Structural and magnetic properties of Mn-doped ZnO nanocrystals

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Structural and magnetic properties of Mn-doped ZnO nanocrystals

M. Bououdina

^a,b,n

, K. Omri

^c

, M. El-Hilo

^b

, A. El Amiri

^d

, O.M. Lemine

^e

, A. Alyamani

^f

, E.K. Hlil

^g

, H. Lassri

^d

, L. El Mir

^c,e

aNanotechnology Centre, University of Bahrain, PO Box 32038, Kingdom of Bahrain

bDepartment of Physics, College of Science, University of Bahrain, PO Box 32038, Kingdom of Bahrain

cLaboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Faculty of Sciences in Gabes, Gabes, Tunisia

dLaboratoire de Physique Fondamentale et Appliquée (LPFA), Faculté des Sciences Ain Chock, Université Hassan II, B.P. 5366, Mâarif, Casablanca, Morocco

eAl Imam Mohammad Bin Saoud Islamic University (IMSIU), College of Sciences, Department of Physics, Riyadh 11623, Saudi Arabia

fNational Nanotechnology Centre, KACST, Riyadh, Saudi Arabia

gInstitut Néel, CNRS–Université Joseph Fourier, BP 166, 38042 Grenoble, France

H I G H L I G H T S

Mn-doped ZnO nanocrystals with crystallite size is in the range of 30-50 nm were successfully prepared using a novel sol-gel method.

Rietveld reﬁnements conﬁrm the formation of pure Mn-doped ZnO for lower Mn concentration.

Above 2% of Mn, the doping is not helping the long range ferromagnetic order in the sample but only enhancing the paramagnetic component.

The estimated magnetic moment via An Ab-initio calculations is also consistent with magnetic analysisThe estimated values for the magnetic moment per Mn atom are found to be in the range of 2-3.5µB/Mn.

The estimated magnetic moment via An Ab-initio calculations is also consistent with magnetic analysis.

a r t i c l e i n f o

Article history:

Received 9 June 2013 Received in revised form 31 July 2013

Accepted 20 August 2013 Available online 31 August 2013 Keywords:

Dilute magnetic semiconductor Mn doped ZnO

DFT calculation Nanocrystal

Magnetic moment analysis

a b s t r a c t

Mn-doped ZnO nanocrystals were successfully prepared using a novel sol–gel method followed by drying in autoclave under supercritical conditions. The estimated crystallite size is in the range of 30–50 nm, in agreement with TEM analysis. Rietveld reﬁnements conﬁrm the formation of pure Mn-doped ZnO for lower Mn concentration. i.e. less than 5%. The lattice parameters increase with increasing Mn content according to Vegard's law due to the larger ionic radius of Mn^2þcompared to that of Zn^2þ. Magnetic analysis reveals that increasing the doping level of Mn above 2% is not helping the long range ferromagnetic order in the sample but only enhancing the paramagnetic component. The paramagnetic susceptibility is found to increase linearly with increasing Mn concentration which suggests the formation of uncoupled magnetic moment. The estimated values for the magnetic moment per Mn atom are found to be in the range of 2–3.5mB/Mn. Ab-initio calculations also have been performed which showed that doping diamagnetic bulk ZnO with Mn induces ferromagnetic at room temperature, the total magnetic momentum increases with increasing Mn content whereas the magnetic moment of Mn is predicted to be in the range of 3–3.5μB/Mn atom which is consistent with the values obtained from magnetic measurements.

1. Introduction

Materials at the nanoscale are attracting more attention due to their fascinating properties making them with improved properties compared to their bulk counterparts due to large surface area and quantum conﬁnement. In the literature, many researchﬁnd- ings indicate that the physical, chemical, and mechanical

properties can be easily tuned by changing only the shape and/

or size of the nanoparticles (NPs)[1].

