Study On Magnetic Properties And Cation Distributions In Co-Cr Ferrites

Ferrites are ferrimagnetic oxides containing iron and other transition metalelements. The ferrite materials have potential applications in various fields, such aspermanent magnets, recording magnetic tapes, hard disc recording media, read-writeheads, magnetic refrigeration, magnetic drug delivery etc.The general formula of spinel ferrite is (A)[B]2O4, which space group belongs toFd3m. There are8formulas per unit cell, in which oxygen ions arrange with facecenter cubic, with64tetrahedral sites and32octahedral sites.8cations occupy thetetrahedral (A) sites and16cations occupy the octahedral [B] sites. Spinel ferrites canbe doped different metal ions in order to adjust its magnetic and electrical properties.In recent decades, there are issues that Cr ions occupy either in (A) sites or in [B] sitesfor spinel ferrite. Therefore, our group proposed a new model for magnetic ordering inferrites (MOIF model). This model, in which the both models of Super-Exchange andDouble-Exchange interactions can be merged and improved, not only can be used toexplain the magnetic ordering mechanism in spinel ferrites with chemical formulaMFe2O4(M=Fe, Co, Ni, Cu), and can be used to explain antiferromagnetic orderingmechanism for oxides MO (M=Mn, Fe, Co, Ni), but also can be used to explain theissues about magnetic structure in Cr doped ferrites. In addition, the new model ismore simple and distinct than the old models. According to the new model, themagnetic moments of Cr and Fe caions at the same sites,(A) or [B] sites in spinelferrites, were antiparallel.In this paper, spinel ferrite samples with composition CoCrxFe2-xO4(0.0≤x≤1.0)and Co1-xCrxFe2O4(0.0≤x≤1.0) have been synthesized by hydrothermal method. Thestructure, morphology, magnetic properties and cation distribution are investigated. Itis found that:(1) X-ray diffraction patterns indicated that all the samples had a single phasecubic spinel structure with a space group of Fd3m. The lattice constant of sampleswith composition CoCrxFe2-xO4(0.0≤x≤1.0) decreased with increasing Cr content x.The lattice constant of samples with composition Co1-xCrxFe2O4(0.0≤x≤1.0) decreasedwith increasing Cr content, when x≤0.4；and then increased with increasing x.(2) According to the magnetic hysteresis loops at10K, it is found that thesaturation magnetizations of all the samples monotonically decreased with increasingCr content x. (3) According to the MOIF model and relative reports from our group, when thecontents of Fe are more than or equal to2, we assumed that at the [B] sites [or (A)sites] of the spinel ferrite, the magnetic moment directions of Cr3+and Cr2+cationswill be antiparallel to those of Fe and Co cations. When the contents of Fe are lessthan2, the magnetic moments of cations have cant angles, which increase withdecreasing Fe contents. At the same time that ionicities of the samples were takenaccount, The cation distribution of the samples was estimated by fitting thedependence of the magnetic moments on x at10K. The results indicated that a part ofCr cations occupy at (A) sites, and that Cr cation content at (A) sites increase withincreasing Cr doped level x, which tendency is accordence with the result obtainedusing neutron diffraction reported in a reference.(4) According to the study of infrared spectra, we find that the absorption peakpositions of B-O-B (Fe-O-Cr and Co-O-Cr) bonds of CrFe2O4and CrCoFeO4arehigher for91and97cm-1than that of Fe-O-Co bonds in CoFe2O4, respectively.However, according reports by some references, the absorption peak position ofNiFe2O4is higher only for9cm-1than that of CoFe2O4. According to the MOIFmodel, the magnetic moment directions of Cr cations are antiparallel to those of Feand Co cations, the shift characterastics of the infrared spectrum peaks can beexplained: due to the magnetic moments of Fe2+, Co2+, Ni2+, Cr2+cations are4、3、2、-4μB，respectively, the magnetic repelling energy of Fe-O-Ni bonds is lower than thatof Fe-O-Co bonds, which is the main reason that the absorption peak position ofNiFe2O4is higher for9cm-1than that of CoFe2O4; the magnetic energy of Fe-O-Crbonds became absorbing energy, result in that the shifting range of th absorption peakof Cr doped samples was far larger than that of NiFe2O4. Therefore, this experimentalrasult is an evidence for our assumptions about the magnetic moment direction of Crcations in spinel ferrites.