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RoleofmultivalentCu,oxygenvacanciesandCuOnanophaseinferromagneticpropertiesofZnO:CuthinfilmsARTICLEinRSCADVANCESJUNE2015ImpactFactor:3.71

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RSC Advances c5ra09002d

PAPER

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1Role of multivalent Cu, oxygen vacancies and CuOnanophase in the ferromagnetic properties ofZnO:Cu thin lms

M. Younas,* Junying Shen, Mingquan He, R. Lortz,Fahad Azad, M. J. Akhtar, A. Maqsood and F. C. C. Ling*

Comprehensive microstructural, electronic and magneticanalyses have been carried out on ZnO:Cu thin lms grownby pulsed laser deposition on c-plane sapphire underdierent oxygen partial pressures. Detailed X-ray diraction(XRD), X-ray photoelectron spectroscopy (XPS) and highresolution transmission electron microscopy (HRTEM)analyses reveal that increase in oxygen growth pressuredegrades the epitaxy of ZnO:Cu thin lms due to inclusion ofnanosize CuO in the ZnO host lattice. HRTEM and

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50Translation errors between word-processor les and typesettPlease pay particular attention to: tabulated material; equationhave not already indicated the corresponding author(s) pleaseshould be minor and not involve extensive changes. Please domanuscript. All corrections must be submitted at the same tim

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magnetization studies suggest that thin lm quality plays aless eective role in governing the magnetic properties ofthese samples. Instead, room temperature ferromagnetism(FM) of these ZnO:Cu thin lm samples are highly tunable bythe simultaneous presence of CuO nanophases andmultivalent Cu and V$$O concentrations, which are in strongcontest with each other. For low oxygen partial pressuregrown sample, the eective Cu2 V$$O Cu1 network is themain contributor to the observed FM and is in completionwith CuO nanophases only when there is a relatively low V$$Oconcentration with a dominant Cu2+ oxidation state. Forvacuum grown samples containing high V$$O concentrationand Cu1+ as dominant oxidation state, the Cu2 V$$O Cu1network becomes less eective and a CuO nanophase (45nm) is the dominent FM supplier. The extrinsic FM in thevacuum grown sample, which is the best epitaxial qualitysample, is further conrmed by the zero eld cooled (ZFC)and eld cooled (FC) magnetization protocols.ART C5RA09002D_GRABS

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Journal: RSC Advances

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Title: Role of multivalent Cu, oxygen vacancies and CuO nanophase in the ferromagnetic properties of ZnO:Cuthin films

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Role of multivale1

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partial pressure grown sample, the eective Cu2 V$$O Cu1 network is the main contributor to the

1. Introduction

The control over the distributioconducting host is crucial formagnetic semiconductors (DMSlenges to avoid secondary phaseclustering tendency of transitionoxides. Under nonequilibriumattractive interaction between Tspinodal decomposition of magTherefore, chemical phase separgap semiconductors such thatanticipated.15 ZnO is an encouconductor having ability to lasewavelengths. Because of its long

3

Received 14th May 2015Accepted 15th June 2015

DOI: 10.1039/c5ra09002d

www.rsc.org/advances

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RSC Advances

PAPERaDepartment of Physics, The University of H

R. ChinabDepartment of Physics, Hong Kong Universi

Bay, Kowloon, Hong Kong, P. R. ChinacSchool of Natural Sciences, National Univ

Islamabad, PakistandEMMG, Physics Division, PINSTECH, P.O.eDepartment of Physics, Air University, E-9 P

This journal is The Royal Society of Cobserved FM and is in completion with CuO nanophases only when there is a relatively low V$$Oconcentration with a dominant Cu2+ oxidation state. For vacuum grown samples containing high V$$Oconcentration and Cu1+ as dominant oxidation state, the Cu2 V$$O Cu1 network becomes lesseective and a CuO nanophase (45 nm) is the dominent FM supplier. The extrinsic FM in the vacuum

grown sample, which is the best epitaxial quality sample, is further conrmed by the zero eld cooled

(ZFC) and eld cooled (FC) magnetization protocols.

n of magnetic ions in a semi-the functionality of diluted

). It is one of the major chal-s and precipitates due to themetal (TM) elements and theirgrowth conditions, a strongM impurities stimulates thenetically robust nanocrystals.ation in TM doped wide bandGaN and ZnO is signicantlyraging wide band gap semi-r spontaneously at ultravioletspin-coherence time at room

temperature (RT), ZnO would possibly behave as a bipolarspintronic material with the ferromagnetism (FM) mediated byboth holes and electrons.68 Although extensive research hasbeen conducted in order to nd a true ZnO-based DMS withhigh Curie temperature (TC), controlled FM with its realisticorigin in ZnO based DMS911 is debatably one of the stimulatingresearch topic in materials science and condensed-matterphysics in these days. There are still open questions that chal-lenge our understanding, such as whether the ferromagneticordering arises from strongmagnetic interactions between well-separated magnetic dopants actually substituting at the hostlattice or from the magnetic secondary phases and metalprecipitates.

A possible way to avoid magnetic secondary phases or metalprecipitates is the doping of nonmagnetic elements, such asCu.12 One of such examples is the ZnO:Cu system as metalliczinc, ZnO, metallic copper, Cu2O, and bulk CuO are not ferro-

13nanophase in theZnO:Cu thin lm

M. Younas,*a Junying Shen,A. Maqsoode and F. C. C. Li

Comprehensive microstructural, e

lms grown by pulsed laser depo

Detailed X-ray diraction (XRD

transmission electron microscopy

degrades the epitaxy of ZnO:Cu

HRTEM and magnetization studie

the magnetic properties of these

ZnO:Cu thin lm samples are hig

multivalent Cu and V$$O concentra

Cite this: DOI: 10.1039/c5ra09002dong Kong, Pokfulam Road, Hong Kong, P.

ty of Science and Technology, Clear Water

ersity of Sciences and Technology, H-12,

Nilore, Islamabad, Pakistan

AF Complex, Islamabad, Pakistan

hemistry 2015nt Cu, oxygen vacancies and CuOferromagnetic properties of

Mingquan He,b R. Lortz,b Fahad Azad,ac M. J. Akhtar,d

g*a

ctronic and magnetic analyses have been carried out on ZnO:Cu thin

ition on c-plane sapphire under dierent oxygen partial pressures.

