| Spinel ferrite with general formula(A)[B]2O4 is magnetic functional material which is composed of iron and one or more other metals. Due to the fact that there are stable physical and chemical properties, spinel ferrites are used to many fields, such as communications, electronics, microwave, medical biology, etc. The cations occupied at the(A) sites and [B] sites are usually 3d transition metal ions, such as Fe, Co, Ni, Cu, Zn, etc. The cations distributions in(A) sites and [B] sites play a vital role in the performance of ferrites.For the typical MFe2O4(M = Fe, Co, Ni, Cu) materials, the cation magnetic moment at an(A) or [B] sublattice was parallel, but the cation magnetic moments between(A) and [B] sublattices were antiparallel each other. In the traditional theory, superexchange interaction can be used to explain the magnetic structure of spinel ferrites. American researchers suggested that cations on the octahedral and tetrahedral sites coupled antiferromagnetically via the magnetic superexchange interaction, while the cations on the tetrahedral sites or the octahedral sites coupled ferromagnetically through the magnetic double exchange interaction. However, there have no satisfactory explanation for the differences of magnetic structure between Ti(Cr, Mn) and Fe(Co, Ni, Cu) doped spinel ferrites, which become a puzzle to the magnetism for a long time.On the basis of the constraints arising from Hund’s rules on the direction of the electron spin, our group firstly proposed that the magnetic moment directions of Cr cations with the numbers of 3d electrons nd≤4, were antiparallel to those of Fe, Co, Ni, Cu cations(nd≥5) whether at the(A) sublattice or at the [B] sublattice of the spinel ferrites, and estimated the cation distributions of Cr doped series spinel ferrites by quantum mechanical potential barrier model(QMPB model) and the magnetic moments of the samples at 10 K. Because the constraints arising from Hund’s rules on the direction of the electron spin, being similar to Cr2+(3d4) and Cr3+(3d3), we proposed that the moment direction of Mn3+(3d4), Ti2+(3d2),Ti3+(3d1) cations are antiparaller those of which 3d election numbers nd≥5, such as Mn2+ cation, bivalent and trivalent Fe, Ni cations in the same sublattices. In order to confirm this idea, we studied the cation distributions for three series of Ti-Ni spinel ferrites and MnxNi1-xFe2O4 spinel ferrites, we also proposed an O2 p electron itineration mechanism. The results of this study were as follows:(1) Three series of Ti-Ni ferrite samples with chemical compositions Ni0.68-0.8xTixFe2.32-0.2xO4(x=0.0, 0.078, 0.156, 0.234, 0.312), Ni0.68+0.26xTixFe2.32-1.26xO4(x=0.00, 0.08, 0.16, 0.24), Ni1-xTixFe2O4(x=0.0,0.1,0.2,0.3,0.4) were prepared by solid phase reaction method. All Ti-Ni samples had a single-phase cubic spinel structure with space group Fd3 m. The lattice parameter, a, of three series samples increased with the Ti doping level x. The average crystallite sizes of all samples were larger than 100 nm, so that the influence of surface effects on the magnetization of all samples could be ignored. In all series of Ti-Ni spinel samples, the saturation magnetization σS decreased gradually with the increasing Ti doping level x. The variation trend of the σ-T curves for the samples with x<0.15 were very close to those for the samples with x = 0.00. However, when the Ti doping level had a value x>0.15, there was two transition temperatures in the σ-T curves, TL and TN, When T < TN, the value of σ decreased markedly with decreasing T; when T = TL, dσ/d T reached a maximum value. The characteristics of the σ-T and dσ/d T-T curves were very similar to the typical antiferromagnetic materials. Such behavior of three series samples indicated that an additional antiferromagnetic structure arose in the traditional spinel phase due to the Ti doping, and also suggested that Ti cation has a magnetic moment, rather than assumption of no magnetic Ti4+ cation in the previous literature. We assumed that the magnetic moments of the Ti cations at 10 K were antiparallel to those of the Fe and Ni cations. Using quantum mechanics potential barrier model successfully fitted the magnetic moment per formula of series Ti-Ni spinel samples at 10 K. In the fitting process, we obtained that the distributions of Ti, Ni, Fe cations at the(A) and [B] sites, and found that about 80% of Ti cations and about 77% of Ni cations with +2 valence entered the [B] sites, which close to the opinion of some researchers. The result shows that the assumption of Ti ion magnetic moment direction and cation distribution is reasonable.(2) Powder ferrite samples with chemical compositions MnxNi1-xFe2O4(0.0≤x≤1.0) were prepared by sol-gel method. The XRD spectrum of MnxNi1-xFe2O4 at room temperature showed that each sample had a single-phase cubic spinel structure with space group Fd3 m. The lattice parameter of samples increased with increasing Mn doping level x. The saturation magnetization σS increased gradually with increasing Mn doped level x.(3) We assumed that the magnetic moments of the Mn3+ cations were antiparallel to those of the Mn2+, bivalent and trivalent Fe, Ni cations, and successfully fitted the magnetic moment per formula of each spinel sample at 10 K using quantum-mechanics potential barrier model. In the fitting process, we obtained that the cation distributions of Mn, Ni and Fe cations at the(A) and [B] sites, and found that more than 70 percent of Mn, Ni cations entered the [B] sublattice, which is close to the results of some of previous authors.(4) To explain the phenomenon that magnetic moment of the cations with 3d electron number nd≤4 are antiparallel to those of the cations with nd≥5 in a sulattice of spinel ferrites,we proposed an O2 p electron itinerant mechanism:(i) Taking into account the ionicity, there existed a part of monovalent oxygen anions in oxides, in which there is a O2 p electron hole.(ii) In a given sublattice, a 2p electron of an oxygen anion could hop to an O2 p hole in an adjacent oxygen anion mediated by the metal cations. The spin direction of the 2p electron as an itinerant electron remained constant. Meanwhile, the O2 p electron hole hops toward the opposite derection.(iii) Due to the fact that the two O2 p electrons in the outer orbit of an oxygen anion have opposite spin directions, and each of them became an itinerant electron in two different sublattice, resulting in the cation magnetic moments at the(A) sites being antiparallel to those at the [B] sites in MFe2O4(M =Fe, Co, Ni, Cu) spinel ferrites.Not only the O2 p electron itineration mechanism can instead of the superexchange interaction and the double exchange interaction to explain the magnetic structures in MFe2O4(M =Fe, Co, Ni, Cu), but also can explain the magnetic structure of Ti, Cr, Mn doped spinel ferrites which can not be explained using the superexchange interaction and the double exchange interaction models. In addition, the O2 p electron itineration mechanism being based on the fundamental principles of atomic shell structure, can give a clear physical images for the causes of these magnetic structures, which were easier to understand. |
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