A review on MnZn ferrites: Synthesis, characterization and applications.

Researchers are taking great interest in the synthesis and characterization of MnZn ferrites due to their wide range of applications in many areas. MnZn ferrites are a class of soft magnetic materials that have very good electrical, magnetic and optical properties. The properties of MnZn ferrites include high value of resistivity, permeability, permittivity, saturation magnetization, low power losses and coercivity. The above mentioned advantageous features of MnZn ferrites make them suitable for the use in various applications. In biomedical field these ferrites are used for cancer treatment and MRI. MnZn ferrites are also used in electronic applications for making transformers, transducers and inductors. These ferrites are also used in magnetic fluids, sensors and biosensors. MnZn ferrite is highly useful material for several electrical and electronic applications. It finds applications in almost every household appliances like mobile charger, LED bulb, TV, refrigerator, juicer mixer, washing machine, iron, microwave oven, mobile, laptop, desktop, printer and so on. Therefore, the present review focuses on different techniques for synthesis of MnZn ferrites in literature, their characterization tools, effect of doping on the properties of MnZn ferrite and finally we will discuss about their applications.

Introduction

A ferrite [[1], [2], [3], [4], [5], [6]] is a ceramic material that is made up of iron oxide (Fe2O4) in large proportion mixed with metallic element such asbarium (Ba), manganese (Mn), nickel (Ni), zinc (Zn) in small proportions. The nature of both the iron oxide and the metal is electrically non-conducting and ferrimagnetic. Ferrimagnetic material is one that possesses unequal opposing magnetic moments which allow such materials to retain spontaneous magnetization. Ferrites are generally classified into two types: hard ferrites [7,8] and soft ferrites [[9], [10], [11], [12], [13], [14], [15], [16], [17]]. Hard ferrites have high coercivity and such materials are difficult to magnetize. Therefore these materials are used in making permanent magnets which are used for applications in refrigerator, loudspeaker, washing machine, TV, communication systems, switch mode power supplies, dc-dc converters, microwave absorbing systems, high frequency applications, refrigerator, loudspeaker etc. [[18], [19], [20], [21], [22], [23], [24], [25]]. On the other hand, soft ferrites have low coercivity as a result of which their magnetization can easily be altered. Soft ferries are good conductors of magnetic field which has led to its wide range of applications in electronic industry such as developing transformer cores, high frequency inductors and as microwave components [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]], see Fig. 1 for more details. Furthermore, advantages of soft ferrites include high resistivity, low cost, time and temperature stability, low loss and high permeability [[43], [44], [45], [46]]. Most common soft ferrites are MnZn ferrites [[47], [48], [49], [50], [51], [52], [53], [54], [55]] and NiZn ferrites [[56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73]]. MnZn ferrites are more preferred as they have high permeability [74] and saturation magnetization [75]as compared to NiZn ferrites. Because of low value of resistivity of MnZn ferrites as compared to NiZn ferrites, these ferrites are used for low frequency applications [[76], [77], [78]]. The properties of MnZn ferrites are essentially dependent on the synthesis methods [55,[79], [80], [81], [82], [83], [84], [85]] and the doping concentrations inside nanoferrites [86,87].

Fig. 1

Applications of soft ferrites

In the past decade MnZn have attracted a large amount of attention in academia due to its advantageous features that make MnZn ferrites suitable to be used in many applications of daily life. The data of the publications of the MnZn ferrites by web of science in the last decade is shown in Fig. 2 . The record of the data shows that there is a regular increase in the publications of the MnZn ferrite documents in the last ten years and much more progress in citations may be seen in years 2018 and 2019.

Fig. 2

Documents on MnZn ferrites in web of science in last 10 years a) number of articles and b) number of citations

Fig. 3 shows various applications of MnZn ferrites. The change in the concentration of cations [[88], [89], [90], [91], [92]] and sintering conditions [93,94] changes the magnetic, electrical properties and structural properties of nanoferrites which lead to its wide range of applications. In addition, the shape, morphology, electrical, magnetic properties are affected by the cation distribution in the MnZn ferrite [95]. Cationic distribution for Mn1-xZnxFe2O4 is described in Table 2. Considering the importance of Mn–Zn ferrites for various applications, a comprehensive review based on 261 references is summarized.

Fig. 3

Applications of MnZn ferrites.

Table 2

Cation distribution of Mn1-xZnxFe2O4.

X	Cation distribution
0.2	(Zn0.2Mn0.4Fe0.4)[Mn0.4Fe1.6]O4
0.4	(Zn0.4Mn0.2Fe0.4)[Mn0.4Fe1.6]O4
0.6	(Zn0.4Mn0.2Fe0.4)[Zn0.2Mn0.2Fe1.6]O4
0.8	(Zn0.6Fe0.4)[Zn0.2Mn0.2Fe1.6]O4

Purpose of the review

The main purpose of the review is to focus on the synthesis, morphology, properties and characterization methods of MnZn ferrites. While the subject of magnetic nanostructures is enormously wide and a large number of good review articles are published on magnetic nanoparticles, MnZn ferrites in particular constitute a special niche of nanoparticles because of immense interest of the scientific community in soft ferrites. In addition, this review critically analyze methods and discusses on the choice of synthesis method for use of MnZn ferrites in a given application. In brief methods such as sol-gel method [[96], [97], [98], [99], [100], [101]], co-precipitation method [85,[102], [103], [104], [105], [106], [107], [108], [109]], conventional ceramic technique [110,111], hydrothermal method [[112], [113], [114]], citrate precursor method [[115], [116], [117]], solid state reaction method [118], auto-combustion method [119] and microemulsion method [120] for the synthesis of MnZn ferrites are discussed. Various advantages and disadvantages of the synthesis methods are shown in Table 1 .

Table 1

Summary of advantages and disadvantages of major synthesis techniques.

Methods	Temperature (°C)	Advantages	Limitations
Co-precipitation	30–140	Simple processAqueous mediaControlled size and morphologyEasily functionalized	Poor crystallinityVery long reaction time requiredBroad size distribution
Hydrothermal	100–200	ScalableControlled sizeAqueous mediaHigh yield	Requirement of special reactorHigh pressure required (>2000PSI)High temperatureLong reaction time
Sol-gel method	20–200	Controlled size and shapeLow cost	Takes longer timeYield is medium
Microwave hydrothermal method	160	Fast heating speedFaster and economicalVery fine nanoparticles producedUniform morphology	–
Combustion method	480	Less time and energy requiredSimple and effective methodVersatile and fastNanoparticles produced are pure and homogeneous	Very high temperature is required
Solid state reaction method	25	No toxic and expensive solvent usedFacile and economic	–
Oxidation process	30	Narrow size distributionUniform size	Irregular and elongated morphology of the product

Morphology of MnZn ferrites

MnZn ferrites have spinel structure [121]. The spinel structure has one major unit cell composed of 8 sub-unit cells having face centered cubic (FCC) structure with two types of sites in each unit cell i.e. tetrahedral (A) site and octahedral (B) site in the complete structure of MnZn ferrite. There are 64 tetrahedral interstitial sites and 32 octahedral interstitial sites. Spinel structure has closed packed oxygen atoms arrangement in which 32 oxygen atoms form a unit cell. Tetrahedral (A) sites are surrounded by four nearest neighbor oxygen atoms and octahedral (B) sites have six nearest oxygen atoms around it as shown in Fig. 4 . In MnZn spinel ferrite lattice, Zn ions are on the tetrahedral sites while Fe and Mn ions occupy both tetrahedral and octahedral sites. Due to this spinel structure, different metallic ions can be introduced that causes change in the electric and magnetic properties of ferrites. The metal ions introduced may enter the spinel crystal lattice by replacing Fe3+ ions and leading to aggregation of these ions on the grain boundary. These morphological features suggest that the properties of MnZn ferrite nanoparticles can be tuned as long as the nanoparticle designer is specifically for a given application choose appropriate synthesis and characterization techniques. In order to know the best advantages of MnZn ferrites for various applications, one has to be aware of different synthesis and characterization techniques.

Fig. 4

Spinel structure(Reproduced by permission from Ref. [241],License Number: 4646900916646, Copyright 2016, Elsevier).

Why do we prefer MnZn ferrites?

MnZn ferrites are preferred over other ferrites due to their low cost and wide range of applications. These ferrites are very important for stress insensitivity and low noise and are generally used for applications where frequency requirements are below 2 MHz. MnZn ferrites are also advantageous due to their almost zero magnetocrystalline anisotropy.