X-ray photoelectron spectroscopy (XPS) and high resolution

(HRTEM) analyses reveal that increase in oxygen growth pressure

in lms due to inclusion of nanosize CuO in the ZnO host lattice.

suggest that thin lm quality plays a less eective role in governing

samples. Instead, room temperature ferromagnetism (FM) of these

ly tunable by the simultaneous presence of CuO nanophases and

ions, which are in strong contest with each other. For low oxygenmagnetic. In addition, the size mismatch between Cu and Znis very small, resulting in a low formation energy for CuZn and asmall lattice distortion. Therefore, the ZnO:Cu system has beenpredicted to be ferromagnetic and so it develops as one of themajor challenges to build a room temperature (RT) DMS usingnonmagnetic dopants. A substantial amount of experimentaldata have already been collected, however there are some

RSC Adv., 2015, xx, 110 | 1

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55controversies regarding true carrier mediated FM in this class ofmaterials.

One possible doubt on the DMS nature of the ZnO:Cu mightbe the mixed valence state of the Cu in the ZnO materials. TheZnO:Cu system containing CuZn

+ (i.e., Cu3+) or CuZn0 (i.e., Cu2+)

is found to possess magnetic moments of 2 mB/Cu and 1 mB/Cu,respectively. However, CuZn

(Cu1+) or CuZn2 (i.e., Cu0) in

n-type samples would not possess any local magnetic momentsas the d electrons are fully lled. Under O-rich conditionsduring sample growth, substituting Cu has the lowest forma-tion energy and most of the Cu would be in the oxidation stateof +2 or +1 in the Cu:ZnO sample as EF is pinned around Cu

2+/Cu1+ conditions.14,15 Under O-poor conditions, the role of iso-lated oxygen vacancies V$$O and zinc interstitials (Zni) are stillcontroversial in the ferromagnetic stability of the ZnO:Cusystem.16,17 However, donoracceptor defect pairs (DAPs) inwhich Cu coupled to V$$O or Zni such as Cu V$$O ,18 and CuZni19encourage the most interest because the overlap betweendelocalized molecular orbitals can play a signicant role on thedevelopment of ferromagnetic ordering in the ZnO:Cu system.Another, most critical and important issue with DMS nature ofthe ZnO:Cu is the formation of CuO nanophases that mayexhibit a ferromagnetic nature.20,21 However, in these reportsthe magnetization of the ZnO:Cu system is solely tuned by thepresence of CuO inclusions, and the eect of multivalent Cuand V$$O on the magnetic behaviour is totally ignored. Moreover,some researchers found that Cu related phases cannot regulatemagnetic properties of the ZnO:Cu system, since there is hardlyany interaction between Cu clusters and host lattice, which isessential for a coupling to arise and clustering is revealed to bedetrimental to the FM.2224

In the above discussed scenario it is still unclear whether theferromagnetism originates from diluted spins of Cu elements orresults from secondary phase segregation or Cu metal clus-tering in the ZnO:Cu system. Therefore, in this research work,we performed a comprehensive structural, electronic andmagnetic analysis on pulsed laser deposition (PLD) grownZnO:Cu thin lms to explore the possible Cu valence states,defect complexes and potential role of secondary phases intuning the FM. To control Cu valence states and V$$O concen-trations, the ZnO:Cu thin lms were grown under dierentoxygen partial pressures. Our nding shows that the CuOnanophases alone cannot generate high magnetization aspreviously thought.20,21 Instead, a mixed Cu oxidation statecoupled with oxygen vacancies V$$O and CuO nanophases aresimultaneously playing a vital role to tune the magnetic prop-erties. We suggest that RT FM in ZnO:Cu is not of single origin,even when CuO secondary phases are present, and otherpossible sources that can contribute should also be consideredin parallel.

2. Experimental

The ZnO:Cu (with Cu0.04Zn0.96O stoichiometric composition ofthe target with 99.999% purity) thin lms were grown on c-planesapphire substrate using a pulsed KrF excimer laser (l 248nm) with a frequency of 2 Hz and energy of 300 mJ. Thin lm2 | RSC Adv., 2015, xx, 110growth was carried out in three dierent oxygen pressures,namely PO2 0.00 Pa (S-1), 0.02 Pa (S-2) and 1.00 Pa (S-3) withsubstrate temperature of 600 C. Thin lms samples used in thepresent study had thickness in the range of 185280 nm. Theroom temperature (RT) Hall Eect measurement was conductedin a van der Pauw conguration (Accenet HL-5500 PC system).Room-temperature X-ray diraction (XRD) measurements wereperformed on a Siemens D5000 diractometer with the CuKaline (0.1541 nm) to examine the structural properties. Thesecondary ion mass spectroscopy (SIMS) measurement wasconducted using the Cameca (Model IMS 4F) dynamicsecondary ion mass spectrometer. The electronic structureswere studied by X-ray photoelectron spectroscopy (XPS) usingthe MgKa line (Kratos Axis Ultra DLS system). The morphologyof the samples was studied by high resolution transmissionelectron microscopy (HRTEM) (2010F TEM, JEOL). Magneticmeasurements were performed using a VSM SQUID magne-tometer (MPMS-5s, Quantum Design). RT magnetic hysteresisloops over 0.5 T have been recorded while magnetoresistance(MR) measurements were carried out from 300 K to 1.5 K undermagnetic eld of 10 T. The zero-eld cooled (ZFC) and eld-cooled (FC) magnetization vs. temperature studies wereperformed from 1.5350 K under 100 Oe magnetic eldsperpendicular to the sample surface.

3. Results and discussion3.1 Structural and electronic properties

Sample S-1 is n-type (carrier concentration 3.32 1018 cm3)while samples S-2 and S-3 do not have sucient conductivity forassessing electrical parameters by Hall measurements andshow insulating behaviour (resistivity 108 U cm by IVmeasurements). The XRD patterns for ZnO:Cu thin lms rep-resented in Fig. 1(a) shows dominant reection from (002) and(004) planes of wurtzite ZnO, and (006) planes of sapphiresubstrate for all three samples. The (002) peak values of thethree samples are at 34.34 (S-1), 34.46 (S-2) and 34.49 (S-3)with corresponding FWHM values of 0.23, 0.24 and 0.28,respectively. The (002) peak for the ZnO:Cu samples is at lowerpositions than those of pristine ZnO thin lms grown under thesame conditions i.e. 34.491, 34.540 and 34.568 for 0.00 Pa,0.02 Pa and 1.00 Pa, respectively (shown by marks inFig. 1(a)). From (002) peak positions, we estimated the c-axislattice parameter values of 5.220 A (S-1), 5.202 A (S-2), 5.198 A(S-3) by employing the reduced Bragg relation c l/sin q, wherel is the X-ray wavelength and q is the diraction angel of (002)peak.24 The c-axis lattice parameters for ZnO:Cu thin lms havehigher values than that of pristine ZnO thin lms (5.199 A, 5.188A and 5.186 A, respectively for 0.00 Pa, 0.02 Pa and 1.00 Pagrown samples). In view of the relevant ionic radii of Cu+(0.60A), Cu2+(0.59 A) and Zn2+(0.74 A),25 the (002) peak positions ofthe ZnO:Cu system should shi towards higher angles withlinear decrease in c-parameters at lower Cu concentrations20

compared to pristine ZnO thin lms. However, we observe shiin (002) peak positions towards lower angles and slightly highervalues of c-lattice parameters of the ZnO:Cu samples comparedto pristine ZnO thin lms. This abnormal trend stipulates theThis journal is The Royal Society of Chemistry 2015

