Results in Physics 6 (2016) 1064–1071

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Influence of Mn doping on the magnetic and optical properties of ZnOnanocrystalline particles

http://dx.doi.org/10.1016/j.rinp.2016.11.0412211-3797/� 2016 Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⇑ Corresponding authors at: Department of Physics, The Hashemite University,13115 Zarqa, Jordan (M. Shatnawi), Department of Physics, Kuwait University,13060 Safat, Kuwait (A.M. Alsmadi).

E-mail addresses: shatnawi@hu.edu.jo (M. Shatnawi), abdel.alsmadi@ku.edu.kw(A.M. Alsmadi).

M. Shatnawi a,⇑, A.M. Alsmadi a,b,⇑, I. Bsoul c, B. Salameh b,d, M. Mathai e, G. Alnawashi a, Gassem M. Alzoubi a,F. Al-Dweri a, M.S. Bawa’aneh f

aDepartment of Physics, The Hashemite University, 13115 Zarqa, JordanbDepartment of Physics, Kuwait University, 13060 Safat, KuwaitcDepartment of Physics, Al al-Bayt University, Mafraq, JordandDepartment of Applied Physics, Tafila Technical University, Tafila, JordaneDepartment of Electrical Engineering, Kuwait University, 13060 Safat, KuwaitfDepartment of Physics, Yarmouk University, Irbid, Jordan

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 September 2016Accepted 18 November 2016Available online 23 November 2016

Keywords:ZnOMn doped ZnODilute magnetic semiconductorsMagnetic propertiesXPS spectroscopyFTIREnergy gapsp-d exchange interactionOxygen vacancies

The structural, optical and magnetic properties of Mn doped ZnO nanocrystalline particles, Zn1-xMnxO,with different percentages of Mn content have been studied. XRD and XPS measurements showed thatall samples with Mn doping up to x = 0.1 possess typical wurtzite structure and have no other impurityphases. The incorporation of Mn ions into the ZnO lattice was also confirmed by FTIR and UV–Vis. spec-troscopy results. Both XRD and SEM results indicated a slight decrease in the grain size with increasingthe Mn doping level. The XPS results indicated an increase in the oxygen vacancies concentration withincreasing the Mn doping level. The magnetization measurements revealed a weak ferromagnetic behav-ior at room temperature and a clear ferromagnetic behavior with relatively large coercive fields at lowtemperature. The ferromagnetic order is improved by increasing the Mn doping. In addition, we observedan increase in the concentration of oxygen vacancies, which is also induced by increasing the Mn dopinglevel. A ferromagnetic coupling of the local moment of Mn dopants through the sp-d exchange interactionand oxygen vacancies, in addition to different magnetic contributions due to different forms of Mn ionsthat coexist in the Mn doped nanoparticles were presented in order to interpret the observed magneticbehavior. We observed a clear red shift in the direct band gap and an increase in the coercive field andsaturation magnetization values with increasing the Mn doping level.� 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Zinc oxide (ZnO) semiconductors have already been activelystudied because of their advantageous electronic and optical prop-erties which in turns lead to a wide variety of device applicationssuch as opto-electronic and luminescent devices, chemical sensors,logistic fuel cells, DNA sequence detectors, and ultraviolet photo-detectors [1–4]. Intensive research is currently being carried outon enhancing these advantageous properties and adding novelfunctionalities into ZnO. For example by making ZnO as a dilutemagnetic semiconductors (DMS) in which a random substitutionof a fraction of the original atoms of the ZnO host lattice is substi-

tuted by one of the transition metal (TM) ions (e.g. Mn2+, Fe2+, Co2+

and Ni2+) could introduce ferromagnetic function while its semi-conducting property is retained [5]. By combining both semicon-ducting and ferromagnetic properties, DMS can lead to the state-of-art spintronic devices that use both spin and charge of electrons[5]. These future devices, such as non-volatile memories, read headand spin valve transistors, are expected to downsize the currentelectronics into nanometers scales. Through different TM dopingZnO, Mn has the advantage because of its high moment, relativelysmall ionic radii difference between Mn2+ and Zn2+, and its exactlyhalf-filled 3d orbitals which facilitates its incorporation into theZnO lattice. In fact, the research on DMS was sparked by the theo-retical work on Mn-doped ZnO by Dietl et al. where room-temperature ferromagnetism (RTFM) was predicted [6].

