Ni-Cu-Zn Ferrite Research : A Brief Review

The temperature at which the ferrite is sintered cr itically depends on the chemical composition. The electromagnetic properties are dep endent on the densification and microstructure. Substitutions and addition of sinte ri g aids is an attractive approach to enhance the electromagnetic properties. Various com positions in the system Ni1-x-yCuxZnyFe2O4 were investigated. Cu is used to decrease the sint ring temperature. However, Cu decreases the resistivity, which is not desirable for its high frequency applications. So, optimization of Cu content is nec essary. Different ranges of electromagnetic properties have been reported with various Zn concentrations. Optimization of Zn concentration with respect to Ni and Cu is es sential to achieve desirable electromagnetic properties. Influence of rare earth s has also been reported. The investigations showed an improved densification in N -Zn and increased permeability in Cu-Zn ferrite by Sm substitution. La substitutions showed an improved resistivity in Ni-Zn ferrites. Similarly, these substitutions may improv e the electromagnetic properties in Ni-CuZn ferrites. V2O5, MoO3 and Bi 2O3 were reported to be the most widely used sintering additives. Bi 2O3-WO3 and V2O5-MoO3 mixed additives were better than the individual additives in Ni-Cu-Zn ferrite, respectively. Furthe r research is needed on the mixed sintering additives in the system V 2O5, Bi2O3 and MoO3 for the ferrites.


Introduction
Ferrites constitute a special branch of ferrimagnetics.The term ferrite denotes a group of iron oxides, which have the general formula MO.Fe 2 O 3 , where, M is a divalent metal ion such as Mn 2+, Fe 2+, Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Mg 2+ or Cd 2+ .The typical ferrite is magnetite, Fe 3 O 4 (on FeO.Fe 2 O 3 ), which has been a well known magnetic oxide since ancient time.The ferrites were developed into commercially important materials chiefly during the year 1933-1945 by Sonek [1] and his associates at the Philips Research Laboratories in Holland.In a classical paper published in 1948, Neel [2] provided the theoretical key to an understanding of the ferrites.
Ferrites are important materials both from application point of view as well as theoretical point of view.One of the reasons of being ferrites are interesting is their high resistivity.The resistivity of ferrites varies from 10 2 to 10 10 ohm-cm which is about 15 orders of magnitude higher than that of iron [3].This outstanding property of ferrites makes them highly demandable for high frequency applications.Other reasons which make Ferrites to be the most important are their applicability at higher frequency, lower price, greater heat resistance and higher corrosion resistance.Along with the technological advances in a variety of areas, the demand for soft magnetic materials increases day by day.
Among the soft magnetic materials, polycrystalline ferrites have received special attention due to their good magnetic properties and high electrical resistivity over a wide range of frequencies; starting from a few hundred Hz to several GHz.Spinel type ferrites are commonly used in many electronic and magnetic devices due to their high magnetic permeability and low magnetic losses [4,5] and also used in electrode materials for high temperature applications because of their high thermodynamic stability, electrical resistivity, electrolytic activity and resistance to corrosion [6,7].Moreover, these low cost materials are easy to synthesize and offer the advantages of greater shape formability than their metal and amorphous magnetic counterparts.Almost every item of electronic equipment produced today contains some ferrimagnetic spinel ferrite materials.Loudspeakers, motors, deflection yokes, electromagnetic interference suppressors, radar absorbers, antenna rods, proximity sensors, humidity sensors, memory devices, recording heads, broadband transformers, filters, inductors, etc are frequently based on ferrites.
Properties of ferrites are dependent upon several factors such as composition, method of preparation, substitution and doping of different cations, sintering temperature and time, sintered density, grain size and their distribution [8,9].Apart from the fact that they have very complex structures, their physical properties themselves are dependent on a number of valence electrons of the divalent or trivalent metal ions of tetrahedral (A) and octahedral (B) sites.Several attempts have been made to enhance the qualities of ferrites by employing various methods.The most general method is the incorporation of same suitable nonmagnetic/diamagnetic impurities at the A or B sites.This method enables them to acquire improved electrical, magnetic and optical properties.
Ni-Cu-Zn ferrite system is a widely used ferrite material system in various electromagnetic devices.Some of the important properties of Ni-Cu-Zn ferrite are high resistivity, high permeability and comparatively low magnetic losses.These properties make this ferrite system very attractive.In this paper we have represented a review of different growth techniques of Ni-Cu-Zn ferrite system and effect of incorporation of different elements and additives.

Different methods of ferrite preparation
Spinel ferrites are prepared through various methods like solid state reaction method [10]; high energy ball milling method [11]; sol-gel method [12]; chemical co-precipitation method [13]; microwave sintering method [14]; auto combustion method [15], conventional ceramic technique [16], conventional two-step synthesis method [17] etc. Ni-Cu-Zn ferrite has been synthesized through solid state reaction method by many investigators [18][19][20][21][22][23][24][25].In this method, different metal oxides are mixed and calcined to get ferrite powders.However, mechanical mixing of different oxides is hardly intimate and homogeneous and hence it results in composition fluctuation at every stage of processing that also persists after sintering [26].Solid state process requires calcination temperature more than 750°C for phase formation and sintering temperature more than 1000°C to achieve better densification.At this high sintering temperature, evaporation of Zn leads to the formation of chemically inhomogeneous material [27].Chemical methods overcome some limitations of solid state reaction method.
