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Carriers-mediated ferromagnetic enhancement in Al-dopedZnMnO dilute magnetic semiconductors

Murtaza Saleema, Saadat A. Siddiqia, d, Shahid Atiqa,⁎, M. Sabieh Anwarb,Irshad Hussainb, Shahzad Alamc

aCentre of Excellence in Solid State Physics, University of the Punjab, Lahore-54590, PakistanbSchool of Science and Engineering (SSE), Lahore University of Management Sciences (LUMS), Opposite Sector U,D.H.A. Lahore Cantt-54792, PakistancPakistan Council for Scientific and Industrial Research (PCSIR) Laboratories Complex, Lahore, PakistandInterdisciplinary Research Centre in Biomedical Materials (IRCBM), COMSATS Institute of Information Technology, Defence Road,Off Raiwind Road, Lahore, Pakistan

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +92 42 35839387E-mail address: shahidatiqpasrur@yahoo

1044-5803/$ – see front matter © 2011 Elseviedoi:10.1016/j.matchar.2011.08.003

A B S T R A C T

Article history:Received 6 March 2011Received in revised form12 August 2011Accepted 12 August 2011

Nano-crystalline Zn0.95 –xMn0.05AlxO (x=0, 0.05, 0.10) dilute magnetic semiconductors (DMS)were synthesized by sol–gel derived auto-combustion. X-ray diffraction (XRD) analysisshows that the samples have pure wurtzite structure typical of ZnO without theformation of secondary phases or impurity. Crystallite sizes were approximated byScherrer formula while surface morphology and grain sizes were measured by fieldemission scanning electron microscopy. Incorporation of Mn and Al into the ZnOstructure was confirmed by energy-dispersive X-ray analysis. Temperature dependentelectrical resistivity measurements showed a decreasing trend with the doping of Al inZnMnO, which is attributable to the enhancement of free carriers. Vibrating samplemagnetometer studies confirmed the presence of ferromagnetic behavior at roomtemperature. The results indicate that Al doping results in significant variation in theconcentration of free carriers and correspondingly the carrier-mediated magnetizationand room temperature ferromagnetic behavior, showing promise for practicalapplications. We attribute the enhanced saturation magnetization and electricalconductivity to the exchange interaction mediated by free electrons.

© 2011 Elsevier Inc. All rights reserved.

Keywords:Diluted magnetic semiconductorsAl-doped ZnMnOSol–gel synthesisElectrical propertiesMagnetic properties

1. Introduction

The addition of magnetic impurities stimulates dramaticchanges in the structural, electrical and magnetic propertiesof ZnO. Charge-based electronics with the additional controlover the spin degree of freedom is the central objective ofthe rapidly progressing field of spintronic devices [1,2]. ZnOis an attractive material for devices based on dilutedmagneticsemiconductors (DMS), possessing an attractive band-gap

9x105; fax: +92 42 99236.com (S. Atiq).

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(Eg=3.37 eV) and the wurtzite-type hexagonal crystal struc-ture. ZnO based DMS are usually produced by substitutingthe cations of the host ZnO with small amounts of magneticions like Fe, Co, Ni and Mn. Theoretical calculations have pre-dicted the existence of a stable ferromagnetic state fortransition-metals doped ZnO without any additional doping[3]. The origin of ferromagnetism has largely remained an out-standing controversy [4–6] in oxide-based DMS. It is, therefore,important to investigate the accurate structural, electrical and

1139.

1103M A T E R I A L S C H A R A C T E R I Z A T I O N 6 2 ( 2 0 1 1 ) 1 1 0 2 – 1 1 0 7

magnetic properties of DMS compounds that can aid in theunderstanding of the origin of ferromagnetism in thesematerials.

