Magnetic, electrical, and dielectric properties of La–Cu substituted Sr-hexaferrites for use in microwave LTCC devices

The Sr_1-xLa_xFe_12-xCu_xO₁₉ (x = 0–0.4) ferrites with appropriate amount of Bi₂O₃·B₂O₃·SiO₂·ZnO (BBSZ) glass were prepared by traditional solid phase method at low sintering temperatures compatible with low temperature co-fired ceramics (LTCC) technology. Detailed effects of La–Cu substitution amount on their crystal structure, magnetic, electrical, and dielectric properties were investigated. The results show that the pure M-type phase is obtained for the ferrites with x ≤ 0.3. With further increasing x, the multiphase structure is inevitably formed, where the LaFeO₃ phase coexists with the M-type phase. In the single M-type phase region, the variation of magnetic properties with La–Cu substitution amount is well explained based on the occupancy effects of La³⁺ and Cu²⁺ in magnetoplumbite structure. Suitable La–Cu substitution with x = 0.25 raises the saturation magnetization to a maximum value of 67.5 emu/g. The intrinsic coercivity decreases from 3.68 to 3.43 kOe with x increasing from 0 to 0.3, which is mainly dominated by the reduced magnetic anisotropy field. Electrical transport behavior of the ferrites is found to follow the impurity semiconductor, and the effect of La–Cu substitution on the direct-current resistivity is observed. Their polarization behavior in test frequency range of 1 kHz–10 MHz obeys the charge polarization mechanism, which happens since the frequency of the hopping of electron exchange between Fe²⁺ to Fe³⁺ is far from the frequency of alternating-current field. - Highlights: • Low temperature sintered Sr-hexaferrites with La–Cu doing are successfully prepared. • Magnetic properties are well explained considering the occupancy effects of La³⁺ and Cu²⁺. • Electrical transport behavior follows the conduction mechanism of semiconductors. • Polarization behavior obeys charge polarization mechanism of ferrite materials. • The ferrite with x = 0.25 has improved physical properties for microwave LTCC applications.