Study of Additive Effects on the Structural, Magnetic and Transport Properties of Cobalt Ferrites

Abstract

A detailed study was carried out on four different series of spinel Co-based ferrites, with composition Co1-xCdxFe2O4 and Co1-xZnxFe2O4 (where x = 0.0 - 1.0 in steps of 0.1), rare-earth (RE) substituted diluted system Co0.2Cd0.8 Fe2-xRExO4 (RE = Ho and Sm; x = 0.05, 0.1) and Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu: x = 0.05, 0.1). The samples were prepared by conventional double sintering ceramic technique and were found to be single-phase cubic spinel structure of Co-Cd and Co-Zn series by X-ray diffraction technique, while all rare-earth (RE) doped samples showed additional peaks other than spinet. The lattice constant of Co1-xCdxFe2O4 and Co1-xZnxFe2O4 ferrites increase linearly with the increase of Cd and Zn content, followed Vegard's law. Bulk density and X-ray density increases significantly with the increase of Cd and Zn content. Curie temperature, Tc decreases almost linearly with increasing x content up to x =0.7. Samples with x > 0.8 show paramagnetic characteristic at T = 5K with no spontaneous magnetization that revealed through Arrot-Belov-Kouvel plots. The continuous decrease of Tc with the substitution of non-magnetic Cd and Zn in Co1-xCdxFe2O4 and Co1-x ZnxFe2O4 system is attributed to progressive weakening of the strength of A-B intersublattice exchange interaction, JAB. Small thermal Hysteresis of initial permeability, μ' is observed below Te. Saturation magnetization, Ms and magnetic moment are observed to increase upto x = 0.4 - 0.5 and decrease thereafter due to the spin canting in B-sites. The change of saturation magnetization with the increase of Cd/Zn substitution has been explained on the basis of Ned's collinear two-sublattice magnetization model and Yafet-Kittel's non-collinear magnetization model. The initial permeability is found to increase with the increase of Cd2 and Zn2 ions upto x = 0.6. Temperature dependence of low field DC magnetization in the field-cooled and zero-field cooled conditions are performed for the dilute Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 (x = 0.7, 0.8, 0.9, 1.0) spinet ferrites. Samples with x = 0.7 and x = 0.8 show re-entrant spin-glass behavior while the samples, x = 0.9 and x = I show the spin-glass behavior. Frequency dependent complex AC susceptibility measurement as a function of temperature as the samples CdFe2O4 and ZnFe2O4 indicates a spin-glass behavior with the manifestation of shift of spin freezing temperature. The samples also show a typical spin-glass behavior with a manifestation of non-equilibrium dynamics, such as aging, rejuvenation, and memory effects. Large magnetic hysteresis effect has been observed at low temperature for the diluted ferrite compositions. Low field B-H loops, at room temperature, were measured at constant frequency. The hysteresis behavior of Co1-x CdxFe2O4 and Co1-x ZnxFe2O4 reveals the softer ferromagnetic nature of the studied materials with the increase of Cd and Zn content. DC electrical resistivity increases significantly with the increase of Cd and Zn content. The AC resistivity and dielectric constant, ε`, of the samples are found to decrease with increasing frequency, exhibiting normal ferrimagnetic behavior. Dielectric constant, c, decreases rapidly with the increase in frequency at lower frequencies and slowly at higher frequencies, which may be due to the Maxwell-Wagner interlacial polarization. The variation of electrical and dielectric properties is explained on the basis of Fe2+ /Fe3+ ionic concentration as well as the electronic hopping frequency between Fe2+ and Fe3+ ions. Rare-earth (RE) doping of Ho3+ and Sm3+ in Co0.2Cd0.8Fe2-xRExO4 ferrite shows some anomalous behavior. A minority second ferromagnetic phase having well defined Tc has been detected Ho3+ doped sample, where Tc is found to increase with Ho3+ content. This ferromagnetic phase may be assigned as ferromagnetic cluster containing Ho3+. Magnetization is found to increase with Ho3+ addition having higher magnetization for higher Ho content while Sm3+ addition reduces magnetization. The decrease of magnetization with Sm3 substitution for Fe3+ may be explained as due to lower free ion magnetic moment of Sm3+. Samples Co0.2Zn0.8Fe2RExO4 (RE = Gd and Eu) show substantial increase of magnetization with Gd3+ which attains higher value with higher content of Gd3+ while decreasing with Eu3+ doped sample. The decrease of magnetization for Eu3+ doping is due to nonmagnetic nature of Eu which has no magnetic moment of the free Eu3+ ion even at low temperature.