In the class of soft ferrites, MnZn ferrites are preferred due to high permeability [[122], [123], [124], [125], [126], [127]], saturation induction [[128], [129], [130]], low power losses [34,[131], [132], [133], [134], [135], [136], [137], [138]] and high magnetic induction [139,140]. MnZn ferrites are of great interest due to their wide range of applications such as hyperthermia applications [141], power applications [[109], [110], [111], [142], [143], [144]], magnetic fluid [145], high frequency power supply [142], memory storage devices, TV sets, biomedicines [146], magnetic resonance, catalysis etc. There is a continuous progress in the size and shape control of MnZn ferrites and also on the morphological and magnetic properties of MnZn ferrites by using different methods [147] of synthesis like sol-gel method [[96], [97], [98]], co-precipitation method [85,[102], [103], [104], [105], [106], [107], [108], [109],143], conventional ceramic technique [110,111], hydrothermal method [[112], [113], [114],[148], [149], [150]], citrate precursor method [115], solid state reaction method [118], auto-combustion method [119,151], microemulsion method [120]. The effect of doping on the structural and magnetic properties of pure MnZn ferrites is also taken into account.

Synthesis methods to prepare MnZn ferrites

There are two approaches to synthesize nanoparticles: top–down and bottom–up. Both these approaches are shown in Fig. 5(a). In top–down, a bulk material is broken down to get nanosized particles. This method has many limitations like generally metal oxides are used, requirement of very high temperature for the reaction, products are inhomogeneous, presence of impurities, crystal defects, broad size distribution and imperfection in surface structure. In bottom–up approach, small atomic building blocks fit together to produce nanoparticles. This is most favorable method for nanoparticles synthesis as the products in this method are homogeneous, highly pure and have narrow size distribution.

Fig. 5

(a) Top–down and Bottom–up approach to synthesize nanoparticles and (b) flow chart for preparation of MnZn ferrite by novel combustion method using subsequent heat treatments (Reproduced by permission from Ref. no. [259], Licence No. 4763520487833, Copyright 2011, Elsevier).

Various synthesis techniques are used to prepare MnZn ferrite nanoparticles [[152], [153], [154], [155], [156], [157], [158], [159], [160]]such as sol-gel mthod [[161], [162], [163], [164]], polyol process [165],co-precipitation method [104,166,167], hydrothermal method [113], citrate precursor method [122], solid state reaction method [118], auto-combustion method, ceramic processing method [139]. Some of the techniques to synthesize MnZn ferrites are shown in Fig. 6 . By doping other elements or oxides [[168], [169], [170], [171]] the structural, electrical and magnetic properties of MnZn ferrite can be enhanced. For instance, Zaspalis et al. [172]observed that there was 17% improvement in the total power loss per volume when doping was done of Nb2O5 in pure MnZn ferrite. After doping there was reduction in the losses related to magnetostriction and stress related hysteresis losses. Also, the eddy current losses related to electrical resistivity were also reduced. Xiang et al. [173] prepared MnZn ferrite particles with Ce3+ doping and observed that no impurity phase was detected in the XRD pattern. It confirmed that Ce ions entirely got dissolved in spinel structure. This also led to an increase in the saturation magnetization and decrease in the coercivity of MnZn ferrites, leading to an overall improvement in the soft magnetic properties of the material. Some methods of synthesis are described below.

Fig. 6

Various synthesis techniques to synthesize MnZn ferrites.

Microwave hydrothermal process

Microwave is a form of electromagnetic energy associated with electromagnetic field. It can be defined as an electromagnetic wave having frequency and wavelength between 300 MHz and 300 GHz in 1 m to 1 mm range respectively. While the study of microwaves started during 1930s, the first work on microwave hydrothermal synthesis of nanoparticles was demonstrated by Dr. Komarneni while distinguishing the traditional hydrothermal synthesis methods [174] from microwave hydrothermal synthesis [[175], [176], [177]]. In microwave hydrothermal method, heat required in the synthesis process is generated by microwaves which have the advantage of high penetrating power. Microwaves can penetrate and heat the sample to a certain depth. Microwave hydrothermal method is beneficial as it has very fast heating rates to allow generation of uniform nanomaterials with fine particle size distribution. Hence, this is faster, cleaner and economical method as compared to traditional hydrothermal method [178]. Praveena et al. [179] synthesized MnZn ferrites by using microwave hydrothermal process. Pure manganese nitrate [Mn(NO3)2.6H2O], zinc nitrate [Zn(NO3)2.6H2O] and ferric nitrate [Fe(NO3)2.9H2O] were dissolved in 50 ml de-ionized water. In this process pH was maintained at about 9.4.Thereafter the mixture was sealed in tetrafluorometoxil (TFM) and was put in microwave oven for 30 min at 160 followed by washing of the solids with de-ionized water and ethanol several times. The resulting wet mixture was dried and then polyvinyl alcohol (PVA) was added that acted as a binder. The powder was then pressed into pellets followed by sintering at 900 for 30 min. Single phase spinel structure was confirmed by the XRD spectra.

Hydrothermal method

The hydrothermal method is used for the preparation of ferrite nanoparticles on a large scale. Essentially, in this method the yield of nanoparticles is very high. If the parameters such as temperature, pressure and reaction time are properly selected, good quality nanoparticles can be synthesized. Phong et al. [112] studied magnetic properties and specific absorption of Mn0.3Zn0.7Fe2O4 nanoparticles. In this work the MnZn ferrites were prepared by a hydrothermal process in a Teflon-lined stainless steel autoclave. The starting materials FeCl3, MnCl2, ZnCl2 were dissolved in HCl solution and NaOH was slowly added to the solution and stirred for 30 min. The solution was transferred to Teflon-lined stainless steel autoclave till it was 80% full. The autoclave was heated at 180 for 12h and then left for cooling to room temperature. After that the products were washed many times with hot de-ionized water and acetone and finally dried in an oven at 80 for 5h. By this method, large quantity of ferrite nanoparticles can be synthesized. The Mn-Zn ferrite nanomaterials prepared by this method have controlled size and this method requires aqueous media for the synthesis. But this method has some limitations that include requirement of special reactor, need of high pressure and high temperature.

Co-precipitation method

Co-precipitation [180] is an easy and conventional method to synthesize nanomaterials. The ferrites prepared using this method are of controlled size, highly pure and have homogeneous structure. Typical co-precipitation method for synthesis of nanoparticles is shown in Fig. 7 . Normally inorganic salts (nitrate, chloride, sulphate, etc.) are used in this method as the starting materials that are dissolved in water or any other medium which is suitable to form a homogeneous solution. The pH of the solution is adjusted to 7–9 and the solvent is evaporated to get nanoparticle precipitates. It should be noted that the concentration of salt, temperature, pH and the rate of pH change are detrimental to crystal growth and aggregation of the particles. After precipitation, the solid mass is collected and washed. This is followed by heating of the residue up to the boiling point of the medium to dry the resultant product and form hydroxides. The hydroxides are then calcined to transform the hydroxide into crystalline oxides. Thakur et al. [181] used co–precipitation method to synthesize MnZn ferrite. In this method, manganese chloride, zinc chloride, iron(III) chloride and sodium hydroxide were used as raw materials. A 3 M solution was prepared in 60 ml of distilled water. This solution was then poured into boiling NaOH solution while stirring for 60 min at temperature 353–358K with a magnetic stirrer, maintaining the pH between 11 and 12. Stirring allowed precipitates of the nanoparticles to settled down and then sample was washed many times with distilled water. After washing, the sample was dried in hot air oven followed by crushing the resultant into powder using mortar pestle. Anwar et al. [9] also synthesized MnZn ferrites by the chemical co–precipitation method by taking solution of Mn(NO3)2.4H2O, Zn(NO3)2H2O and Fe2(NO3)3.9H2O as the starting materials. These were mixed to form homogeneous solution at 358K. Then, ammonia solution was added dropwise with constant stirring maintaining the pH between 10 and 11. The mixture was heated at 353K for 1h. Then after the washing and drying process the ferrite powder was heated at 673 K, 773K and 923K separately and pressed in the form of circular pellets. The chemical reaction during the process was:0.5Mn(NO

Fig. 7

Typical Co–precipitation method for Mn-Zn ferrite nanoparticles synthesis.

