Superparamagnetic magnesium ferrite nanoparticles fabricatedby a simple, thermal-treatment method

Mahmoud Goodarz Naseri a,∗, Mohammad Hossein Majles Ara b, Elias B. Saion c,Abdul Halim Shaari c

a Department of Physics, Faculty of Science, Malayer University, Malayer, Iranb Department of Physics, Faculty of Science, Kharazmi University, Shahid Mofatteh Ave. No. 49, Tehran, Iranc Department of Physics, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

a r t i c l e i n f o

Article history:Received 28 October 2012Received in revised form12 August 2013Available online 2 September 2013

Keywords:Magnetic materialMicrostructureNanoparticleThermal-treatmentPolymer

a b s t r a c t

This study investigated the synthesis of magnesium ferrite (MgFe2O4) nanoparticles with cubic symmetrythat were prepared by a thermal-treatment method by using a solution that contained poly (vinyl alcohol)(PVA) as a capping agent and Mg and Fe nitrates as alternative sources of metal. Heat treatment wasconducted using an electric cylinder furnace in an air atmosphere at temperatures between 673 and 973 K,and magnesium ferrite nanoparticles were produced that had different crystallite sizes ranging from5 to8 nm. The products were well characterized by X-ray diffraction (XRD), transmission electron microscopy(TEM), field emission scanning electron microscope (FESEM), X-ray analysis (EDXA), and Fourier transforminfrared spectroscopy (FT-IR). All the samples calcined from 673 to 973 K exhibited super paramagneticbehavior with unpaired electrons spins, which was confirmed by using a vibrating sample magnetometer(VSM) and electron paramagnetic resonance (EPR) spectroscopy.

& 2013 Published by Elsevier B.V.

1. Introduction

Magnetic materials are one of the most well-known magneticmaterials, and they have been investigated intensively in recentyears from a purely scientific perspective and because of theirunique magnetic, electrical, and optical properties [1,2]. The spinelstructure has the general formula of A2þB23þO4 in which A (thetetrahedral site) represents a divalent metal ion, and B (theoctahedral site) represents a trivalent metal ion [1]. Spinel ferrites,which are a subcategory of the spinel structure, consists of a close-packed oxygen arrangement in which 32 oxygen ions form a unitcell. There are several interstices between the layers of oxygen ionsthat can be separated into two types, i.e., A and B sites, dependingon the coordination of the nearest neighboring oxygen ions. In theunit cell, only eight of 64 tetrahedral sites and 16 of 32 octahedralsites are occupied by metal ions [3]. In the case inwhich B3þ¼Fe3þ ,the resulting spinel ferrites have a general chemical composition ofMFe2O4 (e.g., M¼Mg, Cu, Zn, Ni, Co) and are used extensively asmagnetic materials [4]. Among the spinel ferrite compounds, super-paramagnetism magnesium ferrite nanoparticles has been studiedextensively due toits unique feature of magnetic nanoparticles andhas great relevance to modern technologies including magnetic

resonance imaging contrast agents, data lifetime in high densityinformation storage, ferrofluid technology, and magnetocaloricrefrigeration [5]. Magnesium ferrite is a magnetic bi-oxide ceramicmaterial with a partially inverse spinel structure so that the distribu-tion of the cations in it can be represented by [Mg1�δ

2þ

Feδ3þ]A[Mgδ2þ Fe2�δ3þ]BO4 where δ represents the degree of inver-

sion of cation in the structure [6]. Many approaches have beendeveloped for the synthesis of the MgFe2O4 nanoparticles, such ascombustion synthesis [6], solvothermal method [7], the sol–gelprocess [8], co-precipitation [5], solid-state reaction [9], hydrother-mal synthesis [10], high-energy ball milling approach [11], mechan-ochemistry synthesis [12], microwave processing approaches [13]and microemulsion method [14]. Factors and various precipitationagents were used to produce spinel ferrite nanocrystals of specificsizes and shapes. Examples include the metal hydroxide in co-precipitation method; the surfactant and ammonia in the micro-emulsion method; the organic matrices in the polymeric precursor,sol–gel, and polyol methods; and high-temperature, solid-statereaction. Most of these methods have achieved particles of therequired sizes and shapes, but they are difficult to use on a largescale because of the expensive and complicated procedures involved,the high reaction temperatures, the long reaction times, the toxicreagents used, and the by-products produced that have the potentialto harm the environment. As a result of the present work, thesynthesis of MgFe2O4 nanoparticles and the effects of calcinationtemperature on crystallinity, morphology, microstructure, elementalcomposition, phase composition, magnetic properties and magnetic