Mn doped ZnO have been synthesized by various techniques[7–14] and RTFM has been already observed in Mn doped ZnO bymany research groups [14–21]. However, the mechanism for the

M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071 1065

observed ferromagnetism in Mn doped ZnO, similar to other TMdoped ZnO, still remains controversial and the question whetherit is an intrinsic or extrinsic property is still with no clear answer.Depending on the preparation method, different magnetic proper-ties ranging from paramagnetism to ferromagnetism with differentCurie temperatures were reported. For instance, different magneti-zation mechanisms like the uncompensated spins at the surface ofanti-ferromagnetic nanocrystal of Mn rich-ZnMnO, strong sp-dexchange interaction between acceptor level and Mn2+ impurityionization level, are reported to explain the observed RTFM[16,20]. Direct and indirect interactions among Mn2+ ions werealso introduced to explain the observed RTFM [21]. The concentra-tion of oxygen vacancies, especially single ionized oxygen vacan-cies, were reported to play a crucial role in mediating RTFM inthe Mn doped ZnO [13,14,17–19]. On the other hand, the absenceof ferromagnetism in Mn-doped ZnO is reported by other studies[22–24]. Other researchers found that the observed ferromag-netism in Mn doped ZnO arises from external impurities [25]. Evenfor results that have confirmed the intrinsic ferromagnetic naturein Mn doped ZnO, the explanations for its origin is still diverse.The ferromagnetismwas claimed to be either originating from sub-stitutional spin-polarized Mn atoms sitting in the ZnO lattice withdifferent interactions between the localized Mn ions and throughthe carrier-mediated host atoms or from lattice defects (local mag-netic moment of defects such as the Zn and/or O vacancies).

The discrepancy between these different studies besides show-ing physical properties dependence on the preparation methods,and the lack of an understanding for the magnetic-optical interre-lation in Mn doped ZnO nanoparticles requires the implementationof complementary structural, optical and magnetic study to enablea comprehensive investigation on their physical properties.Although solid state reaction method is known to be simple andreproducible, only few reports were devoted to the study Mndoped ZnO nanoparticles using this method.

In this paper, we have performed a systematic study on theeffect of Mn doping on the physical properties of ZnO nanocrys-talline particles in an attempt to understand their magnetic behav-ior and also to explore the interrelation between the structural,optical and magnetic properties of these particles. The overallresults were discussed in terms of the Mn dopant mechanismand the coupling between the Mn moments through the sp-dexchange interaction and the concentration of oxygen vacancies.

Fig. 1. Standard JCPDS pattern for ZnO (file no: 043-0002) and XRD patterns ofZn1-xMnxO with different doping concentrations.

Experimental procedures

Mn-doped ZnO nanocrystalline particles, Zn1-xMnxO, were fab-ricated by the solid state reaction technique with different percent-ages of Mn content x = 0.0, 0.02, 0.04, 0.06, 0.08, and 0.1. Thisfabrication method is standard and explained in more details else-where [9]. The crystal structures were examined by X-ray diffrac-tion (XRD). The XRD measurements were performed usingSiemens D5000 with Cu Ka radiation (k = 1.5406 Å) diffractometer.The morphology of the obtained nanoparticles was characterizedby scanning electron microscopy (SEM) (Phillips XL-30). The effec-tive concentration and binding-state of the ions in the Mn-dopedZnO nanoparticles were characterized with X-ray photoelectronspectroscopy (XPS) (ESCA-LAB 250 Xi, Thermo-VG Scientific). Infor-mation on the molecular geometry, intermolecular interaction andvibration bonds present in the un-doped and Mn-doped ZnOnanoparticles were obtained from the analysis of the Fourier trans-form infrared (FTIR) spectra. The FTIR spectra were recorded usingBruker-Tensor-37 spectrometer with an experimental resolution ofabout 5 cm�1. The Ultra Violet–visible (UV–Vis.) diffuse reflectancespectra were carried out using the Shimadzu Solid Spec-3700 UVe-ViseNIR Spectrophotometer. The magnetic properties were investi-

