Effects of Substitution in Barium hexaferritesBaFe_(12-x)X_xO_(19)(X=Co,Ti; Sc)

摘要

The short and long-range atomic and magnetic order in BaFe_(12-x)Co_xTi_xO_(19)(x=0.4, 0.7,0.8) and BaFe_(10.4)Sc_(1.6)O_(19)prepared by soft chemistry routes were studied and compared with parentBaFe_(12)O_(19)prepared by solid-state reaction. ForBaFe_(10.3)Co_(0.85)Ti_(O_(19)neutron diffraction andmagnetic measurements revealed that (Co , Ti ) substitution causes significant distortions in thelocal oxygen surrounding of ferric cation sites, while the grain-size effect on the structuralparameters is considerably smaller. The thermal expansion coefficient exhibits a strong anisotropy.The refined magnetic moments for x=0.45 and 0.7 based on the five-cation sublattice block-typecollinear ferrimagnetic structure of uniaxial type known as Gorter type for BaFe_(12)O_(19), areconsiderably lower than the theoretical spin only moments, especially for the 4e and 12k sites,indicating for x = 0.7 a local noncollinearity with short-range ordering. For x 0.85, at 10 K ablock-type conical magnetic structure sets in. For BaFe_(10.4)Sc_(1.6)O_(19),combined neutron diffraction,field-dependent ~(57)FeMossbauer studies and magnetic measurements show that the collinear block-type structure remains effective at 300 K and below it down to about 190 K. In the 190 — 1.6 Krange we observe a temperature dependent incommensurate complex block-type conical structure.