Co-precipitation process was used to synthesis of Ni-Cu-Zn ferrites by many researchers [28][29][30].Hsu et al. [28] obtained crystalline ferrite particles with particle size of about 30 nm.Rahman et al. [29] reported that the average crystallite size of dried ferrite powder was 10 nm and crystallite size influenced the coercivity.
Modak et al. [30] reported that the coercive field for the ferrite with particle size 12 nm was interestingly low (87 Am -1 ) and the saturation magnetization was moderately high (~ 50 Am 2 kg -1 ).
Ghodake et al. [32] reported the synthesis of (Ni x Zn y Cu 1-x -y Fe 2 O 4 ferrites by coprecipitation technique using oxalate precursors.
These chemical methods have some disadvantages.They, have multiple step pathways that are time consuming, require expensive alkoxide precursor material and are highly pH sensitive which require special attention for complex systems like Ni-Cu-Zn ferrites [36].The auto combustion method has the advantages of using inexpensive precursors and low external energy consumption and resulting nano-sized, homogeneous, highly reactive powder.
Several researchers prepared Ni-Cu-Zn ferrite [34,[37][38][39][40] including various other ferrites like Mg-Cu-Zn [41], Mg-Cu [42] and Ni-Zn [43,44] by auto combustion method to produce nano-precursor.Generally, metal nitrate salts are used as reactant and glycine [45], urea [44] and citric acid [37] are used as fuel in auto combustion method.Citric acid is preferred because both glycine and urea contains N 2 and releases extra amount of pollutant N 2 gas during combustion reaction.Auto combustion process has been proved to be a simple and economic way to prepare nano-scale ferrite powders [37].Nanostructured materials offer novel properties.It decreases the free energy during sintering due to increasing surface area which is the main driving force for lowering the sintering temperature of particles.By proper tuning of the particle size, it is possible to optimize the desired properties like electrical, magnetic, optical, thermal, mechanical etc [46].

Improvement of electromagnetic properties
Many investigators have focused their attention on the improvement of electromagnetic properties of the ferrite by divalent ions substitution.Generally, the divalent metal ions (M 2+ ); Ni, Zn, Cu, Mg, Mn, Co or mixtures of these are substituted in different spinel ferrites.These substitutes have different sitting preferences for the two sites ('A' and 'B') in the spinel structure and can change many properties as an effect of modified cation distribution in the ferrite.On the basis of site distribution of M 2+ ions and the strength of the exchange interaction among magnetic ions, the influence of M 2+ substitutions on electromagnetic properties can be explained.

Effect of Zn incorporation
Zn 2+ is used to improve electromagnetic properties as well as densification in the ferrite.It is substituted in spinel ferrite to improve magnetization [47].It also lowers magnetostriction and anisotropy in ferrites [24].Goev et al. [25] stated that initial permeability increased and hysteresis loss decreased with increasing Zn concentration in Ni 0.85-y Cu 0.15 Zn y Fe 2 O 4 ferrite.Both saturation and remancence magnetization had maximum at y = 0.4.An improved electromagnetic properties were obtained by Low et al. [48] at high Zn content of Ni 0.02 xCu 0.02y Zn 0.02z Fe 2 O 4 ; where x+y+z= 50.Ahmed et al. [49] investigated the influence of zinc ion substitution on densification in Ni-Zn ferrite.They found rapid densification with increased Zn 2+ concentration.Sun et al. [50] reported the initial permeability and relative loss factor increased while the cut off frequency decreased with increasing Zn content in Ni 1-x Zn x Fe 2 O 4 ferrite.Zn substituted Cu 1−x Zn x Fe 2 O 4 ferrite had been prepared by Ajmal et al. [51].They found that sintered density increased with increasing Zn concentration.
Both saturation magnetization and magnetic moment increased with increasing Zn concentration up to x = 0.2 and then decreased with further addition of Zn.Jadhav [52] worked on the structural and magnetic properties of Zn substituted Li-Cu ferrites having composition Li x Cu 0.4 Zn 0.6-2x Fe 2+x O 4 and found that the magnetic moment increased with increasing zinc content up to x = 0.15 and then decreased with further addition of Zn.Shaikh et al. [53] investigated the electrical resistivity and Curie temperature of Zn substituted Li x Mg 0.4 Zn 0.6-2x Fe 2+x O 4 ferrites and found that the resistivity initially decreased with Zn content up to 0.15 and then increased with further increasing Zn content.The Curie temperature was also found to decrease linearly with the addition of Zn.At high level of Zn substitution, the 'A' site magnetic ion became so diluted that the coupling between the two lattices was lost and the saturation magnetization dropped.An additional consequence of the weakening magnetic interactions was that the spin coupling could more easily be moved out of alignment by thermal energy vibrations so that, the Curie temperature decreased with Zn substitution.