gated using a Quantum Design magnetometry. The magnetizationdata were recorded in magnetic field up to 3.5 T and at tempera-tures between 300 K and 5 K.

Results and discussion

Structural characterization and surface morphology

Fig. 1 shows XRD patterns of Zn1-xMnxO, with different percent-ages of Mn content including the undoped sample. All the diffrac-tion peaks correspond well to the wurtzite structure for pure ZnOand no evidence of any other phases was found, indicating a suc-cessful incorporation of Mn ions into the ZnO lattice site ratherthan the interstitial ones for Mn doping up to x = 0.1. Similarresults were reported by many research groups for Mn dopedZnO with Mn concentration up to x = 0.1 or even higher[12,14,26]. Fig. 2 shows XRD patterns with full Rietveld refinementanalysis for Zn1-xMnxO, with x = 0.0, and x = 0.08. The results of therefinement demonstrated that both samples are consistent withthe wurtzite ZnO structure. The refinement results for all samplesare summarized in Table 1. The lattice constants and the volume ofthe unit cell for the Mn doped ZnO nanoparticles are found to beslightly large compared the pure ZnO and tends to increase withincreasing the Mn doping. A very small increase in the lattice con-stants for the Mn doped ZnO nanoparticles is expected for a suc-cessful incorporation of the Mn ions by the Zn ions in Zn1-xMnxOsince the Mn2+ radius (0.74 Å) is slightly larger than the Zn2+

(0.67 Å). Similar results were reported by other research groups

Fig. 2. Rietveld refined XRD patterns for Zn1-xMnxO with x = 0.0, and x = 0.08.

1066 M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071

on Mn-doped ZnO nanoparticles that are prepared using chemicalroute and ball milling method [12,19]. The average crystallite sizes(D) were calculated from all high intensity peaks of the XRD pat-tern by using Scherrer’s equation [27] and listed in Table 1. A slightdecrease in the average crystallite size was observed with increas-ing the Mn doping level.

The microstructural characteristics of the Zn1-xMnxO wereexamined by the SEM. Fig. 3 shows representative SEM images cor-responding to morphology and grain size of the Zn1-xMnxO with(x = 0.0, 0.04, 0.08 and 0.1). Large grains composed of differentsizes nanoparticles may be seen in these images. The average par-ticle sizes are found to be between 300 and 350 nm for pure ZnO.For Mn doped ZnO, the grain size is slightly smaller than the pureZnO and decreases as the Mn doping level increases. This is in goodagreement with the XRD results (see Table 1).

X-ray photoelectron and Fourier transform infrared spectroscopyresults

Although the XRD results strongly indicated that all Zn1-xMnxOnanocrystals with Mn doping up to x = 0.1 possess typical wurtzitestructure and have no other impurity phases, other quantitativetechniques like XPS could provide another evidence on how dopedion incorporate into ZnO matrix. In addition, the phase separation

Table 1Results of the refinement of XRD patterns of Zn1-xMnxO nanoparticles. Bragg factor, v2, la

x RB v2 a = b (Å)

