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Powder samples of the ferrites MxMn1-xFe2O4 (M = Zn, Mg, Al) were prepared using a chemical co-precipitation method. X-ray diffraction analysis showed that the three series of samples had a single-phase cubic spinel structure and that there was a decrease in the lattice parameters with increasing x. There were different dependences on the doping level x of the magnetic moments (µexp) for the three series of samples measured at 10 K. We found a non-monotonic behavior for µexp as a function x for the Zn doped samples with a maximum at x = 0.4, while µexp decreased monotonically with increasing x for the Mg and Al doped samples. On the basis of the O2p itinerant electron model, the magnetic moment direction of the Mn3+ cations is expected to be antiparallel to those of the Mn2+ and Fe cations in these samples. With this assumption, the curves of µexp versus x for the three series of samples were fitted using a quantum-mechanical potential barrier model earlier proposed by our group, and the cation distributions were obtained. [ABSTRACT FROM AUTHOR]

Ferrite powder samples of Mn xNi1- xFe2O4 (0.0 ≤ x ≤ 1.0) with single phase (A)[B]2O4 spinel structure were prepared using a sol-gel method. Following the successful proposal by our group, that the magnetic moment directions of Cr2+ (3d4) cations are antiparallel to those of Fe3+ (3d5) and Fe2+ (3d6) cations in a given sublattice of the Cr-doped spinel ferrites due to the constraints imposed by Hund's rules, we extend here the same idea and assume that the magnetic moment directions of Mn3+ (3d4) cations are also antiparallel to those of Mn2+ (3d5) and divalent and trivalent Fe (Ni) cations in a given sublattice of Mn-doped spinel ferrites. We have thereby obtained cation distributions for the samples by fitting the magnetic moments of the samples at 10 K. The results indicate that 72% of the Mn cations occupy the [B] sites in MnFe2O4, which is close to the results for Ni (82%) in NiFe2O4, but is different from the result obtained using the conventional view which yields 80% of the Mn cations in the (A) sites of MnFe2O4. On the basis of the present analyses of the magnetic structure of Mn xNi1- xFe2O4 (0.0 ≤ x ≤ 1.0), we propose here a new model for spinel ferrites that is distinctly different from both the super- and the double-exchange model and that we refer to as the O2p itinerant electron model. Using this model, not only can the magnetic structure of the spinel ferrites MFe2O4 ( M = Fe, Co, Ni, and Cu) be explained better than by using the super- and double-exchange interaction models, but also the magnetic structure and the cation distributions of Cr-, Mn-, and Ti-doped spinel ferrites can be explained. [ABSTRACT FROM AUTHOR]

Powder samples of the spinel ferrites MxNi0.7-xFe2.3O4 (M = Cr, Co and 0.0 ≤ x ≤ 0.3) and CrxNi0.7Fe2.3-xO4 (0.0 ≤ x ≤ 0.3) were synthesized using the chemical co-precipitation method. The XRD spectra confirmed that the samples had a singlephase cubic spinel structure. Magnetic measurements showed that the magnetic moments (μexp) per formula both at 10 K and 300 K increased with Co substitution, while the values of μexp decreased with Cr substitution. Applying the assumption that the magnetic moments of Cr2+ and Cr3+ lie antiparallel to those of the divalent and trivalent Fe, Co, and Ni cations in the same sublattice of spinel ferrites, these interesting behaviors could be easily interpreted. The cation distributions of the three series of samples were estimated successfully by fitting the dependences of μexp, measured at 10 K, on the doping level x, using a quantum-mechanical potential barrier model earlier proposed by our group. The results obtained for the Cr cation distributions at the (A) and [B] sites are very close to those obtained elsewhere using neutron diffraction. [ABSTRACT FROM AUTHOR]

Ti-doped Ni0.68Fe2.32O4 spinel ferrite samples with nominal composition Ni0.68−0.8 xTi xFe2.32−0.2 xO4 (0 ≤ x ≤ 0.312) were prepared using conventional ceramic methods. The samples exhibited a single-phase cubic spinel structure. When the doping level had values x > 0.15, a transition temperature, TN, was found, below which the magnetization of the samples decreased with decreasing measurement temperature. This phenomenon indicates that an additional antiferromagnetic structure arises in the traditional spinel phase of ferrites resulted from Ti doping. Therefore, Ti ions appear in the form of Ti3+ and Ti2+ cations that have magnetic moments, rather than as Ti4+ cations without magnetic moment as assumed by many researchers. The dependence of the magnetic moments on the Ti doping level x at 10 K was fitted successfully using our quantum-mechanical potential barrier model proposed earlier. In the fitting process, we assumed that the magnetic moments of the Ti cations were opposite to the direction of the Fe and Ni moments. According to the fitted results, the Ti2+ cations at the [B] sites constitute about 81% of the total Ti content x for all samples, which is close to the content of the Ni2+ cations at the [B] sites (77%). [ABSTRACT FROM AUTHOR]

A quantum-mechanical method for estimating the cation distribution in spinel ferrites is proposed, by which the ionization energy of the cations and the Pauli repulsion energy is considered, together with the magnetic ordered energy and the tendency toward charge density balance. Using this method, not only can the difference between the observed and the traditional theoretical magnetic moments of the spinel structure ferrites MFe2O4 (M=Mn,Fe,Co,Ni,Cu) be explained, but also the dependence of the magnetic moments of the ferrites M1-xZnxFe2O4 (M=Mn,Fe,Co,Ni,Cu) on the doping level x can be fitted. [ABSTRACT FROM AUTHOR]

Powder samples of the spinel ferrites MxNi0.7-xFe2.3O4 (M = Cr, Co and 0.0 ≤ x ≤ 0.3) and CrxNi0.7Fe2.3-xO4 (0.0 ≤ x ≤ 0.3) were synthesized using the chemical co-precipitation method. The XRD spectra confirmed that the samples had a singlephase cubic spinel structure. Magnetic measurements showed that the magnetic moments (μexp) per formula both at 10 K and 300 K increased with Co substitution, while the values of μexp decreased with Cr substitution. Applying the assumption that the magnetic moments of Cr2+ and Cr3+ lie antiparallel to those of the divalent and trivalent Fe, Co, and Ni cations in the same sublattice of spinel ferrites, these interesting behaviors could be easily interpreted. The cation distributions of the three series of samples were estimated successfully by fitting the dependences of μexp, measured at 10 K, on the doping level x, using a quantum-mechanical potential barrier model earlier proposed by our group. The results obtained for the Cr cation distributions at the (A) and [B] sites are very close to those obtained elsewhere using neutron diffraction. [ABSTRACT FROM AUTHOR]