Diluted magnetic semiconductors (DMS) materials [2] have attracted much interest in recent years because of the combination of both semiconducting and magnetic properties within the same material. Among the potential applications that DMS materials can offer is spintronics [3] (exploiting both the electron charge associated with the intrinsic spin of the electron).

Zinc oxide (ZnO) crystallizes within the hexagonal wurtzite- type structure at room temperature, with a wide bandgap energy (Eg¼3.3 eV at 300 K) [4], and considered as electron (n-type) conductivity, with a large exciton binding energy of60 meV[4].

Contents lists available atScienceDirect

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

Physica E

http://dx.doi.org/10.1016/j.physe.2013.08.024

nCorresponding author at: Nanotechnology Centre, University of Bahrain, PO Box 32038, Kingdom of Bahrain. Tel.:þ97 317437917.

E-mail address:mboudina@gmail.com (M. Bououdina).

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ZnO NPs have been synthesized using various methods including the precipitation[5], sol–gel[6], microwave plasma[7], modi- ﬁed polyol method[8], citrate-assisted hydrothermal[9], electro- magnetic levitational gas condensation method[10], etc.

Moreover, it has been reported that doping paramagnetic ZnO by another transition metal, such as Cr[11], Mn, Fe and Ni[12]and Co[13]; induces important modiﬁcation in its magnetic properties, it becomes ferromagnetic at room temperature thereby a potential candidate for spintronics.

In this paper, Mn-doped ZnO system with various Mn content up to a higher values (0.1–15 at%) has been prepared using a new modiﬁed sol–gel method. Structural and microstructural as well magnetic properties were investigated. The effects induced by Mn addition will be presented and discussed.

2. Experimental 2.1. Sample preparation

Manganese doped zinc oxide (ZnO:Mn) nanopowders were in the ﬁrst step prepared by a sol–gel method using 16 g of zinc acetate dehydrate [Zn(CH3COO)22H2O] as a precursor in 112 ml of methanol. After 10 min of magnetic stirring at room temperature, an adequate quantity of manganese (II) chloride tetrahydride [MnCl24H2O] was added. After 15 min under magnetic stirring, the solution was placed in an autoclave and dried in the supercritical conditions of ethyl alcohol (EtOH) according to El Mir et al.

protocol[14].

2.2. Characterisations

The structural characterizations of pure and Mn-doped ZnO compositions were performed using Rigaku Ultima IV diffract- ometer equipped with Cu-Kα radiation (λ¼1.5418 Å). Structural reﬁnements using the Rietveld method were carried out using PDXL program; lattice parameters (a, c), crystallite size (D) and microstrain (ε) were reﬁned.

Morphological studies have been performed with a transmis- sion electron microscope (TEM, JEM-200CX). The specimens for TEM analysis were prepared by putting the as-grown crystals in EtOH and immersing them in an ultrasonic bath for 15 min, then dropping a few drops of the resulting suspension containing the synthesized materials onto TEM grid.

Magnetic characterization was made by measuring the magnetization curves using a vibrating sample magnetometer type Micro- Mag Model 3900 with a sensitivity of 0.5μemu for a 1 s averaging time scale. Magnetization curves were measured at room temperature (T¼300 K) from þ1 T to1 T using an averaging time of 1 s.

2.3. Electronic and magnetic structure calculations

Electronic structure calculations were performed within the Korringa–Kohn–Rostoker (KKR) method [15]. To match random distribution of Mn²^þions in Zn²^þsites, we have used the coherent potential approximation (CPA). The Vosko, Wilk and Nusair (VWN) parameterization of exchange-correlation energy functional [16]

was used. The form of the crystal potential is approximated by a muffin-tin potential, and the wave functions in the respective muffin-tin spheres were expanded in real harmonics up tol¼2, wherelis the angular quantum momentum defined at each site.