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Journal of Magnetism and Magnetic Materials

0304-8853/$ - see front matter & 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.jmmm.2013.08.032

n Corresponding author: Tel.: þ98 60142698153; þ98 9126868423;fax: þ98 60389454454.

E-mail address: mahmoud.naseri55@gmail.com (M. Goodarz Naseri).

Journal of Magnetism and Magnetic Materials 350 (2014) 141–147

resonance study are reported for the first time, which is a significantextension and enhancement of the thermal-treatment method.

2. Experimental

2.1. Materials

Metal nitrate reagents were used as precursors, poly (vinyl alcohol)(PVA) was used as the capping agent, and deionized water was used asthe solvent. Iron nitrate, Fe (NO3)3 �9H2O, and magnesium nitrate, Mg(NO3)2 �6H2O, were purchased from Acros Organics with puritiesexceeding 99%. PVA (MW¼31000 g/mol) was purchased from SigmaAldrich and was used without further purification.

2.2. Methodology

An aqueous solution of PVA was prepared by dissolving 2.5 g ofpolymer in 100 ml of deionized water at 353 K, before mixing0.2 mmol iron nitrate and 0.1 mmol magnesium nitrate (Fe:Mg¼2:1) into the polymer solution and constantly stirring for2 h using a magnetic stirrer until a colored solution was obtained.A glass electrode was used to determine the pH of the solution,which ranged 1–2. No precipitation of materials was observedbefore the heat treatment. The mixed solution was poured into aglass Petri dish and heated at 373 K in an oven for 24 h toevaporate the water. The dried solid magnesium ferrite thatremained was crushed and ground in a mortar to form powder.The calcinations of the powders were conducted at 673,773, 873and 973 K for 3 h for the decomposition of organic compounds andthe crystallization of the nanocrystals.

3. Characterization

The structure of the MgFe2O4 nanoparticles was characterizedby the XRD technique using a Philips X-pert type instrument withCu Kα radiation and a wave length λ¼1.5405 Å radiation togenerate diffraction patterns from powder crystalline samples atambient temperature in a 2θ range of 101–801. The morphologyand particle size of the nanocrystals were determined fromTEM images that were obtained by using a JEOL 2010 F UHRversion electron microscope at an accelerating voltage of 200 kV.The microstructures of the magnesium ferrite nanoparticles werestudied FESEM using JEOL JSM-6701 F type instrument coupledwith EDX for elemental analysis. FT-IR spectra were recorded usinga PerkinElmer FTIR model 1650 spectrometer. Before recording thespectra, the samples were placed on a Universal ATR SamplingAccessory (diamond coated with CsI) and pressed, and then thespectra were recorded. Magnetization measurements were con-ducted using a VSM (Lake Shore 4700) at room temperature with amaximum magnetic field of 15 kOe. EPR spectra were recorded on

a JEOL JES-FA200 EPR spectrometer (JEOL, Tokyo, Japan) at roomtemperature.

