| Ferrites are metal oxides composed of iron and other metal elements. In spinelferrites with the chemical formula (A)[B]2O4,(A) sites and [B] sites are occupied by3d transition metal ions, such as Fe, Co, Ni, Cu, Zn and so on. The spinel ferrites canbe used as magnetic function materials, therefore, they are widely applied ininformation storage, electron device, microwave absorption, biomedical science, andso on. In spinel ferrites, cation distributions in (A) and [B] sites play an important roleto influence their physical and chemical properties. In recent years, there were issuesthat Cr ions occupy either at (A) sites or at [B] sites in spinel ferrite. Therefore ourgroup proposed a new model for magnetic ordering in ferrites (MOIF model). Thismodel, in which the both models of Super-Exchange and Double-exchangeinteractions can be merged, not only can be used to explain the magnetic orderingmechanism in spinel ferrites with chemical formula MFe2O4(M=Mn, Fe, Co, Ni, Cu),and can be used to explain antiferromagnetic ordering mechanism for oxides MO(M=Mn, Fe, Co, Ni), but also can be used to explain the issues about magneticstructure in Cr doped ferrites. In addition, the new model is more simple and distinctthan the old models. According to the new model, the magnetic moments of Cr and Fecaions at the same sites,(A) or [B] sites in spinel ferrites, were antiparallel.In this paper, ferrite powder samples with nominal composition CuxCr1-xFe2O4(0.0≤x≤0.4) and CuCrxFe2-xO4(0.0≤x≤0.5) were synthesized by chemicalco-precipitation. The microstructures and magnetic properties of the two series ofsamples were characterized by X-ray energy dispersive spectra (EDS), X-raydiffraction (XRD), Fourier transform infrared spectra (FT-IR), scanning electronmicroscope (SEM), and Physical Property Measurement System (PPMS). We foundfollowing phenomena:(1) The samples with nominal composition CuxCr1-xFe2O4(0.0≤x≤0.4) wereprepared by thermal treated at1673K for4h. EDS indicated that the actualcomposition of samples were Cux1Crx2Fe3-x1-x2O4(0.0≤x1≤0.284,1.04≥x2≥0.656), dueto that a few of Cu ions lost in the thermal treatment process. XRD patterns indicatedthat the samples had a single-phase cubic spinel structure with space group Fd3m, thatcell constant a (8.392±0.005), is approximately constant for the various Cu dopinglevels x1(0.0≤x1≤0.284), and that the volume average diameters of crystallites in all samples were above100nm. The magnetic measurements indicated that the saturationmagnetization of samples increased obviously with increasing Cu content x1.It is well known that the magnetic moments of Cu2+and Cr2+ions are1and4μB,respectively. According the traditional view in references, the saturationmagnetization of samples should decrease when Cu substitutes for Cr cation, whichvariation tendency is contrary to our experimental result. According to the MOIFmodel proposed by our group, in which the magnetic moments of Cr and Fe ions areantiparallel, the moments of samples would increased with Cu doping level x1.Therefore, the variation tendency of the saturation magnetization of samples can beexplained. Using the quantum mechanical potential barrier model proposed by ourgroup and the calculation results about the ionicity, to fit the dependency of thesample magnetic moments at10K, we estimated the cations distribution in (A) sitesand [B] sites. Therefore, this experiment result is an evidence to support the MOIFmodel.(2) The samples with nominal composition CuCrxFe2-xO4(0.0≤x≤0.5) wereprepared by thermal treatment in1473K for2h. EDS indicated that the actualcomposition of samples were Cu0.85CrxFe2.15-xO4(0.0≤x≤0.5), due to that a few of Cuelements lost in the thermal treatment process. XRD patterns indicated that thesamples had a single-phase cubic spinel structure with the space group of Fd3m, thelattice constant decrease linearly with Cr doping level x, with the variation range from0.838nm to0.835nm. Using to the MOIF model, the calculation results about theionicity and the quantum mechanical potential barrier model reported by our group, tofit the sample magnetic moments at10K, we obtained the cation distributions in (A)and [B] sublattices successfully.(3) For our two series of samples, estimated dependences of Cr content at (A)sites on the total Cr content were accordance with the results of neutron diffractionanalysis in a reference. Using the Cu, Cr and Fe ion contents at the (A) and [B] sitesestimated by fitting sample magnetic moments at10K, the XRD patterns of our twoseries samples were fitted with the Fullprof-Suite software. The various fittingparameters are all acceptable. Therefore, estimated cation distributions by us arereasonable.(4) FT-IR measurement results indicated that the absorption peak position ofB-O-B (Fe-O-Cu) bonds of Cu0.85Fe2.15O4sample was400.5cm-1, and the absorption peak position of B-O-B (Fe-O-Cr) bonds of Cr1.04Fe1.96O4sample was477.8cm-1.Therefore, the absorption peak position of Fe-O-Cr bonds was higher for77cm-1thanthat of Fe-O-Cu. By the reference, the absorption peak position of Fe-O-Cu bondswas higher for about20cm-1than that of Fe-O-Co bonds only. We can find the similarphenomenon in other references, but there was no reasonable explanation to be found.In this paper, we explained this phenomenon firstly using the MOIF model, in whichthe magnetic moments Cr and the Fe ions were antiparallel: due to the magneticmoments of Fe2+, Co2+, Cu2+, Cr2+ions were4,3,1,-4μB, the magnetic repellingenergy in Fe-O-Cu bonds was smaller than the Fe-O-Co bonds, which is main reasonresulted in the wave number of absorption peak position to increase; the magneticinteraction energy in Fe-O-Cr bonds became attractive energy, which resulted in thewave number of the absorption peak position to increase drastically. Therefore, theFT-IR measurement results were the evidence to support the MOIF model. |
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