Journal of Magnetism and Magnetic Materials 452 (2018) 169–178

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Journal of Magnetism and Magnetic Materials

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Research articles

Impact of Gd3+/graphene substitution on the physical propertiesof magnesium ferrite nanocomposites

https://doi.org/10.1016/j.jmmm.2017.12.0530304-8853/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: atawfik@sci.cu.edu.eg (A.T. Mohamed).

Ebtesam E. Ateia, Amira T. Mohamed ⇑, Kareem ElsayedPhysics Department, Faculty of Science, Cairo University, Giza, Egypt

a r t i c l e i n f o

Article history:Received 5 October 2017Received in revised form 10 December 2017Accepted 15 December 2017Available online 20 December 2017

Keywords:NanoparticlesMagnesium ferriteRare earth ionsGrapheneCoercivityEntropy change

a b s t r a c t

Magnesium nano ferrite with composition MgFe2O4, MgGd0.05Fe1.95O4 and MgFe2O4 – 5 wt% GO wassynthesized using a citrate auto-combustion method. The crystal structure, morphology, and magneticproperties of the investigated samples were studied. High Resolution Transmission ElectronMicroscopy (HRTEM) images show that the substitution of small amounts of Gd3+/GO causes a consider-able reduction of the grain size. Studies on the magnetic properties demonstrate that the coercivity ofGO-substituted magnesium nano ferrites is enhanced from 72 Oe to 203 Oe and the magnetocrystallineanisotropy constant increases from 1171 to 3425 emu Oe/gm at 300 K. The direct effects of graphene onmorphology, crystal structure as well as the magnetic properties reveal that the studied sample are suit-able for turbidity color and removal. The magnetic entropy change is estimated from magnetization datausing Maxwell relation. The calculated Curie temperature from the Curie-Weiss law and the maximumentropy change are in good agreement with each other. Based on UV diffuse reflectance spectroscopystudies, the optical band gaps are in the range of 1.4–2.15 eV. In addition, the combination of smallparticle size and good magnetic properties makes the investigated samples act as a potential candidatesfor superior catalysts, adsorbents, and electromagnetic wave absorbers.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Graphene nanoparticles an attractive carbon material, ispromising for improving adsorption capacity, high chemical stabil-ity, good electrical properties, strong mechanical stability, andlarge specific surface area [1–3]. Therefore it has attracted theattention of scientists and researchers due to its useful applicationsin various fields [4]. Recently, combining of graphene with othernanoferrites were reported to develop novel nanocomposites.

Graphene-based materials with low aggregation and high speci-fic surface areas show high adsorption capacities for organic pollu-tants [5–7]. Therefore, it is important to prevent aggregationbetween the layers. For convenient separation, magnetic particlesare introduced in the adsorbent to form magnetic graphene com-posites. The added magnetic particles also play an important rolein preventing aggregation of the graphene sheets.

The rare earth ions are the promising substitute for theenhancement of the properties of nano ferrite samples. Recently,Manikandan et al. reported rare earth element (REE) lanthanum

doped zinc oxide (La: ZnO) nanomaterials and studied their struc-tural optical and antibacterial studies [8].

The addition of rare earth ions like Tb, Gd and La change thephysical properties of the samples. [9]. Many workers [10–12]investigated the magnesium based ferrites for numerous applica-tions. But the reports on rare earth substituted magnetic propertiesin Mg-Gd ferrite system are rather rare.

The aim of the present study is to synthesize and characterizeMgFe2O4, MgGd0.05Fe1.95O4, and graphene-based MgFe2O4 nanocomposites using citrate auto combustion method. We manufac-tured the graphene oxide (GO) sheets by adopting a modifiedHummer’s method.

2. Experimental

Nanoparticles of doped magnesium ferrite was prepared usingcitrate auto combustion method [13]. In this method, the stoichio-metric quantities of Fe(NO3)3�9H2O, Mg(NO3)2�6H2O and Gd(NO3)3�6H2O were dissolved in double-distilled water and stirred wellusing a magnetic stirrer for about 1 h at 80 �C, followed by dryingat 200 �C. The structure and crystallite sizes were tested by X-raydiffractometer (XRD) using Diano corporation of target Cu-Ka(k = 1.5418 A�). The average nanoparticle sizes were determined

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using Scherrer’s relationship [14]. The morphology of the sampleswas studied by High Resolution Transmission Electron Microscopy(HRTEM) attached with EDAX unit (energy dispersive X-ray analy-ses). The synthesized powder of Mg, Mg/Gd and Mg/Go sampleswere calcined at 600 for 4 h with heating rate of 4 �C/min. Themagnetization (emu/g) was measured at room temperature (300K) and low temperature (100 K) using a vibrating sample magne-tometer (VSM) Model Lake Shore 7410. UV-Visible-NIR spectrom-eter is an effective method used for characterization of opticalband-gap energy of materials. A V-570 Spectrophotometer fromJasco Co., Japan (integrating sphere reflectance unit), was usedfor the present investigation.