Room-temperature (RT) ferromagnetism in ZnO-basedDMS has also motivated numerous research groups world-wide to develop reproducible synthesis techniques for suchmaterials. Mn-doped ZnO materials have been extensivelystudied during recent years and their magnetic propertieshave also been extensively studied. For example, RT ferro-magnetic behavior was reported in ZnMnO samples synthe-sized in bulk and thin film forms [7–9]. Chen et al. [10]reported ferromagnetic interactions in ZnMnO samples sin-tered in an Ar atmosphere and observed that ferromagneticinteractions disappeared when sintered in air. Similarly,Sharma et al. [11] observed the ferromagnetic interactions atroom temperature in bulk doped ZnO samples synthesizedby the solid state reaction and sol–gel routes. Ab initio studieshave shown that the Curie temperature (Tc) of Mn, Fe, Co or Nidoped ZnO can be raised by doping with additional carriers[12,13]. Liu et al. [14] reported that it is possible to achieve fer-romagnetism in ZnO powder doped with Co through addition-al doping with Al that increases the carrier concentration.Existence of ferromagnetic behavior in Zn0.93Co0.05Cu0.02Ohas also been reported in previous work [15].

The doping of Al in ZnMnO has been shown to increase theconcentration of free carriers but without any change in mag-netic properties [16]. Recently, Li et al. reported an enhance-ment in ferromagnetism in chemically synthesized ZnMnOdoped with dilute amounts of Al [17]. Carrier mediation by ad-ditional doping with Al in Mn-doped ZnO DMS thus offers anattractive option for commercial applications. In this work,we report the effect of additional Al-doping on the concentra-tion of free carriers in ZnMnO, which is shown to significantlyenhance the magnetization at room temperature.

Fig. 1 – XRD spectra of (a) Zn0.95Mn0.05O, (b) Zn0.90Mn0.05Al0.05Oand (c) Zn0.85Mn0.05Al0.10O.

2. Experimental Method

Sol–gel is a well established wet chemical technique used tosynthesize single phasematerials possessing excellent homo-geneity [18]. Owing to its exothermic nature, the sol–gel de-rived auto combustion method offers quick formation ofpure single phase crystalline materials. Furthermore, the so-lution based reaction results in better chemical homogeneityand thus an easy transformation into oxides [19].

In the present work, nano-crystalline Zn0.95-xMn0.05AlxO(x=0, 0.05, 0.10) powder samples were synthesized via sol–gel auto-combustion. All chemicals used, including zinc ni-trate [Zn(NO3)2·6H2O], manganese nitrate [Mn(NO3)2·6H2O],aluminum nitrate [Al(NO3)3·6H2O] and citric acid (C6H8O7),purchased from BDH (United Kingdom), were of analyticalgrade and used without further purification. The appropriatemolar ratios of the chemicals were separately dissolved in100 ml of distilled water. The aqueous solutions were thenmixed with vigorous stirring for about 30 min. The pH of theaqueous solutions was continuously monitored and main-tained at 7.0 by adding an appropriate amount of liquid am-monia [NH3]. The xerogel of the solution was attained byconstant stirring and evaporation at 150 °C on a hot plate.

The temperature of the xerogel was then increased to 250 °C,resulting in the self-combustive formation of the powder.The as-synthesized powder was characterized using XRD(Panalytical X'Pert Pro Multipurpose Diffractometer (MPD))for structural analysis. The diffractometer was operated at40 kV and 40 mA using Cu Kα1 radiation having λ=1.540598 Åwith a step scan size of 0.02˚. The morphology of the Al-doped MnZnO samples was observed using a field emissionscanning electron microscope (FESEM, JSM-7500F). Energy-dispersive X-ray (EDX) analysis was performed using a S-3700N (Hitachi, Japan) SEMwhilemagnetic properties were in-vestigated using Lakeshore's 7404 VSM. The samples werethen pelletized using a hydraulic press and sintered at 250 °Cfor 2 h to densify them for subsequent determination of thetemperature dependent electrical properties by the four-probe method.

3. Results and Discussion

Fig. 1 shows the XRD patterns of as-synthesized undoped andAl-doped ZnMnO samples. Both doped and undoped mate-rials show wurtzite-like hexagonal structure which is in ac-cordance with several reports describing the preservation ofthe structure of ZnO under light doping [20–23]. Al doping inZnO-based structures up to atomic percentages of 4 and 10%has previously been reported [24–26] and our work extendsthese studies. The XRD results indicate that the doping ofZnO with Mn or (Mn+Al) does not affect the crystal systemof the host ZnO with space group P63mc. Furthermore, no ad-ditional impurity or secondary phase is detected. Therefore, itis inferred that the Mn and Al ions have substituted the Zn

Table 1 – Quantitative data of EDX for Zn0.95Mn0.05O andZn0.90Mn0.05Al0.05O.