0.0 3.01 0.960 3.24680.02 2.61 0.934 3.24740.04 2.46 0.913 3.24760.06 2.85 0.972 3.24770.08 2.12 1.052 3.24790.10 2.81 0.957 3.2494

in the Mn-doped ZnO may be also chemical were some of Mn ionscould be distributed over cation sites in small amounts which isbelow the detection sensitivity of the XRD. For that, XPS measure-ments were implemented to investigate the chemical compositionand the valence state of all elements in Zn1-xMnxO. The calibrationfor the peak positions was performed according to the C 1s peak at284.6 eV yielded from the samples handling. The results of the XPSmeasurements for the Zn1-xMnxO with x = 0.08 as a representativesample are shown in Fig. 4. The full XPS survey spectrum is shownin Fig. 4(a). The characteristics peaks of Zn, Mn, O, and the adven-titious C elements were observed confirming the presence of Mn inthe ZnO host lattice. No other additional peaks were observed, con-firming the single phase of the sample. The oxidation states werefurther investigated by the high resolution spectra obtained by afine scan around each element. The results are shown in Fig. 4(b), (c) and (d) for the Zn 2p, Mn 2p, and O 1s, respectively. InFig. 4 (b) each of the two strong Zn 2p symmetrical peaks can befitted with one Gaussian envelope located at 1021.3 eV and1044.2 eV corresponding to Zn 2p3/2 and Zn 2p1/2 states, respec-tively. The values of the binding energies and the spin orbit split-ting energy agree very well with the standard value for ZnO[13,14,21,28,29]. This indicates that Zn preserves its single compo-nent 2+ oxidation state without forming other Mn-Zn-O com-pounds or clusters. The observed small increase in the bindingenergy of the Zn 2P3/2 signal (1021.3 eV for the Mn-doped ZnO)compared to the standard value of (1021.1 eV for pure ZnO) isattributed to the substitution of Mn2+ ions, which have a slightlylarge radius compared to Zn2+ ions, in the Zn lattice site to formthe Zn-Mn bonding structure [13]. The binding energies of Mn2p3/2 is found to be around 640.7 eV, which matches the Mn2+

binding energy in MnO [13,30]. This indicates a divalent sate ofthe Mn and confirms the successful substitution of the Mn2+ atthe tetrahedral sites surrounded by the O2� ions. Moreover it alsorules out the possibility for the formation of metallic Mn or otherMn oxides structures as Mn3O4, Mn2O3, and/or MnO2 which have2 P2/3 peaks at 639 eV, 641.2 eV, 641.6 eV and 642.2 eV, respec-tively. The binding energies of Mn 2p1/2 is found to be around651.4 eV and the observed peak around 654 eV is attributed toAuger electrons from the deep level of Zn LM7 and Zn LM8. TheXPS measurements performed on the other samples with differentMn concentrations (data not shown) showed almost similar resultswith no shift in the binding energies affirming that the Mn is occu-pying the Zn substitutional lattice sites in all Zn1-xMnxO samples.Fig. 4 (d) shows the O 1s asymmetric peak which could be decon-voluted to three Gaussian components, a, b, and c. The component(Oa) with low binding energy centered at 529.8 eV is ascribed tothe O2� ions bounded to either Zn2+ or substitutional Mn2+ at thetetragonal sites in the ZnO lattice [28,31], while the component(Oc) with high binding energy centered at 532.4 eV is associatedto the chemisorbed oxygen [28]. The peak (Ob) at 531.2 eV is asso-ciated with oxygen vacancies (VO), in oxygen-deficient regionswithin the matrix of ZnO [32,33]. Interestingly, the ratio of Ob/OT

integrated intensities could reflect the concentration of oxygenvacancies in the Mn doped ZnO nanoparticles. OT represents thetotal area of the O 1s peak. In Table 2, the ratio of Ob/OT is listed

ttice constants a, c, unit cell volume, and crystallite size.

c (Å) V (Å)3 D (nm)

5.2019 47.4929 675.2017 47.5066 575.2022 47.5193 465.2022 47.5202 455.2024 47.5233 475.2050 47.5951 42

Fig. 3. SEM images for representative samples of Zn1-xMnxO with (a) x = 0.0, (b) x = 0.04, (c) x = 0.08, and (d) x = 0.1.

