J Supercond Nov Magn (2012) 25:91–100DOI 10.1007/s10948-011-1212-7

O R I G I NA L PA P E R

Synthesis, Structural, and Magnetic Characterizationof Mn1−xNixFe2O4 Spinel Nanoferrites

Ahmed Faraz · Mudasara Saqib · Nasir M. Ahmad ·Fazal-ur-Rehman · Asghari Maqsood ·Muhammad Usman · Arif Mumtaz ·Muhammed A. Hassan

Received: 2 March 2011 / Accepted: 22 June 2011 / Published online: 27 July 2011© Springer Science+Business Media, LLC 2011

Abstract Nanoferrites of composition Mn1−xNixFe2O4

with x = 0.00, 0.25, 0.50, 0.75, 1.00 were prepared by thechemical coprecipitation method. The prepared nanoferriteswere characterized by infrared spectroscopy (IR), X-raydiffraction (XRD), scanning electron microscopy (SEM),and atomic force microscopy (AFM) and vibrating samplemagnetometer (VSM) to study the compositional, struc-tural, morphological and magnetic changes taking placewith varying Ni concentration in the composition of theprepared nanoferrites. IR reveals the presence of both high-frequency and low-frequency bands due to tetrahedral andoctahedral sites, respectively. The XRD results indicated theformation of single spinel ferrite with crystalline size in therange of 14–26 nm. The lattice parameters (a) decrease withthe increase of the Ni concentration x in the lattice. Fur-ther information about the morphology of the nanoferriteswas obtained from the AFM and SEM results. The mag-netic hysteresis curves clearly indicate the soft nature ofthe prepared nanoferrites. Various magnetic properties suchas saturation magnetization (Ms) and remanence (Mr ) arecalculated from the hysteresis loops and observed to be de-pendent on the composition.

A. Faraz · N.M. Ahmad (�) · Fazal-ur-Rehman · A. Maqsood ·M.A. HassanThermal Transport Laboratory, School of Chemical and MaterialsEngineering, National University of Sciences and Technology(NUST), NUST H-12 Campus, Islamabad 44000, Pakistane-mail: nasir.ahmad@scme.nust.edu.pk

M. SaqibSheikh Zayed Hospital, Federal Postgraduate Medical Institute,Lahore, Pakistan

M. Usman · A. MumtazDepartment of Physics, Quaid-i-Azam University, Islamabad,Pakistan

Keywords Coercivity · Mn1−xNixFe2O4 spinelnanoferrites · Nanostructure · Saturation magnetization ·Remanence

1 Introduction

The nanosize particles of transition-metal oxides are capa-ble to exhibit unique optical, spontaneous magnetism, andelectrical properties when compared with their bulk coun-terparts. One of the best known ferrites is spinel type ironoxide Fe3O4, because of its natural occurrence and sponta-neous magnetism, high magnetic permeability and low con-duction losses [1]. The ferrites are mixed metal oxide withgeneral formula MFe2O4, where M can be a wide variety ofmetal cations, such as Fe, Co, Mn, and Ni. The interestingand useful physical, spectroscopic, electrical and magneticcharacteristics of the spinel ferrites of the type of MFe2O4

depend on several factors. Such factors include Fe2+, Fe3+ions, choice of the other metal cations and their distributionbetween tetrahedral (A) and octahedral (B) sites of the spinellattice, synthesis procedure, composition, sintering temper-ature and time, and concentration of doping [2–4]. For ex-ample, MnFe2O4 has a spinel structure which is mostly ofnormal-type. Contrary to MnFe2O4, NiFe2O4 has a fully in-verse spinel structure to exhibit ferrimagnetism, which orig-inated from magnetic moments of anti-parallel spins be-tween Fe3+ ions at tetrahedral sites and Ni2+ ions at octahe-dral sites [5]. Considering this, ferrites of Mn1−xNixFe2O4

present interesting compositions to investigate since individ-ual ferrites of MnFe2O4 and NiFe2O4 have mostly normalspinel and inverse spinel structures, respectively. Further-more, there is relatively less research carried out to describethis important class of ferrites. The substitution of Ni diva-lent ions in pure ferrites of Mn can lead to the modifica-

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tion of the compositional and structural as well as electri-cal and magnetic properties [6]. The electrical properties ofthese ferrites are currently underway and will be the sub-ject of future work. In the current work, a detailed com-positional, structural, morphological and magnetic proper-ties of Mn1−xNixFe2O4 of varying compositions of Ni con-centration (x = 0.00, 0.25, 0.50, 0.75, 1.00) have been pre-sented. It is worth to mention that in most of the previouslypublished work, properties of MnFe2O4 and NiFe2O4 fer-rites have only been reported separately [7–10]. Nanofer-rites of Mn1−xNixFe2O4 with systematic variation of Niconcentration x were prepared by the chemical coprecipi-tation method. As one of the oldest techniques for the syn-thesis of nanoparticles, metal salt coprecipitation uses pre-cipitating reagents to react with a prepared solution of singleor multiple precursor metal salts. This method is useful forsize-controllable and chemically stable preparation of nano-ferrites [11]. The characteristics of the prepared nanoferriteswere studied by infrared spectroscopy (IR), X-rays diffrac-tion (XRD), scanning electron microscopy (SEM), atomicforce microscopy (AFM), and vibrating sample magnetome-ter (VSM) to study the compositional, structural, morpho-logical and magnetic properties.