Yadav et al. [182] studied the properties of ferrite nanoparticles by co–precipitation method with samarium doping. The ferrites with Sm doping were very pure and had single crystalline spinel phase. Kumar et al. [183] studied the conduction phenomena in indium substituted Mn–Zn nano-ferrites. Mn0.4Zn0.6InyFe2-yO4 (y = 0. 0.035, 0.070, 0.100) were synthesized by oxalate co-precipitation method followed by microwave heating. The raw materials used were manganese sulphate monohydrate (MnSO4.H2O), iron sulphate heptahydrate (FeSO4.7H2O), zinc sulphate heptahydrate (ZnSO4.7H2O), anhydrous indium sulphate (In2(SO4)3) and Diammoniumoxalate monohydrate [(NH4)2C2O4.H2O]. These all starting materials were mixed by rapidly adding Di-ammonium oxalate under continuous stirring at 45 °C for 30 min until precipitates were formed. Precipitates were washed many times and then dried in an oven at 100 °C for 8h. Dried yellow precipitates were used to prepare ferrites by using in-house built microwave heating set up. Aluminium metal powder was used as microwave susceptor. This set up was then put on a commercial microwave oven operated at a frequency 2.45 GHz. The oven was set to raise the temperature to 450 °C. Then, brick was taken out and allowed to cool. Co–precipitation method has several advantages as it uses aqueous medium for synthesis and also the synthesis is very simple. There is a good control on the size and morphology of the nano particles formed. But this method takes long time to synthesize nanoferrites. This method is disadvantageous due to poor crystalline nature of the resultant ferrite powder.

Sol–gel method

Sol–gel method [[184], [185]] is a promising method used for the preparation of ferrite nano materials. It is a chemical solution process used to synthesize nanoparticles. A sol is a colloidal or molecular suspension of solid particles of ions in a solvent and gel is a semi–rigid mass that forms when the solvent from the sol starts evaporating where the particles left behind start to join together in a continuous network. The resultant product which comes out is in the form of colloidal powder or films. This method is advantageous because of controlled microstructure of the final product. The resultant particles formed are of uniform and small size. Also, this technique of nanoparticles synthesis is economical and it involves use of low temperature. Duan et al. [161] synthesized MnZn ferrite nanoparticles taking pure Fe(NO3)3.9H2O, Zn(NO3)2.8H2O and Mn(NO3)2as starting materials. These materials were dissolved in de-ionized water at 60. Also, C6H8O7.H2O was dissolved in above solution to chelate the metal ions with the citrate ions and concentration was adjusted to 0.1–0.4 by adding de-ionized water. PVP was added as a binder to reduce film cracks. The spin coating was done at 3000 rpm for 30s. The samples were heated at 350 for 30min, followed by crystallization at 550 for 60min after each coating. The flow chart of the sol-gel auto combustion method is shown in Fig. 8 . The sol-gel method is also used for depositing structurally and magnetically uniform films for spin thermoelectric generator. Gabal et al. [186] studied Mn–Zn nano-crystalline ferrites synthesized from spent Zn–C batteries using novel gelatin method. The Zn–C batteries were used to synthesize the ferrites by using sol-gel method using gelatin. Jalaiah et al. [187] synthesized nickel doped MnZn ferrites by sol–gel auto combustion method and observed non-collinear magnetic structure. The room temperature conductivity was observed to be higher than pure MnZn ferrite. There was a decrease in dielectric constant and dielectric loss tangent with increase in nickel concentration.This method is advantageous because of better size and shape control but it takes a longer time to complete the synthesis. Sol-gel method is a simple process, require low processing temperature and low cost. The prepared ferrite consists of a pure cubic spinel structure.

Fig. 8

Flow chart for the preparation of samples using sol-gel auto combustion method (Reproduced by permission from Ref. [240], Licence No. 4763510638098, Copyright 2016, Elsevier).

Combustion method

Combustion process is the effective and low cost method to synthesize nano materials. This process is simple, versatile and fast for nano material preparation. This method is advantageous as less time and energy is spent during the synthesis process. The nanoparticles produced are pure and homogeneous. Many researchers synthesized MnZn ferrites by using this method [188,189]. Manganese nitrate [Mn(NO3)2.6H2O], zinc nitrate [Zn(NO3)2.6H2O], iron nitrate [Fe(NO3)3.9H2O] were taken in proper proportions and urea [CO(NH2)2] was used as a reducing agent in this process. Typically a solution is formedby adding these all materials in de–ionized water and heated on a hot plate at 480 in air. Then, it is ignited within 5 s with flame temperature ~1600. Combustion technique methodology is described in Fig. 5. While doping with other elements, a decrease in the lattice parameter was observed which could be attributed to the fact that ions of doped elements get trapped at the grain boundaries. Hence they hinder the grain growth and may cause an increase in strain on the grains that leads to decrease in lattice parameter. Doping of rare–earth metals can be done using combustion method [189] in a single step. The fuel chosen in the combustion methodalso has very important effect on the MnZn ferrites prepared. The fuels that are generally preferred in this method are urea and glycine. By using these fuels uniform nanoferrites with controlled stoichiometry are obtained.

Solid state reaction method

The solid-state reaction method to synthesize nanoparticles has several advantages. In this method, toxic and expensive organic solvents are not used in the reaction and all the materials used to synthesize MnZn ferrite nanoparticles are easily available and cost effective. The synthesis process is performed at room temperature under atmospheric pressure which is facile and economic. Many researchers synthesized MnZn ferrites by using this method [30,132,134,190]. The raw materials MnCO3, ZnO and Fe2O3 in the proper weight were powdered and the powdered samples were calcined at 1100 for 5h in air atmosphere using muffle furnace with heating rate of 10/min and a cooling rate of 5/min. Then, PVA was used as a binder and powder was pettelized into small disks and torroids. Then sintering was done to get the required nano ferrites. Kogias et al. [191] studied MnZn ferrite with low losses at 500 kHz over a broad temperature range by preparing MnZn ferrite using conventional ceramic technique of solid state reaction.Tsakaloudi et al. [192] studied process and material parameters towards the design of fast firing cycles for high permeability MnZn ferrites. In this paper, high permeability of MnZn ferrites was reduced by increasingthe energy consumption in the synthesis reaction due to prolonged sintering process for the production of nanoferrites. Zapata et al. [30] studied effect of zinc concentration on the microstructure and relaxation frequency of Mn–Zn ferrites synthesized by solid state reaction. Rahaman et al. [193] studied synthesis, structural, and electromagnetic properties of Mg doped ferrites. Fig. 9 shows the flow chart of various synthesis techniques.

Fig. 9

Different methods of synthesis for preparation of MnZn ferrite.

Oxidation method

Oxidation method is a chemical method to prepare ferrite nanoparticles. The ferrite particles synthesized are irregular, have elongated morphology. The advantage of this method is that the particles have narrow size distribution and uniform size but by using this method ferrite colloids of small size are formed. Joseyphus et al. [41] prepared MnZn ferrite by using oxidation method. The synthesis procedure of nanoparticles by oxidation method is shown in Fig. 10 . Proper amounts of FeSO4.7H2O, MnCl2.4H2O, ZnSO4.H2O and Fe2(SO4)3 were used as starting materials to synthesize Mn0.67Zn0.33Fe2O4. The weighed amounts of FeSO4.7H2O, MnCl2.4H2O, ZnSO4.H2O and Fe2(SO4)3 were dissolved in 250 mL water and then the mixture was allowed to react with NaOH dissolved in 250 mL of water. Constant stirring was done for 2h to oxidize the metal hydroxide precipitates by adding KNO3. The pH was maintained between 12 and 13. Washing of the precipitates was done many times and then these were allowed to dry in an oven at 333K for 2 days.

Fig. 10

Synthesis techniques of oxidation method.

Nitrilotriacetate precursor method

By using this method we can synthesize MnZn ferrite at a very lower temperature. Tangsali et al. [194] synthesized MnxZn1-xFe2O4 (x = 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7) by using this method. All the metal salts were mixed in proper amounts in aqueous solution of dihydrazinium nitrilotriacetate. Dry precursors of nitrilotriacetate hydrazinate of metal ions were obtained from the solution and ignited. Then the autocombustion of the dry precursors resulted in the formation of metal oxides. Tangsali et al. [195] also studied the effect of sintering conditions on the resistivity of MnZn ferrite nanoparticles prepared by using this method. The resultant products in this technique showed high saturation magnetization and high values of Curie temperature that was between 750K and 380K.

