Journal of Magnetism and Magnetic Materials 323 (2011) 829–832

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

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

Structural, magnetic and transport properties of Ni-doped ZnO films

J.J. Lu a,n, T.C. Lin a, S.Y. Tsai b, T.S. Mo a, K.J. Gan c

a Nano-Technology R&D Center, Kun-Shan University, Tainan, Taiwanb Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwanc Department of Electrical Engineering, National Chiayi University, Chiayi, Taiwan

a r t i c l e i n f o

Article history:

Received 27 September 2010

Received in revised form

23 October 2010Available online 20 November 2010

Keywords:

Diluted magnetic semiconductor

Ferromagnetism

EXAFS

53/$ - see front matter & 2010 Elsevier B.V. A

016/j.jmmm.2010.11.025

esponding author: Tel.: +886 6 2050496.

ail address: jjlu@mail.ksu.edu.tw (J.J. Lu).

a b s t r a c t

In this work, Ni-doped ZnO (Zn1�xNixO, x¼0, 0.03, 0.06, 0.11) films were prepared using magnetron

sputtering. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature dependence

electrical resistance, Hall and magnetic measurements were utilized in order to study the properties of the

Ni-doped ZnO films. XRD and XAS results indicate that all the samples have a ZnO wurtzite structure and

Ni atoms incorporated into ZnO host matrix without forming any secondary phase. The Hall and electrical

resistance measurements revealed that the resistivity increased by Ni doping, and all the Ni-doped ZnO

films exhibited n-type semiconducting behavior. The magnetic measurements showed that for the

samples with x¼0.06 and 0.11 are room-temperature ferromagnetic having a saturation magnetization of

0.33 and 0.39 mB/Ni, respectively. The bound-magnetic-polaron mediated exchange is proposed to be the

possible mechanism for the room-temperature ferromagnetism in this work.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Diluted magnetic semiconductors (DMSs) are referred to asnon-magnetic semiconductors in which a small fraction of hostcations are replaced by magnetic elements. With both spin andcharge degrees of freedom in a single material, DMSs have attracteda great deal of attention in recent years due to their potentialapplications in spintronics [1–3]. For developing future spintronicdevices, the basic requirements are room-temperature ferromag-netism and good optical transmittance. Since Dietl et al. [4] firstpredicted that p-type doped ZnO-based DMSs could exhibit room-temperature ferromagnetism, later Sato and Katayama-Yoshida [5]further suggested theoretically that the ferromagnetism could alsoexist in n-type doped ZnO systems, intense investigations havebeen focused on ZnO-based DMS systems.

In seeking ZnO-based DMS systems, by comparison, fewerpapers have been focused on Ni-doped ZnO systems. Among them,some groups reported that Ni-doped ZnO DMSs are able to exhibitintrinsic room-temperature ferromagnetism [6–11]. However,some researchers reported that their Ni-doped ZnO systems wereonly room-temperature paramagnetic [12–15] or the ferromagnet-ism was due to Ni clusters [16–17]. The reported magneticproperties are discrepant, and the origin of ferromagnetism is stillcontroversial.

To ascertain whether the ferromagnetism results from a singlephase or ferromagnetic clusters, in the present work Ni-doped ZnO

ll rights reserved.

films were deposited on glass substrates by magnetron sputtering.Several measurements, including X-ray diffraction (XRD), X-rayabsorption spectroscopy (XAS), temperature dependence electricalresistance, Hall and DC magnetic measurements were utilized inorder to study the properties of these films.

2. Experimental

The pure ZnO and Ni-doped ZnO films were deposited onto heatedglass substrates (Corning 1737F) by magnetron co-sputtering in amixture of oxygen and argon gases. The targets used in this study aremetals Zn (99.99% purity, 76.2 mm diameter), and Ni (99.999% purity,76.2 mm diameter). The sputtering was performed in a mixed atmo-sphere of Ar (5 N) and O2 (5 N) with a flow rate ratio of 6:1, at a totalpressure 0.5 Pa. The substrate temperature was kept at 350 1C using afeedback-controlled heater, and the substrate holder was rotatedusing a stepping motor during sputtering in order to achieve uniformthickness. The variation of the substrate temperature was maintainedwithin 75 1C during deposition process. A cryo-pump, backed by arotary pump, was used to achieve a background pressure of 1�10�6

Torr before introducing argon gas.The film thickness was measured using a conventional stylus

surface roughness detector (Alpha-step 200, TENCOR, USA). Thethicknesses of films were maintained at about 450 nm. An energydispersive spectrometer (EDS) of a field emission scanning micro-scope (FE-SEM, JEOL, JSM-6700 F) was used to explore the chemicalcomponents of Ni-doped ZnO films in this work. The atomicpercentages of Ni in the films are x¼0.03, 0.06, and 0.11.