2.1. Preparation of MgFe2O4/GO

A stoichiometric amount of magnesium (III) nitrate [Mg (NO3)2-�6H2O] and iron (III) nitrate [Fe (NO3)3�9H2O] were dissolved in 50ml of double distilled water under magnetic stirring. Then 5 wt% ofGO was added to above solution, followed by ultrasonic treatmentfor about 30 min until GO was well dispersed in the solution. Themolar amount of citric acid added was equal to the total molaramount of metal nitrate in the solution. The solution was continu-ously stirred for 1 h at room temperature. Ammonia solution wasslowly added to adjust the pH and also to stabilize the nitrate-citrate solution. The obtained precipitate was collected and fol-lowed by calcination at 600 �C for 4 h to obtain MgFe2O4/GOpowder.

3. Results and discussion

Fig. 1(a and b) illustrates the X-ray diffraction pattern forMgFe2O4, MgGd0.05Fe1.95O4 and MgFe2O4 – 5 wt% GO nano ferritesamples. The diffraction peaks correspond to spinel lattice with acubic structure for the investigated samples. No extra reflectionpeaks are detected in the X-ray diffraction patterns corresponding

Fig. 1. (a and b): The X-ray diffraction patterns for (a) MgFe2O4, MgGd0.

to any impurity phase. Their structure fully matches with powderdiffraction file JCPDS (01-088-1938).

Generally, the major peak in graphene oxide is due to the (0 0 1)reflection at 10.57�, which shows the complete oxidation and exfo-liation of flake graphite precursor [15,16]. However, there is nopeak of graphene oxide in the synthesized composite samples(Fig. 1b). The absence of the characteristic peak (0 0 1) associatedwith graphene oxide in Mg/GO sample shows the complete reduc-tion under condition mentioned in experimental synthesis. Actu-ally, the absence of the characteristic peaks of graphene is alsodue to the intrinsic low crystalline structure of graphene. This rat-ifies that the substituted GO is completely dissolved in the magne-sium nano-ferrite structure.

A slight increase in the peak broadens with the addition of Gd/GO is observed. This is identified with the crystallite refinementand lattice micro strain decrement. The theoretical lattice parame-ters for pure and doped samples are calculated as mentionedbefore [17]. The theoretical lattice parameter, experimental latticeparameter, crystallite size, X-ray density, strain and tolerance fac-tor are summarized in Table 1.

However the substitution of Gd ions induces structural distor-tion due to its large size and yields micro-strain, utilizing theWilliamson-Hall equation [18]. The tolerance factor, T, for pureMg and Mg/Gd samples are calculated according to Rodericket al. [19]. It is found that for the studied samples, value of T isclose to unity signifying defect-free formation of spinel structure.

As shown from the table the Gd3+/GO substitution decrease thelattice parameters. The decrease of the lattice constant supportsthe incorporation of Gd3+/GO into the magnesium ferrite lattice.

The crystallite size is detected from XRD broadening of (3 1 1)peak using Scherrer’s formula [14]. The crystallite size of the puremagnesium nano ferrite decreases with the substitution of Gd3+/GO oxide and the calculated data is shown in the Table. The growthof the MgFe2O4 is restricted by the substitution of Gd3+/GO leadingto a relatively small crystallite size compared with parent magne-

05Fe1.95O4 and G- MgFe2O4 and (b) grapheme oxide and G-MgFe2O4.

Table 1The experimental lattice parameter (aexp), Theoretical lattice parameter (atheo), Crystallite size (D), X-ray density, Strain, and Tolerance factor for the investigated samples.