Stearic acid gel method

Stearic acid or octadecanoic acid (CH3(CH2)16COOH) is a common fatty acid that exists as glycerol ester in animal and plant fat. Many researchers used stearic acid gel to synthesize nanomaterials. Jafarnejad et al. [196] used this method to synthesize MgCr2O4 and Enhessari et al. [197] synthesized CoTiO3 by using stearic acid gel method. Ma et al. [198] synthesized MnZn ferrite with chemical formula MnxZn1-xFe2O4 by using stearic acid gel method. Proper amounts of MnCO3, Zn(NO3)2.6H2O and Fe(NO3)2.9H2O were powdered and mixed with Stearic acid in molten form. Stirring was done for 3–4h after heating the mixture in oil bath at 120. This resulted in formation of a brown gel. The gel was cooled in air and then powdered by grinding it. This was followed by washing of the grinded mixture with water three times followed by drying of this mixture at 100. MnZn ferrites were obtained by heating at 450 for 1 h.

Characterization

The characterizations of MnZn ferrites are done with various instruments such that X-ray diffractometer, scanning electron microscopy [[199], [200], [201]], transmission electron microscopy and atomic force microscope (AFM). The magnetic properties of the ferrites are studied by vibrating sample magnetometer (VSM), magnetization hysteresis (M − H) loops [202] and electron spin resonance (ESR) hysteresis loop measurements. The X-ray investigation is done using X-ray diffractometer with CuKα radiation (λ = 1.5405 Å). Various formulas for the determination of lattice constant, X-ray density and crystallite size are listed below.

Measurement of lattice constant (a): From the analysis of XRD data, the lattice constant can be calculated using the formula:where a is the lattice constant, d is the interplanar spacing, and h, k, and l are the miller indices.

Measurement of X-ray density (d ): Theoretical density can be calculated using the relation:where M is the molecular mass of each component, N is the Avogadro's number (6.023 × 1023 particles/mol).

Measurement of Experimental density (d ): Experimental density can be measured using the formula:

Measurement of crystallite size (D): Crystallite size is calculated by using the Scherrer's formula:where D is the crystallite size, λ = 1.54056 Å is the wavelength of X-ray, is Bragg's angle and β is the FWHM value.

Size and shape

Many techniques are used to determine the shape, size and morphology of magnetic nanoparticles such as XRD, SEM, TEM, HRTEM (High resolution transmission electron microscopy) and FESEM (Field-emission scanning electron microscopy). By using HRTEM we can get information about shape, size, crystallinity and lattice spacing. XRD is used to determine the size by using Scherrer equation. However, SEM is better than XRD as XRD cannot determine the size of very small magnetic nanoparticles.

XRD analysis

Ramiza et al. [102] studied the effect of UV radiations to control particle size of Mn–Zn spinel ferrite nano-particles. From the XRD analysis it was observed that with the UV radiations minimum particle size was obtained i.e. 6.198 nm. The size of the sample that was not UV treated was found to reduce from 90 nm to 50nm and the crystallite size of the pure MnZn ferrites was found to be in the range 25 nm–35 nm in almost all cases as described in Table 3 . The lattice parameter was found to be in the range 8.30–8.570. Praveena et al. [179] observed that the lattice constant lie between 8.302 Å and 8.311 Å according to composition. The bulk density and the X-ray density increased from 4.98 gm/cm3 to 4.90 gm/cm3 and 5.12 gm/cm3 to 4.98 gm/cm3. Thakur et al. [203] studied effect of sintering temperature and observed that the average crystallite size was found to increase with an increase in sintering temperature i.e. from 11.38 nm to 67.42 nm. Also, the lattice constant was found to increase from 8.409 Å to 8.483 Å with increasing sintering temperature. At 1373K, a well crystallized single MnZn ferrite phase was formed. Mirshekari et al. [204] observed from the XRD that the average crystallite size was in the range 43.25 nm–66.7 nm. Small amount of lattice strains were also observed improving its magnetic properties. Anwar et al. [9]studied the effect of sintering temperature and observed that the pure MnZn ferrite had pure spinel structure and at 673K sample had cubic spinel structure. At 923 K, the XRD pattern contained additional reflections which were due to Fe3O4. The crystallite size increased from 7 to 13 nm. The lattice constant decreased from 8.439 Å to 8.431 Å with an increase in sintering temperature from 673K to 923K. The XRD density was found in between 5.21g/cm3and 5.23 g/cm3 as described in Table 3. Gabal et al. [186] observed from the XRD analysis that MnZn ferrites had a single phase cubic spinel structure with characteristic (311) reflection on 2Ө = 34.58 and no diffraction peak due to impurity was observed. The broad diffraction peaks were observed showing ultrafine nature and small crystallite size. The lattice parameter showed a decreasing value from 8.4466 Å to 8.4164 Å with increasing Zn content and the density increased from 5.13 g/cm3 to 5.32 g/cm3. Phong et al. [112] observed that the XRD pattern showed single phase spinel cubic structure with Fd3m space group. The lattice parameter was calculated 8.432 Å and x-ray density was 5.27 g/cm3. The average crystallite size was 14 nm. Further from the XRD patterns [30], it was observed that the cubic spinel phase was formed and slight contraction was observed in lattice parameter from 8.4749 Å to 8.4353 Å as Zn concentration increased because Zn2+ ions (0.082 nm ionic radii) replaced Mn2+ ions (ionic radii = 0.091 nm). The value of sintered density increased from 4.93 g/cm3 to 4.96 g/cm3 with increase in Zn content. Jalaiah et al. [187] observed that the lattice parameter found to vary from 8.4555 Å to 8.5758 Å. The average crystallite size was calculated by the Scherrer's formula and was found to be in the range 10 nm. In the XRD pattern as studied by Angadi et al. [189] observed the Braggs reflections that indicate the crystalline nature of the samples with cubic spinel structure corresponding to Fd3m space group. When Sc3+ concentration was increased, the peak shifted towards the lower 2 angle because of the relative difference between the ionic radii of Sc3+ (0.745 Å) with that of Fe3+ (0.55 Å). A decrease in the lattice parameterwas observed from 8.434 Å to 8.431 Å on Sc3+ doping which could be due to presence of Sc3+ ions at the grain boundaries. In the XRD patterns, peaks showed the cubic spinel structure [190]. The lattice parameter of pure MnZn ferrite increased with increase in sintering temperature from 8.3383 Å to 8.3496 Å and decreased in Mg doped MnZn ferrite from 8.3542 Å to 8.3225 Å with increasing sintering temperature. Bulk density decreased with an increase in sintering temperature from 4.87 g/cm3 to 4.45 g/cm3 in pure MnZn ferrite and from 4.61 g/cm3 to 4.57g/cm3in Mg doped MnZn ferrite due to discontinuous grain growth. Islam et al. [128] studied structural, magnetic and electrical properties of Gd-substituted Mn–Zn mixed ferrites. From the XRD patterns it was concluded that for the sample without Gd doping, the ferrite was perfectly single phase spinel and as there was an increase in the Gd concentration, some un-indexed peak as secondary phase appeared. With the increase in Gd content, the lattice parameter also increased from 8.4645 Å to 8.4750 Å. In the XRD patterns of Al doped MnZn ferrite observed by Haralkar [184], the formation of cubic spinel ferrite structure was observed. It was observed that the lattice parameter decreased from 8.445 Å to 8.385 Å with increasing value of x due to the replacement of Fe3+(0.67 Å) ions by Al3+(0.51 Å). The value of X-ray density also decreased from 5.202 g/cm3 to 4.989 g/cm3 with increase in Al content. The crystallite size decreased from 19 nm to 11 nm with increase in Al content. From the study of Yadav et al. [182], the XRD pattern showed spinel structure without any impurity. Also, the graphs had very broad peaks indicating the ultrafine nature and small crystallite size of ferrites. The lattice parameter increased from 8.4052 Å to 8.4219 Å with increase in Sm3+ concentration. The crystallite size decreased from 12.9 nm to 8.7 nm. X-ray density also increased from 5.172 g/cm3 to 6.295 g/cm3. These all variations were because of the substitution of heavier atomic weight samarium with lower atomic weight iron. The XRD peaks of Indium doped Mn-Zn ferrite as studied by Kumar and group [183] showed the pure spinel phase of space group Fd3m. With the increase in Indium concentration, the XRD peaks shifted towards higher angle. This showed some lattice distortion in the cubic structure. The crystallite size increased from 14.6 nm to 15.9 nm with increasing doping concentration. The lattice constant increased from 8.391 Å to 8.418 Å. Also, the x-ray density increased from 5.315 g/cm3 to 5.445 g/cm3. XRD graphs of MnZn ferrite for various compositions and with Indium doping are shown in Fig. 11 . The characteristics peaks match with the ferrite particles and show the phase group Fd3m and spinel structure having single phase. Hence, it is concluded that the MnZn ferrites have single phase spinel cubic structure with Fd3m phase group; however some distortion in the structure can be observed because of doping.