Zn0.95Mn0.05O Zn0.90Mn0.05Al0.05O

Element Weight% Atomic% Element Weight% Atomic%

C 1.20 3.83 C 1.18 1.70O 20.91 50.03 O 20.21 48.93Mn 4.79 3.34 Al 5.17 4.64Zn 73.10 42.80 Mn 5.69 4.90

Ni 1.34 0.86Zn 66.40 38.97

Total 100.00 Total 100.00

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sites without distorting the host crystal structure. The XRDpatterns, however, indicate different peak intensities, as wellas changes in the broadening and shift of the peak positions,(the inset shows this effect for the (101) peaks). This broaden-ing is expected due to the difference in ionic radii of the dop-ants [27,28]. The elemental composition of doped ZnO elicitedfrom EDX, shown in Fig. 2, also confirms the incorporation ofMn and Al. The EDX spectra, revealing the presence of Zn,Mn, Al and O and their contents, are in close agreementwith the elemental compositions of the dissolved reactants.Quantitative data for EDX analysis for Zn0.95Mn0.05O andZn0.90Mn0.05Al0.05O samples are provided in Table 1. Traces ofC and Ni are from the sample stub.

It is well established that the lattice parameters are strong-ly influenced by the concentration of defects [29,30]. There-fore, doping of Mn and Al at the Zn sites might produce aslight distortion in the ZnO structure. The atomic radius ofAl (0.535 Å) is less than that of Zn (0.74 Å) [31] whichmanifestsas a shrinkage in the lattice parameters ‘a’ and ‘c’, as shown inFig. 3. Lattice parameters were determined using “CELL” soft-ware. The calculated values of these lattice parameters werebetween 3.26–3.23 Å and 5.22–5.18 Å, respectively. The aver-age crystallite size was estimated from the X-ray peak broad-ening, considering the most intense diffraction peak (101) forall the samples, using the Scherrer formula [1]:

Dhkl ¼ kλβcosθ

ð1Þ

Fig. 2 – EDX spectra of (a) Zn0.95Mn0.05O and (b) Zn0.90

Mn0.05Al0.05O.

where θ is the Bragg angle, λ denotes the wavelength(1.5405 Å) of Cu Kα radiation, Dh,k,l represents the average crys-tallite diameter and β is the full width at half maximum in ra-dians. The evaluated crystallite sizes of all the samples wereobserved to vary from 13.09 to 8.58 nm, and are shown inFig. 4. The substitution of ions with different ionic radii affectsthe lattice parameters and there are many reports on the sub-ject [32], however, its subsequent effect on the crystallite sizeis debatable, as discussed by some authors [33]. The slight de-crease in crystallite size as a result of Al substitution at Znsites could be correlated with the decrease in lattice parame-ters, owing to smaller ionic radius of Al that is substitutingZn, thus causing slight distortions in Al-doped ZnO latticeyielding smaller sized particles.

Fig. 5 shows the field emission scanning electron micro-graphs of the selected powder samples. These micrographsreveal the high crystallinity of these samples but the particlesdo not seem to be of uniform size. The wide dispersion in thesize is due to the sample preparation and drying processwhich led to the formation of nanoparticulate aggregates ofvarious sizes ranging from 50 to 500 nm.

The electrical resistivity of all the samples decreased as thetemperature was increased depicting semiconducting behav-ior of the samples, and is shown in Fig. 6. The presence ofsome oxygen vacancies in ZnO, and the interstitial Zn2+ dem-onstrates n-type conductivity, directly proportional to theconcentration of electrons. The Al3+ substitution at the Zn2+

sites treated in the present study introduces one extra valence

Fig. 3 – Variation in lattice parameters ‘a’ and ‘c’ with thedifferent dopant concentrations.

