Contents lists available at SciVerse ScienceDirect

Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 16 (2013) 1804–1807

1369-80http://d

n CorrE-m

afranco

journal homepage: www.elsevier.com/locate/mssp

Room temperature ferromagnetism in combustion reactionprepared iron-doped zinc oxide nanoparticles

A. Franco Jra,n, T.E.P. Alves b,c

a Instituto de Física, Universidade Federal de Goiás, 74001-970 Goiânia-GO, Brazilb Instituto Federal de Goiás, 75131-500 Anápolis-GO, Brazilc Instituto de Química, Universidade Federal de Goiás, Goiânia-GO, Brazil

a r t i c l e i n f o

Available online 27 July 2013

Keywords:Zinc oxideDMSRoom temperature ferromagnetismCombustion reaction

01/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2013.07.008

esponding author. Tel.: +55 62 3521 1014.ail addresses: franco@if.ufg.br,junior@gmail.com (A. Franco Jr).

a b s t r a c t

We present a study of the structural and magnetic properties of Zn1�xFexO (0.00≤x≤0.09)nanoparticles prepared by a combustion reaction method. X-ray diffraction patterns of allsamples showed sharp and intensive peaks of hexagonal wurtzite structure of ZnOwithout any evidence of spurious crystalline phases in all samples. The magneticproperties were measured by using a vibrating sample magnetometer. Hysteresis loopindicated that all samples are ferromagnetic at room temperature and the Curietemperature increased with increasing iron content. Room temperature ferromagnetismcan be explained in terms of intrinsic properties and the presence of defects in Fe- dopedZnO nanoparticles.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In the last decades, diluted magnetic semiconducting(DMS) have gained a subject of intense study due to theirunique combination of both charge and spin degrees offreedom. Zinc oxide (ZnO) base DMS materials, in parti-cular, have been extensively studied as they hold greatpotential for applications in magneto-electronics, spin-electronics and microwave devices due to their unusualproperties such as direct wide band gap (∼3.37 eV) andhigh exciton binding energy (∼60 meV) [1–5]. ZnO crystal-lizes in a stable hexagonal wurtzite structure with latticeparameter a¼3.2495 Å and c¼5.2069 Å; Zn atoms aretetrahedrally coordinated with four O atoms, where theZn d-electrons hybridize with the O p-electrons.

Regardless of theoretical models [6] that predicted thatroom-temperature ferromagnetism (RTFM) could be foundin doped ZnO materials, the origin of RTFM in Fe-doped ZnOis still quite controversial and depends on the history of

All rights reserved.

sample preparation [7–11]. For instance, in doped Fe–ZnOprepared by mechanical alloying, RTFM is intrinsic property[7,8] while samples prepared by the hydrothermal methodRTFM is a result of exchange interaction between local spin-polarized and conductive electrons (such as the electrons ofFe3+ ions) according to the Ruderman–Kittel–Kasuya–Yosida(RKKY) theory. Furthermore, RTFM in doped Fe–ZnO nano-crystals synthesized through a sol–gel method [9], is anintrinsic property and the origin of the ferromagnetism innot due to metallic iron or iron oxides, but given by exchangeand Zn vacancies in samples. Also RTFM has been associatedto defects introduced during sample preparation or to thepresence of spurious phases [10]. Samples prepared by asolid-reaction technique were reported to show room ferro-magnetism [11] while a spin-glass phase was found insamples prepared by a low temperature chemical pyrophoricreaction technique. Thus, the origin of room temperatureferromagnetism in doped ZnO compounds seems to beeither an intrinsic or extrinsic property of the material;which in turn depends on the method used to synthesizethe doped ZnO compounds.

As an alternative, the combustion reaction method isa simple, fast, and inexpensive approach to synthesizing

A. Franco Jr, T.E.P. Alves / Materials Science in Semiconductor Processing 16 (2013) 1804–1807 1805

Fe-doped ZnO nanoparticulate powders since it does notinvolve intermediate decomposition and/or calcining steps[12–15]. In addition, it is easy to control the stoichiometriccomposition and crystallite size, which have importanteffects on the magnetic properties of the compound. Apeculiar characteristic of this method is that the heatrequired to sustain the chemical reaction is provided bythe reaction itself and not by an external source. Theresulting product, is a dry crystalline powders agglomer-ated into very fragile foam with high chemical homoge-neity and purity.

