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Applied Surface Science 356 (2015) 804–811

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

tructural, optical, and magnetic properties of Ni doped ZnOanoparticles: Correlation of magnetic moment with defect density

appaditya Pala, D. Sarkara, P.K. Girib,∗

Department of Physics, Gauhati University, Guwahati 781014, IndiaDepartment of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India

r t i c l e i n f o

rticle history:eceived 9 April 2015eceived in revised form 29 July 2015ccepted 19 August 2015vailable online 21 August 2015

eywords:nO nanoparticlesi dopinghotoluminescenceefectserromagnetismMS

a b s t r a c t

We report on the room temperature ferromagnetism in the Zn1−xNixO (x = 0, 0.03 and 0.05) nanoparticles(NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline,wurtzite ZnO structure for the 3% Ni doped ZnO NPs for higher milling time. HRTEM lattice image andSAED pattern show that the doped NPs are single crystalline with a d-spacing of 2.47 A correspondingto the (1 0 1) plane. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy con-firm the presence of Ni ions inside the ZnO matrix with 2+ valance state. Room temperature magneticmeasurements exhibit the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) andcoercive field (Hc) of 260 Oe. Micro-Raman studies illustrate doping/disorder induced additional Ramanmodes at ∼547, 574 cm−1 in addition to 437 cm−1 peak of pure ZnO. Photoluminescence (PL) spectraand UV–vis absorption measurements demonstrate some modification in the band edge emission andabsorption characteristics, respectively. PL spectra also show defect related strong visible emission, whichis believed to play a significant role in the FM ordering. These observations highlight the effect of changingdefect density on the observed ferromagnetic moment values for the as synthesized Zn1−xNixO NPs. Mag-

netic interaction is quantitatively analyzed and explained using a bound magnetic polaron model andexpected to arise from the intrinsic exchange interaction of Ni ions and OV, Zni defects. Systemic studieson the structural, magnetic, and optical properties reveal that both the nature of the defects as well asNi2+ ions are significant ingredients behind attaining high moment as well as high ordering temperaturein Ni doped ZnO NPs.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

Transition metal (TM) doped ZnO based diluted magnetic semi-onductors (DMS) are potential candidate for the attainment oferromagnetic ground state with a high Curie (Tc) temperature,ollowing the theoretical prediction of room-temperature (RT) fer-omagnetism (FM) both in p-type ZnO and n-type ZnO systems1–3]. It leads to a novel behavior for the use of both chargend spin of electrons by producing spin polarized carriers havinghe prospect of promising applications in semiconductor spin-ronic devices [3–7]. It might have potential applications in theew emerging field of optoelectronics, spin polarized light emit-

ing diodes, magnetic tunnel junctions, photovoltaics and sensors4,8,9]. Among the II–VI semiconductors, ZnO is a versatile multi-unctional candidate, with a direct wide band gap (3.37 eV at 300 K),

∗ Corresponding author.E-mail address: giri@iitg.ernet.in (P.K. Giri).

ttp://dx.doi.org/10.1016/j.apsusc.2015.08.163169-4332/© 2015 Elsevier B.V. All rights reserved.

large excitonic binding energy (60 meV), outstanding electro-optic,piezoelectric properties, and excellent chemical stability, whichcan be useful in electronics and optoelectronics [10,11]. The coex-istence of magnetic, semiconducting, and optical functionalitiesincrease the potential of TM-doped ZnO (ZnO:TM) to be a trulymultifunctional material [3,5].

There is an ongoing debate regarding the origin of FM, kind ofinteraction and the role of TM in ZnO based DMSs. Various studiesindicate that the origin of FM as intrinsic in nature and is due to theexchange interaction between the localized magnetic moment ofTM ion and delocalized charge carriers [1], or through the perco-lation of defect mediated bound magnetic polarons (BMP), formedby the intrinsic defects, related to vacancy or interstitials [12,13].However, in some cases, it is reported that FM is extrinsically origi-nated from segregation of TM ions or from the secondary magnetic

phases [14,15]. Also defect densities, such as oxygen/zinc vacanciesor interstitials [16,17] and cluster of defects [18] could alone trig-ger FM (known as d0 FM) without TM doping. However, in case ofd0 FM, for most of these materials it is difficult to retain the FM at

ce Science 356 (2015) 804–811 805

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dibritlfiN[aAdahNt

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2

2

tc(ndm1pivdtiT

Fig. 1. (a) XRD pattern of Zn1 − xNixO (x = 0, 0.03) NPs for different milling times

B. Pal et al. / Applied Surfa

levated temperatures. Also, it is very difficult to establish a directink between the magnetization and defects due to complexity ofefect states in ZnO and to have its suitable handling for the devicepplication purpose. Hence, the conclusive origin of RTFM is yet toe uncovered, and the mechanism behind the magnetic ordering istill under debate. Descending to the nanoscale dimensions, addi-ional effects related to the high surface to volume ratio, surfaceefect contributions appear, making the whole issue on TM-dopednO DMSs more interesting.