J.J. Lu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 829–832830

The XRD patterns of the deposited films were examined by powderX-ray diffraction (XRD) with a microcomputer controlled X-raydiffractometer (Rigaku, D/MAX-2500) using monochromatic Cu Karadiation (l�1.542 A) at a step scan rate 41 m�1. Both Zn K-edge andNi K-edge Extended X-ray Absorption Fine Structure (EXAFS) X-rayabsorption spectra were carried out at the National SynchrotronRadiation Research Center, Taiwan (NSRRC) on X-ray wiggler beam-line BL17C. The EXAFS data were collected from the fluorescencesignal. General EXAFS data analysis is described in the literature [18].

Magnetic behavior of the Ni-doped ZnO films was measured usinga commercial superconducting quantum interference device (SQUID)magnetometer (Quantum Design MPMS). The magnetization of theglass substrates was separately measured and subtracted from rawdata in order to obtain the magnetization of the films.

The resistivity and carrier concentrations of the films at roomtemperature were measured by a Hall-effect measurement system(LakeShore, Model 7662) using van der Pauw method. The tem-perature dependence of electrical resistance of the films wasmeasured by four-point probe method on an LR-700 AC bridgein a cryostat fully automated for temperature stability and dataacquisition.

Table 1Measured Ni contents, grain size D, resistivity and carrier concentration of the

Zn1�xNixO films.

Ni content

(%)

Grain size D

(nm)

Resistivity

(O cm)

Carrier concentration

(cm�3)

0.0 30.6 8.72�10�2 1.97�1019

3.0 29.8 1.13�10�1 1.12�1019

6.0 27.5 9.76�10�1 2.42�1018

11.0 25.9 1.77�100 8.13�1017

3. Results and discussions

Fig. 1 displays the XRD patterns of the Zn1�xNixO films. As can beseen from Fig. 1, all the films are polycrystalline with a hexagonallattice structure and a strong preferential (0 0 2) oriented growth.No evident Ni metal clusters or secondary phases are able to bedetected in the XRD patterns. The grain size D representinglongitudinal coherence length of the crystal was deduced fromthe (0 0 2) peak by using Scherrer’s formula [19]

D¼0:9l

bcosyBð1Þ

Fig. 1. X-ray diffraction patterns of the Ni-doped ZnO films. The inset shows the

positions of (0 0 2) peak at different Ni contents.

where b is the FWHM of (0 0 2) peak corrected from instrumentalbroadening of the monocrystalline Si diffraction peak, and yB is theBragg angle. The deduced data are presented in Table 1.

The inset of Fig. 1 shows the positions of (0 0 2) peak at differentNi contents. Compared with the pure ZnO film, there is a tendencyfor the (0 0 2) peaks to shift towards larger angles. Similar behaviorwas also observed by Hou et al. [7], who prepared Ni-doped ZnOfilms by magnetron sputtering. They suggested that Ni2 + substi-tutionally replaces Zn2 + because the ionic radius of Ni2 + (0.69 A) issmaller than Zn2 + (0.74 A).

Since an X-ray spectrometer is not sensitive to small amounts of asecondary phase, we still cannot exclude the existence of Ni clustersfrom the XRD results. It is well known that XAS is a powerful tool toprovide chemical states and local structure of incorporated ions inhost compounds, even in a dilute concentration. Therefore, to furtherverify the local structures XAS experiments were explored.

Fig. 2(a) displays the Zn K-edge Fourier transformed EXAFSspectra of the films. Typical Fourier transformed EXAFS spectra areobtained by multiplying the weighting scheme (k3) after back-ground subtraction and normalization. As it is shown in Fig. 2(a), all

Fig. 2. (a) The Zn K-edge EXAFS spectra of the pure and the Ni-doped ZnO films.

(b) The Ni K-edge EXAFS spectra of the Ni-doped ZnO films and Ni powder.

J.J. Lu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 829–832 831

the Ni-doped samples reveal similar characteristics to the pure ZnOfilm. Two major peaks around 1.5 and 2.9 A can be observed. Theformer corresponds to the nearest oxygen atoms; the latter is dueto the Zn atoms in the second shell. Data fitting was performedusing FEFFIT from UWXAFS3.0 in combination with FEFF8.0. Theobtained Zn–O and Zn–Zn bond lengths are all around 2.0 and 3.2 A,respectively, and the coordination numbers for the Zn–O shell areall near 4. These results are in good accordance with those reportedon ZnO films in the wurtzite type structure.

To probe the local environment of Ni atoms, Ni K-edge EXAFSanalysis was also carried out. The Ni K-edge Fourier transformedEXAFS spectra of the Zn1�xNixO films and Ni powder are shown inFig. 2(b). The left main peak is about 1.5 A, which corresponds to anoxygen peak. The next main peak located near 2.4 A corresponds toa Zn peak. All the spectra of the Ni-doped ZnO films have similarfeatures and are consistent with those reported previously [6,20].The best fitting data for the Ni–O shell’s coordination numbers forare all around 3.7, and the Ni–O bond lengths, for the samples withx¼0.03, 0.06 and 0.11, are 2.04, 2.02 and 2.01 A. The shortening ofthe Ni–O distance can be attributed to the smaller radius of Ni2 +.The above-mentioned results indicate that Ni2 + ions replace Zn2 +

ions substitutionally in the host matrix, which is consistent withthe XRD results.