2 Experimental

2.1 Synthesis

The polycrystalline powder of a series of Mn–Ni ferritesMn1−xNixFe2O4 (with x = 0.00, 0.25, 0.50, 0.75, 1.00)were prepared with stoichiometric amounts of the appro-priate metal using chemical coprecipitation method. Allused chemicals were of analytical grades and used as re-ceived. General procedure involved the preparation of aque-ous solutions of Ni(NO3)26H2O, Mn(NO3)2·4H2O andFe(NO3)3·9H2O in alkaline medium of NaOH. NaOH with3 M concentration was used as the coprecipitation agent.Separate solutions of Ni(NO3)2·6H2O and Mn(NO3)2·4H2O (with total concentration of 0.1 M), and Fe(NO3)3·9H2O (0.2 M) were prepared in 100 ml of de-ionized water.An example of preparing ferrites with the composition ofMn0.5Ni0.5Fe2O4 (with x = 0.50) is described below. Forthis composition, the stoichiometric-reagent ratio of 0.05 Mof Mn(NO3)2 and 0.05 M of Ni(NO3)2·6H2O and 0.2 M ofFe(NO3)3·9H2O were used. These reagents were taken sep-arately in 100 ml de-ionized water followed by mixing theprepared solutions in a beaker; heating for about 30 min-utes at 85 °C with constant magnetic stirring until a clearsolution was obtained. Solution of coprecipitating agent ofNaOH (3 M) was made separately in 100 ml of de-ionizedwater and heated up to 85 °C. The solution of coprecipi-tating agent then mixed rapidly with the already prepared

salt solutions. It is important to mention that rapid mixingof coprecipitating agent in metal solution gives relativelysmaller size, monodispersed and chemically homogeneousnanoparticles. For transforming metallic hydroxide into fer-rites, temperature was kept constant at 85 °C. After mix-ing metallic solution with NaOH solution, both solutionswere heated and stirred for 30 minutes. This was followedby stoppage of heat while constant stirring remained con-tinue for further three hours. The pH value was kept between11.00 and 11.80. Precipitates were washed 5 times with de-ionized water. Products were dried with the help of electricoven by these over night at 110 °C. At the end, a black col-ored ferrite powder was obtained. Similarly, all 5 types ofcompositions were synthesized by varying the desired stoi-chiometric amount of salts. Dried samples were sintered ina furnace for about 5-hours at 800 °C. This was followed bycooling of the furnace at 20 °C/min. Samples were removedonce furnace was cooled.

2.2 Analysis and Characterization

The prepared Mn–Ni ferrites Mn1−xNixFe2O4 (with x =0.00, 0.25, 0.50, 0.75, 1.00) were characterized by differ-ent techniques. Compositional analyses were performed us-ing XRF (JEOL JAPAN JSX3201M). Transmission IR spec-tra were obtained by Perkin Elmer FTIR spectrometer us-ing KBr pellet. Structural and related characteristics of thesamples were determined by X-ray diffraction (XRD) us-ing a Rigaku DMAX3 diffractometer to determine the phasestructures. For this purpose, the discs of circular shape wereformed by applying uniform load on the ferrites powderwith the aid of hydraulic press. XRD patterns were takenusing CuKα (λ = 1.5406 Å) radiation at room tempera-ture. The mean crystallite sizes were estimated using thestandard Scherrer equation, and retrieved refinement anal-ysis was then performed on the XRD data to obtain thelattice constants. The average crystallite size, lattice con-stant, a, and measured density, ρm, and XRD density ρx

were calculated using the standard relations as describedelsewhere [7–9]. Morphology and the microstructure of thenanoferrites were studied with the aid of scanning electronmicroscopy (SEM), Jeol JSM 6490A and atomic force mi-croscopy (AFM) Joel JSPM-5200. Since the prepared fer-rites showed very high resistivity, so that SEM could not beperformed as such on the fabricated discs. To overcome thisproblem, circular discs were sputtered with gold coating ina custom made set-up assembly. Surface of the powder par-ticles were obtained by AFM operating in AC or tappingmode (TM), using a commercial silicon probe. Magnetiza-tion measurements were made using a commercial vibratingsample magnetometer (VSM) model, BHV-50, Riken Den-shi Co. Ltd. Japan.