The attainment of high temperature FM, its stability and repro-ucibility in various environments is one of the challenging issues

n these systems. Researchers have tried to synthesize Ni doped ZnOased DMSs, mostly in thin films and by various sol–gel reactionoutes. Recently, Srinet et al. have found RT FM with low momentn Ni doped ZnO NPs by a sol–gel route [19]. Liu et al. observedhe RT FM behavior in Ni-doped ZnO films synthesized by pulsedaser deposition [20]. Defect mediated RT FM is reported in ZnOlms for Ni concentrations of 2%, 4%, and 7% as intrinsic and for 11%i concentration as extrinsic in nature due to precipitation of Ni

21]. Satyarthi et al. also have reported the coexistence of intrinsicnd extrinsic origins of RT FM in Ni ion implanted ZnO films [22].lso, it has been reported that the nature of FM in Ni doped ZnOoes not remain intrinsic upon annealing in various environmentst high temperature because of segregation of TM ions [23]. Sing-al et al. reported that FM in hydrogenated and vacuum annealedi doped ZnO disappears upon long reheating in air at very high

emperatures of 700 ◦C and 800 ◦C, respectively [24].The kind of FM interaction with respect to the experimental

ndings have been little explored in Ni doped ZnO systems asompared to the Co doped ZnO systems. Moreover, synthesis of Nioped ZnO nanostructures showing FM above room temperatureith a high moment and high crystallinity still remains a signif-

cant challenge for future spintronic devices. Under this scenario,agnetism in Ni doped ZnO is still a subject of much interest that

emands careful investigations. In this paper, we have followed anasy approach toward the synthesis of uniform Zn1−xNixO NPs by aall milling technique at ambient conditions. The structural charac-erization and phase identification are carried out by XRD, RAMAN,PS, TEM and EDS analysis. Further, the oxidation state of the Niopants inside the ZnO lattice is revealed by XPS analysis. Photolu-inescence (PL) and UV–vis absorption measurements are carried

ut to check the doping related changes in band edge emission andbsorption spectra, the kind of defects present and their changesith doping. The magnetic properties are measured by a vibrating

ample magnetometer (VSM).

. Experimental details

.1. Synthesis

Zn1−xNixO (x = 0, 0.03 and 0.05 weight percentage) NPs are syn-hesized by a ball milling technique. The starting materials areommercial ZnO nanopowder (99%, Merck) and Nickel (Ni) powder99.8%, Loba Chemie). 3% and 5% Ni powders are doped with the ZnOanopowder during the ball milling process. After mixing, the pow-ers are first ground for 10 min and then milled in a mechanical ballilling machine (Retsch, PM 100) at 350 rpm for duration of 4, 8 and

2 h in a zirconium oxide vial under atmospheric pressure and tem-erature. Zirconium oxide balls having diameter of 5 mm are used

n this experiment to avoid any contamination from the ball andial. The ball to powder mixture weight ratio is taken as10:1. The

ifferent milling parameters like rpm speed of the machine, ballo powder weight ratio and milling time are carefully controlledn order to achieve uniform doping without any contamination.he undoped ZnO NPs and 3% Ni doped ZnO NPs for milling time

(4–12 h) showing wurtzite phase; inset shows the XRD pattern for the 5% Ni dopedsample. Doping induced peak shift is clearly seen for the intense peaks correspond-ing to (1 0 0), (0 0 2), and (1 0 1) planes.

of 4 h, 8 h, and 12 h are named as 0-ZnO, 4-ZnO, 8-ZnO, 12-ZnO,respectively for the convenience of discussion.