Therefore, in the present work, we investigated indetail the magnetic properties such as saturation magne-tization (Ms), coercivity (Hc), remanent magnetization (Mr)and the Curie temperature (TC) of nanoparticulate powdersof Zn1�xFexO (x¼0, 0.01, 0.03, 0.05, 0.07 and 0.09) preparedby a modified combustion reaction method.

Fig. 1. XRD of powders of Zn1�xFexO (0≤x≤0.09) prepared by combustionreaction.

2. Experimental procedure

Nanoparticles of Zn1�xFexO with nominal compositionx¼0, 0.01, 0.03, 0.05, 0.07 and 0.09 were synthesized bythe combustion reaction method using analytical gradeiron nitrate (Fe(NO3)3 �9H2O), zinc nitrate (Zn(NO3)2 �6H2O), and urea (CO(NH2)2) as fuel. All reagents weremanufactured by Across, New Jersey, USA; manipulationsand reactions were carried out in air without the protec-tion of nitrogen or inert gas inside a fume-cupboard. Thestoichiometric composition of each mixture was calculatedsuch that the total oxidizing and reducing valences of theoxidizer and fuel were balanced and the release of energywas maximized for each reaction. According to the ther-mochemical concepts from propellant chemistry [13,14].The elements H, C, Fe and Zn are considered reducingelements with the corresponding valences +1, +4, +3 and+2, respectively. The element oxygen is considered as anoxidizing element with the valence –2. The valence of theelement nitrogen is considered to be zero. Thus, forinstance, the total valences in Fe(NO3)3.9H2O add up to –

15 and in Zn(NO3)2 6H2O add up to �10 and should bebalanced by the total valences in the fuel, which add up to+6. Hence the stoichiometric composition of the redoxmixture in order to release the maximum energy for thereaction to synthesize Zn1�xFexO, requires that 5x+10¼6nor n¼1.67 mol of urea. For example, to prepare Zn0.91

Fe0.09O, the reactants should be combined in a molarproportion of 0.91:0.09:1.74 of Zn(NO3)2.6H2O, Fe(NO3)3.9H2O, and urea CO(NH2)2, respectively. All the reactantswere hand mixed in a wide-mouth vitreous silica basin of300 cm3 and heated up to around 400 1C on a hot blanketinside a fume-cupboard under ventilation. With the rise intemperature, melting occurred and an opaque liquid wasproduced that boiled and thickened. Soon after the thick-ened liquid began frothing, ignition took place character-ized by a rapid increase in temperature; the appearance ofa central point of incandescence that propagates in swiftripples to walls of the basin, and released a large quantityof gases. The reactions lasted a few seconds and produceddry, very fragile foam that transformed into powder at theslightest touch.

The as-prepared powders were characterized by X-raydiffraction using a Shimadzu diffractometer (model 6000)with CuKα (λ¼1.5418 Å) in a wide range of Bragg angle(201≤2θ≤1001) with a scanning rate of 21/min at roomtemperature. The average crystallite size was calculatedfrom X-ray line broadening of the (101) diffraction peakusing Scherrer′s equation [16],

Phkl ¼ 0:89λ=β1=2 cos θhkl ð1Þ

where Phkl is the average crystallite diameter of theparticle assumed to be spherical is λ the radiation wave-length of the X-ray used, θhkl the diffraction peak angle andβ1/2 is the corrected line width at observed half-peakintensity. The value of β1/2 was determined from theexperimental integral peak width by applying Warren′scorrection for instrumental broadening [17]; i.e. β1/2¼[(β1/2)2�(βo)2]1/2, where βo is the instrumental broadeningdetermined through the full width at the half maximum ofthe reflection (FWHM) at 14.21 of SiO2 powder havingparticles larger than 1000 Å and β is the FWHM of the(101) diffraction peak of the sample. Cell parameters werecalculated and further refined using regression procedures(Cell, Powder X in Ref. [18]).

The magnetic properties measurements were per-formed using a vibrating sample magnetometer (ADEMagnetics, model EV-9, Westwood, MA. USA), with hightemperature furnace, controlled under computer control.The experiments were performed over a broad range oftemperature varying from room temperature to 750 K inmagnetic fields up to 2.0 kOe.

