Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process

The Cr-substituted M-type barium hexaferrites, BaFe12−xCrxO19, with x=0.0–0.8x=0.0–0.8 have been successfully prepared by nitrate–citrate auto-combustion process using citric acid as a fuel/reductant and nitrates as oxidants. The resulting precursors were calcined at 1100 °C for 1 h and followed by sintering at 1200 °C for 12 h in oxygen atmosphere. The ferrites were systematically investigated by using powder X-ray diffractometer (XRD), magnetic hysteresis recorder, Mössbauer spectrometer, and scanning electron microscope (SEM). The XRD data show the formation of pure magnetoplumbite phase without any other impurity phases. Both a and c lattice parameters calculated by the Rietveld method systematically decrease with increasing Cr content. The effects of Cr3+ ions on the barium ferrites were reported and discussed in detail. The site preference of Cr3+ and magnetic properties of the ferrites have been studied using Mössbauer spectra and hystereses. The results show that the magnetic properties are closely related to the distributions of Cr3+ ions on the five crystallographic sites. The saturation magnetization systematically decreases, however, the coercivity increases with Cr concentration. The magnetization and Mössbauer results indicate that the Cr3+ ions preferentially occupy the 2a, 12k, and 4fVI sites. The average size of hexagonal platelets obtained by SEM photographs tends to decrease with respect to Cr content.