Table 3

Values of Lattice parameter a (Å), Crystallite size D (nm), x-ray density dx-ray (g/cm−3), Saturation magnetization Ms (emu/g), Coercivity Hc (Oe), absorption bands, Remanent magnetization Mr(emu/g) of MnZn ferrite nanoparticles by different synthesis techniques.

Cation distribution	Method of synthesis	Condition of synthesis		a (Å)	D (nm)	dx-ray (g/cm−3)	Ms (emu/g)	Hc (Oe)	Absorption band		Mr(emu/g)	References
		Sintering temperature	Composition						ⱱ1 (cm−1)	ⱱ2 (cm−1)
Mn0.5Zn0.5Fe2O4	Co-precipitation method	973K	–	8.409	11.38	–	–	–	466.02	564.08	–	[203]
		1173K		8.444	39.02	–	–	–	424.94	519.78	–
		1373K		8.483	67.42	–	–	–	485.59	586.88	–
MnxZn(1-x)Fe2O4	Novel combustion method	753K	x = 0.0–1.0	8.457–8.515	27–37	4.96–5.29	11–62	–	–	–	–	[259]
MnxZn(1-x)Fe2O4	Combustion method	753K	x = 0.0	8.457	36.982	–	11.090	45.383	–	–	0.769	[77]
			x = 0.2	8.466	35.891	–	29.402	57.303		–	2.887
			x = 0.4	8.478	34.630	–	41.017	42.824	–	–	3.415
			x = 0.6	8.489	32.977	–	57.245	54.556	–	–	5.087
			x = 0.8	8.515	31.288	–	61.574	74.826	–	–	6.335
			x = 1.0	8.514	27.374	–	60.868	97.260	–	–	8.451
Mn(1-x)ZnxFe2O4	Microwave- hydrothermal process	1173K	x = 0.0	8.302	–	5.12	53.33	153	–	–	21.70	[179]
			x = 0.2	8.315	–	5.35	59.91	169	–	–	24.55
			x = 0.4	8.319	–	5.41	67.2	156	–	–	27.78
			x = 0.6	8.322	–	5.48	78.26	167	–	–	29.58
			x = 0.8	8.328	–	5.49	17.78	172	–	–	7.28
			x = 1.0	8.331	–	4.98	10.74	149	–	–	4.36
Mn(1-x)ZnxFe2O4	Glycine-nitrate auto-combustion method	–	x = 0.2	–	66.7	–	69	60	–	–	21.25	[204]
			x = 0.4	–	43.25	–	49.6	47	–	–	14
			x = 0.6	–	44.5	–	48	40	–	–	12
			x = 0.8	–	66.3	–	34	45	–	–	8
Mn0.5Zn0.5Fe2O4	Chemical co-precipitation method	353K	–	8.439	7	5.21	–	–	–	–	–	[9]
		673K		8.439	11	5.21	–	–	–	–	–
		773K		8.438	12	5.21	–	–	–	–	–
		923K		8.431	13	5.23	–	–	–	–	–
Mn(1-x)ZnxFe2O4	Sol gel method using gelatin	–	x = 0.2	8.4466	23	5.13	32.9	94.2	510	345	–	[186]
			x = 0.4	8.4625	37	5.15	37.6	85.6	520	350	–
			x = 0.6	8.4321	40	5.25	25.9	67.1	526	354	–
			x = 0.8	8.4164	44	5.32	43.7	80.7	540	355	–
Mn0.6Zn0.4Fe2O4	Sol gel method	As prepared	–	–	11 ± 1	–	18.0	18.2	–	–	–	[192]
		623K		–	10 ± 2	–	22.4	45.6	–	–	–
		873K		–	20 ± 2	–	3.9	30.3	–	–	–
		1273K		–	78 ± 5	–	9.3	20.8	–	–	–
		1473K		–	40 ± 5	–	35.0	54.5		–	–
Mn0.3Zn0.7Fe2O4	Hydrothermal process	453K	–	8.432	14	5.27	27.7	–	–	–	–	[112]
Mn(1-x)ZnxFe2O4	Solid state reaction	1573K	x = 0.59	8.4749	–	4.96	36.22	44.63	–	–	–	[30]
			x = 0.61	8.4401	–	4.95	33.15	45.56	–	–	–
			x = 0.65	8.4353	–	40.93	30.78	42.48	–	–	–
Mn0.85Zn0.15NixFe2O4	Sol-gel auto combustion method	1473K	x = 0.03	8.4555	12.611	–	120.89	0.123	–	–	–	[187]
			x = 0.06	8.4801	17.725	–	118.62	0.125	–	–	–
			x = 0.09	8.5758	10.924	–	114.89	0.201	–	–	–
			x = 0.12	8.5610	10.945	–	122.93	0.216	–	–	–
			x = 0.15	8.4077	12.585	–	137.25	0.246	–	–	–
Mn0.5Zn0.5ScyFe2-yO4	Solution combustion method	–	y = 0.00	8.434	20	–	24.6	68	538	358	3.01	[189]
			y = 0.01	8.422	22	–	22.84	58.92	540	361	5.37
			y = 0.03	8.454	19.3	–	31.48	74.89	544	362	7.74
			y = 0.05	8.431	20	–	23.45	84.32	540	361	6.28
Mn0.5Zn0.4Mg0.1Fe2O4	Solid state reaction	111423K	–	8.354	37	5.28	–	–	–	–	–	[190]
		111473K		8.336	45	5.31	–	–	–	–	–
		111523K		8.322	38	5.34	–	–	–	–	–
Mn0.55Zn0.45GdxFe2-xO4	Double sintering ceramic technique	2023K	x = 0.00	8.465	–	5.15	51.2	–	–	–	–	[128]
			x = 0.01	8.468	–	5.16	46.4	–	–	–	–
			x = 0.03	8.472	–	5.20	45.6	–	–	–	–
			x = 0.05	8.472	–	5.22	44.7	–	–	–	–
			x = 0.08	8.475	–	5.31	40.3	–	–	–	–
Mn0.5Zn0.5AlxFe2-xO4	Sol-gel auto combustion method	873K	x = 0.0	8.445	–	4.412	39	89	445	548	1.3	[184]
			x = 0.1	8.438	–	4.321	35	61	446	550	1.18
			x = 0.2	8.421	–	4.198	29	59	452	550	0.9
			x = 0.3	8.403	–	4.057	22.5	48	451	545	0.75
			x = 0.4	8.391	–	3.996	17.5	35	452	538	0.54
			x = 0.5	8.385	–	3.912	10	19	452	529	0.3
Mn0.5Zn0.5SmxFe2-xO4	Chemical co-precipitation method	–	x = 0.0	8.4052	12.9	5.172	23.95	0	–	–	0	[182]
			x = 0.1	8.4118	10.2	5.275	31.72	0	–	–	0
			x = 0.3	8.4216	9.9	5.875	37.75	180	–	–	3.55
			x = 0.5	8.4219	8.7	6.295	42.10	250	–	–	8.50
Mn0.4Zn0.6InyFe2-yO4	Oxalate-co-precipitation technique	–	y = 0.000	8.391	14.6	5.315	–	–	–	–	–	[183]
			y = 0.035	8.393	15.5	5.368	–	–	–	–	–
			y = 0.070	8.397	15.7	5.418	–	–	–	–	–
			y = 0.100	8.418	15.9	5.445	–	–	–	–	–

Fig. 11

(a) XRD powders pattern of synthesized Mn–Zn ferrite powders with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The XRD pattern shows characteristic peaks of spinel structure and quality of pure phase (Reproduced by permission from Refs. [259], Licence No. 4646020803426, Copyright 2011, Elsevier), (b) X-ray diffraction pattern of MnZn ferrite with Indium substitution (Reproduced by permission from Refs. [183], Licence No. 4763520958822, Copyright 2016, Elsevier).