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3 Results and Discussion

3.1 Synthesis by Coprecipitation Method

Among different techniques, the preparation of ferrites viachemical coprecipitation method perhaps offer many advan-tages due to its relative simple reaction procedure, lessercost, better homogeneous mixing of the constituents andgood control over the particle size and shape [11, 12].Transition-metal ferrite nanoparticles with a wide variety ofcompositions ranging from ternary metal oxides to mixedmetal oxides containing two or even three different divalentmetal ions have been prepared by using the coprecipitationtechnique [11, 12]. The choice of metal salts and reactionsparameters are known to strongly influence the character-istics of the synthesized nanoferrites. In the current work,metal salt Ni(NO3)2·6H2O, Mn(NO3)2·4H2O and an iron(III) salt of Fe(NO3)3·9H2O were used to synthesized nano-ferrites of Mn1−xNixFe2O4 by coprecipitation technique.Compositions of the prepared samples obtained from XRFspectrums are in good agreement with the stoichiometry ex-pected to present in the synthesized nanoferrites. Synthesisprocedure involved preparation of aqueous solutions con-taining divalent metal precursors of nitrates and iron (III)salt (ferric salt) [13] followed by treatment with the pre-cipitating agent of NaOH. Use of ferric precursor over fer-rous salt has advantage as synthesis can be performed with-out performing the partial oxidation of the iron (II) com-pound [14]. Another important aspect of the synthesis pro-cedure was the choice of precipitating agent of NaOH. Thenature of the precipitating agent has been known to influ-ence significantly the morphology of the ferrite nanopow-ders synthesized by the coprecipitation method. There arereports of usage of other precipitating agents such as am-monia solutions and urea [15], but the usage of NaOH incurrent work offered several advantages over others. For ex-ample, urea can undergo a hydrolysis reaction when heatedat temperatures higher than 60 °C with the formation of hy-droxide ions. Similarly, it is possible that when the precipi-tation is performed with NaOH instead of NH4OH, the re-sulting spherical morphological particles possess a relativelygood crystallinity, and narrow size distribution [16]. In addi-tion, since regardless of any morphology, the nanostructuredferrite materials resulting from the appropriate heat treat-ment consist of agglomerated particles. The morphologicalchanges of the as-prepared nanopowders during the anneal-ing process were found to depend on their initial composi-tion. Thus, while in the NaOH medium the transition-metalions precipitate as hydroxides, in the presence of an ammo-nia solution the coprecipitation occurs with the formation oftransition-metal complexes incorporating the NH+

4 ions.Another important parameter of the synthesis of nano-

ferrites was the temperature in the coprecipitation pro-cess. It is known that when the metal ions are precipitated

at room temperature, amorphous intermediate compoundssuch as hydroxides or oxyhydroxides separate from the re-action solution [17]. In case of the presence of hydroxo-intermediates, subsequent heat treatment at around 100 °Cis required to digest the resulting precipitate in order to ob-tain the corresponding nanocrystalline ferrite materials. Thispost-process of thermal treatment can influence the size ofthe ferrite nanoparticles. Considering this, in current work,the coprecipitation reactions were conducted in hot solu-tions (∼85 °C) containing the metal precursors and the pre-cipitating agent. The elevated temperature thus facilitatesthe immediate conversion of the hydroxo-intermediates intonanocrystalline ferrites of single-phase mixed metal oxideswith spinel structure and good crystallinity.

3.2 Infrared Spectroscopy Studies

The study of the vibrational spectra of ferrites providesa useful tool to understand their properties as later de-pends on the precise configuration of the ions in theircrystal lattice. Before further discussion of the IR spec-trums of the prepared nanoferrites, it should be empha-sized that MnFe2O4 is considered as mostly a normal mixedspinel, whose structure is more precisely represented bythe formula MnIIO[FeIII2O3], while NiFe2O4 belongs tothe class of fully inverse spinels, whose structure is moreprecisely represented by the formula FeIIIO[NiIIFeIIIO3],where brackets enclose the ions in the octahedral interstices,and roman numbers in superscripts indicate the metal-atomoxidation state. This means that, during the course of prepa-ration of ferrites of Mn1−xNixFe2O4 from x = 0.00 to 1.00,there is an induction in the structural changes in both theoctahedral and tetrahedral units of considered samples.