Effect of Cu incorporation
Cu is conventionally used in Ni-Cu-Zn ferrite to improve densification as well as electromagnetic properties [30].The beneficial effect of copper ions on the densification of ferrite can be reasonably explained by possible sintering mechanisms that take place through a high atomic mobility of Cu ions at relatively low temperature.The increase of the lattice diffusion usually increases the diffusion path leading to an increase of the rate of cation inter diffusion in the solid solution, which is in agreement with the lattice diffusion mechanism proposed by Gupta and Coble [54,55].During sintering grain boundary diffusion [56] may play an important role in the grain growth because the activation energy for lattice diffusion is higher than that of for grain boundary.Nam et al. [57] investigated the effect of Cu substitution on the electromagnetic properties of Ni 0.5-x Cu x Zn 0.50 Fe 2 O 4 ferrite and found that electrical resistivity was maximum at x = 0.2.They also reported that saturation magnetization was maximum at x = 0.2 in Ni 0.2 Cu x Zn 0.80-x Fe 2 O 4 ferrite [35].
Shrotri et al. [58] reported the effect of Cu substitution on the electromagnetic properties of Ni 0.8-x Cu x Zn 0.2 Fe 2 O 4 ferrite and found that the bulk density, electrical resistivity and initial permeability increased considerably with the copper concentration up to x = 0.2.Saturation magnetization and Curie temperature decreased with Cu addition.Caltun et al. [59] also found improved electromagnetic properties at x = 0.2 and suggested that the specific amount of Cu substitution (x = 0.2) was favorable for the grain growth of (NiZn) 0.8-x Cu x Zn 0.2 Fe 2 O 4 ferrite.
Further, Rahman et al. [29] worked in the same composition range and stated that the partial substitution of Ni 2+ with Cu 2+ (up to x = 0.25) influenced the magnetic parameters due to the modification by cation re-distribution.It was found that the maximum coercivity dependent on average crystallite size.The saturation moment decreased linearly with the decrease in Ni 2+ content replaced by Cu 2+ ions at lower concentrations.At higher concentration deviation from this trend was found and magnetization increased with increasing Cu content.
The same property was found by Hsu et al. [28].They reported that Cu substitution for Ni in Ni-Zn ferrite would enhance the densification of the ferrite and subsequently increased the permeability value as well as saturation magnetization value and decreased the coercivity of the sintered ferrite.Su et al. [23] stated that the presence of Cu ion up to 4 mol% activated the sintering process leading to increase in density and decrease in magnetocrystalline anisotropy constant of Ni-Cu-Zn ferrite.
Yan et al. [60] worked on the effect of CuO and V 2 O 5 additions on the microstructure and relative permeability of low temperature fired Ni-Cu-Zn ferrite.They found high relative permeability value of ~1417 at 10 mol% CuO and 0.20 mol% V 2 O 5 addition.Low et al. [48] reported that the increase in Cu content caused a decrease in the dissociation temperature and was beneficial to the electromagnetic properties of Ni-Cu-Zn ferrite as a result of grain growth and better densification.The range of critical CuO content was determined to be 12-20 mol%.
Dimri et al. [61] studied the effect of compositional variation on structural, dielectric and magnetic properties of the Cu substituted Ni 0.6−x Cu x Zn 0.4 Fe 2 O 4 ferrite.The results showed that the addition of copper promoted grain growth, resulting increase in grain size.However, Curie temperature was understandably lowered with the increase in Cu content.A saturation magnetization value of 92 emu/gm was obtained for the composition x = 0.2.Ferrite with Cu concentration of x = 0.4 showed the highest initial permeability.
Cu is also used in other ferrites as divalent cation for improving electromagnetic properties and for lowering the sintering temperature.Yue et al. [41] worked on the effect of copper on the electromagnetic properties of Mg 0.5−x Cu x Zn 0.5 Fe 2 O 4 ferrites and found that the density, grain size, permeability, Curie temperature increased, resistivity decreased with Cu content up to x = 0.40.
Rezlescu et al. [62] reported the effect of Cu substitution on the physical properties of Mg 0.5-x Cu x Zn 0.5 Fe 2 O 4 + 0.5MgO ferrites.They found that the density increased up to x = 0.30 whereas, resistivity increased up to x = 0.10 and permeability increased with Cu content as well.Rezlescu et al. [63] also reported that the sintered density and resistivity of Mg 0.5-x Cu x Zn 0.5 Fe 2 O 4 ferrite increased up to x = 0.3 whereas, permeability increased up to x = 0.4.
Hoque et al. [64] reported that the maximization of initial permeability and saturation magnetization at x = 0.2 in Ni 1-x Cu x Fe 2 O 4 ferrite, which could be attributed to the maximum sintered density obtained for this composition.Haque et al. [65] worked on Cu substituted Mg-Zn ferrites.They found a remarkable increase in the bulk density with increasing Cu substitution for Mg.By incorporating CuO, the initial permeabilities of the samples increased.Saturation magnetization increased slightly with increasing copper content up to x ≤ 0.30 and then it decreased for x = 0.35.Cu substitution is also used in hard ferrites for improving properties [66].