Spin polarized, relativistic effect and spin–orbit interaction were taken into account. The Zn_1xMnxO system was considered to crystallize in wurtzite hexagonal unit cell structure with the lattice parameters a¼3.27 Å and c¼5.26 Å, as measured for (Zn,Mn)O and reported by Fukumura et al.[17].

3. Results and discussion 3.1. Ab-initio calculations

Atfirst, in order to distinct the influence of Mn addition, we have calculated the total DOS of pure ZnO as seen inFig. 1a. The Fermi level is set to 0 and indicated by dashed line. The band gap is about 3 eV which is close to experimental value of 3.3 eV. The valence electron configuration of Mn^2þ is 3d⁵. Under the crystalfield of

-0.6 -0.4 -0.2 0.0 0.2 0.4

-150 -100 -50 0 50 100 150

DOS

Energy ( Ry ) E_F

Fig. 1.(a) Total DOS of pure ZnO; (b) Total DOS of pure Zn0.9Mn0.1O composition.

d(Mn): DOS for Mn 3d-orbitals; p(O): DOS for O p-orbitals; and total: total DOS.

Fig. 2. Variation of total and Mn magnetic moments with Mn content.

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the surrounding O anions, theses ﬁvefold degenerate d states are splitting into two-lying eg (dz² and dx²y²) and three high-lying t2g

(dxy, dyzand dzx) subsets. According to the Hand′s rule, the Mn^2þ must be in high spin conﬁguration (d⁵¼e²_þt³_þe⁰t⁰). Total DOS of Zn0.9Mn0.1O is reported inFig. 1b. From the total, Mn-3d and p-O DOS, one can note that the majority of Mn-3d bands are occupied while the minority ones are above the minimum of the conduction band which prove the presence of Mn^2þ high spin conﬁguration of Mn. InFig. 2, the variation of Mn magnetic moment in Zn_1xMnxO system versus Mn (x) concentration is reported. It can be noticed that the value of Mn moment increases forxvalue in the range 0–5 at%, then decreases to reach a constant value for higher doping concentrations,x45 at%.

This decrease can be explained by the tendency of anti-ferromagnetic ordering for higher Mn doping concentration. Also, the theoretical magnetic moment for Mn^2þ is predicted to be 5mB, but the reached value in our calculations is about 3.7mB, which is due to the anti ferromagnetic coupling of Mn²^þions. It is important to note that the predicted values for the magnetic moment per Mn atom forx45 are consistent with the values estimated from the paramagnetic susceptibility data (Section 3.3).

3.2. Structural and microstructural analyses

Fig. 3 shows the evolution of X-ray diffraction (XRD) patterns of Mn-doped ZnO system. The formation of the hexagonal wurtzite phase is clearly observed. For lower Mn concentrations up to 5 at%, no additional peaks can be detected thereby excluding the presence of

any impurities and conﬁrming the formation of pure and single phase within the range of Mn doping concentrations, i.e. 1–5 at%. It is important to note that no further annealing is needed in this method compared to conventional chemical routes, where annealing is usually necessary to remove some remaining precursor residues of the formed amorphous phase into crystalline, which also leads to crystal growth accompanied by an important increase of the particle's size. For higher Mn concentrations, higher than 5 at%, new diffraction peaks appeared, which were mainly attributed to some un-reacted residue of precursors and/or Mn oxides.

Rietveld reﬁnements were carried out using PDXL program and the results are reported inFig. 4a and b for 0.1 and 2%, respectively.

The reﬁnement results, including lattice parameters (aandcin Å), crystallite size (D, nm) and microstrain (ε, %), are reported inTable 1.