In the current work, IR spectra of Mn–Ni ferrites wereexamined in the frequency range of 350–1500 cm−1. Fig-ure 1 shows the representative IR spectrums of the preparedferrites of Mn1−xNixFe2O4. Two main broad metal–oxygenbands are important in the IR spectra of all spinels, andspecifically in ferrites [18–20]. The IR band, ν1, usually isobserved in the higher frequency range of 600–550 cm−1.The ν1 band corresponds to intrinsic stretching vibrations ofthe metal atom at the tetrahedral site, Mtetra↔O. The secondband is ν2, a lowest-frequency band which is observed inthe range of 450–385 cm−1. This is assigned to the octahe-drally metal-atom stretching vibrations, Mocta↔O. As shownin Fig. 2, both ν1 and ν2 stretching vibrations were observedwith the normal mode of vibration of tetrahedral cluster ishigher than that of octahedral cluster. This can be attributedto the shorter bond length of tetrahedral cluster and longerbond length of octahedral cluster. Ni2+ ions have the pref-erence to occupy the octahedral site, while Mn+2 and Fe+3

ions can occupy both octahedral and tetrahedral sites andboth ν2 and ν1 bands observed are the characteristics of theprepared Mn–Ni ferrites [18–20].

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Fig. 1 IR spectra of the prepared nanoferrites of Mn1−xNix Fe2O4 with varying Ni concentrations. ν1 and ν2 designate high- and low-energyfrequency bands

Fig. 2 The XRD patterns ofcubic spinel ferrites ofMn1−xNixFe2O4 with varyingNi concentrations

There is also an indication that variation of ν1 and ν2 ofMn1−xNix Fe2O4 with x takes place to some extent. Vari-ation of the ν1 peak position for samples of different x in-dicates structural changes in tetrahedral sites with the com-position and therefore can be correlated to the position ofthe shifting of band with the composition of the preparedferrites. Although the band position and its shape are largely

affected not only by the chemical composition of the sample,but also by a number of other mainly uncontrollable param-eters, such as conditions and methods of preparation, etc.It seems that ν1 band shifts slightly toward the lower wavenumbers with increase in x over the composition range,and indicating weakening of the metal–oxygen bonds in thetetrahedral sites due to the transition between the extent of

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normal spinel and inverse structures. Mn+2 possesses muchlarger preference to tetrahedral geometry than Fe+3. So, bythe increase of the Mn content (and in turn, by the decreaseof the nickel content) in the ferrites, it is expected that thetransition between normal and inverse spinel structure canoccur. Accordingly, the change in the shift of absorptionband can be rationalized having these facts in mind. The for-mation of a normal spinel structure due to the exchange ofthe positions of Fe+3 with Mn+2 ions causes weakening ofthe metal–oxygen bonding in tetrahedral sites. Simultane-ous changes in octahedral sites are reflected in the behaviorof the ν2 band. Due to the same reasons as those mentionedin the case of ν1, one can discuss the variation of the positionof this band just by the observation of general trends. Slightshifts of ν1 and ν2 peak positions for samples prepared in-dicate that changes due to the addition of Mn+2 has slightlyaffected the metal–oxygen force constants in tetrahedral andoctahedral sites. This can be explained by the very small dif-ference in both atomic mass (M) and ionic radii (r) of Mn+2

(M = 54.93801 amu; r = 0.89 Å), Ni+2 (M = 58.69 amu;r = 0.69 Å) and Fe+3 (M = 55.847 amu; r = 0.64 Å).

3.3 Structural Studies

The indexed XRD patterns of the prepared samples ofMn1−xNixFe2O4 are presented in Fig. 2. The presenceof planes (220), (311), (422), (440), and (511) in thediffractograms confirm the formation of cubic spinel struc-ture. As is evident from the figure, all the nanoferrites ofMn1−xNixFe2O4 with varying compositions exhibit single-phase spinel structure without showing any other detectableadditional or impurity phase. All the peaks in the patternsmatch well with the characteristic reflections of Mn and Niferrite found in good agreement with the published work[21–23]. The broad peaks signify lower crystallite size ofthe synthesized samples. The average crystallite size foreach composition was calculated using the standard Scher-rer formula from the line broadening of the XRD peak cor-responding to the total average peaks of the planes of thespinel structure [24]. The crystallite size remains with in

the range 14–25 nm. XRD data were also used to estimatevarious characteristics of the prepared nanoferrites such asaverage crystallite size (t (ave)), lattice constant (a), vol-ume of the cell (V ), X-ray density (ρx ), and porosity (P)

using standard relationships as discussed below [8, 9]. Table1 summarized the values of these characteristics parametersof Mn1−xNixFe2O4 for the various Ni concentrations x.

The values of lattice constant a, for each composition arecalculated and summarized in Table 1. It is observed that thelattice parameters a decreased with the decrease of Mn con-centration, i.e. increase in Ni concentration x in the com-positions of the prepared nanoferrite of Mn1−xNixFe2O4.Similarly, the average crystallite size (t (ave)) was found todepend on the Ni concentration x and decreased with its in-crement in the composition of the Mn1−xNixFe2O4. Theseobservations are attributed to the difference in sizes of theionic radii used in the current work. The ionic radius of Mn(0.89 Å) is relatively larger than that of Ni (0.69 Å) [25]. Thedecrease of a with increase in x is because of the fact that thelarger ionic radii of Mn are replaced by smaller ionic radii ofNi. This observation is in good agreement with several otherstudies where larger metal ions have been substituted withrelatively smaller metal ions [26, 27].