After a detailed study, Murbe et al. [67] found that Ni 0.35 Cu 0.15 Zn 0.5 Fe 2 O 4 provide effective sintering at low temperature (T s ≤ 950 0 C) and provide sufficient permeability and inductance.

Effect of Mn incorporation
Other interesting divalent cation is Mn 2+ .Yue et al. [68] first worked on Mn substituted Ni 0.25-x Mn x Cu 0.25 Zn 0.5 Fe 2 O 4 ferrites and found that the resonant frequency and the Curie temperature decreased with increasing Mn content whereas, the initial permeability increased up to x = 0.06.The same group reported [38] on effect of Mn substitution on electromagnetic properties of (Ni 0.2 Cu 0.2 Zn 0.6 )O.(Fe 2-x Mn x O 3 ) 0.98 ferrites.It was found that Mn content in formulations largely affected the grain size and electromagnetic properties of sintered Ni-Cu-Zn ferrites.With increasing Mn content, the initial permeability was significantly increased, while the electrical resistivity and quality factor decreased.
Further, Li et al. [69] investigated the effect of Mn doping on the magnetic properties and microstructure of the (Ni 0.2 Cu 0.2 Zn 0.6 ) 1.03 (Fe 2 O 3 ) 0.97 +0.97xMnO 2 ferrites.They found that the initial permeability increased with Mn-content, but Mn content more than 0.4 resulted in the formation of a second phase.The high Mn-content increased the grain size of Ni-Cu-Zn ferrites that reduced the fraction of grain boundary with high resistance thereby, lowering the resistivity.Chu et al. [70] studied the magnetic properties of low Mn-doped Ni-Cu-Zn ferrites.The results showed that with the increasing Mn 2+ content up to x = 0.02, saturation magnetization (M s ) of the powder samples decreased.The real part of the permeability of the toroidal specimens increased up to x = 0.01.
Qi et al. [71] prepared Mn substituted (Mg 0.2 Cu 0.2 Zn 0.6 O)(Fe 2-x Mn x O 3 ) 0.97 ferrites.They found that Mn induced higher initial permeability and lowered grain size.It also decreased magnetostriction constant in Mg-Cu-Zn ferrites.
Bhaskar et al. [72] worked on Mn substituted Mg 0.456-x Cu 0.144 Mn x Zn 0.4 Fe 2 O 4 ferrites.They found that the initial permeability and resistivity increased while magnetostriction constant decreased with Mn-content.
Bueno et al. [73] investigated the effect of Mn-substitution on the microstructure and magnetic properties of Ni 0.50−x Zn 0.50−x Mn 2x Fe 2 O 4 ferrites.The results indicated that Mnsubstitution increased induction magnetization (B m ) and remnant magnetization (B r ).The lattice parameter was found to increase with increasing Mn-substitution as well.The addition of Mn enhanced the material's densification.

Incorporation of Co
Co 2+ is used to improve electromagnetic properties in ferrites.Groenou et al. [74] prepared Co-substituted Ni 0.78 Zn 0.22 Fe 2 O 4 ferrite.The results showed that Co 2+ was the anisotropic ion responsible for the magnetic response time.The Co 3+ ions and presumable cation vacancies in association with Co 3+ provide the means whereby, Co 2+ was transported through the lattice.Byun et al. [75] investigated the electromagnetic properties of (Ni 0.2 Cu 0.2 Zn 0.6 ) 1.02-x Co x Fe 1.98 O 4 ferrites.The results showed that the grain size and sintered density changed a little.It also showed the concentration of cation vacancies increased with Co content and the increase in induced anisotropy results in the decrease of initial permeability.The magnetostriction of cobalt ferrite is many times higher than that of Ni-Zn ferrites.
Kulikowski et al. [76] worked on magnetostrictive properties of Co x (NiZn) 1-x Fe 2 O 4 ferrites and shown that the magnetostrictive effect of a core was the result of the effects of the sum of the magnetostrictions of all magnetic domains.
Xiang et al. [77] reported Co substituted Mn 0.4 Zn 0.6−x Co x Fe 2 O 4 ferrites.The results showed that the grain size, saturation magnetization and coercivity increased with increasing Co-content.Rezlescu et al. [78] investigated Co-substituted Ni 0.5 Zn 0.5 Fe 2 O 4 ferrites and found that Co 2+ ions increased the electrical resistivity due to the change in the valence state.Kaiser [79] investigated Ni substitution in Cu 0.8−x Zn 0.2 Ni x Fe 2 O 4 ferrites.The results revealed that the transition (metallic to semiconductor) temperature increased with an increasing Ni concentration.

Incorporation of Mg
Zhong et al. [80] reported the effects of NiO on microstructure and magnetic properties of Zn 0.32 Mn 0.60x Ni x Fe 2.08 O 4 ferrites.The results showed that Ni substitution could cause the lattice constant to decline, grain size to decrease, grain structure to be more compact and porosity to decrease.