4. Results and discussion

4.1. Mechanism of formation of nanoparticles

PVA is a water-soluble and biodegradable synthetic polymer.The metallic ions (Fe and Mg) were dissolved in aqueous PVA. Insolution, these ions are entrapped by ionic-dipole interactionswith hydroxyl groups (–OH) in polymeric chains, as shown inFig. 1. After drying, which eliminated the water from this mixture,the metals ions that were left behind were immobilized in thepolymer scaffold. After heat treatment, ferrite nanoparticles gra-dually formed and PVA and counter anions were removed. Hence,the PVA, in addition to affecting the solution and the drying step,also affects the formation of the nuclei (i.e., nucleation) of themagnesium ferrite nanoparticles in the calcination step. In thisstep, if there were no PVA present, the small nanoparticles thathave high energy levels on their surfaces would become larger viathe Ostwald ripening process [15]; however, when PVA is present,steric hindrance is disrupted, which prevents the aggregation ofthe nanoparticles. Steric hindrance is a phenomenon that isattributed to large molecular weights (410,000) and to therepulsive forces that occur among the polyvinyl groups [16].Theseinteractions are similar to the stabilization of metallic nanoparti-cles, i.e., silver and gold [17,18]. Therefore, it has been substan-tiated that PVA decreases grain growth by inhibiting the metalions from collapsing on the surfaces of the nanoparticles [19].

4.2. Crystallinity, morphology, microstructure, elementalcomposition, and phase composition in MgFe2O4 nanoparticles

Fig. 2 presents the XRD patterns of precursor and magnesiumferrite nanoparticles at different temperatures. Before the calcina-tion procedure, only a broad reflection of the precursor can bedistinguished (Fig. 2a). In this method, spinel-type MgFe2O4

nanoparticles with cubic symmetry were formed when the pre-cursor was calcined in the temperature range of 673–9733 K. Thediffraction lines of these nanoparticles can be indexed readily totetragonal-type MgFe2O4 spinel (ICCD: 01-1114).

Fig. 1. A proposed mechanism of interactions between PVA and metal ions in theformation of magnesium ferrite nanoparticles.

Fig. 2. XRD patterns of precursor and magnesium ferrite nanoparicles calcined at(a) 673, (b) 773, (c) 873 and (d) 973 K.

M. Goodarz Naseri et al. / Journal of Magnetism and Magnetic Materials 350 (2014) 141–147142

However, impure phases of α-Fe2O3 as hematite and MgO asmagnesia exist in all magnesium ferrite nanoparticles fabricatedby the thermal-treatment method. Fig. 2 shows that the intensityof the α-Fe2O3 and MgO impurity phases increased when thecalcination temperature was increased. The appearance of the α-Fe2O3 phase at all calcination temperatures was evidence of thetransfer of Fe3þ ions from B site to A site in these ferritenanoparticles [20]. In addition, the high calcination temperaturesof these nanoparticles could lead to changes in the oxygen contentin the lattice, i.e., with the formation of observable quantities ofMgþ . As a result, the intensity of the MgO-impurity phaseincreased as the calcination temperature increased. The averageparticle diameter of the nanoparticles was estimated from thebroadening of the X-ray diffraction peaks by using Scherrer′sformula

D¼ 0:9λ=ðβ cos θÞ; ð1Þ

where D is the crystallite size (nm), β is the full width of thediffraction line at half the maximum intensity measured inradians, λ is the X-ray wavelength, and θ is the Bragg angle [21].The estimated average diameter ranged from 7 to 12 nm, depend-ing on the calcination temperature (described in Table 1).The value

of the ‘a’ lattice parameter for our MgFe2O4 nanoparticles eval-uated from the XRD spectra are recorded in Table 1 in the range ofapproximately 0.8311–0.8384 nm. In our magnesium ferrite nano-particles, ‘a’ lattice parameter revealed irregular variation withcalcination temperature, while Maensiri et al. [22] observed adecrease in the lattice parameter of nanostructures MgFe2O4 thatwere obtained by electrospinning, when calcination temperatureincreased. The morphology and structure of the MgFe2O4 nano-particles calcined from 673 to 973 K, as observed in the TEMimages, are shown in Fig. 3. The figure shows that the magnesiumferrite nanoparticles obtained by this method are uniform in bothmorphology and particle size distribution. When the calcinationtemperatures were from 673 to 973 K, the particle sizes were from5 to 8 nm, respectively (Table 1). The increase in particle size thatresulted from increasing the calcination temperature suggestedthat the surfaces of several neighboring particles melted and fusedthe particles together, thereby increasing the particle size [23].Increase in particle size due to grain growth have been observedpreviously in nickel ferrite and cobalt ferrite systems at highercalcination temperatures [24,25].