2.2. Characterization

The phase purity and crystal structure of the Zn1−xNixO sam-ples are characterized by X-ray diffraction (XRD) (Rigaku RINT2500 TTRAX-III, Cu K� radiation). The morphology of the samplesis identified using a field emission scanning electron microscopy(FESEM) (Sigma, Zeiss). The high magnification surface morpholo-gies are studied by transmission electron microscopy (TEM),high-resolution TEM (HRTEM), selected area electron diffraction(SAED) pattern and energy dispersive X-ray spectrum (EDS) (JEOL-JEM 2010 operated at 200 kV). The optical absorption spectra arerecorded using a commercial UV-Vis spectrophotometer (PerkinEmler, UV win Lab). The room temperature PL spectra of all thesamples were recorded with a 325 nm He–Cd laser excitation usinga commercial PL spectrometer (Flurolog-3, Horiba) equipped witha PMT (photo multiplier tube) detector. The magnetic properties ofthe samples are examined by using a Lakeshore VSM (Model no.7410). XPS measurement was carried out with a fully automatedXPS microprobe (PHI X-tool, ULVAC-PHI) using Al K� X-ray beam(1486.7 eV) with beam current of 5 mA. Carbon 1s spectrum is usedfor the calibration of the XPS spectra recorded for various samples.

3. Results and discussion

3.1. Microstructure and morphology

Fig. 1(a–d) shows a typical XRD pattern for the Zn1−xNixO (x = 0,0.03) NPs for different milling time (4–12 h). Observed peaks con-firm the single crystalline ZnO wurtzite structure for the 3% Nidoped sample. Although for the samples synthesized with lower

806 B. Pal et al. / Applied Surface Science 356 (2015) 804–811

Table 1Summary of the lattice parameters calculated from the XRD pattern for the Zn1 − xNixO NPs with different milling time.

Sample type Sample code Crystal structure from XRD

2� (◦) Lattice constants Avg. particle size, D (nm)

a (Å) c (Å) v (Å3) d (Å)

Undoped ZnO 0-ZnO 31.69 36.18 3.258 5.219 23.99 2.479 553% Ni doped 4 h 4-ZnO 31.76 36.23 3.249 5.209 23.81 2.478 293% Ni doped 8 h 8-ZnO 31.79 36.26 3.247 5.203 23.75 2.476 203% Ni doped 12 h 12-ZnO 31.85 36.29 3.241 5.199 23.64 2.474 21

mocrpfGiFhtapui0aNZ

FssZopuslaaaitdtbnENrscct

phs4fsm

Fig. 2. (a) TEM image of undoped ZnO NPs. (b) TEM image of the 3% Ni doped ZnONPs, (c and d) HRTEM lattice image and corresponding SAED pattern of a single 3%Ni doped ZnO NP showing c-axis growth. (e) EDS spectra taken on a few Ni dopedZnO NPs.

illing time, small intensity secondary phase related peak isbserved, it disappeared for higher milling time. As a result, theontribution of extrinsic phases has vanished and the material haseached a single phase nature. Further 5% Ni doped sample showsresence of secondary phases (NiO/Ni) (see inset of Fig. 1), evenor higher milling time due to saturated state of doping level [25].enerally for the Ni content of x > 5%, it is beyond the solid solubil-

ty limit of Ni in ZnO, especially for ball milling technique [19,26].ig. 1 also shows a clear shift in the diffraction peaks toward theigher angle with the doping for the three most intense peaks ofhe XRD pattern. The lattice parameters a, c and the cell volume vre calculated from the XRD peak positions and we find that latticearameters decrease with doping; as a consequence, the lattice vol-me decreases with increasing doping concentration (see details

n Table 1). This is quiet expected since the ionic radius of Ni2+ is.55 A, while that of Zn2+ is 0.60 A [27]. The shifting of XRD patternnd corresponding decrease of the lattice parameters suggest thati2+ ions are successfully incorporated into the ZnO lattice at then2+ sites.

Fig. 2(a) shows a TEM image of the undoped ZnO NPs andig. 2(b) shows a TEM image of the 3% Ni doped ZnO NPs synthe-ized after 12 h of milling. The micrograph reveals average particleize of ∼80 nm for the undoped NPs before milling. For the dopednO NPs, the average particle size is ∼30–40 nm. Different kindsf shapes, like irregularly shaped nanoparticles, larger rectangularlatelets and hexagonally shaped nanoparticles are visible for thendoped NPs. However, doped NPs are of regular shapes, mostlypherical and its size is reduced due to the milling effect. HRTEMattice image is presented in Fig. 2(c), which shows that all the NPsre single crystalline. The d-spacing of the crystal plane is calculateds 2.47 A, which shows the preferable crystal growth plane is (1 0 1)nd it is also the highest intensity peak in the XRD pattern as shownn Fig. 1. The crystallinity and preferential orientation of the NPs inhe sample are further confirmed from the selected area electroniffraction (SAED) pattern, as shown in Fig. 2(d), confirming thathe Zn1−xNixO NPs are single crystals. The SAED pattern is obtainedy focusing the beam vertically on a single nanoparticle, hexago-al pattern indicates the single wurtzite structure of the ZnO NPs.DS spectra attached with the TEM set-up taken on a few singlePs of the as-synthesized Zn1−xNixO sample show Zn, Ni, and O

elated peaks besides Cu from Cu grid used in the measurement, ashown in Fig. 2(e). It shows 1.78 at.% of Ni doping in the ZnO NPs,orresponding to the 3 mol.% of the initial cationic precursor, withonsideration of the accuracy of the EDS analysis. This indicateshat Ni ions are successfully inserted inside the ZnO matrix.

