Revista Mexicana de Fısica S58 (2) 167–170 DICIEMBRE 2012

Fe-doped NiO Nanoparticles: Synthesis, Characterization, and MagneticProperties

K.O. Mouraa, R.J.S. Limaa, C.B.R. Jesusa, J.G.S. Duqueb, and C.T. Menesesb

aDepartamento de Fısica, Universidade Federal de Sergipe,Campus prof. Jose Aluısio Campos, 49100-000, Sao Cristovao, SE, Brazil

cDepartamento de Fısica, Universidade Federal de Sergipe,Campus prof. Alberto Carvalho, 49500-000, Itabaiana, SE, Brazil

e-mail: ctmeneses@gmail.com

Recibido el 25 de junio de 2010; aceptado el 23 de abril de 2011

Pure and Fe-doped NiO nanoparticles (NP’s) with different particles sizes have been synthesized by the co-precipitaiton method. We haveobserved that the magnetic and structural properties change significantly with Fe doping. X-ray diffraction analyses combined with Rietveldrefinement have shown a decreasing in the particle size with the iron insertion. Besides, our results also shown that the average nanoparticlessize increase with increasing of synthesis temperature. In contrast with the pure NiO nanoparticles a low temperature peak is clearly observedin the zero-field-cooled magnetization curve of the Fe-doped NiO sample. We have associated it with the increasing of the surface anisotropydriving to the system to the freezing of the surface spins.

Keywords:Chemical synthesis; magnetic nanoparticle; superparamagnetism; NiO, surface anisotropy.

Nanopartıculas (NPs) NiO puras y dopadas con Fe con diferentes tamanos de partıculas fueron sintetizadas usando el metodo de co-precipitacion. Las propiedades estructurales y magneticas cambian significativamente con la inclusion del dopante Fe. Analisis de difraccionde rayos X se combina con el refinamiento Rietveld se ha demostrado una disminucion en lo tamano de las partıculas con la insercion delFe. Estos resultados tambien muestran que los aumentos de tamano medio de las partıculas con lo aumento de la temperatura de sıntesis. Unpicola baja temperatura en las curvas de magnetizacion zero-field-cooledaparece en la muestra dopada con Fe, en contraste con la muestrade NiO puro, en la que asignamos este comportamiento a anisotropıa de superficie causada por la congelacion de los spins frustrados ydesordenados sobre la superficie de la partıculas causada por los iones Fe.

Descriptores: Sıntesis quımica; nanopartıcula magnetica; superparamagnetismo; NiO; anisotropıa superficial.

PACS: 81.16.Be; 75.50.Tt; 75.20-g; 75.50.Ee; 75.30.Gw

1. Introduction

In the last two decades the nanoscience have been an excit-ing area of research as much of application point of view asof fundamental research. This because these materials ex-hibit significant changes in their mechanical, magnetic, elec-tric properties compared with its counterpart material in bulkform [1]. In the case magnetic nanoparticles (NP’s), the NiONP’s have recently been the subject of renewed attention forpresenting unusual features, mainly to NiO nanoparticles be-low 10 nm [2]. Some works reports that magnetic propertiesof such nanoparticles exhibit anomalous behavior due to fi-nite size effects [2,3]. This fact has been related to the largemoments originated from uncompensated surface spins thatfreeze into spin-glass state. Winkler et al [3] have shown thatlarge finite effect and surface effects in NiO nanoparticles of3 nm for interacting systems. On the other hand, Khadar etal have showed that NiO nanoparticles (3-5 nm) present asuperparamagnetic and superantiferromagnetic behavior forparticles between 13-18 nm [4].

To study these materials is necessary a good control ofthe size, size distribution and morphology of particles. Dueto this several methods have been employed to control theseproperties. Han et al suggest that microemulsions methodis an effective pathway to prepare ultrafine particles. How-

ever, this method is not efficient to control the size distri-bution [5]. In this sense, the thermal decomposition usingorganometallic precursor [6,7] has been successfully used tocontrol the particles size distribution. However, this methodgenerally uses high cost chemical reagents. So, the co-precipitation method appears as an intermediate method tosynthesize nanoparticles for different materials.