Fig. 4 – Crystallite sizes of the samples measured usingScherrer formula. Fig. 6 – Temperature dependent resistivity of Zn0.95Mn0.05O,

Zn0.90Mn0.05Al0.05O and Zn0.85Mn0.05Al0.10O.

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electron into the system. The quantummechanical study car-ried out by Maldonado et al. [34] showed that Al doping in ZnOcreates large radius electron polarons. These electrons en-hance the n-type electrical conductivity and the solid can betreated as a polarizable continuum with conduction electronsmoving throughout the lattice. It is known [35,36] that thesubstitution of Al at Zn sites increases the free carrier concen-tration resulting in low value of resistivity. The ZnMnO

Fig. 5 – FESEM micrograph of (a) Zn0.95Mn0.05O (b) Zn0.90

Mn0.05Al0.05O.

system doped with additional dilute amounts of Al can betreated in a similar manner.

The magnetic hysteresis (M–H) loops achieved through vi-brating samplemagnetometrywere obtainedat anappliedmag-netic field of ±8 kOe, and are shown in Fig. 7. Room temperatureferromagnetic behavior is evident from the M–H loops in boththe undoped and Al-doped ZnMnO DMS. It has been reportedthat ferromagnetism in Mn-doped ZnO DMS may originatefrom the Mn3O4-like secondary phases [37]. However, the originof RT ferromagnetism in ZnMnO DMS is still somewhat contro-versial. Recently, Yang et al. [38] confirmed that the origin ofRT ferromagnetism could not be derived from any impurity orsecondary phase. The substitution of Mn ions at the regular Znsites could be the main cause for the presence of these ferro-magnetic interactions. According to the RKKY (Ruderman KittelKasuya Yoshida) theory [11,39], these ferromagnetic interac-tions in DMS materials arise from the exchange interactionsbetween local spin-polarized electrons (for instance, the elec-trons of Mn2+ ions) and conductive electrons. Free carriers (elec-trons) play a vital role in establishing the magnetic phase andhence the ferromagnetism in ZnO dopedwithmagnetic impuri-ties [18,40,41]. In addition, it has also been previously reported

Fig. 7 – Field dependent magnetization of Zn0.95Mn0.05O,Zn0.90Mn0.05Al0.05O and Zn0.85Mn0.05Al0.10O.

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that magnetization is enhanced significantly in magneticelements-doped ZnO with additional doping of Al3+ [42,43].The saturation magnetization (Ms) values evaluated from theM–H loops of three samples varied between 0.045 and0.094 emu g−1. Diamagnetic contributions observed at highmagnetic fields are attributed to the sample holder [44] andparent ZnO [21], and these contributions were subtracted fromthe data. The Ms value was observed to increase significantlyin Al-doped ZnMnO DMS. The effect of Al doping on Ms ofZnMnO DMS can be confidently attributed to the variation ofcarrier concentrations, as observed in the electrical propertiesof the samples, described earlier. This is a direct evidence ofthe presence of RKKY interactionsmediated by the free carriers.

4. Conclusions

Free carriers andmagnetization enhancementwith thedoping ofAl in ZnMnO has been investigated. The undoped and Al-dopedZnMnO samples were prepared from an inexpensive sol–geltechnique and characterized using XRD, EDX, SEM, temperaturedependent resistivity measurements and VSM for the study ofstructural, compositional, morphological, electrical and magnet-ic properties, respectively. It was observed that thewurtzite-typehexagonal structure of ZnO was not disturbed by the dilutedopants, besides variations in the unit cell dimensions. Theadditional doping of Al in ZnMnO strongly influenced theconcentration of free carriers as is evident from the decrease intemperature dependent resistivity. Consequently, ferromagneticinteractions observed at room temperature,mediated by the freecarriers, were enhanced due to doping of Al in ZnMnO.

Acknowledgment

The authors would like to thank the Higher Education Com-mission (HEC), Islamabad, Pakistan for funding. We are grate-ful to Dr. Riaz Ahmad, Muhammad Kamran, Dr. ShahzadNaseem and Dr. Saira Riaz for their help in experimentalmeasurements.

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