Raman scattering is a versatile technique for detecting the incor-oration of dopants, the resulted defects and lattice disorder in theost lattice [28]. Fig. 3(a–d) shows the room temperature Ramanpectra of the undoped and the 3% Ni doped ZnO NPs in the range

00–800 cm−1. One sharp and strong peak at 437 cm−1 can be seenor all the samples and it is assigned to E2 (high) mode, which is thetrongest mode in wurtzite crystal structure, associated with theotion of oxygen atoms in ZnO lattice [29]. Note that as compared

to undoped NPs, the doped NPs show additional strong Ramanbands at around 547 and 574 cm−1. The peak at ∼574 cm−1 is theE1(LO) mode and can be attributed to defects due to O vacanciesand/or Zn interstitial defect states [30,31]. Wang et al. [28] andCheng et al. [32] also observed this mode in (Co, Al) and Ce-dopedZnO. According to the literature, the additional modes are inducedby host lattice defects; the host lattice defects in ZnO are acti-vated and amplified with the doping content and then these modesappears. In the present case, due to the smaller ionic radius of Ni2+

2+

than that of Zn , more oxygen vacancies and Zn interstitials areexpected to form in the Ni2+ doped ZnO sample [33].

B. Pal et al. / Applied Surface Scie

Fm

3

ded

[37]. The highest binding energy peak at nearly 532.7 eV corre-

FN

ig. 3. (a–d) Raman spectra for the undoped and 3% Ni doped ZnO NPs for differentilling times (4–12 h).

.2. Local structure of dopant and defects

The valance state, substitution and elemental contents of Nioped ZnO nanostructures can be determined by the bindingnergies of each element from the XPS spectra. The peak positionsepend on the local structure of the different ions and can provide

ig. 4. XPS spectra of the (a) Zn 2p3/2 and 2p1/2 states and (b) Ni 2p3/2 and 2p1/2 states of

i doped ZnO NPs.

nce 356 (2015) 804–811 807

the information about the chemical state of different elements.Fig. 4 shows the XPS spectra of (1) Zn 2p, (2) Ni 2p and (3) O 1sstates of pure and 3% Ni-doped ZnO NPs. Fig. 4(a) shows two strongpeaks centered at around 1021.54 eV and 1044.61 eV, which arein agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2,respectively. Their spin-orbital splitting energy is around 23.07 eV,confirming that Zn is present as Zn2+. Fig. 4(b) shows that the peaksof Ni 2p3/2 and Ni 2p1/2 core levels are centered around 855.51 and873.01 eV, respectively, for the 3% Ni doped ZnO NPs (12 h milled).Whereas the corresponding satellite structures are clearly observedat 861 and 879 eV. The ionic state and the phase information ofthe Ni ions within the ZnO matrix can be explained on the basisof the energy difference between Ni 2p3/2 and Ni 2p1/2 core levels.For Ni2+ states inside ZnO matrix, the spin-orbital splitting energydifference is around 17.50 eV, which is different from the value(17.27 eV) of the metallic Ni phase [25,34]. We observed the spin-orbital splitting energy as 17.51 eV for the 3% Ni doped ZnO NPs for12 h of milling. Also, observed Ni 2p3/2 (855.51 eV) position is quitedifferent from that of the metallic Ni (852.7 eV), NiO (853.8 eV)and Ni2O3 (856.7 eV) [35]. Furthermore, the shape of peaks forNi metallic with satellites structure is narrow and different fromthe observed spectrum of Ni-doped ZnO NPs [35]. These resultsprovide clear evidence that Ni ions are successfully substituted intotetrahedral sites of the ZnO wurtzite structure without formingany detectable secondary phases. The O1s peak in the surfaceis coherently fitted by Gaussian components, centered at 530.4,531.5 and 532.7 eV, respectively, as shown in Fig. 4(c and d) for theundoped and 3% Ni doped samples. The low binding energy peakat nearly 530.4 eV is attributed to O atoms at regular lattice site(OL), between O2− and Zn2+ (or substitutional Ni2+) ions [36]. Themedium binding energy peak at nearly 531.5 eV is associated withO atoms in the oxygen deficient regions (OV) within the ZnO matrix

sponds to interstitial O atoms (Oi) or surface oxygen in forms of –OHgroups. From the relative intensity of the XPS peaks, it is inferredthat the presence of Oi related defect is minimum in the both