The effect of the variation in composition of the preparednanoferrites on their density has also been studied by es-timating both the measured density (ρm) and X-ray den-sity (ρx ). The measured density, ρm can be typically cal-culated from the mass m and volume V of the sintered sam-ples according to the simple relation ρm = m/V . The X-ray densities, ρx of the nanoferrites of Mn1−xNixFe2O4 canbe calculated using the standard relationship [28]. The val-ues of measured density (ρm) and X-ray density (ρx ) aregiven in Table 1. It is evident that both the densities in-crease as Ni concentration x increases from 0 to 1 in theprepared nanoferrites. The ρm and ρx increased from 3.34 to4.78 g cm−3 and 5.21 to 5.47 g cm−3, respectively, with con-centration x in the prepared nanoferrites. This observation isin agreement with similar studies carried out in which twodifferent metals are used. Since Ni has a larger atomic mass(58.6934 amu) than the Mn (54.9380 amu) atoms, this sub-

Table 1 The average crystallitesize (t (ave)), lattice constant(a), volume (V ), X-ray density(ρx ), measured density (ρm),and porosity (P ) of the preparednanosized Mn1−xNixFe2O4ferrites with varying Niconcentrations. Values inparenthesis are the average sizeof the particles measured fromSEM analyses

Ferrites MnFe2O4 Mn0.75Ni0.25Fe2O4 Mn0.50Ni0.50Fe2O4 Mn0.25Ni0.75Fe2O4 NiFe2O4

composition

Parameter x = 0.00 x = 0.25 x = 0.50 x = 0.75 x = 1.00

t (aver) (nm) 25 (23) 23 (20) 22 20 14 (13)

a (Å) 8.43 8.42 8.39 8.38 8.35

V (Å3) 599 597 591 588 582

ρx (g cm−3) 5.21 5.31 5.36 5.42 5.47

ρm (g cm−3) 3.34 3.63 4. 37 4.46 4.78

P (fraction) 0.359 0.327 0.195 0.188 0.137

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Fig. 3 Variation in the porosity (P ) and X-ray density (ρx ) ofMn1−xNixFe2O4 nanoferrites

sequently influenced the density due to the increase in masswhich overtakes the decrease in volume of the unit cell [34].

The effect of the varying Ni concentrations of x inMn1−xNixFe2O4 on the porosity (P) of prepared nanofer-rites has also been investigated. The porosity (P ) is calcu-lated using the standard relation described elsewhere [28].The influence of the varying Ni concentration x on theporosity (P) along with the X-ray density is shown in Fig. 3.It is observed that the porosity of the nanoferrites decreaseswith increase in Ni concentration x in Mn1−xNixFe2O4,while the X-ray density decreases. Since particle size andsurface area decrease with increase in x, this can decreasethe grain boundaries of the particle and subsequently de-crease the porosity. This in return can produce density in-crement with the increase in x value. The smaller grains canproduce a large number of grain boundaries. It is very diffi-cult to distinguish between the effects which are due to grainsize and porosity.

3.4 Morphological Studies

The morphology and the microstructure of the nanoferriteswere studied by scanning electron microscope (SEM) andatomic force microscopy (AFM). AFM and SEM providedirect information about the size, structure and morphologyof the particles. The representative SEM microphotographsof Mn1−xNixFe2O4, with the Ni concentration x = 0.25,are presented in Fig. 4 at low resolution (a) and high res-olution (b) of the prepared nanoferrites. SEM micrographsexhibit single phase as indicated by XRD patterns, and didnot show any other phase due to side reaction or interdif-fusion between phases. However, there were clusters of ag-glomeration of the ferrite particles as found frequently insuch ferrites [29–31]. Nonetheless, it seems that the sam-ples possess spherical nanosize grains. The size distribution

Fig. 4 Representative scanning electron microscopy (SEM) of thenanoferrites of Mn1−xNixFe2O4 with Ni concentration of x = 0.25:(a) low resolution; (b) high resolution

is found to be almost uniform in the samples. As given in Ta-ble 2, the average size determined from SEM was observedto be between 23–13 nm for the prepared nanoferrites. Theconventional synthetic methods usually employ to prepareferrite powders exhibit non-uniform grain size distribution.In the present case uniform size distribution of nanoferritesare observed and this signifies the importance of the co-precipitation method used for the synthesis of the ferrites.The developments of the uniform and fine grain size mag-netic materials are vital for many technological applications.Since discs were used for SEM analyses, the pores were ob-served on grain boundary but not on individual grains. Inaddition, variation in sizes was noted for different composi-tions of prepared ferrites with higher Ni concentration in theferrites produced relatively smaller size of the particles. Fur-thermore, average size of the nanoferrites with varying com-position is in excellent agreement with the results obtainedfrom XRD analysis. The SEM results of the prepared nano-