Morphological structure

Various techniques such as AFM, TEM, and SEM etc. are used to investigate the morphological structure of the ferrite nanoparticles. SEM is widely used for it but TEM is better than SEM because of poor resolution of SEM. AFM is a technique that can be used in different conditions like air, vacuum, liquid and moist conditions. Winiarska et al. [205] observed that the TEM gave core shell type structure formation. Mirsekari et al. [204] found from the SEM micrographs that the morphology of MnZn ferrite was porous, sponge like and agglomerated with an average particle size of 2 μm. Anwar et al. [9] observed from the SEM micrographs that the particles were spherical in shape. Gabal et al. [206] studies showed that TEM morphology showed very strong agglomeration of the cubic particles, having some particles in one line. From the SEM micrographs [192], mean grain diameter was observed 7.880.4 μm. In Phong's observation [112] of the TEM images showed that the ferrites had homogeneous structure and spherical in shape. Particles showed agglomeration due to slow particle growth. In the SEM analysis of the Ni doped ferrite done by Jalaiah [187], the presence of aggregates of small grains at the surface of the higher nickel containing samples was observed. In the TEM analysis done by Angadi et al. [189] the particles were lightly agglomerated due to the slow growth of particles during the preparation. The particle size of pure Mn0.5Zn0.5Fe2O4 was about 20 nm. The TEM images showed that the electron diffraction pattern consisted of concentric rings with spots over the rings showing that the samples were crystalline in nature. The particle size lies between 20 and 23 nm. From the SEM images [190], it was concluded that polyhedral morphology with nonuniform grains were displayed for both pure and doped MnZn ferrites. Pure MnZn ferrites sintered at 1150 had average grain size of 2.10 μm having well defined grain boundaries and the sample sintered at 1200 and 1250 had grain size of 2.84 m and 3.13 μm. In case of Mg doped ferrites, grain size increased with sintering temperature from 2.00 to 3.10 μm. From the TEM images [84], it was concluded that the molecules were spherical in shape and particles were aggregated. From the SEM analysis by Yadav et al. [182], it was observed that particle size increased with Sm content but bigger particles were formed by the agglomeration of ultra fine particles. The TEM images showed that all the particles were nearly spherical in shape and average particle size was 10–20 nm. SEM analysis [183] showed the uniform, spherical shaped and loosely agglomerated particles. The shape of the MnZn ferrites is usually spherical and having particle size in the 9–23 nm range. The SEM images of the pure MnZn ferrites are shown in Fig. 12 that shows spherical structure of the ferrite nanoparticles and in Fig. 13 SEM images shows elongated nature of the ferrite nanoparticles. Also, the TEM images of pure MnZn ferrite nanoparticles are shown in Fig. 14 .

Fig. 12

(a) SEM images of Ti/Sn substituted MnZn ferrite, (Reproduced by permission from Ref. [125], Licence No. 4763511309362, Copyright 2015, Elsevier), (b) microstructure of MnZn ferrites with Gd concentration 0.0, 0.01, 0.03, 0.05 and 0.08 (Reproduced by permission from Refs. [128], Licence No. 4763520066588, Copyright 2013, Elsevier).

Fig. 13

(a)SEM micrographs show the as-synthesized MnxZn1-xFe2O4 powders: (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8, (f) x = 1.0 (Reproduced by permission from Ref. [259], Licence No. 4646020803426, Copyright 2011, Elsevier), (b) SEM images of MnZn ferrites doped with Sm and Gd (Reproduced by permission from Refs. [240], Licence No. 4763520638098, Copyright 2016, Elsevier).

Fig. 14

TEM image of Mn1-xZnxFe2O4 system prepared using gelatin method (Reproduced by permission from Refs. [186], Licence No: 4646030657048, Copyright 2012, Elsevier), (b)TEM micrographs of Mn–Zn ferrites annealed at 300 °C and 500 °C (Reproduced by permission from Ref. [165], Licence No: 4646030272206, Copyright 2011, Elsevier), (c) TEM and HRTEM images of Mn–Zn ferrite with Sc doping (Reproduced by permission from Ref. [189], Licence No: 4644680497245, Copyright 2017, Elsevier).

FT-IR analysis

FT-IR stands for Fourier transform Infrared, the method that is used for infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some is passed through or transmitted. The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. Islam et al. [128] recorded the FTIR spectra of MnZn ferrite nanoparticles in the range from 250 cm−1 to 4000 cm−1. In the FTIR spectra [184], the value of the absorption band ⱱ1 around 600 cm−1 remained almost constant whereas the value of absorption spectra ⱱ2 around 400 cm−1 decreased from 548 cm−1 to 528 cm−1. This is because of the difference in Fe3+-O2- distance for tetrahedral and octahedral sites. The absorption bands in the region 1200 cm−1-1500 cm−1 correspond to NO3 − ions, absorption band at 1700 cm−1 showed carboxyl group COO− and at 2300 cm−1 correspond to hydrogen bonded O–H groups. In the FTIR spectra [189], two prominent absorption bands nearly at 540 cm−1(ⱱ1) and 360 cm−1(ⱱ2) observed were attributed to the tetrahedral and the octahedral complexes. The difference between these two values was due to the relative changes in bond length (Fe–O) at tetrahedral (A) sites and octahedral (B) sites.The FTIR spectra recorded by Gabal [206] in the range 600 cm−1-200cm−1 showed high frequency band (ⱱ1) increased with increasing Zn content due to vibrational spectra of metal ion-oxygen complex in the tetrahedral sites, while value of lower frequency band (ⱱ2) due to vibration in the octahedral site, slightly changed. FTIR of all compounds showed the formation of spinel phase. Ciocarlan et al. [103] synthesized Mn ferrite along with Ni ferrite, Zn ferrite and Co ferrite and studied their various properties. Formation of spinel phase is observed from FT-IR spectra of all the compounds as shown in Fig. 17(b).

Fig. 17

(a) FT-IR spectra of MnZn ferrites with Sm and Gd doping (Reproduced by permission from Ref. [240], Licence No. 4763520638098, Copyright 2016, Elsevier), (b) FT-IR spectra of prepared nanoparticles (Reproduced by permission from Ref. [259], Licence No. 4763520231621, Copyright 2011, Elsevier), (c) FTIR spectra of MnZn ferrites with Gd doping (Reproduced by permission from Refs. [128], Licence No: 4763520066588, Copyright 2013, Elsevier).

Power loss

MnZn ferrites are the magnetic materials with very low power loss so that they can be used in many electronic applications. Aiping et al. [123] synthesized MnZn ferrites using conventional ceramic processing technique and studied the effect of SnO2 addition on the magnetic properties of the prepared ferrite. It was observed that there is an overall decrease in the loss factor with increase in SnO2 concentration. Also, power loss and minimum power loss decreased with increase in the doping of SnO2 as shown in Fig. 16 (c). Jalaiah et al. [187] studied structural, magnetic and electrical properties of nickel doped Mn–Zn spinel ferrite. The nickel substituted Mn–Zn ferrite Mn0.85Zn0.15NixFe2O4 (x = 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared using sol gel auto combustion method. The position of the dielectric loss maxima shifted towards the lower frequency with increase in Ni concentration as dipole-dipole interaction becomes stronger at lower frequency causing hindrance to the rotation of the dipoles. The ac conductivity increased with increasing frequency. The room temperature conductivity of Mn0.85Zn0.15NixFe2O4 (x = 0.03,0.06,0.09,0.12 and 0.15) ferrites was higher than pure spinel ferrite. Sun et al. [125] studied cation distribution and magnetic properties of Ti/Sn-substituted MnZn ferrites. Solid state reaction method was used to prepare Manganese–Zinc ferrites with composition Mn0.782-xZn0.128Mx 4+Fe2+ 2-2xO4 (x = 0; M = Ti, x = 0.04; M = Sn, x = 0.04). The core losses measured at 100 kHz and 200 mT showed that the core losses for all samples decreased firstly and then increased with increasing temperature further. The power loss for unsubstituted and Ti4+ sample was higher at room temperature. Also, temperature of minimum in PL ~ T curve shifts to lower temperature for Ti4+ and Sn4+ substituted samples. At low frequencies the there was power losses only due to eddy current loss Pe and hysteresis loss Ph. The Ph decreased with increasing temperature firstly up to 80 °C and then increased with further increase in temperature and it was minimum for Sn4+ doping. The Pe of all samples were relatively low and there was a slight change at low temperature but there was a sharp increase at high temperature as shown in Fig. 16 (a), (b). Wei et al. [129] studied effect of TiO2 and Nb2O5 additives on the magnetic properties of cobalt-modified MnZn ferrites. Traditional ceramic process was used to prepare MnZn samples with composition (Mn0.673Zn0.246Fe2.073Co0.006O4) by using Fe2O3, Mn3O4, ZnO and Co2O3 as the starting materials. The power loss vs. temperature plot showed that the power loss decreased firstly and then increased with an increase in temperature showing lowest loss point between 60 °C and 100 °C. Power loss reduced as the concentration of additives was increased. Also, both the hysteresis loss and the eddy current loss decreased with increase in concentration of additives and after reaching minima for concentration of TiO2 and Nb2O5, 0.03 wt% and 0.02 wt% increased further. This is because as small amount of Ti4+ and Nb5+ ions were entered into the grains, it causes an increase in Fe2+ ions, which lead to positive K1 values and decrease the hysteresis loss. Further increasing the dopant concentration cause excessive increase in Fe2+ ions, which make K1 value more positive and increase the hysteresis loss. When the concentration of TiO2 and Nb2O5 was less than 0.03 wt% and 0.02 wt%, the grain and grain boundary resistivity both increased and hence, the eddy current loss decreased as eddy current loss is inversely proportional to resistivity. Further increase in additives concentration decreased the resistivity, causing the eddy current loss to increase. Anwar et al. [9] studied the effect of sintering temperature on various structural, electrical and dielectric parameters of MnZn ferrites using the co-precipitation method for the synthesis of Mn0.5Zn0.5Fe2O4. The dielectric constant decreased very sharply in low frequency region and slowed down in high frequency region almost approaching to frequency independent nature. It exhibit dielectric dispersion. From the plot of loss tangent vs. frequency it was observed that the loss tangent decreased initially with increase in frequency and then showed a relaxation peak. It is observed from all this data that MnZn nanoferrites have very low power loss to be used suitably in making various electronic appliances.

