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Materials Science and Engineering B 171 (2010) 144–148

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

tructure and magnetic properties of nickel–zinc ferrite microspheresynthesized by solvothermal method

ei Yana, Wan Jianga, Qinghong Zhangb, Yaogang Lia,∗, Hongzhi Wangb,∗

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, People’s Republic of ChinaCollege of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 10 December 2009eceived in revised form 18 March 2010

a b s t r a c t

Monodisperse Ni–Zn ferrites (NixZn1−xFe2O4) microspheres have been synthesized via solvothermalmethod. X-ray diffraction pattern (XRD), transmission electron microscope (TEM), field emission-

ccepted 31 March 2010

eywords:i–Zn ferritesanostructure

scanning electron microscopy (FE-SEM) and vibrating sample magnetometry are used to characterizethe shape, structure, size and magnetic properties of the as-synthesized magnetic microspheres. Thepowder XRD patterns revealed the formation of the single phase spinel structure for the synthesizedmaterials. TEM and FE-SEM show the size and morphology of the as-synthesized sample in detail. Themaximum magnetic saturation value of the Ni0.2Zn0.8Fe2O4 microspheres can reach 60.6 emu g−1. Thesemagnetic NixZn1−xFe2O4 microspheres are expected to have wide applications in bionanoscience and

logy.

olvothermal methodagnetic properties electronic devices techno

. Introduction

Ferrite nanoparticles can be used in heat transfer devices, drugelivery systems and medical diagnostics because of their highlectrical resistivity, chemical stability, mechanical hardness andeasonable cost [1–3]. Typically, the MFe2O4 (M = Co, Ni, Mn, Zn, Fe,tc.) spinel ferrite nanoparticles exhibit interesting magnetic andagneto-optical properties that are potentially useful for a broad

ange of applications [4–6].As an important member of ferrite family, Ni–Zn ferrites have

ttracted significant research interest based on its fascinating mag-etic and electromagnetic properties. The physical properties ofhe Ni–Zn ferrites are very sensitive to the method of prepara-ion, the amount and the type of substitutions [7,8]. Many methodsave been developed to prepare the NixZn1−xFe2O4 nanoparticles,uch as co-precipitation method [9], hydrothermal method [10],itrate precursor method [11,12], sol–gel method [13,14], and self-ropagating high-temperature method [15,16].

Lima et al. prepared Ni–Zn ferrites by using the citrate precursorethod and the resulting products were used as radar-absorbingaterials (RAM) [17]. Akther Hossain et al. investigated struc-

ural, electrical, and magnetic properties of NixZn1−xFe2O4 (x = 0.2,.4) samples sintered at various temperatures [18]. Zahi fabricatedi–Zn ferrites using two different methods: the sol–gel method,nd the conventional solid-state reaction using different starting

∗ Corresponding authors. Tel.: +86 21 67792342; fax: +86 21 67792340.E-mail addresses: yaogang li@dhu.edu.cn (Y. Li), wanghz@dhu.edu.cn (H. Wang).

921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2010.03.088

© 2010 Elsevier B.V. All rights reserved.

materials [19]. Grimal et al. studied the influence of iron, nickeland zinc stoichiometry on dynamic magneto-elastic properties ofNi0.5Zn0.5Fe2O4 spinel ferrites [20]. Mangalaraja et al. reported thata Ni–Zn ferrite powder of composition Ni0.8Zn0.2Fe2O4 was synthe-sized by microwave-assisted flash combustion technique [21].

However, to the best of our knowledge, no group has syn-thesized Ni–Zn ferrite by using glycol (C2H6O2) as solvent insolvothermal method. Unlike the other methods mentioned above,the solvothermal method requires neither expensive starting mate-rials and environment-unfriendly solvent nor extremely hightemperature. This work is to demonstrate a general, efficient, onestep and environmentally friendly synthetic strategy for obtainingNixZn1−xFe2O4 microspheres via a simple solvothermal method.Here, we prepared NixZn1−xFe2O4 magnetic microspheres by asolvothermal method and the maximum magnetic saturation valueof the microspheres can reach 60.6 emu g−1. The forming mecha-nism of the spinel Ni–Zn ferrite microspheres was investigated indetail. The as-synthesized microspheres display a great potentialapplication in integrated nanoscale electronic systems due to theirhigh magnetic sensitivity.

2. Experimental

2.1. Materials

All the reagents (C2H6O2, FeCl3·6H2O, Zn(NO3)2·6H2O,Ni(NO3)2·6H2O, NaAc, PEG-200) used in the synthesis of fer-rite were of analytical grade and were purchased from SinopharmChemical Reagent Co., Ltd.