The variation of the microstructral parameters, for instance crystallite size (D) and microstrain (ε) with Mn content is reported in Fig. 5. It is clearly observed that both crystallite size and microstrain increase with increasing Mn content. This may be attributed to various factors: (i) Mn²^þhas larger ionic radius than Zn^2þ (0.80 Å and 0.74 Å) hence its substitution during synthesis

Intensity (a.u)

20 30 40 50 60 70 80

2 Theta (degree)

(100) (002) (101) (102) (110) (103) (112) (201)

Fig. 3.X-ray diffraction spectra of ZnO:Mn nanoparticles for different manganese doping concentrations: (a) Zn0.999Mn0.001O; (b) Zn0.98Mn0.02O; (c) Zn0.95Mn0.05O;

(d) Zn0.92Mn0.08O; and (e) Zn0.90Mn0.10O.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

-1667 -667 333 1333

Intensity (counts)

2-theta (deg)

Intensity (counts)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

20 30 40 50 60 70 80

-1507 -507 493 1493

Intensity (cps)

2-theta (deg)

Intensity (cps)

Fig. 4.Ritevled reﬁnements of (a) Zn0.999Mn0.001O and (b) Zn0.98Mn0.02O compounds. Solid blue curve: calculated pattern; dots: experimental data; solid red curve: intensity difference. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

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will create some strains/stresses within the crystal lattice; (ii) Mn can promote both densiﬁcation and grain growth; (iii) increased grain boundaries or surface diffusion; etc.

The variation of the lattice parameters with Mn content can be clearly observed inFig. 6a and b. It can be observed that the lattice parameters behave similarly and obey to Vegard's law: a linear increase with increasing Mn content. This indicates and conﬁrms that Mn²^þions dissolve into ZnO crystal lattice and occupy Zn^2þ tetrahedral sites. It is important to note that the values of the lattice parameters reported in literature vary considerably: for 2 at

% of Mn content (i) the values obtained in this study are in good agreement with the values reported by Anghel et al.[18], 47.777 and 47.8 Å³, respectively; (ii) whereas Usman et al. reported

slightly lower values, 47.545 Å³[19]. This reﬂects clearly the effects of the type of the synthesis route, the starting precursors and post treatment on structural (lattice parameters, stoichiometry, etc.) and microstructural (particle size, microstrain, etc.) parameters of the synthesized nanocrystals. It is important to note that botha andcvaries monotically with Mn content. The relative increase of the lattice parameters with increasing Mn doping concentration can be attributed to the larger ionic radius of Mn²^þ (0.08 Å) Table 1

Variation of crystallite size (D), microstrain (ε), lattice parametersaandcwith Mn content as obtained by the Rietveld analysis.

Composition Crystallite size D(nm)

Micostrainε(%) Lattice parameters a(Å)c(Å)

Zn0.999Mn0.001O 30 0.023 3.2506 (4) 5.2074 (5)

Zn0.98Mn0.02O 34 0.097 3.2503 (5) 5.2066 (7) Zn0.95Mn0.05O 42 0.066 3.2509 (5) 5.2074 (6) Zn0.92Mn0.08O 33 0.302 3.2516 (3) 5.2080 (5) Zn0.90Mn0.10O 49 0.295 3.2520 (20) 5.2080 (30)

10 20 30 40 50 60

Crystallite size D (nm)

Mn content (at.%)

(%)

4 6 8 10

0 2

0 2 4 6 8 10

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Mn content (at.%)

Fig. 5.Evolution of (a) crystallite size (D) and (b) microstrain (ε) with Mn content.

V(Å3)

47.61 47.62 47.63 47.64 47.65 47.66 47.67 47.68 47.69 47.70 47.71

Mn content (at.%)

0 2 4 6 8 10

3.2500 3.2502 3.2504 3.2506 3.2508 3.2510 3.2512 3.2514 3.2516 3.2518 3.2520 3.2522 3.2524

Lattice parameter a (Å) Lattice parameter a (Å)

Mn content (at.%)

5.2062 5.2064 5.2066 5.2068 5.2070 5.2072 5.2074 5.2076 5.2078 5.2080 5.2082 5.2084

Fig. 6.Variation of lattice parameters with Mn content: (a) fora(empty triangles) andc(ﬁlled traingles) and (b) for volume.