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Table 2 Magnetic parametersof saturation magnetization(Ms ), remanent magnetization(Mr ), squareness ratio(Mr/Ms ), and coercivity (Hc)of the nanosizedMn1−xNixFe2O4 ferrites withvarying Ni concentrations

Ferrites MnFe2O4 Mn0.50Ni0.50Fe2O4 Mn0.25Ni0.75Fe2O4 NiFe2O4

composition

Hc (Oe) 240 214 286 210

Mr (emu/g) 2.07 2.22 3.30 3.48

Ms (emu/g) 7.41 11.01 15.57 21.59

Mr/Ms (S/R) 0.27 0.20 0.21 0.16

Fig. 5 Magnetic hysteresiscurve of Mn1−xNixFe2O4 for0 ≤ x ≤ 1. The inset specificallyrepresents the variation in Hc

and Mr of the loops

ferrites indicate that particles distribution is quite uniform.The average particle size obtained from SEM analyses were23, 20, and 13 nm for x = 0.00, 0.25 and 1.00, respectively.The SEM micrographs of a few samples were taken with ap-proximately 300,000 magnifications, which is the maximumpossible on this instrument. Considering such a smaller sizeof the prepared nanoferrites, samples were further analyzedusing AFM as this is one of the best techniques to study par-ticles on the nanometric scale. AFM study of the preparednanoferrites of Mn1−xNixFe2O4 with varying Ni concen-tration x = 0.00, 0.25, 1.00 was carried out. In the currentwork, surface of the powder particles were obtained by AFMoperating in AC or tapping mode, using a commercial sili-con probe. Tapping mode was chosen because contact mode(CM) produced noisy images, which suggested that particleswere being removed by the probe. For each sample, the sev-eral particle sizes were measured directly on AFM imagesand the average diameter of the particles, which were ap-proximately spherical in shape with narrow size distribution,was taken as the mean of the largest and the smallest dimen-sions. It is again observed that the Mn acts as contributorsto the enhancement of the particle size. The average particlesize decreases as the concentration of Mn decreases in com-position, with the average minimum particle sizes obtained

being 7.96 nm, 1.80 nm and 1.73 nm for x = 0.00, 0.25 and1.00, respectively.

3.5 Magnetic Properties

Figure 5 shows the variation of magnetization M (emu/g)versus the applied magnetic field H(Oe) at room tempera-ture for the prepared spinel nanoferrites Mn1−xNixFe2O4.The hysteresis curve is obtained using the vibrating samplemagnetometer (VSM). The narrow loops for all the samplesindicate the soft nature of the ferrite nanomaterials. All thecurves appear to behave normally, and the magnetizationis found to increase with increasing the applied magneticfield and attains its maximum value for applied field of 1.25Tesla. The magnetization for the prepared Mn1−xNixFe2O4

samples exhibits a clear hysteretic behavior under appliedmagnetic field. Dependence of the various magnetic parame-ters such as saturation magnetization (Ms ), remanence (Mr )and coercivity (Hc) are calculated from the hysteresis loopand all the parameters calculated from the hysteresis curvesare given in the Table 2. Figure 6 represents the various mag-netic parameters as a function of Ni concentration x in theprepared samples of nanoferrites.

The saturation magnetization Ms for each sample was de-termined by the magnetization curve at Hmax. The depen-dence of Ms on Ni concentration x is illustrated in Fig. 6(a).

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Fig. 6 Magnetic properties of Mn1−xNixFe2O4 for 0 ≤ x ≤ 1: (a) saturation magnetization (Ms ); (b) remanent magnetization (Mr ); (c) coercivity(Hc); (d) squareness ratio (Mr/Ms )

It is noticed that Ms increases with x in Mn1−xNixFe2O4.The remanent magnetization Mr for each sample was de-termined by the magnetization curve at H = 0.0 Oe. Thedependence of Mr on the Ni concentration x is illustratedin Fig. 6(b). It is observed that Mr increases with x. Itis evident from the results given in Table 2 and presentedin Figs. 6a and 6b that both saturation magnetization, Ms ,and remanence, Mr , start to increase from 7.41 emu/g to21.59 emu/g and 2.07 emu/g to 3.48 emu/g, respectively, asthe Ni concentration increased. The increase in the value ofMs and Mr with increase in Ni concentration x can be ex-plained on the basis of Néel’s theory [32]. The results ob-tained are also in reasonable agreement to previous stud-ies of ferrites containing Mn or Ni ions [33, 34]. For ex-ample, it was observed that for the nanosized ferrites of

Mn0.5−xNixZn0.5Fe2O4, (with x = 0.0, 0.1, 0.2, 0.3), themaximum magnetization decreases up to x = 0.2, whilethere is a rise in the maximum magnetization value of thesample with composition x = 0.3 [33]. This is an interestingobservation, and agrees well to the current results.

