Ni–Co ferrite has been studied with Mössbauer spectroscopy and x-ray diffraction. The crystal structure for this system is spinel, and the lattice constant is in accord with Vegard’s law. The Mössbauer spectra consist of two six-line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Néel temperature increases linearly with Ni concentration, suggesting the superexchange interacion for the Ni–O–Fe link is stronger than that for the Co–O–Fe link. It is found that Debye temperatures for the A and B sites of CoFe2O4 and NiFe2O4 are found to be θA=734 K, θB=248 K, and θA=378 K, θB=357 K, respectively. The intensity ratio of the A to B patterns is found to increase at low temperatures with increasing temperature due to the large difference of Debye temperatures of the two sites and to decrease at high temperatures due to migration of Fe3+ ions from A to B sites. Atomic migration of CoFe2O4 starts near 400 K and increases rapidly with increasing temperature to such a degree that 69% of the ferric ions as the A sites have moved over to the B sites by 780 K. It is noted that, as the Ni concentration in cobalt ferrite increases, the Debye temperatures tend to decrease the migration at the A and B sites is slow.

Ultrafine Ni0.65Zn0.35Cu0.2Fe1.8O4 particles were fabricated by a sol–gel method. The magnetic and structural properties of the powders were investigated with x-ray diffraction, vibrating sample magnetometer, and Mössbauer spectroscopy. Ni–Zn–Cu ferrite powders that were fired at and above 823 K have only a single phase spinel structure and behave ferrimagnetically. Powders annealed at 523, 623, and 723 K have a typical spinel structure and are simultaneously paramagnetic and ferrimagnetic in nature. The magnetic behavior of Ni–Zn–Cu ferrite powders fired at and above 623 K showed that an increase of the annealing temperature yielded a decrease of the coercivity and an increase of the saturation magnetization. The maximum coercivity and the saturation magnetization of Ni–Zn–Cu ferrite powders were Hc=96 Oe and Ms=68 emu/g. Mössbauer spectra of powder annealed at 1223 K were taken at various temperatures ranging from 12 to 675 K. As the temperature increased toward TN, a systematic line broadening effect in the Mössbauer spectra was observed and was interpreted as originating from the different temperature dependencies of the magnetic hyperfine fields at various iron sites. The isomer shifts indicated that the iron ions were ferric at the tetrahedral [A] and the octahedral site [B]. The Néel temperature was determined to be TN=675±2 K.

Sr0.75Ba0.25Fe12O19 hexagonal ferrite has attracted much attention due to its large (BH)MAX values and workability. We have prepared sheet magnets by the Dr. Blade method. To examine the effects of additives, such as SiO2, TiO2, Al2O3, and Cr2O3, on magnetic properties of sheet magnets, we used VSM magnetometer, x-ray diffraction, and Mössbauer spectrometer. The crystal structure is found to be a magnetoplumbite of typical M-type hexagonal ferrite, but the α-Fe2O3 phase develops with increasing additives concentration. Using our refined computer program, we have analyzed the Mössbauer spectra in the temperature range from 13 to 800 K. The Mössbauer spectra indicate that the line intensity for the 12k site is reduced with increasing SiO2 concentration, which is different from the reports of Fe-substituted Ba ferrite. This suggests that the developing α-Fe2O3 phase is related to 12k sites. The isomer shifts show the charge states of Fe ions is ferric. When the additives concentrations increase, the Curie temperatures, Tc go down. One sextet for α-Fe2O3 phase still persists above Tc, so it suggests that the high-Tc values do not result from α-Fe2O3. While Al2O3 and Cr2O3 additives increase coercive force Hc, the other additives reduce it.

No abstract