3% Ni doped ZnO NPs, (c) O 1s states of undoped ZnO NPs, and (d) O 1s states of 3%

808 B. Pal et al. / Applied Surface Science 356 (2015) 804–811

Fig. 5. UV–Vis absorption spectra for the undoped and 3% Ni doped ZnO NPs,showing blue shift of band edge absorption with doping. (For interpretation of thert

u(

3

ptaiccnhtwpstITtteoZdT

dotaarwpaTrNbep

Fig. 6. (a and b) PL spectra of undoped and 3% Ni doped ZnO NPs. Strong UV and

eferences to color in this figure legend, the reader is referred to the web version ofhis article.)

ndoped and doped samples. This is consistent with the PL analysisdiscussed below), where OV related green emission is dominant.

.3. Optical properties

The UV–vis absorption spectra have been used primarily to findossible modification in the energy band structure of ZnO due tohe Ni doping. Fig. 5 shows the UV–vis absorption spectra takent RT for the undoped and doped samples. A distinct blue shifts observed in the absorption edge for the Ni doped ZnO NPs asompared to the undoped ZnO NPs. The observed blue shift indi-ates minor modification in the band structure in the Ni doped ZnOanostructures due to the incorporation of Ni ions. Various waysave been used to determine the band gap Eg from optical absorp-ion measurements [38]. A practical method is to equate Eg with theavelength at which the absorption is 50% of that at the excitoniceak (or shoulder). Since there is a lack of sharp edge in the dopedamples, we followed the derivative method to extract the inflec-ion point and calculate the Eg. Calculated Eg is shown in Table 2.t shows a blue shift in the calculated band gap (Eg) with doping.he observed changes in the absorption edges indicate a change inhe band structure due to doping and resulting defect [39]. Sincehe dimensions of the synthesized nanostructures are larger thanxcitonic Bohr radius in ZnO, size effect is unlikely to cause thebserved blue shift. The sp–d exchange interaction between thenO band electrons and localized d-electrons associated with theoped Ni2+ cations may cause the change in band structure [26,40].he interaction leads to a modification in the energy bands.

PL spectroscopy is very effective to characterize the presence ofefects, which have been shown to play a crucial role in the devel-pment of FM ordering in ZnO based DMSs. Here, RT PL studies forhe pure and 3% Ni doped ZnO NPs reveal a strong UV emission peaknd a broad visible emission band ranging from 410 nm to 650 nm,s shown in Fig. 6(a and b). Observed broad PL peak in the visibleegion indicate existence of multicomponents which are extractedith the fitting of multiple Gaussian peaks. The extracted individualeak positions are tabulated in Table 2. Undoped ZnO NPs exhibitn excitonic emission band centered at 380 nm (3.23 eV) (peak I).his near band-edge emission (NBE) is generally attributed to theecombination of free excitons present in the ZnO nanostructures.

i-doped ZnO NPs show a blue shift in this NBE to 3.26 eV. Thelue shift of the UV emission peak can be attributed to the strongxchange interaction between the d-electrons of Ni ions and the s,

electrons of the host band.

visible PL bands related to defects can be seen. Different PL peaks (I to IV) are fittedwith Gaussian functions.

Next, PL band (peak II) is a blue emission centered at ∼425 nm(2.91 eV) and it is commonly observed in the ZnO nanostructures,originating from the presence of Zni related surface defects [41].As the emission attributed to Oi (in the red and near-infraredregions) is not observed here and the blue emission correspond-ing to the Zni is observed, so our sample might be Zn rich. Hence,Zni and OV defects are expected to be responsible for the visibleemission. An intense green emission peak (peak III) is observed cen-tered at ∼515 nm (2.40 eV). The origin of this peak is considered asthe presence of singly ionized oxygen vacancy states (OV) [41,42].OV might have three different charge states, as F0 (doubly occu-pied), F+ (singly occupied), and F2+ (unoccupied) in the ZnO lattice[43,44]. However, the F0 and F2+ vacancies have spin-zero groundstates; thus, they do not induce FM in ZnO [12,43]. The F+ vacancycontributes magnetic moment and can activate bound magneticpolarons (BMPs) in DMSs [44]. It is hard to distinguish ZnV andOV from the PL spectrum only, due to the slight difference of theiremission peaks. So, lower density of ZnV, Oi related defects mightbe present, as a long tail of visible emission has been observed upto higher wavelengths. Next, a yellowish green emission peak (IV)centered on 550 nm (2.25 eV) is observed. It might appear from theantisite oxygen (OZn) related defect states. Also, small amount of Oidefects and hydroxyl group might be present on the surface of ZnOnanostructures, as the same has been observed in the XPS spectra[45].