In this work we have used co-precipitation method to ob-tain pure and Fe-doped NiO nanoparticles in order to studythe size dependence on the structural and magnetic proper-ties using X-ray diffraction and magnetic measurements asfunction of magnetic field and temperature.

2. Experimental procedure

Pure and Fe-doped NiO nanoparticles were prepared by co-precipitation method following the procedures of Menesesetal [8]. The precursors were chemically obtained at room tem-perature by mixing a nickel nitrate Ni(NO3)2.6H2O aqueoussolution (and iron nitrate; Fe(NO3)2.3H2O, in the case fordoped samples) and an aqueous sodium hydroxide NaOH so-lution was used to control of pH∼ 13. The resulting gelswere washed and centrifuged several times until remove com-pletely the Na ions, and dried in air at 80◦C. Finally, theprecursors were synthesized at different temperatures for 3

168 K.O. MOURA, R.J.S. LIMA, C.B.R. JESUS, J.G.S. DUQUE, AND C.T. MENESES

hours. We labeled the samples as N1 and N2 to refer to theNiO samples synthesized at 350 and 450◦C, respectively, andF1 and F2 to refer to the 10 % Fe-doped NiO samples syn-thesized at 350◦C and 450◦C, respectively. To visualize themorphology and particle size distribution we have used a fieldemission gun scanning electron microscopy (Zeiss Supra 55VP FEG).

The crystalline structures of the samples were investi-gated by X-ray diffraction using a Rigaku powder diffrac-tometer with Bragg-Brentano geometry modeθ-2θ in therange of 30-70 ˚ (CuKα operated at 40 kV, 40 mA). Rietveldrefinement was carried out by DBWS9807 with interface db-wstools using modified pseudo-voigt function as profile func-tion [9]. In these analyses were extracted information on thefull width at half maximum (FWHM) for{1 1 1}, {0 0 2},{0 2 2} crystallographic families to be used to estimate thecrystallite size calculated by Williamson-Hall equation [10].Magnetic measurements as function of the field and temper-ature were carried out using a SQUID magnetometer (Quan-tum Design MPMSevercoolsystem).

3. Result and discussion

Figure 1 shows the Rietveld refinement to X-ray diffractionpatterns for all samples studied in this work. We have identi-

FIGURE 1. Rietveld refinement and X-ray diffraction patterns tosamples pure and Fe-doped NiO nanoparticles synthesized at 350 e450◦C.

fied one identical structure to the cubic NiO without the pres-ence of impurity phases even to Fe-doped samples. However,it is notable the dependence of the synthesis temperature toboth systems in the process of growth of the particles consid-ering the broadening of Bragg peak. We have estimated thecrystallite size for N1 and N2 samples as 16(2) nm and 38 (5)nm, respectively. To the case of Fe-doped samples, F1 andF2, the sizes were 9(2) nm and 23(4) nm, respectively. Theseresults show us that the Fe-doped samples present a decreas-ing in the crystallite size when compared with the pure NiOsample prepared under the same synthesis conditions. Dif-ferently of crystallite size, the particle microstrain increasewith the Fe insertion and with decreasing of the particle size.We associate these modifications to the small difference be-tween the ionic radii of Ni2+ (0.69 A) and Fe3+ (0.64 A),which provoke a disorder in the crystalline structure. Thisfact can also be associated with changing in the lattice param-eter. We have verified a decreasing in the lattice parameter asfunction of the iron insertion, which is enhanced for increas-ing the synthesis temperature. Unlike the results reported byMalik et al [1] all XRD patterns showed in Fig. 1 have thesame structure of cubic NiO without the presence of spuriousphases.

FIGURE 2. SEM images for samples synthesized at 450o (a) NiOand (b) Fe-doped NiO nanoparticles, 100 nm bars.

