Structural analysis, magnetic and electrical properties of samarium substituted lithium–nickel mixed ferrites

Abstract

A series of Sm-doped Li–Ni ferrites with formula of (Li_0.5Fe_0.5)_0.4Ni_0.6Sm_yFe_2−yO₄, where 0.0≤y≤0.1 were prepared by double sintering ceramic technique. The structure was characterized by X-ray diffraction, which has confirmed the formation of single-phase spinel structure. The samarium concentration dependence of lattice parameters obeys Vegard's law. The octahedral site radii increased with Sm content while the tetrahedral site radii decreased. Deviation from the ideal crystal structure (Δ) is found to decrease with Sm substitution, and the hopping length on the octahedral site is found to increase with Sm content. Hall measurement confirmed p-type conductivity behavior for Sm-doped ferrite and the main charge transport mechanism is hopping of halls between Ni²⁺ and Ni³⁺. Sintering at 1300 °C resulted in low resistivity ferrite, which was found to increase with Sm content. Resistivity is governed by both charge carrier mobility and carrier concentration. It decreases with frequency, and this behavior with frequency is discussed according to Koop's theorem. The dielectric constant is found to decrease more rapidly at low frequencies than at higher frequencies while the dielectric constant increases with Sm content. The decrease in ε″ with frequency agrees with Deby's type relaxation process. Maximum in ε″ is observed when the hopping frequency is equal to the external electric field frequency. The variation in tan δ with frequency shows a similar nature to that of ε″ with frequency. The magnetization under applied magnetic field for the samples exhibits a clear hysteretic behavior. The scanning electron microscope (SEM) studies showed that the domain walls may tend to be trapped (pinned) by non-magnetic inclusions, precipitates and voids. The saturation magnetization (M_S) increases with the sintering temperature, while the coercivity (H_Ci) is found to decrease.

Introduction

Lithium ferrites are low cost materials and have important magnetic and electrical properties for the applications of microwave devices, such as isolators, circulators and phase shifters. It is known that the intrinsic parameters of ferrites, such as magnetization, low dielectric losses, Curie temperature and resistivity, depend on the chemical composition, heat treatment and type of additive or substituted ions. In addition, the rectangularity of B–H loop is also found to increase with increasing defects that hinder the magnetic domain wall motion within the grains [1]. It is interesting to note that the electromagnetic properties of spinel ferrites can be tailored by controlling the type and amount of transition metallic substitutes. Until now, several investigations have been carried on to make further improvements in the electric, dielectric and magnetic properties of substituted lithium ferrites. The rare earth oxides are becoming promising additives for the improvement of ferrite properties. It is known that rare earth (RE) ions have unpaired 4f electrons, which have the role of originating magnetic anisotropy because of their orbital shape. The magnetocrystalline anisotropy in ferrite is related to the 4f–3d couplings between the transition metal and rare earth ions; thus doping rare earth ions into spinel lithium ferrite can improve their electrical and magnetic properties. Many researchers have studied the influence of various rare earth doping atoms on the properties of, Li–Ni, Ni–Zn, Mn–Zn, Mg–Cu, Cu–Zn ferrites, etc. [2], [3], [4], [5], [6]. The results of these researches show that different rare-earth ions behave differently in spinel ferrite. In general, the permeability of ferrite (e.g. Ni–Zn) was reported to decrease with the substitution of R₂O₃. An increase of about 60% relative permeability in Sm-doped Cu–Zn ferrite is reported by Sattar et al. [1]. Jalli et al. [7] report on the Samarium (Sm) doped M-type strontium ferrite single crystals. Dwevedi et al. [8] report on the structural, electrical and magnetoreactance of Dy, Gd and Nd-doped Ni ferrites. Some literature reports the decrease in relative density of ferrites with rare earth addition. On the other hand, others report on its increase. It has been accepted that the rare-earth ions commonly reside at octahedral sites and have limited solubility in the spinel lattice due to their large ionic radii. Nevertheless, the precise value of their solubility in the spinel lattice is not well known [9], [10].

In the present work, the magnetic, electric and dielectric properties of Sm-substituted Li–Ni spinel ferrite are synthesized and characterized as a function of composition, frequency and sintering temperature. This may provide interesting properties due to the cations redistribution in the tetrahedral A and octahedral B—sites of the spinel structure. Both mechanisms of dielectric polarization and domain walls motion are discussed in detail in this work.