3. Results and discussion

X-ray powder diffraction (XRD) patterns of all samplesshowed sharp and intensive peaks of hexagonal wurtzitestructure of ZnO (JCPDS # 36-1451) without any evidenceof spurious crystalline phases in all samples, except forx¼0.07 and 0.09; exhibit small peaks corresponding toZnFe2O4 (JCPD # 82–1042) and Fe3O4 (JCPD # 89–2355), asshown in Fig. 1. This spurious phases have been alsodetected by Liu et al. [9] for samples of x¼0.10 but

Fig. 2. TEM image of the Zn0.93Fe0.07O nanoparticulate powders showingrough and irregular spherical morphology.

Fig. 3. Typical hysteresis loops of the Zn1�xFexO (0≤x≤0.09) nanoparti-culate powders, obtained at room temperature. The inset shows thedetails of hysteresis curves; typical of ferromagnetic materials.

Fig. 4. The temperature dependence of magnetization of Zn1�xFexO(0≤x≤0.09) nanoparticulate powders for H¼200 Oe.

Fig. 5. The temperature dependence of the inverse of susceptibility (1/χ)of Zn1�xFexO (0≤x≤0.09) nanoparticulate powders.

A. Franco Jr, T.E.P. Alves / Materials Science in Semiconductor Processing 16 (2013) 1804–18071806

synthesized by the different method. The average crystal-lite size determined from the prominent (101) peak of thediffraction using Scherrer′s equation was ∼32 nm for alldoped Fe–ZnO samples with lattice parameters ratio c/a(1.60270.003). Transmission electronic microscopy (TEM)micrograph of all samples shows that powders consist ofagglomerated of ∼46 nm nanoparticles of rough, irregularbut generally spherical shape, Fig. 2. The observed par-ticle size is consistent with those measured by means ofXRD data.

Fig. 3 shows the hysteresis loop for nanoparticulatepowders of Zn1�xFexO (x¼0, 0.01, 0.03, 0.05, 0.07 and0.09); typical of magnetic behaviors, indicating that thepresence of an ordered magnetic structure can exist in thewurtzite structure of doped Fe–ZnO compounds at roomtemperature. Here and throughout this paper the diamagneticmagnetization for undoped-ZnO samples was subtracted fromthemeasuredmagnetization curves. The hysteresis loops closeat relative low fields, and the saturation magnetization,

Ms was determined by extrapolating a graph of M versus 1/Hto 1/H¼0, being ∼0.16 (∼0.006 μB), 0.60 (∼0.04 μB), 1.65(∼0.08 μB), 4.3 (∼0.18 μB) and 4.5 (� 0.19 μB) emu/cm3 forx¼0.01, 0.03, 0.05, 0.07 and 0.09, respectively. This results (Ms)are comparable to those results for iron implanted ZnO films[19], iron doped ZnO by mechanical alloying [7], and irondoped ZnO rod arrays prepared by the hydrothermal method[8], increase with increasing the amount of doping. Thecoercivity and remanent magnetization increased withincreasing x as shown in the inset of Fig. 3. It is clear thathysteresis loops are symmetric about zero fields, whichexcludes the possibility of existence of exchange bias char-acteristics as reported by Liu et. al. [9].

Temperature dependence of magnetization for all sam-ples were obtained by applying a magnetic field of 200 Oeand heating the samples from room temperature throughhigher temperatures at a rate of 5 1C/min. As expected, themagnetization decreased with increasing temperatureapproaching to zero at ∼590 K and 740 K for x¼0.01 and0.09, respectively, as shown in Fig. 4. The Curie tempera-ture was obtained by plotting the inverse of susceptibility(1/χ) versus temperature (T); being ∼571 K and 734 K forx¼0.01 and 0.09, respectively, Fig. 5. This ensures that allsamples are magnetically ordered above room tempera-ture. These higher TC values may be attributed to the

A. Franco Jr, T.E.P. Alves / Materials Science in Semiconductor Processing 16 (2013) 1804–1807 1807

presence of clusters of defects such as Fe3O4 (TC¼858 K)and ZnFe2O4 (TC¼634 K) [15] in Fe-doped ZnO nanoparti-culate powders, especially for samples with x¼0.07 and0.09 as shown in Fig. 1.