0 200 400 600 800 1000 1200

Zn 2

s

Zn 3

d Zn 3

pZn

3s

O 1

sM

n 2p

Zn 3

p 3/2

Zn 3

p 1/2

(a. u

.)

a) Zn1-x

MnxO

x = 0.08

C 1

s

1021 1028 1035 1042 1049

Zn 2P3/2

Zn 2P1/2

b)

635 640 645 650 655

c)

Mn 2P3/2

Mn 2P1/2

Inte

nsity

Binding 528 530 532 534 536

Energy (eV)

α

β

O 1Sd)

γ

Fig. 4. XPS spectra for ZnxMn1-xO, with x = 0.0.8. (a) XPS survey scan spectra, (b) high resolution Zn 2p core level XPS spectra, (c) high resolution Mn 2p XPS spectra, and (d)high resolution O 1s XPS spectra.

M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071 1067

for different Mn doped ZnO nanoparticles. As we can see from thistable, a clear increase in the relative area ratio of Ob/OT, indicatingan increase in the oxygen vacancies concentration (VO), by increas-

ing the Mn doping level. This result could be connected with theXRD results, where we observed a decrease in the average crystal-lite sizes indicating more lattice defects (oxygen vacancies) with

Table 2XPS calculation results for areal ratio of Ob/OT of Zn1-xMnxO nanoparticles. The lastcolumn represents the saturation magnetization (Ms) measured at 10 K.

x Ob/OT (%) Ms (emu/g)

0.02 7.8 0.210.04 21.2 1.250.06 36.9 1.320.08 36.8 1.420.10 47.3 1.53

1068 M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071

increasing the Mn doping level. More lattice defects with increas-ing the Mn doping level were previously reported on Mn-dopedZnO nanoparticles [19].

Fig. 5 shows the room temperature FTIR spectra in thewavenumber range of 300–4000 cm�1 for Zn1-xMnxO with x = 0.0,0.04, 0.08, and 0.10. The observed strong band centered at around450 cm�1 in all samples under investigation is assigned to theasymmetric stretching vibration of O-Zn-O in the octahedral coor-dination, while the peak at around 620 cm�1 is ascribed to the Mn-O stretching modes. These bands confirm the wurtzite structure inboth ZnO and Mn doped ZnO nanoparticles [34,35]. The broadabsorption in the 3350–3650 cm�1 range is attributed to the –OHgroups of H2O, indicating the existence of water absorbed on thesurface of ZnO nanograins. The two weak absorption peaksobserved at around 1400 and 1600 cm�1 are corresponded to thestretching of the asymmetric and symmetric C@O stretching bond.The deformation bands of C@O can also be observed around980 cm�1.

Magnetic properties

Magnetization curves for the Zn1-xMnxO samples with differentmanganese content, x, were measured at 300 K and at 10 K. Theresults for few representative samples are shown in Fig. 6 (a) and(b). At 300 K the pure ZnO showed diamagnetic behavior in goodagreement with previous results [36,37]. For Mn doped ZnO sam-ples, the room temperature magnetization curves at high fieldsregion show almost a linear curve indicating paramagnetic/superparamagnetic behavior. However, a closer examination of the mag-netization curves in the low field region visualizes a weak hystere-sis in all samples under investigations. The expanded view of theM-H curves (not shown here) showed clearly a small coercivityfield of values ranging from 30 Oe for x = 0.02–60 Oe for x = 0.1.

500 1000 1500 2000 2500 3000 3500

Tran

smitt

ance

(a. u

.)

Wavenumber (cm-1)

x = 0.00

x = 0.10

x = 0.08

x = 0.04

Zn-O

Mn-O

Fig. 5. FTIR spectra for ZnxMn1-xO, with x = 0.0, x = 0.04, x = 0.08, and x = 0.1measured at room temperature.