Fig. 7.Typical TEM photograph showing the general morphology of Mn doped zinc oxide nanopowder.

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compared to that Zn²^þ (0.74 Å) hence leading to a lattice expansion.

TEM image as shown inFig. 7clearly indicates the formation of well deﬁned nanocrystals with prismatic shapes with a narrow particle size distribution depending on Mn concentration. For ZnO doped with 2% of Mn (Fig. 7), the size varies in the range 20–40 nm in very good agreement with the results of crystallite size deduced from XRD analysis, i.e. 20–50 nm.

3.3. Magnetic analysis

Fig. 8shows the measured room temperature magnetization curve for the 0.1% Mn substituted ZnO compound. The data show that the measured magnetization curve has two components, one is diamagnetic and another is ferromagnetic. After removing the diamagnetic contribution (χd¼2.3510⁷emu/g Oe), the magnetization curve exhibits ferromagnetic behavior with very small saturation magnetizationMs¼0.0006 (emu/g). The origin of the observed ferromagnetic behavior at this extremely low substitution level is still unclear. It is well known that pure bulk ZnO is diamagnetic, however recent study showed that pure MgO or ZnO can exhibit ferromagnetic order below certain ﬁlm thicknesses [20]. Other workers[21,22]have also reported room temperature ferromagnetism (RTFM) in pure ZnO or MgO. The observed RTFM in pure MgO or ZnO is attributed to vacancy defects which can induce a magnetic moment in the insulator.

For the 0.1% Mn examined sample, if the 0.1% Mn ions are assumed to be responsible for the observed saturation magnetization (Nμ¼Ms), then a very mall magnetic momentμ¼0.01μB/Mn will be obtained.

Accordingly the observed ferromagnetic behavior for this sample may arise from some defects induced magnetic moment that could formed during the preparation of ZnO compound.

Fig. 9shows the measured room temperature magnetization curve for the 2% Mn doped ZnO. The data show a clear ferromagnetic order, where the hysteresis loop is quite visible and closes at aﬁeldH¼0.2 T. The continuing rise of the magnetization at high ﬁeld is almost linear which is considered a paramagnetic component that arises from uncoupled moments. After removing the paramagnetic component (χp¼4.210⁷emu/g Oe), the hysteresis loop showed a saturation magnetizationMs¼0.005 emu/g and a coercivityHc¼75 Oe.

Fig. 10shows the measured room temperature magnetization curve for the 5% Mn doped ZnO. The data show an increase in the Fig. 8.The measured room temperature magnetization curve for the 0.1% Mn doped ZnO. Blue curve: as-measured magnetization; Red curve: magnetization after removal of diamagnetic/paramagnetic component. (For interpretation of the references to color in thisﬁgure legend, the reader is referred to the web version of this article.)

Fig. 9.The measured room temperature magnetization curve for the 2% Mn doped ZnO. Blue curve: as-measured magnetization; Red curve: magnetization after removal of diamagnetic/paramagnetic component. (For interpretation of the references to color in thisﬁgure legend, the reader is referred to the web version of this article.)

Fig. 10.The measured room temperature magnetization curve for the 5% Mn doped ZnO. Blue curve: as-measured magnetization; Red curve: magnetization after removal of diamagnetic/paramagnetic component. (For interpretation of the references to color in thisﬁgure legend, the reader is referred to the web version of this article.)

Fig. 11.The measured room temperature magnetization curve for the 8, 10, and 15% Mn doped ZnO.

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contribution of the paramagnetic moments (uncoupled spins) where χp¼2.9110⁶emu/g Oe. After removing the paramagnetic contribution, the magnetic behavior still exhibits a ferromagnetic behavior but with lower saturation magnetization (Ms¼0.0026 emu/g) compared to the sample with the 2% of Mn which can be attributed to the possible antiferromagnetic order as the Mn concentration increases.