Note that the relative intensity of the UV emission to the visibleemission peak has increased from 1.8 to 3.2 for the 3% Ni dopedNPs (12-ZnO) as compared to the undoped ZnO NPs. So, the relativeintensity of the defect modes have reduced in the doped NPs dueto milling induced doping effect. Interestingly, magnetic momentvalue has reduced with the reduction of defect density for the Nidoped sample with higher milling time. So, defects are believedto play an important role in the FM interaction of the doped Ni

ions. However, as both the milled and unmilled pure ZnO NPs donot show any detectable FM moment despite of the presence ofsuch defects, it indicates that the defects alone are not sufficient to

B. Pal et al. / Applied Surface Science 356 (2015) 804–811 809

Table 2Summary of the PL peaks with UV and visible emission bands fitted with Gaussian line shapes. Absorption peaks are measured from the respective UV–visible absorptionspectrum.

Sample Peak I (eV) Peak II (eV) Peak III (eV) Peak IV (eV) Relative peak intensity (IUV/ID) Absorption band gap (eV)

Hydro

poPm

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0-ZnO 3.23 2.85 2.45 2.25

12-ZnO 3.26 2.88 2.45 2.25

Peak identity NBE Zni OV, F+ OZn , Oi

romote FM ordering in the as-synthesized sample. Thus, dopingf Ni2+ ions through milling effect is important to tune the visibleL properties of the ZnO nanostructures and this in turn affects theagnetic properties of the doped ZnO NPs.

.4. Magnetic properties

We observe distinct ferromagnetic behavior at RT in the dopedn1−xNixO samples measured by VSM. Fig. 7(a–c) shows theagnetic hysteresis (M–H) loop for the 3% Ni doped ZnO NPs for dif-

erent milling times, measured at room temperature. The observedaturation magnetic moment (Ms) is of the order of 2.6–1.6 emu/gnd it varied systematically with milling time duration. The detailedariation of the coercivity, retentivity and other magnetic proper-ies are included in Table 3. It is well known that for the wide rangef applications, the DMS material should have a Tc, well above RT300 K). It is clear from the RT M–H loop that these materials canetain FM nature up to far above room temperature, as a result Tc

ill be high. Note that the observed Ms value is decreasing withncreasing milling time for the 3% Ni doped ZnO samples. As morei ions are incorporated inside the ZnO matrix for higher milling

ime, as a result the overall FM moment value has decreased andhe material has reached a single phase nature. In general the kindf FM interactions for the Ni ions in extrinsic phase and the Ni

ons inside the ZnO matrix are of different nature. The change in

oment value for higher milling time indicates that the observedM interaction is from substituted Ni ions, as 12-ZnO sample is ofingle phase nature. Also a preference of antiferromagnetic over

Fig. 7. Room temperature M–H plot showing the hysteresis loop for the (a–c)

1.8 3.283.2 3.32

xyl groups

ferromagnetic interactions might contribute partially due to thereduction in average interatomic distance of doped Ni ions [39].

However, the change in magnetic moment value(2.9–2.8 emu/g) is not very significant for the 5% Ni dopedZnO samples (see Fig. 7(d–f)), as extrinsic phase is seen in the XRDpattern even for higher milling time. These findings suggest that3% doping is an optimum choice for the ball milling technique, ashigher concentration (5%, 7%) of doping shows saturated nature ofTM substitution. Also, we have reported in our earlier work aboutthe choice of low concentration of magnetic doping and transitionproblem in the M–T curve for higher doping concentration [26].Thus, 3% doping of TM seems to yield an optimum choice for theobserved high temperature FM in ball milling technique. Note that,despite the presence of intrinsic defects no measurable magneticmoment is observed for the undoped milled sample. This confirmsthat the defects alone are not sufficient to account for the observedstrong FM in case of our samples. Further, in our earlier workon Co doped (low concentration) ZnO NWs synthesized by vapordeposition, we found very low magnetic moment and its nature asparamagnetic due to the low defect concentration in the growthprocess [46]. So, we believe that the presence of TM ions and thedefects (Zni, OV, OZn) promote the FM interaction to attain highmoment value and high ordering temperature.