Fig. 16

(a-b) Temperature dependence of hysteresis loss and eddy current loss of MnZn ferrite (Reproduced by permission from Ref. [125], Licence No: 4763511309362, Copyright 2015, Elsevier), (c) Temperature dependance of power loss with SnO2 addition (Reproduced by permission from Refs. [123], Licence No: 4763501423793, Copyright 2006, Elsevier).

Magnetic properties

Most common techniques for determining the magnetic properties of ferrite nanoparticles are VSM (vibrating sample magnetometer), magnetization hysteresis (M − H) loops and electron spin resonance (ESR) hysteresis loop measurements. We can calculate saturation magnetization, remenant magnetization and coercivity by using these characterization techniques.

Saturation magnetization

Saturation magnetization is the saturation value of magnetization of a ferromagnetic body. The inside of the magnetic body is normally divided into many number of domains, but as the external magnetic field increases, domain walls may move and magnetization may rotate within domains, so the magnetic body comes in single-domain state. The magnetization saturation is reached if the easy magnetization axis and the external magnetic field direction match and the value of the magnetization at this time is called the saturation magnetization. The value of this saturation magnetization of MnZn ferrites is high [141,162]in comparison to other ferrites. Syue et al. [188] observed the value of saturation magnetization increased from 11 to 62emu/g with increasing Mn2+ content and saturates further.The value of saturation magnetization remained in the range 11.090 emu/g to 60.868 emu/g [188] when combustion method is used without subsequent heat treatments. Praveena et al. [179] observed that the value of saturation magnetization (Ms) increased from 53.33 Am2/Kg to 78.26 Am2/Kg with increase in zinc content and then decreased to 10.74Am2/kg if zinc content is further increased. From the hysteresis curves Mirsekari et al. [204] observed that the saturation magnetization decreased from 69 to 34 emu/g. Gabal et al. [186]observed the value of saturation magnetization Ms increased from 32.9emu/g to 37.6emu/g, then decreased to 25.9emu/g and again increased to 43.7emu/g.The value of saturation magnetization was found to be 27.7emu/g in Phong's [112] experiment showed the properties of super spin glass and supermagnetism behavior. The study by Zapata [30] showed that the Ms value decreased from 36.22emu/g to 30.78emu/g with increase in Zn concentration. This was due to the fact that increased Zn content decreased the ferric ions on the A sites and this reduced the A–B interaction. The saturation magnetization (Ms) [187] decreased firstly from 120.896emu/gm to 114.888emu/gm with increase in Ni concentration and then increased to 137.246emu/gm with further increase in Ni concentration as shown in Fig. 13. This decrease was due to the occupation of Ni2+ ions in octahedral B sites. Angadi et al. [171] observed from the M − H loops recorded by VSM that the variation of saturation magnetization(Ms) increased with increasing Sc3+ doping from 24.6emu/g to 31.48emu/g and then decreased further to 23.45emu/g. Hence, the Sc3+ doped Mn–Zn ferrites are useful for modern technological applications as well as low and high frequency applications. Islam et al. [128] observed that the value of saturation magnetization decreased from 51.2emu/gm to 40.3emu/gm with an increase in Gd content. By doping Al in pure MnZn ferrite [184] the saturation magnetization decreased with increase in Al content. Hysteresis loops measurements by Yadav et al. [182] showed that the value of saturation magnetization increased from 23.95emu/gm to 42.10emu/gm. Due to this, high value of magnetization MnZn ferrites are used in the field of power applications.

Remenant magnetization

Remanent magnetization is the value of magnetization that remains in the absence of an induced magnetic field. Mirshekari et al. [204] studied structural and magnetic properties of Mn–Zn ferrite. From the hysteresis curves, it was observed that remenant magnetization decreased from 21.25emu/g to 8 emu/g. Syue et al. [188] studied magnetic properties of MnZn ferrites and found that the value of remenant magnetization remained in the range 0.769 emu/g to 8.451emu/g and it was observed that it was lowest for pure zinc ferrite and highest for pure manganese ferrite. Praveena [179] studied magnetic properties of MnZn ferrite and found that remenant magnetization showed increase in value from 21.70Am2/kg to 29.58Am2/kg and then decreased to 4.36Am2/kg with increasing x value as described in Table 3. Gabal et al. [186] showed that the remenant magnetization also increased from 5.5emu/g to 6.7emu/g firstly and then decreased to 3.6emu/g and again increased to 7.3emu/g. After doping Al [84,184] the remanence magnetization Mr varied from 0.5emu/g to 1.32emu/g with Al content. Yadav et al. [182]observed that remanence magnetization increased from 0emu/g to 8.50emu/gm with increasing value of Sm content from 0.0 to 0.5. Fig. 15 shows the results of VSM characterization of MnZn ferrite having high value of saturation magnetization and low coercivity.

Fig. 15

(a) Magnetic hysteresis loops for Mn0.85Zn0.15NixFe2O4 (x = 0.03, 0.06, 0.09, 0.12 and 0.15) (Reproduced by permission from Ref. [187], Licence No: 4656371369727, Copyright 2017, Elsevier), (b) Hysteresis loop of MnZn ferrite with Samarium doping where x is Sm concentration having (a) x = 0.0, (b) x = 0.1, (c) x = 0.3 and (d) x = 0.5 (Reproduced by permission from Refs. [182], Licence No. 4763520720419, Copyright 2015, Elsevier).

Coercivity

The coercivity is also called as coercive field and coercive force. It is defined as the ability of a ferromagnetic material to withstand an external magnetic field without demagnetizing it. In case of a ferromagnetic material, it is defined as the intensity of applied magnetic field that is required to reduce the magnetization to zero after the saturation state of the magnetization. The materials which have high coercivity are called hard materials and the materials with low value of coercivity are soft materials. The hard materials are used to make the permanent magnets and soft materials are used for making transformers, inductor cores and microwave devices. Praveena et al. [179] found the coercivity value varying in the range 0.0149Oe-0.0172Oe for Mn1-xZnxFe2O4 (x = 0.0–1.0). From the hysteresis curves Mirsekari et al. [204] observed that the coercivity decreased from 60Oe to 45Oe with increasing x from 0.2 to 0.8. The coercivity as calculated by Gabal [186] showed decreasing trend. It decreased from 94.2Oe to 67.1Oe for x = 0.2–0.6 and then increased to 80.7Oe for x = 0.8. Phong et al. [112] studied the properties of MnZn ferrites and found that the coercivity was 130 Oe at 10K. Jalaiah et al. [187] observed from his experiment that the coercivity values of the Mn0.85Zn0.15NixFe2O4 (x = 0.03, 0.06, 0.09, 0.12 and 0.15) samples increased with nickel concentration from 0.123Oe to 0.24Oe because of the decrease in the porosity with increasing dopant concentration. Also, the coercivity decreased from 87Oe to 11Oe with increasing Al3+ concentration [184]. The low value of coercivity of MnZn ferrite put these ferrites in the class of soft ferrites and these are used in applications like making transformer cores, microwave devices and inductors.