Many researchers had worked on Mg substitution in different ferrite systems and found Mg substitution has a significant effect on the structural, electrical and magnetic properties.MgO is a very stable oxide that can be used in a large range of technological applications [81].Mg containing composition is preferred to avoid the presence of divalent iron to obtain high resistivity.With increasing Mg content only octahedral Fe 2+ ions are substituted by Mg 2+ and it decreased with increasing Mg content [82].
In the iron-deficient ferrites, the dominant conduction mechanism is due to holehopping.At high concentration of Mg 2+ ions, the conductivity is low due to the obstruction of Mg 2+ ions to the hopping process between iron ions [83].Mg is also preferred to increase the rate of densification and to avoid the tendency of discontinuous grain growth [84].It is believed that increase of initial permeability can be obtained by decreasing magnetostriction constant.Magnetostriction constant of Mg-Cu-Zn ferrites is lower than that of Ni-Cu-Zn ferrites [85].MgO has low dielectric loss and low dielectric constant.It is doped with ferroelectric material for high frequency device application [86].So, Mg containing ferrites would obtain higher magnetic properties.
Bhosale et al. [87] investigated the effect of Mg in Mg-Cu-Zn ferrite and found that density increased and permeability decreased with Mg 2+ content.But later they [88] reported the ferritization temperature of this system varies with increasing Mg 2+ .It was also been found that lattice parameter decreased and density, initial permeability increased with increased Mg 2+ content up to x = 0.20 in Mg x Cu 0.5-x Zn 0.5 Fe 2 O 4 ferrites.Hiti [89] studied the effect of frequency, temperature and composition on the dielectric behavior of Mg x Zn 1-x Fe 2 O 4 ferrites.The relaxation frequency was found to be shifted to higher values as the temperature increased.

Inclusion of rare earth ions
The rare earth substituted different ferrites are becoming the promising materials for different applications.Addition of small amount of rare earth ions to ferrite samples produces a change in their magnetic and electrical as well as structural properties depending upon the type and the amount of rare earth elements used.
Rare earth ions can be divided into two categories: one with the radius closes to Fe ions; while the other with ionic radius larger than Fe ions [90].The difference in their ionic radii will lead to micro strains, which may cause domain wall motion resulting in deformation of the spinel structure [91].It has been stated that the rare earth ions commonly reside at the octahedral sites by replacing Fe 3+ ions and have limited solubility in the spinel lattice due to their large ionic radii [92].
Ferrimagnetism in ferrite is largely governed by Fe-Fe interaction (the spin coupling of the 3d electrons).If the rare earth ions enter the spinel lattice, the RE-Fe interactions also appears (4f-3d coupling), which can lead to changes in the magnetization and Curie temperature [93].
The rare earth oxides are good electrical insulators and have resistivities at room temperature greater than 106 Ω-cm [94].Rare earth ion forms the orthoferrite phase (REFeO 3 ).The occupation of RE ions on 'B' sites impedes the motion of Fe 2+ in the conduction process in ferrite, thus causing an increase in resistivity [50].
Many investigators have been carried out on the influence of different rare earth atoms (La, Sm, Gd, Nd, Dy, Tb, Ce, Th, Y, Eu) on the properties of ferrites.The results of these researches show that different rare earth atoms behave differently in spinel ferrite.
Rezlescu et al. [95] investigated the effect of Fe replacement by RE (Yb, Er, Sm, Tb, Gd, Dy and Ce) ions on the properties of Ni 0.7 Zn 0.3 Fe 2 O 4 ferrite.The results showed that the electrical resistivity of a ferrite increased by substituting a small quantity of Fe 2 O 3 with RE 2 O 3 .
Sun et al. [50] investigated the effects of rare earth ions on the properties of (Ni 0.5 Zn 0.5 )Fe 1.98 RE 0.02 O 4 (RE = Y, Eu or Gd) nominal compositions.The partial substitution of Fe 3+ with a small amount of RE ions increased the electrical resistivity and relative loss factor, whereas, it slightly decreased the Curie temperature.Results had shown that Y and Eu substitution tend to decrease µ i while Gd did not reduce µ i Jacobo et al. [96] worked on (Zn 0.5 Ni 0.5 )RE 0.02 Fe 1.98 O 4 ferrites, with RE = Y, Gd and Eu.The results showed a small increase in the hyperfine field parameters and a strong decrease of the total resonant area with respect to the pure Ni-Zn ferrite.Curie temperatures decreased and coercive fields increased with substitution.By adding much large ionic radii rare earth ions resulted in local distortion and disorder, enough to induce a softening of the network (s electron density).
Zhao et al. [97] reported the influence of Gd on magnetic properties of (Ni 0.7 Mn 0.3 )Gd x Fe 2-x O 4 ferrites.It was found that the crystallite sizes decreased when Gd ions were doped into Ni-Mn ferrites.With Gd-substitution, when x > 0.06 all Gd ions could not enter into the ferrite lattice but resided at the grain boundary.The maximum content of Gd 3+ ions in ferrite lattices was substituted when x = 0.06.The values of H c and M s were zero for all the samples calcined at 600°C.In addition, the H c and M s values of the samples calcined at 800°C were larger than those sintered at 850°C, with Gd contents less than 0.08.Whereas, when the Gd contents at x = 0.08 and x = 0.10, the H c and M s values of the samples increased with the calcination temperatures.