W. Yan et al. / Materials Science and En

Fx

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ig. 1. The XRD patterns of NixZn1−xFe2O4 microspheres: (a) x = 0.2; (b) x = 0.4; (c)= 0.6; (d) x = 0.8.

.2. Synthesis of NixZn1−xFe2O4 microspheres

A series of NixZn1−xFe2O4 microspheres could be synthesizedith composition (x = 0.2, 0.4, 0.6, 0.8). Powders of NixZn1−xFe2O4icrospheres were prepared by the solvothermal method using

he glycol as solvent. The typical preparation procedure ofi0.2Zn0.8Fe2O4 nanospheres is as follows: 5 mmol FeCl3·6H2O,mmol Zn(NO3)2·6H2O and 0.5 mmol Ni(NO3)2·6H2O were dis-

olved in the dispersion. Then, 3.6 g of NaAc, 0.8 ml of polyethylenelycol 200 (PEG 200) were added to the dispersion under vigoroustirring at room temperature for 30 min. Subsequently, the mixtureas sealed in a teflon-lined stainless steel autoclave (70 ml capac-

ty) and maintained at 200 ◦C for 12 h. Then, the mixture was cooledo ambient temperature. The brown product was collected by mag-et and rinsed with deionized water and ethanol until there wereo chloride ions in the solution. The brown final product was dried

n vacuum at 60 ◦C for 12 h.

.3. Characterization

Powder X-ray diffraction pattern (XRD) was carried out on aapan Rigaku D/max 2550 V X-ray diffractometer using Cu K� irra-iation (� = 0.154056 nm). The operation voltage and current wereept at 40 kV and 300 mA. The size and morphology of the as-ynthesized products were determined at 20 kV by a JSM6700F fieldmission-scanning electron microscopy (FE-SEM). Transmissionlectron microscope (TEM), high-resolution transmission electronicroscope (HRTEM) images and select area electron diffrac-

ion pattern (SAED) were collected using a JEOL-2100F electronicroscope to determine further details of the microspheres. Thei2+/Zn2+/Fe3+ atomic ratio of the NixZn1−xFe2O4 was confirmedia energy-dispersive X-ray (EDX) spectrum, which recorded onn OXFORD ISIS spectroscope attached to the JEOL-2100F electronicroscope. Magnetic characterization was conducted on a vibrat-

ng sample magnetometer (VSM, PPMS Model 6000) in the field ofkOe at room temperature.

. Results and discussion

The crystallinity and structure of the NixZn1−xFe2O4 micro-pheres at the reaction temperature 200 ◦C and the reaction time2 h were confirmed by powder X-ray diffraction. As shown in Fig. 1,he peak position and relative intensity of all diffraction peaks forhe products match well with standard powder diffraction data,

gineering B 171 (2010) 144–148 145

and no impurity peak was observed, which indicated that highpurity crystalline NixZn1−xFe2O4 was synthesized. The (3 1 1) peakwas chosen for calculating the average particle size. The Scherrerformula analysis showed that the grain sizes of Ni0.2Zn0.8Fe2O4,Ni0.4Zn0.6Fe2O4, Ni0.6Zn0.4Fe2O4, and Ni0.8Zn0.2Fe2O4 were about6.94, 7.71, 8.89 and 9.45 nm, respectively.

Fig. 2a and b shows the FE-SEM photomicrographs of thesample (Ni0.2Zn0.8Fe2O4). It can be seen that the particles wereapproximately spherical in shape and most microspheres are inthe sizes of 100–150 nm, but there are small amount of micro-spheres in the sizes of 50–100 nm. The feature of the morphologyis that large numbers of nanoparticles are immobilized onto thesurface of the microspheres. The NixZn1−xFe2O4 nanocrystallitesgrow to form nanospheres via a growth model called orientedaggregation [22,23]. When nanoparticles are not restricted byfunctional surfactants, they grow randomly and aggregate in anisotropic way to form smaller spheres. The average sizes of theNi0.4Zn0.6Fe2O4, Ni0.6Zn0.4Fe2O4 and Ni0.8Zn0.2Fe2O4 microspheresare similar to that of Ni0.2Zn0.8Fe2O4. The size and morphologyof the as-produced sample in detail were investigated by TEM.From the TEM image in Fig. 2c, we can see the morphology of themicrospheres, which demonstrate that microspheres with uniformshape was produced, it also can be clearly seen that the magnetictiny Ni0.2Zn0.8Fe2O4 nanoparticles were agglomerated. As shownin Fig. 2d, the crystallographic orientation of the Ni0.2Zn0.8Fe2O4was investigated by HRTEM. The Ni0.2Zn0.8Fe2O4 nanoparticleswere clearly observed. The prominence of the lattice fringe ofd ≈ 0.292 nm agrees well with the separation between the (2 2 0)lattice planes (0.297 nm). The corresponding SAED pattern of theselected region indicates the product is well crystallized. The resultfrom EDX spectra in Fig. 2e shows that as-prepared Ni0.2Zn0.8Fe2O4microspheres contain Ni, Zn, Fe and O, and no contamination ele-ment was detected. The atomic ratio of Fe:Ni:Zn is about 10:1:4,indicating that the chemical formula of the as-prepared samples isconsistent with the experimental stoichiometric.