Rev. Mex. Fis. S58 (2) (2012) 167–170

FE-DOPED NiO NANOPARTICLES: SYNTHESIS, CHARACTERIZATION, AND MAGNETIC PROPERTIES 169

Figure 2 shows SEM micrographs for samples synthe-sized at 450◦C. These results reveal that pure sample consistof NP’s with non-homogeneous size and shape with averagesize around 40 (9) nm. On the other hand, the Fe-doped NiOsamples present crystallite size nearly uniform tending to ananorod-like shape with average size around 12x60 nm2. In arough approximation, the SEM results are in good accordingwith from results for average size estimated by X-ray diffrac-tion. Finally, we can state that the iron doping produces achange in the sample shape and, as compared with pure NiOsample, a more disperse system is obtained.

Figure 3 shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measured under applied field of100 Oe for the pure and Fe-doped NiO samples synthesizedat 350o. It is evident that the ZFC-FC curves for both sam-ples exhibits a typical superparamagnetic (SPM) behavior inthe high temperature regime and a blocked state in the lowtemperature regime below around 145±3 K (Fig. 3b) to theFe-doped NiO sample and 155±3 K (Fig. 3a) to the puresample. This small shift can be related to the average sizeof the particles. In addition, the curves also indicate a widedistribution of energies barriers that can be associated to theparticles sizes distribution for both systems. However, theFe-doped sample presents irreversibility temperature lowerthan the pure sample indicating a change in the particle sizedistribution due the iron insertion.

Interestingly, the ZFC measurement shown in the Fig. 3bto Fe-doped sample display an additional peak atT = 11 K.

FIGURE 3. ZFC-FC magnetization curves measured atH = 100Oe to (a) NiO and (b) Fe-doped NiO nanoparticles synthesized at350◦C.

FIGURE 4. Hysteresis curves measured at 5 K and 300 K and takenin ZFC mode for samples obtained at 350◦C.

This kind of behavior was also observed previously forboth crystalline as well as amorphous nanoparticles sys-tems [3,12,13]. This unusual magnetic behavior have beeninterpreted as an effect of the particle finite sizes, that is, thebreaking of a large number of exchange bonds which givesrise at surface particles drives the spins to a strongly frus-trated state. So, the enhancement of low temperature maxi-mum observed in the Fe-doped sample can be associated tothe presence of small magnetic clusters on the particle sur-face. This reinforces our idea that such behavior only ap-pears to the particles system with reduced size and there islattice imperfection (microstrain) increasing the effect of theuncompensated magnetic sublattice [1].

Figure 4 shows theM − H curves recorded at tempera-tures 2 K and 300 K for samples (pure and Fe-doped) synthe-sized at 350◦C. These results confirm that these particles dis-play a ferromagnetic-like behavior at low temperature whichis increased for Fe-doped sample. It is also evident that, toFe-doped sample, the hysteresis curves measured close atlow temperature, the coercive field (HC) and remanent mag-netization (MR) are slowly narrowed. In fact, these resultsare well established and corroborated to the uncompensatedspins systems on the surface of the nanoparticles.

4. Conclusions

In conclusion, XRD analysis confirm that single phases of theNiO and Fe-doped NiO nanoparticles have been successfullysynthesized using co-precipitation method. Both results ofXRD and SEM show that average particles size decrease andthere is a change in particles morphology as function of Feinsertion. Magnetization results measured under ZFC con-dition display a broad blocking temperature with maximumaround 145 K and 155 K for samples Fe-doped NiO and pure,respectively, in agreement with our XRD analysis of the par-ticles size of samples. The irreversibility temperature, thatis, the splitting of the ZFC and FC curves was 280 K for

Rev. Mex. Fis. S58 (2) (2012) 167–170

170 K.O. MOURA, R.J.S. LIMA, C.B.R. JESUS, J.G.S. DUQUE, AND C.T. MENESES

Fe-doped and up to 400 K to the pure NiO sample indicatinglikely the change in the size distribution. At low temperaturesregime, we have observed the appearing of a peak which is re-inforced to the Fe-doped sample. This result associated withmagnetization results as function of magnetic field taken atT = 2 K can be understood taking account the increasing ofthe Fe ions on the particle surface.

Acknowledgements

The authors are grateful for access to the facilitiesLME/LNLS for the SEM micrographs (proposal FEG-8888).This research was supported by CNPq funding agency(Project 577512/2008-0, 477114/2008-3) and FAPITEC.

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