Study Of Magnetic Structure And Cation Distributions In Cr（Co） Doped Spinel Ferrites Ni_0.7Fe_2.3O₄

In recent years, spinel ferrites have received considerable atention due to their various applications in magnetic memory devices, transformers, magnetic refrigeration etc. There are eight molecules per cell of a spinel ferrite. The oxygen anions form a close-packed face-centered-cubic structure with the metal cations occupying the two types of interstitial positions. The tetrahedral（A） sites and the octahedral [B] sites are occupied by 8 cations and 16 cations, respectively. On the basis of traditional theory, for spinel ferrites with formula MFe₂O₄（M =Fe、Co、Ni、Cu）, M cations are considered to be divalent cations, while Fe cations are regarded as trivalent cations. There are three classes of spinel ferrites according to the cations distribution in the（A） and [B] sites:（i） Normal spinel structure, a tendency toward charge density balance forces all of the M2+ cations enter the（A） sites and all of the Fe³⁺ enter the [B] sites.（ii） Inverse spinel structure, the Pauli repulsion energy demands that the（A） sites are occupied by the Fe³⁺ cations with the smaller effective ionic radius, and that the [B] sites are occupied by the rest of Fe³⁺ cations and the M2+ cations with the larger effective ionic radius;（iii） Mixed spinel structure, subjected the above two restrictions, there are both M2+ and Fe³⁺ cations at the（A） sites. Since the cation distributions in the spinel ferrites can affect their microstructures, electrical property and magnetism, it is necessary to explore the cation distributions in the spinel ferrites.So far, the influence of Cr-doping on the crystal structure and physical properties of spinel ferrites have been reported in many studies, but the Cr ion distributions have been disputed. For materials with compositions MFe₂O₄（M = Cr, Mn, Fe, Co, Ni, Cu）, when M =Fe, Co, Ni, and Cu, the observed magnetic moments per formula are 4.2, 3.3, 2.3, 1.3 mB, which are slightly higher than the magnetic moments of Fe2+（4 mB）, Co2+（3 mB）, Ni²⁺（2 mB） and Cu2+（1 mB） cations; For M=Mn, the observed magnetic moment is 4.6 mB, which is slightly lower than the magnetic moments of Mn²⁺ cations（5 mB）. For M=Cr, however, the observed magnetic moment is 2 mB, which is only half of the Cr²⁺ magnetic moments（4 mB）. There have no satisfactory explanation for magnetic structure of Cr doped spinel ferrites except the result reported by our group.On the basis of restrictions by Hund’s rules for the spin direction of electrons, our group has proposed an O2 p itinerant electron model, according to which 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. This is due to that in the same sublattice, the spin direction of the itinerant electrons remains constant when it moves to any cation, and the spin directions of 3d electrons（including itinerant and local 3d electrons） are constrained by Hund’s rules. Hence, the magnetic moment directions of the two cations with 3d electron number, nd￡4（or nd35）, will be parallel to each other; however, when one cation with nd￡4, and the other with nd35, their magnetic moment directions will be antiparallel to each other. Considering this restrictions, applying a quantum- mechanical potential barrier model earlier proposed by our group, the cation distributions of Mx1Nx2Fe₂O₄（M, N =Cr, Mn, Fe, Co, Ni, and Cu） samples were estimated successfully by fitting the dependences of the sample magnetic moments, mexp, measured at 10 K, on the doping level x.In this work, ferrite samples with nominal compositions CoxNi_0.7-xFe_2.3O₄（0.0≤x≤0.3）, CrxNi_0.7-xFe_2.3O₄（0.0≤x≤0.3）, and Cr_xNi_0.7Fe_2.3-xO₄（0.0≤x≤0.3） were synthesized by the chemical co-precipitation method. The samples were characterized by X-ray diffractometer, Physical property measurement system and UV-VIS Spectrophotometry. The structure and magnetic property of the three series of samples were studied. We obtained the following results:（1） All the samples had a single-phase cubic spinel structure with a space group Fd3m; the lattice constant（a） increases with the doping level（x） for the Cox Ni_0.7-xFe_2.3O₄ and CrxNi_0.7-xFe_2.3O₄ samples, while a decreases with increasing x for the Cr_xNi_0.7Fe_2.3-xO₄ samples. We found that the volume averaged crystallite sizes of all samples are larger than 100 nm so that surface effects are expected to be very weak.（2） The specific saturation magnetization of the samples was measured at 10 K. We therefore obtained the magnetic moments（mexp） per formula of the sample at 10 K. We found that Co substituted for Ni result in the mexp to increase with increasing x, Cr substituted for Ni or Fe resulted in the mexp to decrease with increasing x.（3） Appling the traditional theory, it is difficult to explain why Cr substituted for Ni result in the mexp to decrease with increasing x, due to the magnetic moment of Cr²⁺（4 mB） are larger than that of Ni²⁺（2 mB）. Applying the O2 p itinerant electron model, 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, it is easy to interpret the dependencies of the mexp on x for all three series of samples. Furthermore, the cation distributions of the three series of samples were estimated successfully by fitting the dependences of mexp on x, using a quantum-mechanical potential barrier model earlier proposed by our group. It is found that about 40 percent of Cr cations entered the（A） sites, which is close to those obtained by A. K. Ghatage using neutron diffraction.（4） It is very important to determine electron transition energies（Etr） between anions and different cations in order to understand the electrical transport and magnetic properties of a material. Many authors have analyzed UV-vis absorption spectra using the curve（ahn）2 vs E, where a is the absorption coefficient and E（=hn） is the photon energy. Such an approach can give only two band gap energies for spinel ferrites. In this paper, using differential UV-vis absorption spectra, da/d E vs E, we have obtained electron transition energies（Etr） between the anions and cations, Fe2+ and Fe³⁺ at the（A） and [B] sites and Ni²⁺ at the [B] sites for the three series of samples and Fe3O4. Therefore, more information can be obtained using the differential UV-vis absorption spectra than the conventional method, which make the electrical transport and magnetic properties of a material to be more easily understanded.