This indicates a partial long range ferromagnetic ordering whichis over dominant by the paramagnetic phase. This coexistence ofthe ferromagnetic and paramagnetic phases persisted even forthe magnetization curves recorded at 10 K (Fig. 6 (b)) but withmore significant ferromagnetic contribution evident by theiralmost S-shaped M-H curves. Still, the saturation magnetizationwas not attainable even in 3.5 T applied magnetic fields due tothe paramagnetic component contribution which is dominant inthe high field region. Such attitude has also been reported forMn- and other TM- doped ZnO systems [12,14,15,19,37]. The con-tribution of the ferromagnetic ordering at low temperature may bequantified by studying the coercivity fields for different concentra-tions of Mn. The results are shown in Fig. 6(c). For all Mn concen-trations, the coercivity field values are higher than the 30 Oe (theroom temperature value) indicating an increase in the ferromag-netic ordering with lowering temperature as expected. In addition,we observed an increase in the coercivity field values with theincrease of Mn doping. We also observed an increase in the satura-tion magnetization values with increasing the Mn doping level. Thesaturation magnetization measured at 10 K is obtained from thelow of approach to saturation [38] and the values are listed inTable 2 for different Mn doping. It is interesting in this context toobserve the direct correlation between the concentration of oxygenvacancies and the saturation magnetization, which could help inexplaining the origin of the ferromagnetic order in Mn dopedZnO nanoparticles. This result suggests that the observed ferro-magnetic order in Mn doped ZnO is enhanced by the concentrationof oxygen vacancies.

Since both XRD and XPS results clearly demonstrated that allMn-doped ZnO under investigation have a single wurtzite phase,with no additional phases, we expect that the observed ferromag-netic behavior in the Mn doped ZnO nanoparticles is from anintrinsic exchange interaction of Mn magnetic moments. Differentmagnetic contributions due to different forms of Mn ions thatcoexist in the Mn doped nanoparticles can be introduced in orderto interpret the observed magnetic behavior. A ferromagnetic cou-pling of the local moment of Mn dopants through the sp-dexchange interaction and lattice defects (oxygen vacancies) couldbe responsible for the ferromagnetic behavior at low temperature.The absence of saturation in the magnetization curves at low tem-perature even in high magnetic applied of 3.5 T may be attributedto the presence of a paramagnetic exchange interaction betweenisolated Mn ions and possible antiferromagnetic coupling betweenMn neighboring ions. The ferromagnetic contribution from possi-ble local magnetic moment of intrinsic defects such as the O andZn vacancies seems to play a major factor in obtaining the RTFMin Mn doped ZnO nanoparticles [13,32,33].

Fig. 6 (d) shows the zero-field cooled (ZFC) and field cooled (FC)magnetization curves of Zn1-xMnxO nanoparticles, with x = 0.1 as arepresentative sample, as a function of temperature measured atsmall magnetic field of 100 Oe. Similar measurements were per-formed on all other samples with different Mn concentrations(data are not shown) and almost similar behavior were observed.A small thermal irreversibility between the ZFC and FC curvescan be clearly seen for almost the whole measured temperaturerange and the FC magnetization is always above the ZFC, support-ing the presence of RTFM in all investigated samples [39]. Thesmall cusp in the ZFC curve around 15 K is consistent with thepresence of multi domain particles at low temperature with similarsource of pinning mechanism.