Cu-2p3/2 peak at 932.6 eV and Cu-2p1/2 peak at 952.5 eV corre-sponding to the peak positions of Cu1+ in Cu2O),26 and thetting results are presented in Table 1. By employing XPSintegrated areas and Cu-2p3/2/Zn-2p3/2 ratio aer propernormalization of corresponding atomic sensitivity factor, weestimated the Cu at.% of 2.31, 2.57 and 1.55 for the S-1, S-2 andS-3 samples, respectively. For S-1 sample, the relative peakintensity for Cu1+-2p3/2 compared to Cu

2+-2p3/2 is high, withoutany satellite structure. Strong satellite peaks due to electronshakeup are commonly found between 940 eV and 945 eV forCu2+ (Cu-3d9)18 and no satellites are expected for Cu0 and Cu1+.

The S-1 sample with relatively high Cu1+-2p3/2 peak intensityand absence of a satellite structure suggest the presence of Cu1+

Fig. 1 (a) XRD patterns for all three as-grown samples of ZnO:Cu thinlms. The inset shows normalized (002) peak intensity. The marksshows (002) peak positions for all three pristine ZnO samples grownunder same conditions, and (b) XRD patterns after annealing all threeZnO:Cu samples at 750 C. Secondary phase peaks are represented byblack squares.

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55possible evaluation of microstructural disorder and secondaryphase inclusions in the ZnO matrix that cause an expansion inthe host lattice without altering the overall wurtzite crystalstructure. On comparing the normalized intensities (insetFig. 1(a)), we observe an 81% reduction in (002) peak intensityfor the S-3 sample (grown in 1.00 Pa), compared to that of S-1(grown in 0.00 Pa). Moreover, a maximum shi in the (002)peak position with highest value of FWHM for S-3 sample is alsonoticed. Since the epitaxial and structure quality of the samplehas positive relations to the intensity, sharpness and position ofthe (002) peak in XRD spectrum, the preliminary XRD obser-vations show that increasing oxygen partial pressures duringgrowth would degrade the epitaxial and structural quality of thethin lm samples. Therefore, the relatively reduced intensityand highest value of FHWM of the (002) peak for the S-3 samplecompared to the S-1 and S-2 samples are suggested to be due topoor epitaxial and crystalline quality.

Along with the dominant peaks, we also observe two extrapeaks at 36 and 65 in the S-2 (PO2 0.02 Pa) sample, butno such peaks are found in the S-1 and S-3 samples within thedetection limit of XRD. The observed peaks at36 and65 inthis case are very close to the reections from (111) at 35.5and (022) at 65.7 planes of CuO (JCPDS#720629).26 Prior toassigning these peaks to CuO secondary phases, we carried outpost-growth annealing on the S-1, S-2 and S-3 samples at 750 Cin Ar for 30 min. Aer Ar annealing, we again observe theseextra peaks at same positions (at 36 and 65) only in theoxygen grown samples (S-2 and S-3), but no such peaks areobserved in the vacuum grown sample (S-1) when annealedunder the same conditions (Fig. 1(b)). This infers the possiblerole of the oxygen supply during the thin lm growth in formingCuO secondary phase. This may be due to the fact, that the hostcrystalline ZnO structure would be interrupted by those insertedsecondary phases and we observed degraded intensities of (002)peak with increasing PO2 (i.e. for S-2 and S-3 samples). There-fore, in the S-2 sample, we associate the peaks at36 and65to reections from (111) and (022) planes of CuO, respectively.Within the detection limit of the XRD we are unable to detectCuO related peaks in vacuum (S-1) and high oxygen partialpressure grown (S-3) samples. Relatively larger c-parameters forthese samples compared to pristine ZnO thin lms demandadditional deep analysis to completely rule out the existence ofCuO phases in S-1 and S-3 samples.

To further explore the Cu related secondary phases in the S-1, S-2 and S-3 samples, we performed an XPS study. It is nor-mally agreed that XPS can dierentiate between Cumetal, Cu2Oand CuO.27 Therefore, the Cu valence states are obtained fromthe high resolution Cu-2p core level XPS spectra shown in Fig. 2.For all the three samples, the respective Cu-2p3/2 and Cu-2p1/2peak positions lying in the range of 932.8933.2 and 952.6953.5 eV show the presence of the mixed Cu valence states,which is in agreement with literature.28,29 Therefore, each XPSspectrum is deconvolved into two components of Cu2+ and Cu1+

by Gaussian curve tting. The ttings are accomplished byemploying a Cu2+ component (by xing Cu-2p3/2 peak at 933.6eV and Cu-2p1/2 peak at 953.5 eV corresponding to the peakpositions of Cu2+ in CuO) and a Cu1+ component (by xingThis journal is The Royal Society of Chemistry 2015 RSC Adv., 2015, xx, 110 | 3

samples are tted using three Gaussian curve ttings. Theasymmetric features observed in O-1s spectra are deconvolvedby several subspectral components: (i) the OL (530.50 0.10 eV)peak shows typical behaviour of O2 ions arranged in a wurtzitelattice of hexagonal Zn2+ ions (ii) the V$$O (531.30 0.20 eV) peakindicating oxygen decient regions within defective ZnO:Cumatrix with intensity directly related to V$$O concentration and(iii) the OS (532.10 0.20 eV) peak normally associated to theloosely bound adsorbed oxygen at the surface or oxygen inter-stitial.34 The V$$O (531.10 0.20 eV) peak intensity graduallydecreases (Fig. 3) with increasing oxygen pressure duringgrowth. This indicates relatively higher V$$O concentrations insample S-1 compared to other samples, in agreement with theCu-2p XPS spectra where we observed Cu1+ as more dominantoxidation state for the same sample. The OS (532.10 0.20 eV)peak more or less has same intensity for S-2 and S-3 samplesshowing that oxygen supply during growth of these samplesenhances the surface oxidation (Fig. 3 le and right insets)when compared to S-1 (vacuum grown sample). These XPSresults reveal an enhanced surface oxidation of Cu speciesduring oxygen growth compared to vacuum growth conditions.One of the intuitive reasons accounting for the observation isthat more and more oxygen molecules are available foroxidizing Cu that can form larger size CuO phases for S-2 and