The remanence ratio (Mr/Ms ), which expresses thesquareness of the hysteresis loop, is illustrated in Fig. 6(c)as a function of Ni concentrations x. It is obvious thatthe squareness decreased with increasing Ni concentra-tion x. The low Mr/Ms value obtained for the preparedMn1−xNixFe2O4 may indicate that a significant amountof nanoparticles are still superparamagnetically fast re-laxing at room temperature, when the external magneticfield is turned off. The coercive force (Hc) data of theprepared nanoferrites as a function of Ni concentration x

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are presented in Fig. 6(d). The coercivity represents thestrength of the magnetic field that is necessary to surpassthe anisotropy barrier and allow the magnetization of thenanoparticle. A lower coercivity Hc is observed by loweringthe anisotropy of a nanoparticle as this lowers the activationenergy barrier and results in a lower applied field requiredfor spin reversal [35]. Temperature also plays a crucial roleto influence the coercivity Hc , and, as has been observed insuperparamagnetic specimens, the coercivity tends to van-ish with increasing temperature [36]. However, one can notethat it is present even at room temperature, thus indicatingthe anisotropy of the prepared Mn1−xNixFe2O4 nanoparti-cles (see the inset of Fig. 5). The coercive force (Hc) valuesobtained in the present study are found to be in between285–210 Oe, and seem to be not significantly different forvarious Ni concentrations in the prepared Mn1−xNixFe2O4

nanoferrites. The reasons for the approximate similarity ofHc of the prepared nanoferrites are not clear at the moment.It is possible that variations in porosity and/or size of theprepared nanoferrites do not occur to such an extent as tosignificantly influence coercivity Hc . Furthermore, the mag-netic properties observed for the nanoparticles are a com-bination of several anisotropy mechanisms, such as magne-tocrystalline anisotropy, surface anisotropy and interparti-cle interactions. It is known that surface anisotropy resultsfrom the low coordination symmetry for the spin-orbit cou-plings at the surface of the nanoparticles. Since the preparednanoparticles of Mn1−xNixFe2O4 have different sizes, den-sities, and porosities, they may differently affect the contri-butions of the surface anisotropy and interparticle interac-tions to the net anisotropy to be unable to induce notabledifferences in the Hc values.

4 Conclusions

The coprecipitation method has been successfully used forthe synthesis of nanoferrites of Mn1−xNixFe2O4 with Niconcentration x = 0.00, 0.25, 0.50, 0.75 and 1.00. Variouscharacterization techniques were used to study the charac-teristics of the prepared nanoferrites. Both the typical high-frequency and low-frequency bands due to tetrahedral andoctahedral sites of the prepared spinel ferrites were observedusing IR spectroscopy. The slight variation in the bands is at-tributed to the relative smaller size differences between Mnand Ni metal ions. Structural analysis through XRD con-firmed the formation of a single-phase spinel Mn–Ni fer-rite structure with size of the nanoferrites in the range of25–13 nm. The lattice constant and porosity were found todecrease with decrease in Mn concentration. These observa-tions are attributed due to the relative larger ionic radii ofMn2+ as compare to Ni+2. Results from SEM also found tobe in agreement with the XRD, and indicated the contribu-tion of Ni+2 to reduce the size of the prepared nanoferrites.

The observed results suggest that the structural, composi-tional and morphological characteristics of the synthesizednanoferrites can be controlled by varying the composition ofMn–Ni ferrites of Mn1−xNixFe2O4. Furthermore, hystere-sis curves obtained from magnetic measurement indicatedthe soft nature of the prepared nanoferrites. Both satura-tion magnetization (Ms ), and remanence (Mr ) are calculatedfrom the hysteresis loop and are found to increase with Niconcentration x in the nanoferrites prepared.

Acknowledgements Authors are thankful to Mr. Zafar Iqbal,Mr. Shahid Ameer, Mr. Noor Ahmed, Mr. Shmasudin, and Mr. AftabAhmed, School of Chemical and Materials Engineering (SCME),NUST for their support to carry out characterization of the samplesusing FTIR, SEM, AFM and XRF. Financial support of Pakistan Sci-ence Foundation (PSF) through Project No. 147 is also acknowledged.