Sample aexp (oA) atheo (oA) D (nm) X-ray density (gm/cm3) Strain Tolerance factor

Mg Fe2O4 8.397 8.381 27.6 4.488 0.0043 1.0Mg Fe1.95Gd0.05O4 8.394 8.400 15.4 5.077 0.0077 0.99GO-MgFe2O4 8.373 – 12.7 – 0.0093 –

Fig. 2. (a–f): (a, b and d) HRTEM micrograph with the histogram for MgFe2O4, MgGd0.05Fe1.95O4 and MgFe2O4-graphene respectively, (c and e) the d-spacing for MgGd0.05-Fe1.95O4 and MgFe2O4-graphene respectively and (f) is the HRTEM for graphene oxide.

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sium nano ferrite. The bond energy of Gd3+AO2�, is higher com-pared to Fe3+AO2�. This means that more energy is needed toincorporate Gd3+ ions into the octahedral sites [20]. In this process

the required energy is supplied at the expense of crystallizationand therefore hinders the growth of the crystallites and a smallercrystallite size is obtained for the Gd3+ doped samples.

Fig. 2 (continued)

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Fig. 2(a–f) shows the HRTEM images for the investigated sam-ples. It is clear that the MgFe2O4 sample shows sphere-like mor-phology with uniform size distribution [21]. The lattice fringeswith interplanar distances of 2.519 nm correspond to the (3 1 1)crystal plane of MgFe2O4. Moreover the doping of Gd3+ ionsdecreases the size of the grains. A clear difference is observedbetween the un-doped and doped samples. This result is in goodagreement with the size reduction detected by XRD analysis.

On the other hand, MgFe2O4 nanoparticles are adsorbed regu-larly on the paper- like GO, which can inhibit the stack of GO layers

(Fig. 2d). Hence there is no possibility of interaction betweenMgFe2O4 and GO. This reveals the same interlayer spacing, indicat-ing that no change occurred in the lattice structure of MgFe2O4

after addition of GO. In addition some wrinkles and folds areappeared on the graphene sheets which cause the intensificationof the contact area with Mg nanoferrite phases [22].

The selected-area electron diffraction (SAED) pattern is shownin the inset of Fig. 2(b and d). It is clear that the image exhibitsbright dots arrangement which ratify the feature of nano poly crys-talline structure of the investigated samples. The size distribution

(a)

(b)

(c)

Fig. 3. (a–d): EDAX for (a) MgFe2O4, (b) MgGd0.05Fe1.95O4 (c) MgFe2O4-graphene and (d) grapheme oxide. The inset table gives the quantitative estimation of elementsobtained directly from the spectrum through its weight and atomic percentages.

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of the studied samples is shown in Fig. 2(a, b and d) and the aver-age particle sizes are in the range of 18–37 nm.

Fig. 3(a–d) shows the energy-dispersive X- ray spectroscopy(EDAX) analyses for the parent and Gd/GO doped samples. The

Fig. 4. (a–c): The hysteresis loops for (a) MgFe2O4, (b) for MgGd0.05Fe1.95O4 and (c)MgFe2O4-graphene.

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characteristic peaks in the spectrum comprise Mg, Fe, Gd, C and O.The atomic percentage (at%) and weight percentage (wt%) of con-stituent elements, are calculated theoretically from the given for-mula MgFe2O4, Mg Gd0.05Fe1.95O4 and MgFe2O4 – 5 wt% GO. Theobtained data from EDAX elemental analysis is shown as inset ofthe figure.

As shown from Fig. 3d carbon and oxygen are presented in thepure graphene oxide sample, which confirm the purity of the usedmaterial. Fascinatingly the EDAX data shows carbon peaks that arenot noticeable in the XRD chart (Fig. 1).

Fig. 4(a and b) shows the hysteresis loops of the investigatedsamples using a vibrating sample magnetometer (VSM) at (300and 100 K). From the figure, it is clear that the magnetizationincreases with the applied magnetic field until reaching saturationtrend. From the hysteresis loops, the coercivity (HC), saturationmagnetization (MS), remnant magnetization (Mr), squareness(Mr/Ms) andmagnetic moment (nB) are calculated and summarizedin Table 2. Generally, the magnetic moment nB is calculated fromthe saturation magnetization (MS) value at room temperature asmentioned in the previous work [23]. The saturation magnetiza-tion is expressed by means of the following relation [24]:

nB ¼ Mw �MS=5585 ð1Þ

where Mw is the molecular weight of a particular ferritecomposition.