Zhao et al. [98] worked on the effect of substitution of Fe 3+ by Nd 3+ ions on structure and magnetic properties of CoFe 2-x Nd x O 4 ferrites.It was stated that the value of saturation magnetization for Nd 3+ doped samples was less than that of the pure cobalt ferrite whereas, the coercivity increased with Nd 3+ content.
Rezlescu et al. [99] investigated the influence of rare earth ions like Yb, Er, Dy, Tb, Gd, Sm substitution on structure, magnetic and electrical properties of (Li 0.3 Zn 0.4 )Fe 1.96 RE 0.04 O 4 ferrites.They found that RE 2 O 3 facilitated the formation of secondary phases at grain boundary which suppressed the grain growth.The results also showed that the Curie point shifted to lower temperature and increased the electrical resistivity.
Ahmed et al. [100] reported the electrical properties of (Mg 1+x Ti x )RE y Fe 2-2x-y O 4 ferrites doped with rare earth ions like Er, Ce and Nd.They found that the resistivity increased by introducing rare earth ions into ferrite structure.
Ahmed et al. [49] worked on (Ni 1−x Zn x )La y Fe 2−y O 4 ferrite system and reported La 3+ could not enter into the octahedral site but form small aggregates on the grain boundary.
Sun et al. [101] reported the effect of Fe substitution by La and Gd on the structure, magnetic, and dielectric properties of (Ni 0.5 Zn 0.5 Fe 2-x )RE x O 4 ferrites frequency, decrease the initial permeability and magnetic loss tangent (tanδ) which could be explained by a combination of low density, small grain size, secondary phase (REFeO 3 ) formation, and more lattice defects.The low tanδ values resulted mainly from the reduction in eddy current loss due to the higher electrical resistivity with increasing RE ion.
Ahmed et al. [93] reported the effect of rare earth ions on the structural, magnetic and electrical properties of (Mn 0.5 Zn 0.5 )RE 0.05 Fe 1.95 O 4 ferrites where RE = Tb, La, Ce and Th.Rare earth ion formed orthoferrite (REFeO 3 ) phase and the formation of these secondary phases in ferrite during sintering process was governed by the type and the amount of RE 3+ ion used.It was found that the molar magnetic susceptibilities for rare earth substituted samples were smaller than pure ferrite.The Curie temperature and electrical resistivity increased with increasing rare earth ions in Mn-Zn ferrites.
Rezlescu et al. [42] worked on (Mg 0.5 Cu 0.5 )Fe 2-x RE x O 4 ferrites, in which RE = La, Gd, Y and x = 0 and 0.2, respectively.XRD analysis evidenced that the compounds containing Y 3+ and La 3+ ions are pluri-phasic whereas, the others are mono-phasic.The electrical resistivity decreased by La and Y incorporation and increased by Gd incorporation.
Costa et al. [102] investigated the effect of samarium on the microstructure, relative density and magnetic properties of (Ni 0.5 Zn 0.5 )Sm x Fe 2-x O 4 ferrites.Results showed that the increased relative density and decreased average grain size with rare earth substitutions.It also showed the increase in coercive field and decrease in permeability with rare earth substitution.They also stated about the formation of secondary phase.
Sattar et al. [103] investigated Cu-Zn ferrites doped with rare earth ions like La, Sm, Nd, Gd, and Dy.They found that all samples were of high relative density and low porosity.The magnetization of the samples with Sm and La were higher than that of undoped.
On the other hand, samples with Gd and Dy had lower values of magnetization than that of the undoped ones.The magnetization values of the sample with Nd may be higher or lower than that of the undoped ones depending on the applied magnetizing field.The samples with La, Sm and Nd had higher values of µ r than that of undoped samples while those with Gd and Dy had lower values of µ r than that of the undoped ones.The most important result was that the relative permeability has increased by about 60%, 35.5% and 25%, in case of Sm, La and Nd, respectively.
Mahmoud et al. [104] studied the cation distribution and spin canting angles variation in Cu 0.5 Zn 0.5 Fe 2 O 4 ferrites substituted with Sm and Nd using Mossbauer spectroscopy.They observed slight decrease in saturation magnetization M s of the specimen doped with Nd and significant increase in M s for the specimen doped with Sm.

Use of sintering aids and additives
The sintering temperature of ferrites can be lowered by using sintering aids [104].These additives form liquid phase which either due to the melting of the additives or eutectic liquid phase formation between the additives and ferrites.Amount of liquid phase increases with increasing amount of sintering aids and results in increased densification.
Additives components may also play an important role in the contribution to the magnetic properties of the sintered ferrites.Excessive amount of sintering additives may deteriorate magnetic properties of ferrites.
