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

The Sr1-xLaxFe12-xCuxO19 (x = 0-0.4) ferrites with appropriate amount of Bi2O3 center dot B2O3 center dot SiO2 center dot 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. (C) 2016 Elsevier B.V. All rights reserved.