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35as more dominant oxidation state compared to Cu2+. Since S-1sample is grown in oxygen free environment that maycontains higher Cu1+/Cu2+ ratio thereby suppressing the satel-lite structure in the XPS spectra. Therefore, the Cu1+ as moredominant oxidation state and its fully lled 3d orbitals (3d104s0)may be one of the possible reasons for the absence of CuOsecondary phases in XRD and Cu-XPS results of S-1 sample, asdiscussed previously. However, samples grown in oxygen partialpressures (S-2 and S-3) show two clear satellite peaks (Fig. 2 le& right insets) with binding energies (BE) in the range of 940943 eV and 961963 eV, indicating the presence of Cu2+ at thetetrahedral zinc site.28,30 The dominant divalent state of the Cuions results from the strong CuO bonding in CuO4 tetrahe-

Fig. 2 Cu-2p XPS spectrum for S-1 sample. The left and right insetsshow Cu-2p XPS spectra for S-2 & S-3 samples, respectively.dron, which can suppresses the Cu solubility in ZnO and maylead to the CuO formation.31 The CuO phase is normally char-acterized by high-intensity shake-up satellites at higher BE (10eV) than that of main Cu-2p3/2 and Cu-2p1/2 peaks, which areconsiderably broader than that of bulk Cu2O and Cumetal.30,32,33 Observation of satellite peak positions 8.3110.31eV above the main Cu-2p peaks in S-2 and S-3 samples clearlypoints towards possible CuO secondary phases in thesesamples.

In order to see the eects of V$$O on the bonding nature ofoxygen with Cu sitting at Zn site, we performed O-1s XPSmeasurements shown in Fig. 3. The O-1s XPS peaks for all

Table 1 XPS tting parameters for all three thin lm samples

Cu-2p3/2 (eV) FWHM (eV) Cu-2p1/2 (eV

S-1 933.02 0.02 1.33 952.81 0.0S-2 933.16 0.02 1.88 952.93 0.0S-3 932.83 0.01 1.77 952.72 0.0a SP stands for satellite peak.

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) FWHM (eV) SP-1a (eV) SP-2a (eV)

5 1.49 5 2.09 943.47 0.3 963.00 0.43 2.14 940.88 0.3 961.03 0.2

Fig. 3 O-1s XPS spectra for S-1 sample. The left and right insets showO-1s XPS spectra for S-2 & S-3 samples, respectively.This journal is The Royal Society of Chemistry 2015

S-3 samples in agreement with XRD results discussedpreviously.

3.2 Microstructural properties

XPS results for S-2 and S-3 samples show a possible presence ofCuO phases, but discard their presence in the S-1 sample. XPS isa surface sensitive technique and has some detection limitproblems. Moreover, the S-1 sample is grown in vacuum, whichis not a favourable condition for the growth of large and stableCuO secondary phases. The CuO nanophases that are very smallin size, present either at the surface or within the bulk of thehost material, might remain undetected by XPS. Therefore, inorder to investigate the defect structure, phase segregation andCu related secondary phases; we performed a detailed HRTEManalysis on all three as grown samples. Fig. 4 shows a cross-sectional HRTEM image at the lmsubstrate interface for theS-1 sample. The HRTEM image in Fig. 4 shows a perfect ZnOgrowth along the (002) plane and the interface is atomicallysharp. The diraction pattern from the lmsubstrate interface(inset Fig. 4) shows absence of spots apart from those belongingto the lm and substrate. The native ZnO:Cu lm in thismicrograph shows some image contrast in the form of a lattice

Hence, the observed defects along the (001) planes (white circlein Fig. 4) are likely to be of a dislocation nature in the S-1sample.

To further conrm the origin of these dislocations, wecarried out a low magnication TEM analysis of native ZnO:Cuthin lm of S-1 sample as shown in Fig. 5. The micrograph inthis gure reveals randomly dispersed Moire fringes, as seen bythe bending/wrinkling of the lattice fringes (white circle inFig. 5) with average size in 34 nm range. Moire fringes nor-mally appear between two crystal structures with parallel orrotated sets of planes.36 The high magnication image pre-sented in the le inset of Fig. 5 shows a lattice distortion causedby such a Moire fringe. This lattice distortion is primarilycaused by lattice mismatch between the host material (ZnO:Cu)and CuO secondary phase. The estimated interplanar distancefrom such a distorted lattice is 0.25 nm, which corresponds tothe reection from (111) planes of monoclinic CuO(JCPDS#720629). Furthermore, the interplanar distance of0.26 nm corresponds to the (002) planes of wurtzite ZnO. This isin good agreement with XRD results of the S-1 sample where the(002) plane is dominant in the ZnO:Cu lms. A very smalldierence (0.01 nm) between the interplanar distances of thedistorted lattice and the undistorted wurtzite ZnO indicates theparallel overlapping of the (111) planes of CuO to the (002)planes of wurtzite ZnO. This is further conrmed by the electron

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55distortion (white circle in Fig. 4). By employing HRTEM, Klenovet al.35 reported that CuO planar clusters (with cluster concen-trations more than 2 atom%) can be incorporated in (001)cationic planes of ZnO at Zn atom positions, with oxygen atomssituated in the adjacent anion planes resulting in the appear-ance of a dislocation loop. Moreover Zn and Cu atoms arealmost equal to each other in their scattering properties, so thesubstitution of Cu for Zn does not aect the image contrast, andthe associated oxygen lattice distortion provides additionalcontrast in the HRTEM images. XPS and XRD results did notreveal any Cu clustering, and the Cu atomic concentrations isalso higher than 2 atom% for S-1 sample (i.e. 2.3 atom%).

Fig. 4 Cross-sectional HRTEM image at the lmsubstrate interfacefor S-1 sample. The inset shows an electron diraction pattern fromthe lmsubstrate interface of the same sample.This journal is The Royal Society of Chemistry 2015diraction pattern (right inset of Fig. 5), for which we are unableto observe separate diraction spots for the CuO secondaryphase. Instead, relatively elongated diraction spots are seenhere, probably due to a very small dierence in the interplanardistances of the overlapping (111) planes of CuO to the (002)planes of wurtzite ZnO. Thus, some of the Cu atoms alloyed withZnO exist as planar nanophases of CuO (34 nm), intermingled

Fig. 5 Low magnication image for the S-1 sample. The left insetshows an HRTEM image of the selected Moire fringes. The right insetshows an electron diraction pattern from the native ZnO:Cu lm ofsample S-1.RSC Adv., 2015, xx, 110 | 5

with the (001) cationic planes of ZnO resulting in the observeddislocation loop.