Several researchers worked on different additives (V 2 O 3 , MoO 3 , Bi 2 O 3 , WO 3 , Glass and PbO) in different spinel ferrite systems to investigate the effect on lowering the sintering temperature and improving electromagnetic properties.
One of the most effective additives is V 2 O 5 .Hsu et al. [105] introduced V 2 O 5 as sintering aids in Ni-Cu-Zn ferrite to reduce the sintering temperature.They found that V 2 O 5 content up to 0.25 mol% improved the magnetic properties.However, a small amount of V 5+ cations entered the crystal lattice of the ferrite and higher amount of V 2 O 5 deteriorated the magnetic properties.0.5 mol% of V 2 O 5 was required to form the liquid phase for enhancing the densification of this composition at around 875°C.
Yan et al. [60] investigated the effect of CuO and V 2 O 5 addition on the microstructure and relative permeability of Ni-Cu-Zn ferrite.They found that these additives contributed to the grain growth and densification of matrix in the sintering process.10 mol% CuO and 0.20 mol% V 2 O 5 compositions showed the highest initial permeability.
Lebourgeois et al. [106] worked on the effect of V 2 O 5 on the densification of Ni 0.24 Cu 0.2 Zn 0.55 Co 0.02 Fe 1.99 O 4 ferrite.They found that with small amount of V 2 O 5 (0.6-1.2 wt %) as a sintering aid was enough for densification at around 850°C.EDAX analyses showed the presence of vanadium inside the ferrite grains.Magnetic properties were deteriorated with increasing the V 2 O 5 content.The permeability decreased and the core losses increased for V 2 O 5 content higher than 0.6 wt%.It was shown that V 2 O 5 content 0.20-0.25 mol% (~0.15-0.20 wt%) improved electromagnetic properties in Ni-Cu-Zn ferrite.
Mirzaee et al. [107] investigated the influence of V 2 O 5 on the microstructure development and magnetic properties of (Ni 0.64 Zn 0.36 )Fe 2 O 4 ferrites.The results showed that the permeability, relative density and grain size initially increased up to 1.6 wt% and then decreased with further addition.The specific resistivity increased continuously.The Curie temperature and saturation magnetization showed peak values at 0.8 wt% and 1.2 wt% of V 2 O 5 , respectively.Mirzaee et al. [108] also worked on the influence of MoO 3 and V 2 O 5 co-doping on the magnetic properties and microstructure of Ni-Zn ferrites.The results showed that 0.05 wt% of MoO 3 in addition to 0.2 wt% of V 2 O 5 increased grain size and sintered density and also decreased power loss noticeably.
Another important sintering aid is MoO 3 .Seo et al. [109] focused on the effect of MoO 3 addition on the sintering behavior and magnetic properties of (Ni 0.2 Cu 0.2 Zn 0.6 ) 1.02 .(Fe 2 O 3 ) 0.98 ferrite.Results showed that MoO 3 addition reduced the sintering temperature and magnetic loss of Ni-Cu-Zn ferrite and it also increased the bulk density and initial permeability up to the additives content of 0.2 wt %.
Su et al. [23] was investigated on the amount of CuO contained in Ni-Zn ferrite and a small additive of MoO 3 added in (Ni 0.29 Cu 0.08 Zn 0.62 )(Fe 2 O 3 ) 1.01 ferrite.Results showed that the initial permeability of the core increased but the Curie temperature decreased a little with 4 mol% CuO content.The highest initial permeability ≈2480 and relatively higher Curie temperature ≈118°C could be obtained at 0.12 wt% MoO 3 additive in Ni-Cu-Zn ferrite.
Gu et al. [110] reported the effect of MoO 3 and TiO 2 additions on the magnetic properties of Mn-Zn ferrite.It was found that the magnetic properties of Mn-Zn power ferrites fluctuate with the increase of MoO 3 content and could be considerably improved with suitable amount of MoO 3 addition.The sample doped with 600 ppm MoO 3 had a decreased power loss of 13.5%, and increased initial permeability of 7.2% than undoped samples.
A few authors also worked on WO 3 as sintering aid.Earlier, Park et al. [111] investigated the effect of WO 3 addition on the magnetic properties of Ni-Cu-Zn ferrites.The results showed that enhanced electrical and magnetic properties obtained from WO 3 addition upto 0.6 wt% into Ni-Cu-Zn ferrites.Later, Su et al. [112] reported that appropriate WO 3 additive could improve grain size of the specimen and removed closed pores.
Recently, Su et al. [113] studied the effects of WO 3 addition on properties of Ni-Cu-Zn ferrites.The results showed that to obtain a single-phase spinel ferrite WO 3 addition should be less than 0.28 wt%.With proper WO 3 addition, the average grain size and the initial permeability of the ferrites increased.The maximum initial permeability was observed at a WO 3 content of 0.16 wt%.
Lead oxide and glass also act as sintering aid in Ni-Cu-Zn ferrites.Earlier, Jean et al. [18] investigated the effect of PbO addition on the densification, microstructural properties of Ni-Cu-Zn ferrites.Results showed that with a small amount of PbO (0.5-2 wt.%) in Ni-Cu-Zn ferrite increased the densification rate and final sintered density when the activation energy of densification was significantly reduced.However, interfacial reaction causes segregation of CuO onto the grain boundary and dissolution of PbO into the ferrite grains, yielding a reduction in initial permeability.
