. Notably, Ti⁴⁺ exhibited the highest diffusion coefficient (1.60 ± 0.05 × 10^-12 cm²/s). Unidentified phases on the NZF side effectively restricted Ba²⁺ and Ti⁴⁺ diffusion. The BTO-NZF heterojunction consisted primarily of nanoscale amorphous layers containing Ba²⁺ and coherent crystalline regions. The dense amorphous barrier layer not only ensured strong interfacial bonding but also significantly suppressed interdiffusion. High-quality heterojunction formation was influenced by four primary factors, including the presence of similar ionic radii and interplanar spacings, amorphous barrier layers, and a certain concentration of large-radius ions. Magnetic characterization showed that NZF had a saturation magnetization (<i>M</i>_s) of 70.29 ± 0.50 emu/g, a coercivity (<i>H</i>_c) of 8.89 ± 0.50 Oe, and a remanent magnetization (<i>M</i>_r) of 1.01 emu/g. The relative dielectric constant (ε_r) of BTO ranged from 1490 to 2500 (at 1 MHz), while the dielectric loss tangent (tanδ) varied between 4 × 10^-3 and 1.25 × 10^-2. These findings offer valuable guidance for selecting cofired laminated substrates and designing high-performance heterojunctions. Moreover, the resulting cofired laminates exhibit excellent electromagnetic properties, demonstrating strong potential for magnetoelectric coupling applications.