Sample S-2 and S-3 grown under oxygen partial pressuresshow dierent results in the TEM analysis. In the low magni-cation image of the S-2 sample presented in Fig. 6, we can stillobserve some image contrasts andMoire's fringes in the form ofwrinkling of the lattice (indicated by the white circle). In theHRTEM image (le inset of Fig. 6) of the S-2 sample, a sharp andparallel arrangement of lattice fringes can be seen very close tothe interface. However, away from lmsubstrate interface, adeviation from epitaxial growth can also be observed in theform of un-patterned lattice fringes. Moreover, similar to the S-1sample, dislocation loops of 56 nm size (indicated by whitecircles) are also noticed for the S-2 sample. The electrondiraction pattern from the lmsubstrate interface of the S-2sample (right inset of Fig. 6) shows epitaxial growth of the(002) plane. However, the diraction spots are not sharpenough as observed for the S-1 sample and some very dull extraspots (white arrows in le inset of Fig. 6) are also found. Sincethe atomic weights of CuO and ZnO are very close to each other,it is relatively dicult to observe separate clear diraction spotsfor CuO if these phases are of very small size and present alongthe ZnO growth planes, as discussed earlier.

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55Therefore, extra spots observed in the electron diractionpattern of S-2 samples are signatures for CuO secondary phasesas normally interpreted in the literature.32 However; the otherimportant point regarding extra spots that appeared in electrondiraction pattern of the S-2 sample is their potential link to thedeviation from epitaxial growth of thin lm. The extra spotsdetected in the electron diraction pattern are normally linkedto only Cu related secondary phases and information on theepitaxial quality of lm is not available, even when multiplepeaks appeared in the XRD pattern of the ZnO:Cu thin lm.32

The possible increase in number and size of these inserted CuO

Fig. 6 Low magnication image for the S-2 sample. The left insetshows an HRTEM image at the lmsubstrate interface for the samesample. The right inset shows an electron diraction pattern from thelmsubstrate interface of sample S-2.6 | RSC Adv., 2015, xx, 110nanophases would interrupt the crystalline host ZnO structure.Therefore the extra spots related to CuO in the electrondiraction pattern demonstrate a deviation from epitaxialgrowth for the S-2 sample. The lower epitaxial quality for thissample is also evident by reduced (002) peak intensity withincreased FWHM compared to S-1 sample in XRD results dis-cussed earlier.

However, high oxygen partial pressure during growth (S-3sample) seems to drastically change the epitaxial quality of thelm as seen by the columnar structure of native ZnO:Cu in thelowmagnication TEM image (Fig. 7). These observed columnarstructuresmight be related to the conversion of epitaxial regionsto polycrystalline regions due to localized epitaxial disorder. TheHRTEM image taken from the lmsubstrate interface (le insetof Fig. 7) reveals un-patterned and intermingled lattice fringesthat appeared to be amorphous regions in direct vicinity to theinterface (indicated by white brackets). To investigate the pres-ence of any type of clustering at the lmsubstrate interface thatmay cause these amorphous regions, we performed SIMS on S-3sample. The SIMS results for sample S-3 shown in Fig. 8demonstrate uniform depth proles of Zn, Al, O and Cuelements. Cu doping seems to be uniformly distributed withoutany type of Cu clustering at the lmsubstrate interface.Therefore, these intermingled lattice fringes are associated withdislocation loops caused by inclusion of CuO nanophases. Highoxygen pressure during growth facilitates small sized CuO tocluster into larger sized CuO (810 nm) nanophases that wouldspread throughout the host lattice at the expense of localizedepitaxial disorder. Apart from the lmsubstrate interfaceregion, we can still see parallel lattice fringes at the nativeZnO:Cu lm and deviation from epitaxial order is only restrictedto some localized regions, while the overall lm structuremaintains its epitaxial growth. The localized level poly-crystallinity of S-3 can also be seen from multiple spots thatappear in the electron diraction pattern from the lmsubstrate interface (right inset of Fig. 7). Although the oxygensupply during growth lowers the epitaxial quality of ZnO:Cu thinlms, the S-2 and S-3 samples did not fully convert to poly-crystalline thinlms andwe observe only localized level epitaxialdisorder. These samples still show epitaxial thin lm growth inagreement with the XRD results where we observed dominantreection from (002) and (004) planes indicating wurtzitestructure with its c axis orientation normal to the surface.

From XRD, XPS and TEM observations, we establish a rela-tion among oxygen partial pressure during growth, evolution ofCuO nanophases and epitaxial quality of thin lm samples.Introduction of oxygen during growth of ZnO:Cu thin lmsenhances the CuO nanophase formation and the nanophaseinclusion aects epitaxy and crystal quality of the thin lm.However, thin lm growth without oxygen shows perfect epitaxywith much better crystalline quality and even small size CuOnanophases interspersed along the (001) plane of the host ZnOhas no eect on the crystal quality compared to the S-2 and S-3samples. XPS is a surface sensitive technique shows CuOsecondary phases only for S-2 and S-3 samples, but TEM anal-ysis shows existence of CuO nanophases in the matrix of ZnOeven close to the lmsubstrate interface for the S-1, S-2 and S-3This journal is The Royal Society of Chemistry 2015

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10samples. Furthermore, the Cu1+ along with high V$$O concen-trations in sample S-1 and more dominant Cu2+ states in S-2and S-3 are found responsible for highly conducting and insu-lating behaviour of the respective samples. It seems that CuOnanophases are playing no role; instead Cu multivalent statesand V$$O are tuning the conducting and insulating behaviours ofthese samples.

3.3 Magnetic properties

One of the goals of this investigation is to correlate themagneticproperties of ZnO:Cu lms with their micro-structural

Fig. 7 Low magnication image for sample S-3. The left inset showsan HRTEM image at the lmsubstrate interface of the same sample.The right inset shows an electron diraction pattern from the lmsubstrate interface of the same sample.

Fig. 8 The SIMS depth proles of Zn, O, Cu and Al of sample S-3.

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55characteristics. Fig. 9 shows DC magnetization loops at xedtemperature (MH) for all three samples measured at 300 K, aerproper correction for the diamagnetic term arising from thesubstrate. These hysteresis curves provide clear evidence for theferromagnetic ordering at room temperature. From these MHloops, we extracted the saturation magnetization (Ms) values of0.22 106 emu g1, 0.46 106 emu g1 and 0.04 106emu g1 and coercive eld (Hc) values of 160 Oe, 53 Oe and400 Oe for S-1, S-2 and S-3, respectively. The Ms values of allthese samples are relatively lower than the reported values ofZnO:Cu thin lms and nanostructures3739 indicating extrinsicorigin of magnetization in these samples. Generally, two keytheoretical models, the RudermanKittelKasuyaYosida(RKKY)-type indirect exchange interactions and bound-magnetic polarons (BMPs) are used to explain the ferromag-netic ordering in DMS materials when the carrier concentrationis well below the impurity concentration.40,41 Presences of CuOsecondary phases in all three samples and the highly resistivenature of samples S-2 and S-3 would not energetically favorRKKY interactions and BMPmodel alone cannot promote FM inthese samples. Therefore, we anticipate that the weak ferro-magnetic behaviour is not of a single origin, instead V$$O , mixedCu oxidation states and CuO secondary phases contributesimultaneously in ZnO:Cu thin lm samples reported in thepresent study. The existence of CuO planar nanophases maygive rise to the ferromagnetism in ZnO:Cu20 and the magneticsusceptibility of these CuO nanophases increases rapidly whentheir size decreases. When the CuO sizes are small, the residualspins of the Cu ions at the interface can couple ferromagneti-cally with planar spins to generate larger values of themoment.20,42 On the other hand, in oxygen decient samples thedonoracceptor defect pairs (DAPs) such that Cu V$$O , can playa signicant role on the development of ferromangetic orderingin the ZnO:Cu system.18,37