Later, Wang et al. [114] used PbO-SiO 2 , PbO-B 2 O 3 and Bi 2 O 3 flux systems to lower the sintering temperature of (Ni 0.38 Cu 0.12 Zn 0.50 )Fe 2 O 4 ferrite and found that the PbO-SiO 2 glass system to be the most effective additive to obstruct the grain boundary movement and prevent the exaggerated grain growth among these flux systems.Ferrites sintered with PbO-SiO 2 possess higher resistivity, higher Q and higher H c compared with those obtained using other systems.The results also showed that the addition of Bi 2 O 3 considerably deteriorated the quality factor of Ni-Cu-Zn ferrites, though benefited the densification and permeability.Thus, Bi 2 O 3 is another most interesting sintering aid.
Hsu et al. [115] reported Bi 2 O 3 and lead glass as sintering aids for Ni-Cu-Zn ferrite.Results showed that both additives formed liquid phases in the grain boundaries during the sintering process to enhance the densification.But Bi 2 O 3 exhibited better effect than the lead glass to lower the sintering temperature.
Wang et al. [116] worked on the mixing of (Ni 0.38 Cu 0.12 Zn 0.50 ).Fe 2 O 4 powders with Bi 2 O 3 using the solid-state mixing as well as wet chemical coating processes such as ammonia precipitation coating, urea precipitation coating, and solution coating.Ferrites prepared from the wet chemical coating processes could be densified at a lower sintering temperature without significant impact on the micro structural evolution compared with that prepared by solid-state mixing.In addition, samples prepared from the wet chemical coating process have a higher B r and B m and a lower H c compared with that from the solid-state mixing process.Considering both effects of sintering temperature and sintered density (>95% theoretical density), ferrites with 1.5 wt% Bi 2 O 3 addition by ammonia precipitation coating process followed by sintering at around 900°C could provide the best permeability and quality factor.
Jeong et al. [117] investigated the effect of Bi 2 O 3 addition on the microstructure and electromagnetic properties of Ni-Cu-Zn ferrites.The grain size and bulk density gradually increased with the increase in Bi 2 O 3 content.Above 0.5 wt% Bi 2 O 3 , abnormal grain growth was observed.Specimen with 0.25 wt% of Bi 2 O 3 showed the highest initial permeability with good quality factor and a uniform microstructure.
Su et al. [112] investigated the effects of mixed Bi 2 O 3 -WO 3 additives.The results showed that appropriate additives of mixed Bi 2 O 3 -WO 3 could enhance densification of the specimens, improve the initial permeability and the Q-factor of Ni-Cu-Zn ferrites.The maximum initial permeability was achieved with 1.5 wt.% Bi 2 O 3 and 0.3 wt.% WO 3 additives.
Kawano et al. [118] fabricated Bi, Si oxides-doped Ni-Cu-Zn ferrite composite materials.Analysis showed that the composite materials has mainly Ni-Cu-Zn ferrite and Bi 4 (SiO 4 ) 3 phases.The optimum percentage of Bi 2 O 3 content to improve the electromagnetic properties in Ni-Cu-Zn ferrite is 0.25 wt.%.
Kong et al. [119] reported the effects of concentration of Bi 2 O 3 on MgFe 1.98 O 4 ferrite.It was found that 3 wt.% Bi 2 O 3 could result in fully sintered ferrite.Low concentration of Bi 2 O 3 increased the static permeability of the ferrite owing to the improved densification and grain growth, while too high concentration led to decrease in permeability owing to the incorporation of the non-magnetic component (Bi 2 O 3 ) and retarded grain growth.
Murbe et al. [67] found that the addition of Bi 2 O 3 further enhances the densification and has dramatic influences of the microstructure and permeability.With the addition of 0.35-0.5 wt.% Bi 2 O 3 a bimodal microstructure with large (25-30µm) grains surrounded by small-grained (1-2 µm) regions is observed.
Saha et al. [120] revealed that sintering temperature could be reduced by 150 0 C when 0.8 wt.% of Bi 2 O 3 is added to Ni-Cu-Zn ferrites.The results also showed that the addition of Bi 2 O 3 considerably benefited the densification and permeability.

Conclusion
Ni-Cu-Zn ferrite system is an established well-known ferrite material to be used in various electromagnetic devices due to their high resistivity, high permeability and comparatively low magnetic losses.Extensive research on Ni-Cu-Zn ferrite shows better performance with Ni 0.35 Cu 0.15 Zn 0.50 Fe 2 O 4 .It shows better electrical, structural and magnetic properties.Small amount of Bi 2 O 3 addition can reduce sintering temperature considerably and thus providing advantage in view of energy saving and increase of productivity by reducing the sintering time in the manufacture of ferrites.It has also been found that the addition of Bi 2 O 3 to Ni-Zn ferrites increases the initial permeability and improves the magnetic loss factor.