The microstructure of the magnesium ferrite nanoparticles wasinvestigated by FESEM, as shown in Fig. 4. In the MgFe2O4

Table 1Average particle sizes (nm) of MgFe2O4 nanoparticles determined from XRD, TEM, FESEM, and lattice parameter, wave-number obtained from FT-IR spectroscopy.

SpecimensMgFe2O4

Calcinationtemperature (K)

Average particle sizeXRD (nm)

Average particle sizeTEM (nm)

Average particle sizeFESEM (nm)

Lattice parameter a(nm)

Wave number(cm�1)

ν1 ν2

MgFerrite 1 673 7 5 9 0.8311 402 557MgFerrite 2 773 8 6 11 0.8324 396 556MgFerrite 3 873 10 7.5 14 0.8384 395 551MgFerrite 4 973 12 8 15 0.8377 390 558

Fig. 3. TEM images of magnesium ferrite nanoparticles calcined at (a) 673, (b) 773, (c) 873 and (d) 973 K.

M. Goodarz Naseri et al. / Journal of Magnetism and Magnetic Materials 350 (2014) 141–147 143

nanoparticles, the FESEM micrographs show that the microstruc-ture of the materials was affected by the calcination temperature.Table 1 demonstrates that the values of the estimated diameters ofthe MgFe2O4 nanoparticles calcined at 673, 773 and 873 K were 9,11, and 14 nm, respectively. The MgFe2O4 nanoparticles calcined at973 K consisted of large grains with diameters of 15 nm. All ofthese findings are in relatively good agreement with the TEMresults. The appearance of some agglomerated areas in the FESEMimages was due to the naturally-occurring interaction betweenmagnetic nanoparticles. In many cases of nanocrystalline spinelferrites, it has been observed that there is a tendency for thenanoparticles to agglomerate [26]. Heat treatment resulted inagglomeration of the nanoparticles as a function of calciningtemperature, which is typical for spinel ferrites. Therefore, somedegree of agglomeration at the higher calcination temperatureappears to be unavoidable [27].

The composition of the nanoparticles that were formed wasdetermined using EDXA, and the pattern that was obtained isshown in Fig. 5. The EDXA spectrum of magnesium ferritenanoparticles calcined at 673 K revealed the presence of Mg, Fe,and O peaks in the sample, which is in agreement with the XRDresults discussed earlier in connection with Fig. 2. In this experi-ment, the Au peak was assigned to the gold substrate that wasused to hold the sample.

FT-IR spectroscopy is one of the most useful techniques forinvestigating multi-component systems, because it provides infor-mation on the blend composition as well as on the polymer–polymer interactions. Our discussion is focused principally on thehydroxyl and carbonyl stretching vibration bands in Fig. 6, sincethey were expected to be affected by the hydrogen bondinginteractions. Fig. 6(a) shows the FT-IR spectrum of the precursorin the wave-number range between 250 and 4000 cm�1. Theappearance of the band at 1491 cm�1 was associated with thebending vibration related to the CH2 groups [28]. Also, the band at

1711 cm�1 was attributed to the stretching vibration of thecarbonyl group, C¼O, from the aldehyde group [29]. Finally, thelarge band observed at 3318 cm�1was linked to the stretching ofO–H from the inter-molecular and intra-molecular hydrogenbonds [28]. It can be seen that the FT-IR spectra of the calcinedMgFe2O4 nanoparticles (Figs. 6b–d) indicate the presence ofresidual organic compounds at 1491 and 3324 that are associatedwith the bending vibration related to the CH2 groups and thestretching of O–H from the inter-molecular and intra-molecularhydrogen bonds respectively [28]. The vibrational spectra of theabsorption bands of pure spinel magnesium ferrite nanoparticleswere observed at 390 and 558 cm�1 for the sample calcined at973 K (Fig. 6(e)). Fig. 6e confirmed that, in fact, there are noresidual organic compounds at 973 K. The FT-IR spectra of allmagnesium ferrite nanoparticles revealed two principle absorp-tion bands in the range of 350–600 cm�1, with the first band (ν1)

Fig. 4. FESEM micrographs of the magnesium ferrite nanoparticles calcined at (a) 673, (b) 773, (c) 873 and (d) 973 K.