Optical properties

The origin of the magnetic behavior of the TM doped ZnOnanoparticles is strongly correlated to their optical properties[40], therefore it is essential to understand the optical properties

-0.1

-0.05

0

0.05

0.1

-12 -8 -4 0 4 8 12

Zn1-x

MnxO

T = 300 K

a)

M (e

mu/

g)

Magnetic Field (kOe)

0.100.06

x = 0.0

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

-30 -20 -10 0 10 20 30

0.10

x = 0.02

M (e

mu/

g)

Magnetic Field (kOe)

T = 10 Kb)

0

100

200

300

400

500

0.02 0.04 0.06 0.08 0.1

HC (O

e)

Concentration (x)

c)

T = 10 K

0.001

0.002

0.003

0 40 80 120 160 200 240 280Temperature (K)

100 Oe

FC

ZFCM

(em

u/g)

d)x = 0.1

Fig. 6. (a) Magnetization hysteresis curves of the Zn1-xMnxO samples with x = 0.0, x = 0.06, and, x = 0.1 measured at 300 K, (b) Magnetization hysteresis curves of theZn1-xMnxO samples with x = 0.02, x = 0.04, x = 0.08, and x = 0.1 measured at 10 K, (c) The measured coercive fields of the Zn1-xMnxO samples as a function of Mn concentration(x), (d) ZFC and FC magnetizations of Zn1-xMnxO as a function of temperature measured at 100 Oe for x = 0.1.

M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071 1069

of these materials. The room temperature UV–Vis diffuse reflec-tance spectra of pure and Mn doped ZnO nanoparticles with differ-ent Mn concentration in the wavelength range 300–800 nm isshown in Fig. 7 (a). The pure ZnO sample shows a high reflectancein the visible region while a strong reduction in the reflectance isobserved after doping with Mn. Moreover a progressive decreasein the reflectance in the visible region is observed by increasingthe Mn concentration. For the Mn doped samples, additional broadabsorption peaks are observed in the visible region as shown in theinset of Fig. 7 (a). The positions of these peaks do not change withincreasing the Mn concentration. Similar absorption peaks hasbeen reported by ellipsometry and by optical absorption measure-ments of Mn-doped ZnO in the wavelength range (400–685 nm)[41,42]. These peaks correspond to electronic transitions whichare known to occur when Mn2+ exist in a tetrahedral environmentwith lattice distortion [42,43]. Since the wurtzite structure of ZnO(P63/mmc space group) consists of tetrahedrally coordinated O2�

and Zn2+ ions, the observation of these peaks in the visible lightregion indicate the successful incorporation of Mn2+ ions the Zn2+

sites of the ZnO matrix. Moreover, these results are in good agree-ment with the magnetization measurements as a room tempera-ture ferromagnetic behavior is observed because of the high-spinstates occupation. The Tauc’s plots were used to calculate the bandgap energies of all doped ZnO samples according to the followingrelation [44]:

FðRÞhm ¼ Aðhm� EgÞn

Where FðRÞ ¼ ð1�RÞ22R is Kubelka–Munk function, R is the reflec-

tance, m is the frequency, A is a proportionality constant andn = 1/2 for direct band gap semiconducting materials. By extrapo-lating the linear portion of the Tauc Plots to the (F(R)*ht)2 = 0,the energy gap, Eg, is determined as shown in Fig. 7 (b) for repre-sentative samples. For the un-doped ZnO, the band gap is 3.24 eVwhich is similar to the reported value of ZnO nanoparticles

[45,46]. The calculated energy gap versus the Mn concentration(x), of Zn1-xMnxO nanoparticles is shown in Fig. 7 (c). We observeda red shift in the band gap due to Mn doping in ZnO. This narrow-ing of the band gap by increasing the Mn content is attributed tothe exchange interaction between the ‘d’ electron of Mn atomand ‘s’ and ‘p’ electrons of the host Zn atom (s–d and p–d interac-tions). Similar behavior has been observed in doped ZnO nanopar-ticles which show ferromagnetic behavior [45,47,48]. Since theexchange interaction is relatively weak the band gap does not havesignificant decrease with the increase in Mn doping.