Fig. 3 shows the magnetic hysteresis loops of the Zn1�xNixOfilms at 300 K. The magnetization curve of the sample with x¼0.03exhibits a paramagnetic-like characteristic. But for a paramagneticmaterial, the hysteresis curve should be completely linear in theregion of applied field. Such a magnetization behavior is more likelyto be due to superparamagnetism, which is caused by uncorrelatedsmall ferromagnetic clusters. With a further increase in the Nicontent, the hysteresis loops resemble ferromagnetic behaviorwith a small coercivity field and low remanence. The saturationmagnetization, for samples with x¼0.06 and 0.11, are 0.33 and

Fig. 3. The magnetic hysteresis loops of the Ni-doped ZnO films at 300 K.

0.39 mB/Ni, respectively. The coercivity and remanence are likely toarise from the pinning effect of the domain wall. The cause of theformation of ferromagnetic domains is still not fully understood.Several theories, such as bound magnetic polaron (BMP) model [21]and carrier-mediated exchange interaction [22], are often used toaccount for the formation of the domains. To clarify the room-temperature ferromagnetism in our system, we further studiedtheir transport property.

Room-temperature Hall measurements revealed that all thefilms exhibit n-type conductivity. The resistivity and carrierconcentrations of the films are listed in Table 1. Compared to thepure ZnO film, all the Ni-doped ZnO films have higher resistivityand lower concentrations. Fig. 4 shows the temperature depen-dence of the normalized electrical resistance (R(T)/R(300 K)) for thesamples between 80 and 300 K. All the films exhibit a typicalsemiconductor-type conducting behavior. From the above results,the conducting behavior of the Zn1�xNixO films is depressed by Nidoping. Since the transport property can be thought as a directmeasure to the spin–spin correlations, it also offers anotherevidence for the ferromagnetism of the system.

In some reported Ni-doped ZnO systems, the conducting behaviorwas enhanced by Ni doping [8–10]. The mechanism of the intrinsicferromagnetism is suggested to arise from the carrier-mediatedexchange interaction. However, in a magnetic semiconductor orinsulator such as our system, the origin of ferromagnetism is unlikelyto be due to the carrier-mediated exchange interaction. Instead, theBMP Model or F-center-mediated exchange is often proposed to beresponsible for the ferromagnetism. Because the charge valence ofNi2+ is equal to that of Zn2+ in the samples, the F-center-mediatedexchange seems unsuitable in this work. Therefore, the BMP Modelis the more probable mechanism because the non-equilibriumprocess during co-sputtering might increase the defects and oxygenvacancies. According to the BMP theory, an electron associated with

Fig. 4. The temperature dependence of the normalized electrical resistance (R(T)/

R(300 K)) of the samples between 80 and 300 K.

J.J. Lu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 829–832832

a particular defect or oxygen vacancy will be bound in a hydrogenicorbit so as to form BMPs. The BMPs can couple the 3d moments of thenearby Ni ions to form a local ferromagnetic region with a large orbitalradius. Once theses ferromagnetic regions become sufficiently largeenough to overlap, it will lead to ferromagnetic coupling betweenthem and give rise to ferromagnetism in the sample. From the energyband point of view, the BMPs overlap to produce a spin–split impurityband. Because the electrons are able to transfer from the BMPimpurity band to an unoccupied 3d state of Ni ions at the Fermilevel, it will establish the ferromagnetism of the system.

However, the evidence to confirm that the mechanism is BMPrather than other theories is still not sufficient in our present work.Further experiments, such as neutron scattering, are needed toascertain the proposal.

4. Conclusions

The XRD and XAS measurements reveal that all the samples arec-axis oriented hexagonal wurtzite-type structures, and no sec-ondary phases or Ni clusters can be detected. The results alsoindicate a possible substitutional incorporation of Ni2 + into Zn2 +

sites in our present work.The magnetic hysteresis measurement suggests the samples

(x¼0.06 and 0.11) have room-temperature ferromagnetism. Theconducting behavior of the Ni-doped films exhibits essentiallyn-type semiconducting behavior, with higher resistivity and lowercarrier concentration than the undoped ZnO films. Because of lesscarrier concentration, the origin of the ferromagnetism is unlikelyto be due to free carrier-mediated exchange interaction. Judging

from the obtained results, the possible mechanism for the ferro-magnetism in our system is prone to the BMP theory.

Acknowledgement

This work was supported by National Science Council of theRepublic of China under contract no. NSC 97-2112-M-168-001-MY2.

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