Fig. 5. EDXA spectrum of magnesium ferrite nanoparticles calcined at 673 K.

M. Goodarz Naseri et al. / Journal of Magnetism and Magnetic Materials 350 (2014) 141–147144

around 400 cm�1 and the second band (ν2) around 560 cm�1

(Table 1). These two main broad metal–oxygen bands correspondto the intrinsic stretching vibrations of the metal at the tetrahedralsite, Mtetra2O (observed from 551 to 558 cm�1) and octahedral-metal stretching, Mocta2O (observed from 390 to 402 cm�1) [30].The negligible difference in the frequencies between the character-istic vibrations ν1 and ν2 may be attributed to the long bond length ofthe oxygen–metal ions in the octahedral sites and the shorter bondlength of the oxygen–metal ions in the tetrahedral sites [31].

4.3. Magnetic properties and magnetic resonance study

Fig. 7 shows the magnetic hysteresis loops of the nanocrystallineMgFe2O4 calcined at different temperatures, which were measuredat room temperature in the range of approximately �10 toþ10 kOe. All of the calcined magnesium ferrite nanoparticlesexhibited super paramagnetic behavior. The curves disclosed thatthe saturation magnetization (Ms) increased as the calcinationtemperature increased, as indicated in Table 2. The variation ofsaturation magnetization with calcination temperature can be dueto cation inversion, which originates from thermal and mechanicaltreatment [6]. Due to heat treatment conditions, some of the Fe3þ

cations transferred from the B site to the A site (the same number ofMg2þ migrated from A sites to B site), increasing the accumulationof Fe3þ ions at the A site and Mg2þ ions at the B site [14].However, the FeA3þ–FeB3þ super-exchange interactions increased,

and this can lead to an increase in saturation magnetization in theMgFe2O4 nanoparticles [20]. Inter-sub-lattice, super-exchangeinteractions of the cations on the (A–B) are much stronger thanthe (A–A) and (B–B) intra-sub-lattice exchange interactions [32].Furthermore, the variations of Ms with particle size for magnesiumferrite nanoparticles are listed in Table 2, which shows that thetendency of Ms to increase is consistent with the enhancement ofparticle size in the MgFe2O4 samples that were prepared by thethermal-treatment method [24]. The external parts of the nano-particles seems to be composed of some distorted or canted spinsthat prevent the core spins from aligning with the direction of thefield; consequently, the saturation magnetization increase forlarger sizes and decrease for smaller sizes [6].

According to the demonstrated results of Ms in Table 2, thelargest saturation magnetization in MgFe2O4 nanoparticles wasfound to be approximately 11.74 emu g�1, about 65% smaller thanthe value of 33.4 emu g�1 for bulk MgFe2O4 [22]; this may be due tothe existence of an inactive magnetic layer or a disordered layer onthe surfaces of the nanoparticles [20]. In fact, the surfaces distortionof the nanoparticles, due to the interaction of transition metal ionswith the oxygen atoms in the spinel lattice (as discussed earlier) canreduce the net magnetic moment in the particle. This effect isespecially prominent for the ultrafine particles due to their largesurface to volume ratio [27]. Moreover, the formation of the impurephases α-Fe2O3 and MgO for the calcined samples can reduce themagnetic properties of these nanoparticles because of the presenceof the these phases favor the antiparallel alignment of the iron ionswithin the magnesium ferrites which is responsible for the decreaseof magnetization [8].