It is clear from the table that the magnesium upon the additionof 5 wt% of graphene demonstrate an increase in magnetic satura-tion (Ms) as compared with the MgFe2O4 and MgGd0.05Fe1.95O4

samples. The improvements in magnetic parameters in the pres-ence of rare earth ions (Gd3+) and graphene (GO) can be influencedby numerous parameters (extrinsic and intrinsic). The extrinsicparameters depend on the crystallite size, morphology, and densityof the nanoparticles [25]. While the intrinsic parameters, such asthe relative distribution of metals cations at the tetrahedral andoctahedral sites [26]. However, the incorporation of graphene inthe MgFe2O4 nanocomposites increases the tetrahedral and octa-hedral distribution of the metal cations. Also it changes the surfacemorphology, and the crystal strains [27].

On the other side, the ionic radius of Gd3+ (0.938 �A) ions is lar-ger than that of Fe3+ (0.645 �A) ions in B site. This leads to a reduc-tion in the symmetry of the system and hence generates aninternal stress inside the material [28,29]. Moreover, the grainboundary between domains increases due to small crystallite sizeof Mg/Gd3+ samples. Therefore, the area of disordered arrangementfor ions on the grain boundaries may fix and hinder the domainwalls motion, leading to an increase in the coercivity of the sam-ples [29]. However the variation of the coercive field (Hc) is directlydepends on the surface spin disorder [30].

The increase in Hc for the graphene-basedmagnesiumnanocom-posites confirms that GO increases the surface spin disorder.

Finally, in this study, the Mg/GO ferrite nanoparticles and theMg Gd0.05Fe1.95O4 nano ferrite samples have shown higher mag-netic saturation levels as compared to the parent sample. Thiscan be attributed to better morphology and the increase of thedomain wall migration of the MgFe2O4/graphene nanocompositesand MgGd0.05Fe1.95O4 [31,32]. Furthermore, it is observed thatthe coercivity of the Mg/GO is enhanced by 2.8 times relative tothe parent and Gd doped samples.

However, the low temperature has a great effect on the mag-netic properties of the studied samples. The observed increase inmagnetization between 100 K and 300 K can be explained on thebasis of competition between the thermal and anisotropy energiesduring the magnetization process. In general the thermal energyacts against the work done by the magnetic energy in aligningthe moments. This cause a decrease in magnetization of the sys-tem. At low temperature, the effect of thermal energy is reduced.

The increase in coercivity at low temperature is due to increasein magnetocrystalline anisotropy [33]. At high temperature, thethermal energy is more effective in reducing the effects of magne-tocrystalline anisotropy energy. As shown from the table the valueof anisotropy constant for Gd sample (K = 6135 emu. Oe/g) at 100 Kis almost 6 times greater than the value at 300 K. This means that,this sample will be a guaranteeing hopeful for technological appli-cations at or below room temperature.

The variation of the molar magnetic susceptibility (vM) withabsolute temperature as a function of the magnetic field intensity(H = 1010, 1340, and 1660 Oe) for the studied samples is shown inFig. 5(a and b). Reasonable decrease in (vM)with increasing temper-ature is detected until a definite temperature (Tc) after which itreaches to its slightest value. Inset of the graph shows dvM/dT fromwhich one can determine the Curie temperature of the investigatedsamples. In the high temperature region the inverse susceptibility 1/v (=H/M) follows the Curie-Weiss law [34]. The Curie temperature(Tc), Weiss constant (h), and the effectivemagnetic moment are cal-culated from the relation between the reciprocal molar magneticsusceptibility 1/v and the absolute temperature (not present here).The obtained data is recorded in Table 3.

Table 3The Curie temperature (Tc) from Curie Weiss law and from maximum entropy change, Weiss constant (h), and the effective magnetic moment meff.

Sample Magnetic field (Oe) Tc (K) From Curie-Weiss Tc (K) at maximum entropy H (K) meff (BM)

MgFe2O4 H = 5000 Oe 625 620 620 1.1

Mg/Gd H = 1010 642 644 630 2.889H = 1340 642 644 633 2.854H = 1660 642 644 635 2.873

Mg/GO H = 1010 718 725 710 2.493H = 1340 720 725 710 3.239H = 1660 722 725 718 3.318

Fig. 5. (a and b): Magnetic susceptibility versus temperature for (a) MgGd0.05Fe1.95O4 and (b) MgFe2O4-graphene. The inset shows the 1st derivative of magnetic susceptibilitywith temperature.

Table 2Saturation magnetization (Ms), Remnant magnetization (Mr), Coercive field (Hc), Energy loss, Squareness (Mr/Ms), Anisotropy constant (K), and magnetic moment (nB) for theinvestigated samples.