Li believed that the Ni–Zn ferrite formation reaction precededan intermediate process via the hydrolysis of halide ferrite, fol-lowed by the reaction between Fe(OH)3 and Ni2+, Zn2+ ions [22,23].As shown in Fig. 3, there were four main steps: at the first stage,OH− ions were obtained from the glycol, they reacted with Fe3+

and formed tiny Fe(OH)3 particles in the dispersion (see Eq. (1)).Then, many [Fe(OH)]3 particles can assemble to form colloid coresof [Fe(OH)3]m (here, m is the number of Fe(OH)3 molecular), whichhas a large surface area and a strong absorption tendency (see Eq.(2)). According to the Fajans rule for ionic bonding, colloid cores([Fe(OH)3]m) adsorbed preference to those ions that can form thaw-less materials with Fe, so the Ni2+ and Zn2+ were absorbed on thecolloid cores surface. With the reaction time passing by, hydrolysisof metal ions takes place, then the hydroxyl resembling the metalions are obtained (see Eq. (3)). The reaction in these steps may bewritten as follows:

Fe3+ + 3OH− → Fe(OH)3 (1)

mFe(OH)3 → [Fe(OH)3]m (2)

2[Fe(OH)3]m + xmNi2+ + (1 − x)mZn2+ + mOH−

→ [Fe(OH)3]2m·mNixZn1−xOH− (3)

In the last step, as the reaction temperature increases, the interme-diate is slowly transformed to Ni–Zn–Fe2O4, and the reaction can

be written as follows:

[Fe(OH)3]2m·mNixZn1−xOH− → mNixZn1−xFe2O4 + 3mH2O + mH+

(4)

146 W. Yan et al. / Materials Science and Engineering B 171 (2010) 144–148

F SEM ii

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ig. 2. Morphologies of the Ni0.2Zn0.8Fe2O4 microspheres: (a) low magnification FE-nset is the corresponding SAED pattern; (e) EDX spectrum of the sample.

Fig. 4 shows the schematic procedure of the preparation ofixZn1−xFe2O4 microspheres. As indicated, the NixZn1−xFe2O4

pheres are comprised of numerous smaller crystallites. Currently,he main driving force for oriented aggregation of nanocrys-als is attributed to the tendency to decrease the high surface

mage; (b) high magnification FE-SEM image; (c) TEM image; (d) HRTEM image. The

energy. Compared to those in the outer surfaces, the crys-tallites located in the inner cores have high surface energies,because they can also be visualized as a smaller sphere havinga higher curvature and higher surface energies, thus easy to bedissolved.

W. Yan et al. / Materials Science and Engineering B 171 (2010) 144–148 147

Fig. 3. Schematic illustration of the formation of NixZn1−xFe2O4 nanocrystals: (a) [Fe(OH)3]m·NixZn1−xOH1+ gel core; (b) amorphous complex group; (c) spinel nuclide; (d)unit cell of the Ni–Zn ferrite spine.

orma

s(ttmtpA[btmNmpuwtiiNmbm4w

the net magnetization for collinear spin arrangement at tempera-ture T could be expressed as M(T) = MB(T) − MA(T); where MA andMB are the magnetic moments of A and B sites, respectively. Whenthe substitution of Zn content (on the A site) increase, it will leadto decrease the Fe3+ ion on A site and increase the Fe3+ ions on

Fig. 4. Schematic illustration of the f

The magnetic property of the as-prepared NixZn1−xFe2O4 micro-pheres was investigated with a vibrating sample magnetometerVSM) at room temperature. It shows the magnetic properties ofhe samples are affected by the composition and cation distribu-ion. Various cations can be placed in A and B sites to tune its

agnetic properties [24,25]. It is reported that Zn2+ ions preferhe occupation of tetrahedral (A) sites; Ni2+ ions prefer the occu-ation of octahedral (B) sites while Fe3+ ions partially occupy theand B sites [26,27]. The cation distribution of Ni–Zn ferrite is