Fig. 8 shows that the EPR spectra of the samples calcined at(a) 673, (b) 773, (c) 873, and (d) 973 K exhibited broad, symme-trical signals. Peak-to-peak line width (ΔHpp), resonant magneticfield (Hr), and g-factor are three parameters that characterize themagnetic properties. It is worth noting that MgFe2O4 displaysmagnetism in spite of the face that Mg2þ ions are non-magnetic.This may be due to the partially inverse spinel structure ofMgFe2O4 nanoparticles [33]. The EPR spectra indicated that, infact, the broad signals at g¼2.1235, 2.1262 and 2.1288 (listed inTable 2) were attributed to the presence of isolated Fe3þ ions at Bsite. Sengupta et al. [34] observed the broad signals at g¼4.2 andg¼2.00 were assigned to Fe3þ (3d5, 6S5/2) ions at A and B sitesrespectively. It is obvious from Table 2 that the g-values increasefrom 2.1235 to 2.1288 when the calcination temperature andparticle size increased, while Hr decreases from 305 to 302 Oe,which were confirmed by

g¼ hv=βHr; ð2Þ

Fig. 6. FT-IR spectra of precursor and magnesium ferrite nanoparticles calcined at(a) 673, (b) 773, (c) 873 and (d) 973 K.

Fig. 7. Hysteresis loops of magnesium ferrites nanoparticles calcined at (a) 673,(b) 773, (c) 873 and (d) 973 K.

M. Goodarz Naseri et al. / Journal of Magnetism and Magnetic Materials 350 (2014) 141–147 145

where h is Planck′s constant, ν is the microwave frequency, β is theBohr magneton (9.274� 10�21 erg Oe�1), and Hr is the resonantmagnetic field (that should decrease when the g-value increases,whereas ν is constant in EPR spectroscopy).

Basically, the addition of Fe3þ ions located at the A site, as wasdiscussed in the last part, causes an increase in the super-exchangeinteractions, contributing to the increase of the internal field and theg-value and to the decrease of the resonance magnetic field [20].

In our magnesium ferrite nanoparticles, ΔHpp revealed irregularvariation with calcination temperature, as recorded in Table 2. Gen-erally, in ferrites, strong dipole interactions give a large ΔHpp, andstrong super-exchange interactions produce a small ΔHpp [29]. TheEPR line-shapes, Hr, g-values, and ΔHpp provide useful informationabout the microscopic magnetic interactions inside the system.Ordinarily, the variations of three principle parameters, i.e., Hr, g-value,and ΔHpp, in ferrites are caused by microscopic magnetic interactionsinside the materials, mainly the inter-particle, magnetic dipole–dipoleinteraction; intra-particle exchange interaction; and site symmetry[23,35]. These provide information about molecular motion, paramag-netic properties, and the symmetry of the ions [36].

5. Conclusions

Well-ordered MgFe2O4 nanoparticles with a cubic crystalstructure were fabricated successfully using a simple, thermal-treatment method. The influence of calcination temperature ondegree of crystallinity, morphology, microstructure, and phasecomposition was investigated by different characterization tech-niques, i.e., XRD, TEM, FESEM, and FT-IR, respectively. An increasein particle size from 5 to 8 nm was observed when the calcinationtemperature was increased from 673 to 973 K. PVA was utilized asa capping agent for stabilizing the particles, controlling the growthof the nanoparticles, preventing their agglomeration, and creatinga uniform distribution of particle sizes. EDXA was used tocharacterize the composition of the samples, and it confirmedthe presence of Mg, Fe, and O in the sample. Magnetic studieswere executed by VSM and ESR, which substantiated the

Superparamagnetic behavior and the unpaired electron spin,respectively. For the first time, we fabricated MgFe2O4 nanoparti-cles by a simple method that is cost-effective, uses relatively lowreaction temperatures, and is environmentally friendly in that itproduces no by-product effluents. The method can be extended tothe fabrication of other spinel ferrite nanoparticles of interest innanotechnology.

Acknowledgments

This work was supported by the Ministry of Higher Educationof Malaysia and Malayer University of Iran under the FRGS grantand Universiti Putra Malaysia under the RUGS grant.

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Table 2Magnetic parameters of MgFe2O4 nanoparticles observed by VSM and EPR techniques.

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(Oe)

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Fig. 8. EPR spectra of the samples calcined at (a) 773, (b) 873, (c) 773 and (d) 973 K.

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