Sample Ms (emu/g) Mr (emu/g) Hc (Oe) Energy loss (erg/g) Mr/MS K (emu. Oe /g) nB (exp)

MgFe2O4 15.71 1.67 72 4272 0.11 1171 0.562MgGd0.05Fe1.95O4 300 K 15.28 1.85 64 2846 0.12 1011 0.56

100 K 23.61 4.35 249 11,241 0.18 6135 0.87

G-MgFe2O4 300 K 16.18 1.98 203 7184 0.12 3425 0.58100 K 16.25 2.60 240 9756 0.16 4062 0.58

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The increase in the Curie temperature for Mg/GO nano ferritesample can be explained on the basis of A–B exchange interac-tions. The decrease in the grain size and increase of A–B super

exchange interaction causes spin arrangement that increasesthe magnetic characteristics of the present samples as shownin the table.

Fig. 6. (a and b): The change of entropy with temperature at different magnetic fields for (a) MgGd0.05Fe1.95O4 and (b) MgFe2O4-graphene.

Fig. 7. (a–d): (a) The UV-visible DRS of the investigated samples recorded in the wavelength range from 300 to 2000 nm and (b–d) The energy gap from the plot of (F(R1)hm)2 versus (hm) for the investigated sample.

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Table 4Values of the band gap energy for the investigated samples.

Sample Energy gap (eV)

Direct transition Indirect transition

MgFe2O4 2.09 1.55MgGd0.05Fe1.95O4 2.11 1.5MgFe2O4-G 2.15 1.4

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Estimating the magnetic entropy change DSM (T, H) from mag-netization data is commonly done by the use of the following Max-well relation [35]:

DSM ¼Z Hf

H1

@M@T

dH ð2Þ

Which directly implies that the maximum disorder of the givensystem (max. DS) is obtained at the Curie temperature where theferromagnetic-paramagnetic phase transition takes place. Fig. 6(aand b) illustrates the magnetic entropy change as a function oftemperature for the investigated sample. The calculated Tc fromthe Curie-Weiss law is agree well with Tc obtained from the max-imum entropy change as shown in Table 3.

The photo reflection behavior of the synthesized MgFe2O4,MgGd0.05Fe1.95O4 and GO-MgFe2O4 are investigated by UV-Visspectroscopy (Fig. 7a). All the samples have strong reflection peakin the range 400–800 nm. This peak can be attributed to the elec-tron excitation from the O-2p level into the Fe 3d level in spinel-type compounds [36]. Furthermore, the broadening reflection peakdetected at 800 nm is attributed to Mg2+ ions [37].

The band gap energy Eg as a function of wavelength is obtainedby extrapolating the linear part of the Tauc’s plot to (hvF(R))n

against hv as shown in Fig. 7(b–d) and according to the followingequation [38].

ðhmFðR1ÞÞn ¼ Aðhm� EgÞ ð3Þwhere h is the Plank’s constant, t is the frequency of vibration, A is aconstant and Eg is the band gap. Exponent n depends on the type oftransition and n = 1/2 or 2 for indirect and direct allowed transitionsrespectively. The band gap values at different conditions arerecorded in Table 4.

The band gap energy of MgFe2O4-G sample is higher than theobtained values for MgGd0.05Fe1.95O4 and MgFe2O4 this can beattributed to the effect of grain size as reported by Manikandanet al. [39].

Finally, reflectance in the utilized wavelength range indicatesthat the G-MgFe2O4 sample is suitable for turbidity color andremoval. In addition to this, the synthesized nanoparticles alsoexhibit good magnetic properties.

4. Conclusion

1. Rare earth gadolinium doped magnesium ferrite nano-particles(MgGd0.05Fe1.95O4) and MgFe2O4 – 5 wt% GO nanocompositesare successfully prepared by citrate auto combustion method.

2. The impact of the substitution of graphene and Gd3+ ions on thestructural and magnetic properties is systematically studied.The XRD chart reveals a mono-phasic cubic spinel structureand good crystalline nature of the prepared samples.

3. The HRTEM data indicates the distribution of the MgFe2O4 fer-rite constituent elements across the graphene surface.

4. The graphene-based MgFe2O4 magnetic nanocompositesdemonstrated an improvement in its physical properties. Theseimprovements of Mg/GO nano composites provide the basis fora broad range of applications.

5. The coercivity of GO-substituted magnesium nano ferrites isalmost 2.8 times greater than coercivity of parent sample.

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