The Ionicity And Its Effect On Cation Distribution And Magnetic Properties In Spinel Ferrites

The （A）[B]₂O₄spinel ferrites being the typical functional materials have attracted muchattention due to their physical properties and applications, such as ferromagnetism,ferroelectric property, catalytic properties, semiconductive property, high-frequency property,et.al. In spinel ferrites, oxygen ions arrange with a close-packed face-centered-cubic structure,and the metal ions at （A） and [B] sites are surrounded by four and six nearest neighbouroxygen ions at equal distances, respectively. In the spinel ferrites, all cation magneticmoments in （A） or [B] sites are parallel at low temperature, and the cation moments in the （A）sites are antiparallel to those of the [B] sites.In this paper, on the based of experiment results and related theory, used the firstprinciple and quantum-mechanical method, we calculated ionicities of some ferrites withspinel structure, and proposed a new model of magnetic ordering in ferrites, and calculatedcations distribution and magnetic moments, while we explained some physical phenomenathat are difficult in explanation using traditional theories. In addition, we studied the structuraland magnetic properties in ferrites with the nominal composition Li_xFe_3-xO_4-δand Li_xMn₂O₄obtained by improving the prepared method of the samples. It can be found that theparamagnetic sample Li_1.067Fe_0.933O₂was changed into room temperature ferrimagneticmaterial, subjected a heat treatment process under an appropriate applied field.1). The ionicities of some simple compounds and spinel ferrites MFe2O4（M=Mn, Fe, Co,Ni, Cu, Zn, Cr） have been estimated. The average ratio of3d electron number in the ironcations at （A） and [B] sites of Fe₃O₄,5.975/6.089, was obtained by the Cambridge Serial TotalEnergy Package （CASTEP） program. Combined with this ratio and the experimental magneticmoment of Fe₃O₄, the numbers of3d electrons in the iron cations at （A） and [B] sites werecorrected, and were finally confirmed as5.584and5.692. Assuming that other3d and4selectrons are all obtained by oxygen ions, the content of Fe3+ions per formula is calculated asonly1.032, the average valence of oxygen anions is-1.758, and the ionicity of Fe₃O₄ferrite is0.879.In II-VI compounds, assuming that the second electron of a part of cations are ionized,by fitting the ionicity （0.926） of SrO from Phillips, the ionicities of II-VI can be estimatedusing the quantum mechanical model reported by us, which approach to the results fromPhillips. According to this method, by fitting the ionicity （0.879） of Fe₃O₄, the ionicity ofM₃O₄（M=Mn, Co, Ni, Cu, Zn, Cr） is easy to be estimated as0.8293,0.8314,0.8129,0.7990, 0.7822,0.8726, and the ionicity of MFe₂O₄（M=Mn, Fe, Co, Ni, Cu, Zn, Cr） is estimated as0.8293,0.8790,0.8314,0.8129,0.7990,0.7822,0.8726, respectively.2). A model of magnetic ordering in ferrrites （MOIF rule） was proposed. On the basis ofthis model, the cations distribution of the spinel ferrites MFe₂O₄（M=Mn, Fe, Co, Ni, Cu, Cr）was calculated by the quantum mechanical model. MOIF rule includes two parts. Firstly, thereare two2p electrons with opposite spin directions in the outer orbit of the oxygen anions,which serve as intermedium of itinerant3d electrons between cations. Therefore, the metalcations around an oxygen anion should be divided into two groups with opposite spindirections. Secondly, by Hund’s rules, the spins of electrons in a subshell of a free atom tendto align in one direction until the maximum multiplicity is attained. After that, the spins ofelectrons will align in the opposite direction. Thus in the3d subshell of the transition metalfree atoms, maximum of five electrons can have their spins aligned in one direction.Therefore, in the spinel ferrites, when an itinerant electron hops to a cation with local3delectron number, n_d≤4, the spin direction of this itinerant electron will be parallel to their local3d electrons （major spin）. However, when an itinerant electron hops to the cation with nd≥5,the spin direction of this itinerant electron will be antiparallel to their local3d electrons（major spin）.On the basis of MOIF rule, considered the ionization energy of the cations and the Paulirepulsion energy, together with the magnetic ordered energy, the tendency toward chargedensity balance, the cation distributions in spinel ferrites MFe₂O₄（M=Fe, Co, Ni, Cu, Mn, Cr）were estimated using the quantum-mechanical model to fit the observed magnetic moments ofthese materials. Therefore, the difference between the observed and the traditional theoreticalmagnetic moments of the spinel ferrites MFe₂O₄, and the argument with Cr ion distribution inspinel ferrite for many years were explained3). The cation distributions in the Zn-doping spinel ferrites were calculated by thequantum mechanical model proposed by us. Doping nonmagnetic ions into the magneticsubcrystal lattice will destroy the parallel arrangement of the magnetic moment of magneticions, resulting in the angle between the magnetic moment increases with the Zn-doping level,which decreases the moments of the （A） and [B] subcrystal lattices; According toquantum-mechanical model for estimating ion distributions in spinel ferrites and the effect ofnonmagnetic ions, the dependence of the magnetic moments of the ferrites Ni_1-xZn_xFe₂O₄onthe doping level x can be fitted, which can be used to explain why there is a maximum of themagnetic moments for these ferrites when the Zn-doping level x is between0.4and0.5.4). The effect of substitution Li for Fe on the structural and magnetic properties in ferrites with the nominal composition Li_xFe_3-xO_4-δ, and a new ferrimagnetic oxideLi_1.067Fe_0.933O₂was discovered in special. Li_xFe_3-xO_4-δ（x=0.6,0.8,1.0,1.2,1.4,1.6） ferriteshave been prepared by Sol-Gel method with the highest heat treatment temperature being1223K. The XRD patterns indicate that when the doping level x≤1.4the samples have twophases. The first phase is （A）[B]₂O₄type Li0.5Fe2.5O4spinel phase （space group P4332）. Thesecond phase is α-NaFeO₂type LiFeO₂ferrite （space group Fm3m）. The content of the secondphase increases with Li doping level x increasing. However, the sample Li_1.6Fe_1.4O₄-δhas onlya single phase with α-NaFeO₂type like-LiFeO₂ferrite （space group Fm3m）, which shouldtherefore be represented by Li_1.067Fe_0.933O₂. The magnetic properties examinations show thatthe specific saturation magnetization of the samples decreases with increasing Li doping levelx. If the mass of the nonmagnetism phase LiFeO₂was taken off, the specific saturationmagnetization of the samples approach to Li_0.5Fe_2.5O₄; the sample Li_1.067Fe_0.933O₂as preparedwas paramagnetic between10K and404K. However, the sample occurred magnetictransition from paramagnetic to ferrimagnetic phase in the measurement process of thetemperature dependence of magnetization from404K to950K. When the sample undergoes8cycles of increasing and decreasing temperature measurement under different appliedmagnetic field （the highest applied magnetic field is3.0T）, the saturation magnetization at300K reached9Am²/kg, the Curie temperature reached864K, and the structure has a slightdistortion from a cubic to an orthorhombic structure.5). Li_xMn₂O₄（0≤x≤1） oxides have been prepared by Sol-Gel method with the heattreatment temperature being773,973and1223K. The XRD patterns and magneticmeasurement show that the samples with0.2≤x≤0.8possess two phases being Mn₃O₄andLiMn₂O₄phases. The samples with x=0and x=1have Mn3O4and LiMn2O4single phase,respectively. The Curie temperature of Mn3O4phase is at42K. LiMn₂O₄phase isparamagnetic between10K and300K.