4. Possible origin of FM interaction

The magnetic properties of the final system depend on manyparameters, like, the concentration and distribution of the TM ions,type and concentration of defects, n-type doping, p-type doping,

3%, and (d–f) 5% Ni doped ZnO NPs for different milling times (4–12 h).

810 B. Pal et al. / Applied Surface Science 356 (2015) 804–811

Table 3Saturation magnetization (Ms), coercivity (Hc), and retentivity (Mr) determined from M–H curve; transition temperature (Tc) determined from differentiated M–T curve. M0,meff , �m, N are determined from the BMP fitting of the M–H curves for different Zn1 − xNixO (x = 0, 0.03) samples.

Sample type M–H parameters Fitting parameters extracted from BMP model

Ms (emu/g) Hc (G) Mr (emu/g) M0 (emu/g) meff × 10−17 (emu) �m × 10−6 cgs N × 1017 (cm−3)

0-ZnO Nil Nil Nil . . . . . . . . . . . ..76

.98

.76

msZteminpaLooi3

iDoZ[bitpAat(cBdpaiZ

aTdeosBdm[

M

soBLt

In our previous work, we presented a schematic diagramshowing how the various defects play important role in the FMinteraction of TM doped ZnO through the formation of BMPs and

4-ZnO 2.56 280 0.88 28-ZnO 1.84 260 0.65 112-ZnO 1.6 274 0.56 1

orphology of the host lattice etc. Further, on account of the nano-tructure nature, the surface effects are very pronounced for thenO NPs with high surface to volume ratio. There are two interac-ing subsystems in DMS materials: the delocalized conduction bandlectrons, valance band holes, and the diluted system of localizedagnetic moments associated with the TM ions. Ferromagnetic

nteraction couples the spins of s-like electrons or p-like holesear the band edges to the d-shell spins of the TM ions. The firstroposed model was the Zener model driven by the exchange inter-ctions between carriers and localized spins (s–d interaction) [47].ater Dietl et al. have used the Zener description to realize possiblerigin of RT FM as hole-mediated exchange interactions and the-retically predicted a curie temperature above room temperaturen p-type ZnO with 5% Mn dopants and a carrier concentration of.5 × 1020 holes cm−3 [1].

The presence of the intrinsic defects (VO, Zni, VZn, etc.) playmportant role in the FM interaction for TM doped ZnO basedMS systems [39,48,45]. The long-range interaction is necessary tobtain high temperature FM in dilute concentration of TM dopednO DMS systems and it can be mediated by defect induced states6,12,49]. According to the bound magnetic polaron (BMP) model,ound electrons (holes) in the defect states can couple with TM

ons and cause the ferromagnetic regions to overlap, giving riseo long range FM ordering [12,13]. When donors or acceptors areresent, the sp–d interaction often leads to the formation of BMPs.

BMP is a particular type of complex near an occupied donor orn acceptor. It consists of the bound electron (hole) together withhe spins of the TM ions within a hydrogenic Bohr orbit of radius rHfor ZnO, rH = 0.76 nm). Due to the sp–d interaction, the latter spinsan have a significant net ferromagnetic alignment. In this case, theMP resembles a “ferromagnetic ball” embedded in the host latticeoped with TM ions that surround it [13]. Percolation of these BMPsromotes high magnetic moment and high Curie temperature. Inn earlier work on Co doped ZnO NPs, we have demonstrated themportant role of BMPs to achieve RT FM with high moment for thenO based DMS system [26].

Intrinsic defects, such as OV, Zni are inherently present in thes synthesized Zn1−xNixO NPs due to the stabilization of structure.he evidence for the same was obtained from the PL analysis. Theseefects promote the FM ordering by affecting the occupancy ofxtended Ni ions, the interaction of the intrinsic carriers with thexygen vacancies plays significant role in the BMP formation. Ourystematic study shows that formation of defect (OV, Zni) mediatedMPs and their percolation is one of the most promising candi-ates for the observed FM. To understand the suitability of the BMPodel, we attempt to fit the M versus H data using the BMP model

13]. The formula used for the fitting of BMP model:

= M0L(x) + �mH (1)

The first term in Eq. (1) represents the contribution of the BMP’s,econd term is the matrix contribution. Here M0 = Nms, N = Number

f BMPs involved, and ms=‘effective’ spontaneous moment perMP, �m is the susceptibility of the matrix. L(x) = coth(x) − 1/x,angevin function with x = meffH/(kBT), where meff is the true spon-aneous moment per BMP, and at higher temperature it can be