The decrement in the optical band gap is interpreted as strongsp-d exchange interaction between the band electrons of ZnO andthe localized d electrons of Mn ions substituting for the Zn+2 ions[17,37,49]. The difference between the Mn2+ 3d electrons and theZn 4s electrons results in free charge carriers that mediate theinteraction between the Mn localized magnetic moments. Thepresence of a ferromagnetic phase in the Mn doped ZnO nanopar-ticles at room temperature is mainly due to the ferromagnetic cou-pling of the local moment of Mn dopants through the sp-dexchange interaction. This sp-d exchange interaction (hybridiza-tion) has a duel effect. It is needed to mediate the couplingbetween the Mn moments in order to form the long-range ferro-magnetic order, but increasing the sp-d hybridization also causesdelocalization of d electrons and the subsequent destabilizationof Mn magnetic moments [13]. The sp-d exchange interaction canbe also tuned by thermal energy and this could explain the coexis-tence of both ferromagnetic phase, which is dominate at low tem-perature and the paramagnetic phase, which is dominate at roomtemperature in the Mn doped ZnO nanoparticles. Since the RTFMbehavior in the Mn doped ZnO is observed because of the high-spin states occupation and since the magnetization data indicatedan increase in the magnetic moment and coercive field values withMn doping, it is expected to observe an increase in the ferromag-netic ordering, which is mediated by the sp-d interaction and by

Fig. 7. (a) The UV–Vis diffuse reflectance spectra of Zn1-xMnxO nanoparticles withdifferent Mn doping as a function of wavelength, the insert shows the broadreflection minima in the visible range for the samples with 0.08% Mn, (b) The TaucPlots, (F(R)*ht)2 versus ht, of Zn1-xMnxO nanoparticles with different Mn doping, (c)The energy gap as a function of Mn concentration (x).

1070 M. Shatnawi et al. / Results in Physics 6 (2016) 1064–1071

oxygen vacancies, by increasing the Mn concentration. Theincrease in the sp-d interaction and the concentration oxygenvacancies by increasing Mn doping are supported by the opticalreflectance measurements and the XPS measurements,respectively.

Conclusion

In conclusion, we have prepared Zn1-xMnxO nanocrystallineparticles with different percentages of Mn content by the conven-tional solid state reaction technique. The successful incorporationof the Mn2+ ions into the ZnO lattice was confirmed by XRD, XPS,FTIR and UV–Vis diffuse reflectance spectra. The lattice constantsand the volume of the unit cell for the Mn doped ZnO nanoparticleswere found to be slightly large compared the pure ZnO and tendsto increase with increasing the Mn doping. The XPS results indi-

cated an increase in the concentration of oxygen vacancies withincreasing the Mn doping level. At room temperature a signaturefor a weak ferromagnetic phase was observed for all Mn-dopedZnO nanoparticles, which coexist with a strong paramagneticphase. At low temperature, the contribution of the ferromagneticphase becomes dominant over the paramagnetic phase. Differentmagnetic contributions due to different forms of Mn ions thatcoexist in the Mn-doped nanoparticles were presented in orderto interpret the observed magnetic behavior. A ferromagnetic cou-pling of the local moment of Mn dopants through the sp-dexchange interaction and oxygen vacancies, paramagnetic Mn-Mn super exchange interaction between isolated Mn ions, in addi-tion to antiferromagnetic interaction betweenMn neighboring ionswere introduced. In addition, we observed a strong correlationbetween the magnetic and optical properties of the Mn-dopedZnO nanoparticles. A red shift in the absorption band gap and animprovement in the ferromagnetic order with increasing Mn dop-ing were observed. The sp-d exchange interaction responsible forthis red shift could also explain the increase in the ferromagneticphase by increasing the Mn doping.

Acknowledgments

We gratefully acknowledge the support of Kuwait Universityunder the Semiconductor research facility, research administration(project GE 01/08) and the general facility of the faculty of science(Projects GS 01/05, GS 02/10 and GS 02/08). We would like also toacknowledge the support from the Hashemite university for per-forming the magnetic measurements by the PPMS.

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