In MH loops shown in Fig. 9, the S-1 sample shows inter-mediate values of Ms and Hc compared to the S-2 and S-3samples. The HRTEM analysis on S-1 showed perfect epitaxialthin lm growth with a very sharp and clear interface excludingthe possibility of magnetization coming from the interfacedistortion or metal clustering at the interface. Moreover, we areunable to identify other extrinsic sources for the observed weakferromagnetism, and these lms are found to be free fromspurious impurities as undoped ZnO lms prepared underidentical conditions failed to detect any ferromagnetism (notshown here) ruling out the deposition chamber and samplehandling as a potential cause of contamination. Possible sour-ces for the observed magnetization in the S-1 sample are smallsize CuO nanophases (34 nm), mixed Cu oxidation states orhigh V$$O concentration. The isolated V

$$O are known to destroy

the FM in ZnO:Cu system, but V$$O coupled with mixed Cuoxidation sates can endorse ferromagnetic ordering.18,37 Due tomixed oxidation states of Cu in sample S-1, the eectiveCu2 V$$O Cu1 defect complexes may give rise to magneticordering. The electrons bound to V$$O would align in an anti-parallel spin conguration with the Cu ions, thus resulting ina parallel spin conguration of the neighboring Cu ions leadingto an indirect double exchange ferromagneticRSC Adv., 2015, xx, 110 | 7

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30Cu2 V$$O Cu1 network.18 However, S-1 is conducting(carrier concentration 3.32 1018) and a large amount of V$$Oconcentrations may convert Cu2+ to Cu1+ thereby reducingeective Cu2 V$$O Cu1 complexes. Therefore, due to thehighly conducting nature of sample S-1, the indirect doubleexchange ferromagnetic Cu2 V$$O Cu1 network becomesless eective in mediating ferromagnetic ordering. On the otherhand, small size CuO crystals oer higher coercivity and createnoticeable changes in the magnetic properties, since CuOCu

Fig. 9 Room-temperature DC magnetization loop for all threesamples. The upper left inset shows zero eld cooled and eld cooledfor sample S-1. The lower right inset shows magnetoresistance curvesfor S-1 sample.superexchange is strongly dependent on both, bond angels andbond lengths on size reduction.42,43 Therefore, the intermediateMs and high Hc of sample S-1 compared to S-2 is mainly origi-nating from ferromagnetic CuOCu superexchange due touncompensated Cu spins at the surface of small size CuOnanophases. These observations are in agreement with previousreports on doped ZnO and GaN1,5,44 where TM based nano-phases were clearly detected and found responsible for theobserved FM. Moreover, we suggest that if an indirect doubleexchange ferromagnetic Cu2 V$$O Cu1 network is playingany role in tuning the magnetism in sample S-1, it is dominatedby the pronounced ferromagnetic signal coming from thesurface of CuO nanophases embedded in the (001) plane of theZnO:Cu thin lm host.

In the case of sample S-2, we notice the highest value of Mscompared to the S-1 and S-3 samples. Although we observerelatively lower epitaxial quality of S-2 sample compared to S-1sample, the HRTEM analysis still showed epitaxial thin lmgrowth along with the presence of CuO nanophases. Thisreveals that the high Ms value is not coming from the interfacedistortion or metal clustering at the interface. Core level XPSstudy for sample S-2 showed a more dominant Cu2+ signal thanCu1+ with small amount of V$$O concentration. This low V

$$O

concentrations leads to high Cu2+/Cu1+ ratio and the V$$O have

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55better chances to couple with Cu2+ to generate more eectiveCu2 V$$O Cu1 complexes. Furthermore, the S-2 sampleshows the lowest value of Hc compared to the S-1 and S-3samples, which suggests much less contribution of CuOCuferromagnetic superexchange interaction of the CuO nano-phases to the observed FM. Therefore, the observed lowest Hcand highest Ms values for sample S-2 suggest dominantbehaviour of ferromagnetic Cu2 V$$O Cu1 indirect doubleexchange interactions over CuOCu ferromagnetic super-exchange interaction of CuO nanophases in tuning the RT FM.Finally, sample S-3 shows a very weak and distorted magneti-zation signal with the highest value of Hc. As discussed previ-ously, we observe a relatively larger size of CuO (810 nm)nanophases and poor crystal quality of this high oxygen partialpressure grown sample. The XPS analysis also indicated asubstantial decrease in V$$O concentration resulting in acomprehensive reduction of the Cu2 V$$O Cu1 networks.Therefore, the increased Hc might be due to more pronouncedeect of large size CuO nanophase inclusion and the lowest Msis originating from a reduced number of ferromagnetic indirectdouble exchange Cu2 V$$O Cu1 defect complexes.

In the above discussed scenario, the HRTEM analysis unveilsa systematic localized epitaxial disorder, but changes in the MHloop are not regular with increase in oxygen pressure during thethin lm growth. We observe intermediate values of Ms and Hcfor the best quality sample S-1 and the highest value of Ms forthe relatively low quality sample S-2. HRTEM and MH loopsstudies suggest that the thin lm quality seems to play very littleeective role in governing the FM. On the other hand, themagnetic properties of ZnO:Cu thin lm samples are highlytuneable by the presence of CuO nanophases, multivalent Cuand V$$O concentrations.