Due to useful magnetic, electrical and optical properties of ferrite nanoparticles, researchers are taking interest in the synthesis of ferrite nanoparticles and making their use in a lot of applications that include medical field, information technology, antenna, microwave absorbing materials, biosensors and many electronic applications [[207], [208], [209], [210], [211], [212], [213], [214], [215], [216]]. Many reviews are there about the synthesis, properties and applications of ferrites in biomedical [[217], [218], [219]], catalyst [220,221] and wastewater treatment [200,[222], [223], [224], [225]]. MnZn ferrites have a broad area of applications due to high saturation magnetization [226], high initial permeability [50,227], low power loss [228]. The application area of MnZn ferrites include power applications [[229], [230], [231], [232], [233], [234], [235]], microwave devices [236], magnetic fluid [145,237], radar absorbing system, high frequency applications [238,239], bio-medical [240], water purification [241,242]etc. Use of MnZn ferrites in the field of power application attracted great attention in the research areas. From last many years the MnZn ferrites are synthesized to be used in power applications for making current convertors [227], power inductors with magnetic cores [130], electronic transformer cores [243], high frequency applications [143], electronics and communication [244].

Microwave devices

Ferrite nanoparticles have low electrical conductivity and low dielectric losses [245], so they can be used in microwave devices. MnZn ferrites are most suitable for their use in the microwave devices because of high permittivity, high resistivity, high stability, high value of saturation magnetization, high curie temperature with low eddy current and low magnetic losses [246,247]. Due to the use of ferrite nanomaterials, electronic devices can be mechanically hard, chemically stable and permit the materials to operate properly at a wide frequency range [248]. There are a lot of advantages of the use of MnZn ferrites in the microwave devices. There is a decrease in the emission of unwanted EM waves from the device and also it absorbs the incoming EM waves that may harm the microwave device. MnZn ferrites are used in microwave systems because of their low loss and high saturation magnetization. Wang et al. [249] synthesized MnZn ferrite nanoparticles and the result showed that because of high reflection loss and broad absorbing band in low frequency (10 MHz to 1 GHz) these ferrites can be used in electromagnetic microwave absorbing field.

Radar absorbing devices

The radiations emitting from radar results in the increase in electromagnetic radiation pollution in the environment. These radiations reduce the efficiency and performance of electronic instruments and thus decrease their lifetime and safety. As MnZn ferrite belongs to the class of soft ferrites having high electrochemical stability, high permeability, high saturation magnetization and low power losses, it is used in many electronic applications [65,79,128,167,199,209,210]. Ferrite nanoparticles can be used in the radar absorbing devices due to their high value of Curie temperature and temperature stability [[250], [251]]. Also the ferrite nanoparticles are environmentally safe that make their use easier in the radar absorbing devices. The application of MnZn ferrites in radar absorbing system is also attracting the researchers. Praveena et al. [252] synthesized Ni0.4Zn0.2Mn0.4Fe2O4nano ferrites for radar absorbing. The high value of Curie temperature indicated homogeneity and temperature stability. The EPR spectra showed reduction in the peak width and increase in relaxation with increase in sintering temperature. These all results showed that the ferrite nanoparticles can be used for radar absorbing from few MHz to 2 GHz and also these materials are environmentally safe.

Image based diagnostics

A one-pot thermal decomposition method was used to synthesize a series of Zn2+ doped nanoparticles of (ZnxMn1-x)Fe2O4 and (ZnxFe1-x)Fe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.8). By carefully controlling Zn2+ doping level, nanoparticles of size 15 nm with single crystallinity and size monodispersity (s<5%) and having high magnetization value (175 emu/g) were obtained. The nanoparticles provided the large MRI contrast effects (r2 = 860 mm−1s−1) with an eight to fourteen fold increase in MRI contrast and a fourfold enhancement in hyperthermic effects compared to conventional iron oxide nanoparticles. This enhancement was significant for clinical purposes as the nanoparticle probe dosage level can be progressively lowered when using probes that have improved contrast enhancement effects. For (ZnxMn1-x)Fe2O4 nanoparticles, Zn2+ ions mainly occupy tetrahedral sites of the spinel matrix which was confirmed by using extended X-ray absorption fine structure (EXAFS) analysis to examine the Zn and Fe K-edges. To detect small sized pathogenic targets precisely at an early stage, MRI contrast agents are often used to highlight those specific areas of interest. Due to high imaging contrast effects, magnetic nanoparticles can increase the difference between pathogenic targets and normal tissues via MRI. One of the most appropriate ways to increase the MR contrast effects is the optimization of saturation magnetization (Ms) that is directly related to the relaxivity coefficient (r2). The relaxivity coefficient (r2) is determined by a slope of R2 against nanoparticle concentration and often used as an indicator for contrast effects. The relaxivity coefficient (r2) of contrast agents can be tuned and further enhanced by engineering magnetic parameters [253].

Electronic devices

MnZn ferrite nanoparticles are used in making many electronic devices due to their enhanced electrical properties such as high value of resistivity, low ac conductivity, low power losses etc. Dobak et al. [105] studied miniaturization of components due to low loss MnZn ferrites. Also, Sun et al. [138] studied effect of ZrO2 addition on the microstructure and various properties of MnZn ferrites and found that the optimal values of initial permeability (2322), saturation magnetization (522 mT) and power loss (386 kW/m3) make it suitable for switch mode power supply applications. Due to suitable electrical and magnetic properties of the Sc3+ doped Mn–Zn ferrites, these were useful for modern technological application as well as for low and high frequency application. MnZn ferrites are also used to construct power inductors [254,255], wireless power transfer applications [256] and for making inductive components [39].

Telecommunication and others

One of the major use of MnZn ferrites is in telecommunication and high frequency applications [180]. MnZn ferrites have applications in the field of bio–medical and hyperthermia [112]. Hurtado et al. [257]synthesized MnZn ferrite along with activated carbon composite for use in bio–medical applications. MnZn ferrites can be used to make ferrofluid [182]due to high value of saturation magnetization. Arulmurugan et al. [76] synthesized Co–Zn and Mn–Zn ferrite nanoparticles and found that because of low Curie temperature and high value of thermomagnetic coefficient, these ferrites can be used for preparing temperature sensitive ferrofluid. Praveena et al. [258] synthesized Mn–Zn ferrite nanoparticles for high frequency applications. The ferrites had low power loss in frequency range 10Hz-1MHz. The constructed transformer with the ferrite material showed high efficiency and low surface temperature rise at frequency 1 MHz making it suitable for operating at high frequencies.

MnZn ferrites for onging COVID-19 pandemics

As nanomaterials are making a global impact on healthcare and socioeconomic development so are the viruses during pandemics. Nanoparticles of MnZn have unique physical and chemical properties that have associated benefits in development of potential therapeutic drugs, nanomaterial based environment friendly antiviral sprays, drug delivery and to develop anti-viral surface coatings in home appliances. This is attributed to the fact that the choice of synthesis method provides size and charge tunability properties to the MnZn ferrites. The size tunablity ensures that large amount of drug can be delivered into anatomically privileged sites of the virus while charge tunability would facilitate entry of drug in to charged parts of the virus [260]. In addition, biosensors for the early detection of viral strains such the COVID 19 can also be developed with MnZn ferrites. For instance MnZn ferrites can readily be used to develop Giant magnetoresistance based sensors which have previously been used for virus detection [261].

Outlook

The synthesis of MnZn particles has increased in the last ten years and most progress can be seen in the year 2016. Due to the fascinating properties of MnZn ferrites among the class of soft ferrites like high value of saturation magnetization, low value of coercivity, high initial permeability, narrow size distribution of the ferrite particles, low remenant magnetization, the researchers are taking interest in the synthesis of these ferrites.The co-precipitation and sol-gel method are the best for getting the fine crystallite size among all synthesis techniques. The XRD pattern of the MnZn ferrites has characteristic peaks showing the cubic spinel phase having Fd3m phase group. The shape of the prepared ferrite is nearly spherical but some distortion may be observed after doping. FTIR spectra confirmed the spinel phase of the ferrite nanoparticles having tetrahedral and octahedral sites.The value of saturation magnetization is highest when we synthesize the MnZn ferrites with proper amount of nickel doping by using sol–gel auto combustion method. Also, for getting the low value of coercivity sol–gel method is preferred. Generally, MnZn ferrites have a lot of applications including biomedical field, electronic devices, for making radar absorbing materials, for making ferrofluids etc. For enhancing the applications and advantageous properties of MnZn ferrite nanoparticles, further studies are required.The electrical and magnetic properties of MnZn ferrites can be enhanced by doping other metals such as cobalt, zinc, magnesium to make them suitable for use in agricultural and electrical applications. In the context of use of nanoparticles in the pandemic outbreak, such as in the recent COVID-19, MnZn soft ferrites can play a significant role in the development of high contrast imaging dyes for viral strains in body fluids. Perhaps MnZn can also serve as a candidate nanomaterial for developing nanomaterial based medicines and therapeutics.