The ferromagnetism in our doped Fe–ZnO nanoparti-culate powders may be attributed to two mechanismsdepending on the iron content. First, for samples of x¼0.01–0.05; increasing the Fe content results in an increase in themagnetization response (Fig. 3). At low Fe content (x¼0.01)the average distance between Fe ions must be longer com-pared to those of higher Fe content (x¼0.05) samples. Forlong distance between Fe contents, so the ferromagnetisminteraction is quite weak. As the Fe content is increased, theferromagnetic interaction between Fe ions become stronger;so the magnetism is enhanced. Thus the observed roomtemperature ferromagnetism is an intrinsic property of dopedFe–ZnO nanoparticulate powders. Second, as the Fe content(x¼0.07 and 0.09) is increased the magnetization response isincreased as well. In general, the iron in ZnO is a deepacceptor; if Fe substitutes Zn, the valence state of Fe ionsis +2; thus with more Fe ions incorporated into the ZnOlattice structure through substitution of Zn atoms (up to thecationic percolation concentration threshold), the higher willbe the magnetization, as shown in Fig. 3. Further studiessuch as Mössbauer spectroscopy and XPS are in progressto better understand the origin of the room-temperatureferromagnetism in doped Fe–ZnO nanoparticulate powders,prepared by the combustion reaction method.

4. Conclusions

We have synthesized Fe-doped ZnO nanoparticulatepowders by a modified combustion reaction method.Room-temperature ferromagnetism was observed for allsamples with Fe content ranging from 0.01 to 0.09. Theorigin of room-temperature ferromagnetism was attribu-ted to intrinsic property of Fe-doped ZnO nanoparticles forsamples with lower iron content ranging from 0.01 to 0.05

and to the secondary phases, such as clusters of ironoxides and zinc ferrites, present in samples with higheriron contents; x¼0.07 and 0.09.

Acknowledgments

We are grateful to the financial support provided byCNPq (Grant no. 552526/2011-8). One of us (A.F.J.) is aCNPq fellowship under Grant no. 308183/2012-6.

References

[1] H. Ohno, Science 281 (1998) 951–956.[2] G.A. Prinz, Science 282 (1998) 1660–1963.[3] M. Hung, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo,

P. Yang, Science 292 (2001) 1897–1899.[4] C. Liu, F. Yun, H. Morkoç, J. Mater. Sci.: Mater. Electron. 16 (2005)

55–62.[5] F. Pan, C. Song, X.J. Liu, Y.C. Yang, F. Zeng, Mater. Sci. Eng. R 62 (2008)

1–36.[6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287

(2000) 1019–1022.[7] Y. Lin, D. Jiang, F. Lin, W. Shi, X. Ma, J. Alloys Compd. 436 (2007)

30–33.[8] C. Xia, C. Hu, Y. Tiam, P. Chen, B. Wan, J. Xu, Solid State Sci. 13 (2011)

388–393.[9] H. Liu, J. Yang, Y. Zhang, Y. Wang, M. Wei, Mater. Chem. Phys. 112

(2008) 1021–1023.[10] G.Y. Ahn, S.I. Park, I.B. Shim, C.S. Kim, J. Magn. Magn. Mater. 282

(2004) 166–169.[11] S.K. Mandal, T.K. Nath, A. Das, R.K. Kremer, Appl. Phys. Lett. 8 (2006).

(162502–162501).[12] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243–248.[13] S.R. Jain, K.C. Adiga, V.R. Pai Vernecker, Combust. Flame 40 (1981)

71–80.[14] A. Franco Jr., E.C.O. Lima, M.A. Novak, P. Well Jr., J. Magn. Magn.

Mater. 308 (2007) 198–202.[15] A. Franco Jr., F.C.e. Silva, J. Appl. Phys. 113 (2013). (17B513–17B516).[16] P. Scherrer, Cöttinger Nachr. Ges. 2 (1918) 98.[17] B.E. Warren, X-Ray Diffraction, Dover, New York251.[18] C. Dong, H.M. Chen, F. Wu, J. Appl. Crystallogr. 32 (1999) 168–173.[19] B. Zhang, Q.H. Li, L.Q. Shi, H.S. Cheng, J.Z. Wang, J. Vac. Sci. Technol.

A 26 (2008) 1469–1972.