Zn2+1−xFe3+

1+x][Ni2+xFe3+

1−x]; where the term within the squarerackets indicates the octahedral (B) sites and the first term isetrahedral (A) sites. Fig. 5 shows M–H loops of NixZn1−xFe2O4

icrospheres with different Ni content. The inset shows theixZn1−xFe2O4 magnetic microspheres were attracted towards theagnet located in right-hand side of the sample vials over a short

eriod, demonstrating the high magnetic sensitivity of our prod-cts. The magnetization of NixZn1−xFe2O4 microspheres increasesith external magnetic field strength, however, it does not reach

he saturation state yet under a high magnetic field of 5 kOe. Andt shows the present of remanent magnetization and coercivityn M–H curve which indicates the ferromagnetic nature of theseixZn1−xFe2O4 microspheres at room temperature. The saturation

agnetizations of the NixZn1−xFe2O4 microspheres were obtained

y extrapolating M vs. 1/H plot to 1/H = 0 for the NixZn1−xFe2O4icrospheres of x = 0.2, 0.4, 0.6 and 0.8 as 60.6, 57.8, 53.5 and

8.9 emu g−1, respectively. It indicated that the Ms values increasedith the increasing of Zn content. This result is expected because

tion of NixZn1−xFe2O4 microspheres.

Fig. 5. Room temperature magnetic hysteresis of NixZn1−xFe2O4 microspheres: (a)x = 0.2; (b) x = 0.4; (c) x = 0.6; (d) x = 0.8. The inset is the sample separated fromsolution by an external magnetic field.

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[29] Y. Ichiyanagi, T. Uehashi, S. Yamada, Physical Status Solidi C 12 (2004)

48 W. Yan et al. / Materials Science a

he B site, thus the magnetization of the A site will decrease and Bite will increase. Accordingly, the net magnetization will increaseith increasing Zn content. Costa et al. also described this effect

n samples of NiFe2O4 ferrite nanopowder doped with Zn2+ [28].esides, hysteresis loops for the microspheres display zero coerciv-

ty, with no remanence, as would be expected for superamagneticehavior.

Ichiyanagi et al. prepared Ni0.2Zn0.8Fe2O4 mixed ferriteanoparticles encapsulated with amorphous-SiO2 by a wet chem-

cal method. The diameters of these particles were estimated from-ray diffraction patterns as 6.2 nm and the Ms values of theample was 42.1 emu g−1 [29]; Akther Hossain et al. synthesizedi0.2Zn0.8Fe2O4 by solid-state method, the saturation magnetiza-

ions of the samples was about 60 emu g−1 [24]. Moreover, theaturation magnetization of these samples is smaller than thatbserved for the bulk (Ms is over 70 emu g−1) [18]. These differencesn the magnetization value (such as saturation magnetization,emanent magnetization and coercivity) between nanoferrites andulk ferrites can be attributed to the finite size effects [30]. In thisase, the decrease of Ms in nanoparticles can be attributed to theanted spins in the surface layers due to a decrease in the exchangeoupling which is caused by the lack of oxygen mediating superxchange mechanism between nearest iron ions at the surface [31].hey can also be attributed to the enhancement of the surfacearrier potential due to the distortion of crystal lattice caused byhe atoms deviating from normal positions in the surface layers32].

. Conclusion

In summary, the solvothermal method has been used to suc-essfully synthesize NixZn1−xFe2O4 magnetic microspheres. Thispproach developed a simple, efficient, and reproducible route toynthesize the tiny magnetic NixZn1−xFe2O4 nanocrystals. The sat-ration magnetization value of the product (Ni0.2Zn0.8Fe2O4) is0.6 emu g−1. The microspheres display a great potential applica-ion in manipulation and this aspect is very important for futureechnological and high frequency applications.

cknowledgements

We gratefully acknowledge the financial supports by Ministryf Education of the People’s Republic of China (No. NECT-05-0419),hanghai Municipal Education Commission (No. 07SG37), Natural

cience Foundation of China (Nos. 50772022, 50772127), Shanghaieading Academic Discipline Project (B603), the Cultivation Fundf the Key Scientific and Technical Innovation Project (No. 708039),nd the Program of Introducing Talents of Discipline to UniversitiesNo. 111-2-04).

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gineering B 171 (2010) 144–148

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