XRD and HRTEM analyses on sample S-1 show the bestepitaxial quality for this sample. To further conrm the exis-tence of magnetic CuO phases in sample S-1, we carried out ZFCand FC magnetization (M) versus temperature measurementswith 100 Oe applied eld through a temperature range of 1.5 to350 K. One of the important signatures of nanosized magneticparticles is the separation of ZFC and FC curves below thecharacteristic blocking temperature (TB). Fig. 9 (upper le inset)shows a clear bifurcation of the ZFC and FC data and the onsetof irreversibility starts at room temperature. The splittingbetween the FC and ZFC data curves strongly suggests thepresence of superparamagnetic blocking in the magneticnanophase. The ZFC magnetization curve shows a maximum atTB z 210 K, which is the blocking temperature correspondingto the smallest crystal size. The bulk CuO crystallizes in amonoclinic structure and experiments demonstrate an incom-mensurate antiferromagnetic phase transition between TN1 230 K and TN2 213 K, with a commensurate AFM phase belowTN2 due to a magnetolattice coupling eect.45 Small size CuOparticles are known to acquire ferromagnetic order due toresidual spins of the Cu ions at the cluster interface42 and thelattice distortion, as evidenced in HRTEM, can promote FMsuperexchange interaction in CuOCu complexes.46 Therefore,the bifurcation in the ZFC and FC curves and the evolution of abroad peak around TB z 210 K in the ZFC curve suggestThis journal is The Royal Society of Chemistry 2015

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55presence of CuO nanophases that carry the basic magneticmoment in sample S-1. Furthermore, a large separationbetween onset of irreversibility (z300 K) and blockingtemperature (TB z 210 K) in the ZFC curve demonstrate thatCuO nanophases have widely distributed sizes. The blockingtemperature can be related to the volume (V) of the smallestmoment-carrying entity in the system by the relation 25kBTB KV.43 Here kB is the Boltzmann constant and K is the anisotropyconstant. Using TB 210 K, and K 2 106 erg cm3 for typicaltransition metal oxides,47,48 the eective average crystal size is7 nm for sample S-1. This eective average crystal size (7 nm)is larger than that obtained by TEMmeasurements (34 nm) forthe S-1 sample. The larger eective crystal size measured by theZFC/FC magnetization may possibly arise from relevant inter-particle magnetic interactions due to presence of a high densityof CuO nanocrystals in sample S-1 as evidenced by TEM anal-ysis.49 The large dierence between the crystallite sizesmeasured from TEM and ZFC/FC curves also indicates the widedistributions of CuO crystallite sizes, as evident by broad peakat TB 210 K in ZFC magnetization curve.

Apart from the magnetization measurements, a further andimportant way to check the existence of magnetic order is themeasurement of the magnetotransport properties. Our HRTEMand ZFC/FC measurements have revealed that sample S-1 isinhomogeneous and consists of regions of dierent magneticproperties. Depending on the size, distribution, and density ofthe inhomogeneous regions, their eect should be reected inthe transport properties.4 We observed a negative magnetore-sistance (Fig. 9 lower right inset) in the S-1 sample. Themagnetoresistance (MR) is dened as [R(H) R(0)]/R(0), whereR(H) and R(0) are the resistances with and without an appliedmagnetic eld, respectively.50 The MR strongly depends ontemperature, changing from large negative MR (15%) at lowtemperatures to a small negative MR at high temperatures inthe low eld regions. The strongly negative MR is the indicationof an extrinsic origin of FM and absence of carrier mediatedferromagnetism. The negative MR has also been observed inseveral homogenous and inhomogeneous magnetic ZnOsystems.5153 In case of inhomogeneous magnetic ZnO systemsjust like our S-1 sample, the negative MRmay be related to spin-dependent tunnelling between or across CuO regions carryingferromagnetic ordering.4,53

4. Conclusions

We have studied PLD grown ZnO:Cu thin lms prepared underdierent oxygen partial pressures. All the samples contain CuOsecondary phases irrespective of the growth conditions. Anincrease in oxygen partial pressure during growth leads to thereduction in the (002) peak and enhancement in FWHM of thispeak suggesting degradation in the epitaxial quality of theZnO:Cu lm. Core level XPS result shows existence of mixed Cuoxidation states and CuO secondary phases. Detailed HRTEManalysis reveals that an increase in oxygen growth pressuredegrades the epitaxy of these thin lm samples due to theinclusion of nanosize CuO in the ZnO host lattice. Themagneticproperties of ZnO:Cu thin lm samples are highly tuneable byThis journal is The Royal Society of Chemistry 201517 T. F. Shi, Z. G. Xiao, Z. J. Yin, X. H. Li, Y. Q. Wang, H. T. He,J. N. Wang, W. S. Yan and S. Q. Wei, Appl. Phys. Lett., 2010,96, 211905.

18 T. S. Herng, D. C. Qi, T. Berlijn, J. B. Yi, K. S. Yang, Y. Dai,Y. P. Feng, I. Santoso, C. S. Hanke, X. Y. Gao, A. T. S. Wee,13 Q. Ma, D. B. Buchholz and R. P. H. Chang, Phys. Rev. B:Condens. Matter Mater. Phys., 2008, 78, 214429.

14 L. H. Ye, A. J. Freeman and B. Delley, Phys. Rev. B: Condens.Matter Mater. Phys., 2006, 73, 033203.

15 L. M. Huang, A. L. Rosa and R. Ahuja, Phys. Rev. B: Condens.Matter Mater. Phys., 2006, 74, 075206.

16 M. H. N. Assadi, Y. B. Zhang and S. Li, J. Phys.: Condens.the simultaneous presence of CuO nanophases, multivalent Cuand V$$O concentrations. The very broad peak in the ZFCmagnetization and the strongly negative MR signal that origi-nates from ferromagnetic nanosize CuO regions furtherconrms the extrinsic FM in the high quality S-1 sample.

Acknowledgements

The work presented here was supported by the Research GrantCouncil of HKSAR under the GRF scheme (HKU703612P), aswell as HKUST DAG12SC05-4, FSGRF12SC13 and HKU SeedFunding Program for Basic Research (201111159037).

References

1 Bonanni, A. N. Quezada, T. Li, M. Wegscheider, Z. Matej,V. Holy, R. T. Lechner, G. Bauer, M. Rovezzi, F. D. Acapito,M. Kiecana, M. Sawicki and T. Diet, Phys. Rev. Lett., 2008,101, 135502.

2 K. Sato, H. Katayama-Yoshida and P. H. Dederichs, Jpn. J.Appl. Phys., Part 2, 2005, 44, L948.

3 M. Younas, L. L. Zou, M. Nadeem, Naeem-ur-Rehman,S. C. Su, Z. L. Wang, W. Anwand, A. Wagner, J. H. Hao,C. W. Leung, R. Lortz and F. C. C. Ling, Phys. Chem. Chem.Phys., 2014, 16, 16030.