Fig. 11shows the measured room temperature magnetization curve for the 8%, 10% and 15% Mn doped ZnO compositions.

The data show a constant increase in the paramagnetic component of the magnetization with increasing Mn concentration. For these compositions, removing the paramagnetic component gives almost the same ferromagnetic component with a saturation magnetizationMsE0.0017 (emu/g). The obtained values for the paramagnetic susceptibility and saturation magnetization for the ferromagnetic component are summarized inTable 2.

Since the observed values for the saturation magnetization of the ferromagnetic component are very small (less than 0.005 emu/g), then it is expected that the number of Mn atom (NFM) that are responsible for the ferromagnetic order to be very small in comparison with those responsible for the paramagnetic component (Np). Thus by assuming thatNTENpand using χp¼Npm²/3 kT, the estimated values for the magnetic moment of the paramagnetic Mn atoms are found to be in the range 2–3.5mB/Mn (Table 2) which are consistent with the values obtained from the ab-initio calculations (Section 3.1). Previous reports on Mn doped ZnO thinﬁlms have quoted values of 0.16mB/Mn[23]

and 4.8mB/Mn[24]for the observed magnetic moment per Mn ion.

Fig. 12 shows the obtained variation of the paramagnetic susceptibility and estimated magnetic moment per Mn atom for the examined compositions. The data show that χp almost increases linearly with increasing Mn concentration which indicates that adding more Mn atoms is not contributing to the

ferromagnetic order where the highest saturation magnetization is attained at 2% Mn. The variation for the estimated magnetic moment showed that above 5% Mn, the magnetic moment per Mn atom is not changing appreciably and remain around 3mB/Mn.

However the saturation magnetization above 2% Mn decreases which may indicate the possible formation of anti-ferromagnetic order. The calculated values for the Mn moment using ab-initio calculations are also shown inFig. 12. This data show that above the 5% Mn concentration, both the calculated and the estimated values for the Mn magnetic moments are consistent which validate the ab-initio calculations.

4. Conclusion

Nanocrystals (30–50 nm) of Mn-doped ZnO were synthesized with high doping concentration. Up to 5%, Rietveld analysis conﬁrms the formation of pure phase, whereas for higher concentration, additional minor peaks appear, thereby indicating the existence of solubility limit. Both crystallite size and lattice parameters increase with increasing Mn content. Ab-initio calculations reveal the appearance of ferromagnetism attributed mainly to the substitution of Mn²^þ to Zn²^þ within ZnO crystal lattice.

Magnetic analysis have showed that above 2% of Mn, increasing the Mn concentration is not contributing to the long range ferromagnetic order in the sample but only enhancing the paramagnetic component. Moreover, increasing Mn concentration above 2% results in a reduction to the Ms value due to the possibility of the formation of anti-ferromagnetic order. Analysis for the magnetic moment per Mn atom gives a value 2–3mB/Mn which is found to be consistent with the Ab-initio calculations.

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Table 2

Paramagnetic susceptibility, saturation magnetization, number of Mn ions and magnetic moment per Mn atom for the samples examined.

Mn (%) χp(emu/Oe g) Ms(emu/g) NT(Mn/g) μper Mn (μB) Mass (g)

0.1 – 0.0001 7.39910¹⁸ 0.0347

2 4.210⁷ 0.005 1.48310²⁰ 2.02 0.0355

5 2.9110⁶ 0.0026 3.72310²⁰ 3.36 0.0489

8 4.010⁶ 0.0018 5.98010²⁰ 3.11 0.0355

10 5.3810⁶ 0.0015 7.49510²⁰ 3.22 0.0305

15 8.3710⁶ 0.0017 1.13110²¹ 3.27 0.0298

Fig. 12.Variation of the paramagnetic susceptibility and obtained magnetic moment per Mn atoms for the examined samples. The calculated values of Mn moment obtained from the ab-initio calculations are also shown in theﬁgure.