Ga-, Y-, and Sc-substituted M-type ferrites for self-biasing circulators in LTCC microwave modules

The partial substitution of Fe3+ by diamagnetic Me3+ in BaMexFe12-xO19 with Me = Ga and Y (0 <= x <= 2) was studied. Samples were synthesized by the mixed-oxide route and their properties were compared to BaScxFe12-xO19 ferrites. It was shown by XRD that the hexagonal lattice parameters with x for Me = Ga and Sc vary continuously. For Me = Y no single-phase ferrites were obtained indicating that the solubility limit for Y in Ba ferrite is very limited. For Me = Ga and Sc, the saturation magnetization at room temperature is reduced with x. The coercivity as well as the ferromagnetic resonance frequency remain almost unchanged for Ga-substituted ferrites, whereas for Sc-substituted ferrites the coercivity and ferromagnetic resonance frequency decrease with x. Thick films of Sc-ferrite (x = 0.5) were screen-printed onto LTCC tape substrates and post-fired at 900 degrees C. Application of a magnetic field during drying allows fabrication of textured ferrite layers. Alternatively, ferrite tapes and LTCC tapes were successfully co-fired at 900 degrees C.