References

1. Goldman, A.: Handbook of Modern Ferromagnetic Materials. TheSpringer International Series in Engineering and Computer Sci-ence. Springer, New York (1999)

2. Smart, L., Moore, E.: Solid State Chemistry: An Introduction.Fong & Sons Printers Pte Ltd., Singapore (1992)

3. Greenwood, N.N., Earnshaw, A.: Chemistry of the Elements.Pergamon, Oxford (1984)

4. Willard, M.A., Kurihara, L.K., Carpenter, E.E., Calvin, S., Harris,V.G.: Int. Mater. Rev. 49, 125 (2004)

5. Sun, D.H., Sun, D.X., Hao, Y.: Mat. Sci. Forum 663–665, 1325(2010)

6. Sattar, A.A., El-Sayed, H.M., El-Shokrofy, K.M., El-Tabey,M.M.: J. Mater. Eng. Perform. 14, 99 (2005)

7. Yu, H.-F., Yang, S.-W.: J. Alloys Compd. 394, 286 (2005)8. Sattar, A.A., El-Sayed, H.M., Agami, W.R., Ghani, A.A.: Am. J.

Appl. Sci. 4, 89 (2007)9. Parker, R., Lords, H.: Proc. Phys. Soc. A 79, 383 (1962)

10. Carta, D., Casula, M.F., Falqui, A., Loche, D., Mountjoy, G., San-gregorio, C., Corrias, A.: J. Phys. Chem. C 113, 8606 (2009)

11. Gee, S.H., Hong, Y.K., Erickson, D.W., Park, M.H., Sur, J.C.:J. Appl. Phys. 93, 7560 (2003)

12. Arulmurugan, R., Jeyadevan, B., Vaidyanathan, G., Send-hilnathan, S.: J. Magn. Magn. Mater. 288, 470 (2005)

13. Joshi, H.H., Pandya, P.B., Modi, K.B., Jani, N.N., Baldha, G.J.,Kulkarni, R.G.: Bull. Mater. Sci. 20, 93 (1997)

14. Kuo, P.C., Tsai, T.S.: J. Appl. Phys. 65, 4349 (1989)15. Khoudiakov, M., Gupta, M.C., Deevi, S.: Nanotechnology 15, 987

(2004)16. Ryu, B.-H., Chang, H.-J., Choi, Y.-M., Kong, K.-J., Lee, J.-O.,

Kim, C.-G., Jung, H.-K., Byun, J.-H.: Phys. Solid State 201, 1855(2004)

17. Sharma, R.K., Suwalka, O., Lakshmi, N., Venugopalan, K., Baner-jee, A., Joy, P.A.: Mater. Lett. 59, 3402 (2005)

18. Waldron, R.D.: Phys. Rev. 99, 1727 (1955)19. Brabers, V.A.M.: Phys. Status Solidi 33, 563 (1969)20. Kavas, H., Baykal, A., Toprak, M.S., Koseoglu, Y., Sertkol, M.,

Aktas, B.: J. Alloys Compd. 479, 49 (2009)21. Kavas, H., Kasapoglu, N., Baykal, A., Köseoglu, Y.: Chem. Pap.

63, 450 (2009)22. Yu, H.-F., Yang, S.-W.: J. Alloys Compd. 394, 286 (2005)23. Ramalho, M.A.F., Gama, L., Antonio, S.G., Paiva-Santos, C.O.,

Miola, E.J., Kiminami, R.H.G.A., Costa, A.C.F.M.: J. Mater. Sci.42, 3603 (2007)

100 J Supercond Nov Magn (2012) 25:91–100

24. Berry, C.C., Curtis, A.S.G.: J. Phys. D, Appl. Phys. 36, R198(2003)

25. Kurihara, L.K., Suib, S.L.: Chem. Mater. 5, 609 (1993)26. More, S.S., Kadam, R.H., Kadam, A.B., Mane, D.R., Bichile,

G.K.: Cent. Eur. J. Chem. 8, 419 (2010)27. Cong, C.J., Hong, J.H., Liu, Q.Y., Liao, L., Zhang, K.L.: Solid

State Commun. 138, 511 (2006)28. Cullity, B.D.: Elements of X-ray Diffraction. Addison-Wesley,

Reading (1956)29. Gul, I.H., Maqsood, A.: J. Alloys Compd. 465, 227 (2008)30. Karche, B.R., Khasbardar, B.V., Vaingankar, A.S.: Physica C, Su-

percond. 387, 290 (2003)

31. Cristina, A., Costa, F.M.: J. Magn. Magn. Mater. 256, 174 (2003)32. Nell, L.: C. R. Acad. Sci., Ser. 1 Math. 230, 375 (1950)33. Venkataraju, C., Sathishkumar, G., Sivakumar, K.: J. Magn. Magn.

Mater. 322, 230–233 (2010)34. Kavas, H., Baykalb, A., Toprakc, M.S., Koseoglua, Y., Sertkol, M.,

Aktas, B.: J. Alloys Compd. 479, 49–55 (2009)35. Pankhurst, Q.A., Pollard, R.J.: Phys. Rev. Lett. 67, 248 (1991)36. Duquea, J.G.S., Souzaa, E.A., Menesesa, C.T., Kubota, L.: Phys-

ica B 398, 287–290 (2007)