1.2 9.79 1.291.32 6.31 0.841.07 8.25 0.92

approximated to ms = meff. We have analyzed the M–H curve byusing Eq. (1). The parameters m0, meff and �m are variable in thefitting process. The experimental data along with fitted data areshown in Fig. 8 for the 3% Ni doped ZnO NPs. The fitted data closelyfollows the experimental data and the fitted parameters are tabu-lated in Table 3. The spontaneous moment per BMP, meff is found tobe of the order of 10−17 emu. By assuming ms = meff, we have esti-mated the concentration of BMP to be of the order of 1017 cm−3

considering the effective Bohr radius of the BMPs as 0.76 nm (seeTable 3).

However, the calculated BMP concentration is relatively smallcompared to the necessary concentration of 1020 cm−3 in orderto have long range percolation [12]. Thus, the BMP concentrationcannot fully account for the observed high magnetic moment inthe doped ZnO NPs. Further some of the isolated Ni ions mightinteract by AFM interaction, resulting in overall decrease of BMPconcentration [39]. Also, defect related surface spins might con-tribute partly for the observed FM in the Ni doped ZnO NPs. Dueto a large surface area, a lot of defects are present in the 3% dopedsample, as evidenced from the XPS and PL spectra. Sanchez et al.have reported that the uncompensated surface spins enhance thespin polarization induced by substitute Co ions and even in theabsence of magnetic ions, it might promote the formation of p-derived extended magnetic states [50].

Fig. 8. Initial curve (0–Hmax) of the M–H plot is fitted with BMP model (Eq. (1)) forthe 3% Ni doped ZnO NRs. Symbols are for the experimental data and the solid lineis a fit with the BMP model. Extracted parameters are shown in Table 3.

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heir percolation in the ZnO lattice [51]. It was explained that defectediated BMP formation within its hydrogenic radius helps to

romote long range FM ordering with the increase of BMP con-entration. The doped cations provide an extra random potential,hich extends the localized region as its concentration increases.verlap between a hydrogenic electron and the cations within itsrbit leads to ferromagnetic exchange coupling between them, pro-ided the hydrogenic orbital radius is sufficiently large [12,52].hus, BMP formation enhances the long range FM ordering in thei doped ZnO NPs.

Note that, the defect related FM moment for the undopedanostructures as well as for many reports on TM doped ZnOanostructures are of much lower magnitude (micro or mili emu/g)53,54]. We observed at least three orders of magnitude higher

oment in the Ni doped ZnO NPs. Thus, it appears that both TMs asell as defects are important ingredients to attain high moment asell as high Tc. These FM NPs might have important applications inractical spintronic devices owing to its high ordering temperature.

. Conclusion

In summary, Zn1−xNixO NPs showing RTFM with high magneticoment have been synthesized with a low concentration (3%) of Ni

oping. Through systematic study, we find that 3% doping is an opti-um choice for the ball milling technique, as higher concentration

5.7%) of doping shows appearance of secondary phases (e.g., seenset of Fig. 1). The structural, optical, and magnetic properties areignificantly changed after the incorporation of Ni2+ ions in the Zn2+

attice site. XRD, Raman, XPS, TEM and HRTEM analyses confirm thebsence of extrinsic phases in the 3% Ni doped ZnO NPs for higherilling time. HRTEM lattice image and the SAED pattern show that

ll the NPs are of single crystalline phase. XPS spectra also confirmhe presence of Ni ions in 2+ states within the host lattice. RT mag-etic measurements exhibit FM behavior with saturation momentf 2.6–1.6 emu/g. The FM moment decreases at higher milling timeue to decrease of defect density with milling time. PL and XPS spec-ra confirm the nature of defects in the undoped and doped samples.L and UV–vis measurements show doping induced modificationn the band edge emission and absorption spectra. It is believedhat the observed FM is occurring by the intrinsic exchange inter-ction of Ni2+ ions and Zni, OV defects through BMP formation andheir percolation. Both the Ni ions as well as defects are signifi-ant ingredients to attain high moment as well as high orderingemperature.

cknowledgments

We thank CSIR for providing senior research fellowship andnancial support through project grant (059/0050/2k13/EMR-I) inarrying out this work. We also thank Central Instruments FacilityCIF), IIT Guwahati and Dr. Soumen Dhara, Department of Electricalnd Electronic Engineering, Kobe University, Japan to accomplishart of this work.

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