4 M. Tay, Y. Wua, G. C. Han, T. C. Chong, Y. K. Zheng,S. J. Wang, Y. C. and X. Pan, J. Appl. Phys., 2006, 100, 063910.

5 T. Dietl, Nat. Mater., 2010, 9, 965.6 K. R. Kittilstved, W. K. Liu and D. R. Gamelin, Nat. Mater.,2006, 5, 291.

7 K. R. Kittilstved, N. S. Norberg and D. R. Gamelin, Phys. Rev.Lett., 2005, 94, 147209.

8 M. Venkatesan, C. B. Fitzgerald, J. G. Lunney andJ. M. D. Coey, Phys. Rev. Lett., 2004, 93, 177206.

9 P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma,R. Ahuja, J. M. Osorio-Guillen, B. Johansson andG. A. Gehring, Nat. Mater., 2003, 2, 673.

10 J. M. D. Coey, M. Venkatesan and C. B. Fitzgerald, Nat.Mater., 2005, 4, 173.

11 M. Venkatesan, C. B. Fitzgerald, J. G. Lunney andJ. M. D. Coey, Phys. Rev. Lett., 2004, 93, 177206.

12 L. M. Huang, A. L. Rosa and R. Ahuja, Phys. Rev. B: Condens.RSC Adv., 2015, xx, 110 | 9

W. Ku, J. Ding and A. Rusydi, Phys. Rev. Lett., 2010, 105,207201.

19 K. R. Kittilstved, D. A. Schwartz, A. C. Tuan, S. M. Heald,S. A. Chambers and D. R. Gamelin, Phys. Rev. Lett., 2006,97, 037203.

20 C. Sudakar, J. S. Thakur, G. Lawes, R. Naik and V. M. Naik,Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 75, 054423.

21 D. Huang, Y. J. Zhao, D. H. Chen and Y. Z. Shao, Appl. Phys.Lett., 2008, 92, 182509.

22 Q. Ma, D. B. Buchholz and R. P. H. Chang, Phys. Rev. B:Condens. Matter Mater. Phys., 2008, 78, 214429.

23 L. H. Ye, A. J. Freeman and B. Delley, Phys. Rev. B: Condens.Matter Mater. Phys., 2006, 73, 033203.

24 H. Liu, F. Zeng, S. Gao, G. Wang, C. Song and F. Pan, Phys.Chem. Chem. Phys., 2013, 15, 13153.

25 Y. Tian, Y. Li, M. He, I. A. Putra, H. Peng, B. Yao, S. A. Cheongand T. Wu, Appl. Phys. Lett., 2011, 98, 162503.

37 L. Hu, J. Huang, H. He, L. Zhu, S. Liu, Y. Jin, L. Sun and Z. Ye,Nanoscale, 2013, 5, 3918.

38 G. Z. Xing, J. B. Yi, J. G. Tao, T. Liu, L. M. Wong, Z. Zhang,G. P. Li, S. J. Wang, J. Ding, T. C. Sum, C. H. A. Huan andT. Wu, Adv. Mater., 2008, 20, 3521.

39 L. Hu, L. Zhu, H. He and Z. Ye, Appl. Phys. Lett., 2014, 105,072414.

40 Dietl, H. Ohno and F. Matsukura, Phys. Rev. B: Condens.Matter Mater. Phys., 2001, 63, 195205.

41 M. J. Calderon and S. D. Sarma, Ann. Phys., 2007, 322, 2618.42 A. Punnoose, H. Magnone, M. S. Seehra and J. Bonevich,

Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 64, 174420.43 B. D. Cullity and C. D. Graham, Introduction to Magnetic

Materials, Wiley, Hoboken, 2nd edn, 2009.44 L. C. Nistor, C. Ghica, V. Kuncser, D. Pantelica, J. J. Grob,

G. Epurescu and M. Dinescu, J. Phys. D: Appl. Phys., 2013,46, 065003.

45 H. Yamada, X. G. Zheng, Y. Soejima and M. Kawaminami,Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 104104.

RSC Advances Paper

1

5

10

15

20

25

30

35

40

45

50

55

1

5

10

15

20

25

30

35

40

45

50

5526 J. Sohn, S. H. Song, D. W. Nam, I. T. Cho, E. S. Cho, J. H. Leeand H. I. Kwon, Semicond. Sci. Technol., 2013, 28, 015005.

27 S. Poulston, P. M. Parlett, P. Stone and M. Bowker, Surf.Interface Anal., 1996, 24, 811.

28 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben,Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer,Eden Prairie, 1992.

29 D. Chakraborti, J. Narayan and J. T. Prater, Appl. Phys. Lett.,2007, 90, 062504.

30 M. C. Biesinger, L. W. M. Laua, A. R. Gerson andR. S. C. Smart, Appl. Surf. Sci., 2010, 257, 887.

31 M. S. Park and B. I. Min, Phys. Rev. B: Condens. Matter Mater.Phys., 2003, 68, 224436.

32 T. S. Herng, S. P. Lau, S. F. Yu, H. Y. Yang, X. H. Ji, J. S. Chen,N. Yasui and H. Inaba, J. Appl. Phys., 2006, 99, 086101.

33 M. Scrocco, Chem. Phys. Lett., 1979, 63, 52.34 L. M. Kukreja, P. Misra, J. Fallert, D. M. Phase and H. Kal, J.

Appl. Phys., 2012, 112, 013525.35 D. O. Klenov, G. N. Kryukova and L. M. Plyasova, J. Mater.

Chem., 1998, 8, 1665.36 B. Fultz and J. M. Howe, Transmission Electron Mircoscopy

and Diractometry of Materials, Springer, Berlin, 2nd edn,2002.10 | RSC Adv., 2015, xx, 11046 D. B. Buchholz, R. P. H. Chang, J. H. Song and J. B. Ketterson,Appl. Phys. Lett., 2005, 87, 082504.

47 A. Dinia, G. Schmerber, V. P. Bohnes, C. Meny, P. Panissodand E. J. Beaurepaire, J. Magn. Magn. Mater., 2005, 286, 37.

48 B. Ghosh, M. Sardar and S. Banerjee, J. Phys. D: Appl. Phys.,2013, 46, 135001.

49 R. Torchio, C. Meneghini, S. Mobilio, G. Capellini,A. G. Prieto, J. Alonso, M. L. F. Gubieda, V. T. Liveri,A. Longo, A. M. Ruggirello and T. Neisius, J. Phys.: Conf.Ser., 2010, 200, 72100.

50 M. Khalid and P. Esquinazi, Phys. Rev. B: Condens. MatterMater. Phys., 2012, 85, 134424.

51 P. A. Lee and V. Vramakrishnan, Rev. Mod. Phys., 1985, 57,287.

52 H. Ohno, H. Munekata, T. Penney, S. Von Molnar andL. L. Chang, Phys. Rev. Lett., 1992, 68, 2664.

53 M. Paranjape, J. Mitra, A. K. Raychaudhuri, N. K. Todd,N. D. Mathur and M. G. Blamire, Phys. Rev. B: Condens.Matter Mater. Phys., 2003, 68, 144409.This journal is The Royal Society of Chemistry 2015