Cryst. Res. Technol. 43, No. 10, 1046 – 1051 (2008) / DOI 10.1002/crat.200811179

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Co nanoclusters as origin of ferromagnetism in sol-gel synthesized

Zn1-xCoxO (x = 0.05, 0.10 and 0.15) samples

V. K. Sharma and G. D. Varma*

Department of Physics, Indian Institute of Technology Roorkee, Roorkee-247 667, India

Received 2 February 2008, revised 23 May 2008, accepted 21 July 2008

Published online 25 August 2008

Key words annealing, nanoclusters, XRD, FESEM-EDAX, RTFM.

PACS 75.50.Pp

We report on the structural and magnetic properties of the polycrystalline samples of Zn1-xCoxO (x = 0.05,

0.10 and 0.15) synthesized via sol-gel route. The air sintered samples of all compositions exhibit

paramagnetic behaviour at room temperature, on the other hand the same samples on annealing in Ar/H2

atmosphere show room temperature ferromagnetism (RTFM) with enhanced magnetization. The value of

magnetization increases with the Co concentration (x) in both the air sintered and Ar/H2 annealed samples.

The observed ferromagnetism in the Ar/H2 annealed Zn1-xCoxO (x = 0.05, 0.10 and 0.15) samples is attributed

to the presence of Co nanoclusters as detected by XRD and FESEM-EDAX.

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Diluted magnetic semiconductors (DMSs), where transition metal atoms are introduced into cations of the semiconducting host, have attracted tremendous attention because of the potential use of both charge and spin of electrons for spintronics devices [1, 2]. Soon after the theoretical prediction of RTFM in transition metal doped ZnO by Dietl et al. [3] and Sato and Katayama-Yoshida [4] lot of research work started on ZnO based DMS. The first report on RTFM in Co-doped ZnO was made by Ueda et al. [5] in thin films of compositions Zn1-xCoxO (x = 0.05 - 0.25) prepared by PLD on sapphire substrates. Later, many other groups studied the magnetic properties of Co-doped ZnO by growing the materials in thin film and bulk forms under varying growth conditions, and reported wide variety of results [6-9]. From the results of various groups it appears that the ferromagnetism in the sample relies very much on the growth conditions such as synthesis temperature and gaseous environment during synthesis. Deka et al. [10] reported ferromagnetism in hydrogenated Zn0.95Co0.05O sample, synthesized by solution combustion method and showed that ferromagnetism is induced due to the formation of Co metal during reduction process. Co metal clusters are found to be responsible for ferromagnetism by many groups [11, 12]. Manivannan et al. [13] found ferromagnetism in hydrogenated Zn0.95Co0.05O samples because of the oxygen vacancies created by hydrogenation. It has been reported by several groups [14-17] that oxygen vacancies and interstitial Zn defects plays crucial role in introducing ferromagnetism in Co doped ZnO.

Bhatti et al. [18] observed RTFM in chemically synthesized nanocrystalline Zn0.90Co0.10O samples and explained the observed ferromagnetism on the basis of bound magnetic polarons (BMPs) model [19]. Quesada et al. [20] suggested that interfaces of ZnO and Co3O4 grains plays an important role in introducing RTFM in Co doped ZnO. Rubi et al. [21] found that ferromagnetism in ZnO:Co can be switched on and off by consecutive low temperature annealing in N2 and O2, respectively suggesting that n-type defects are important to induce ferromagnetic coupling in ZnO:Co. The role of microstructure in determining the ferromagnetic interactions in Co doped ZnO is also studied by some groups [22,23]. Very recently, several groups [24-26] ____________________

* Corresponding author: e-mail: gdvarfph@iitr.ernet.in

Cryst. Res. Technol. 43, No. 10 (2008) 1047

www.crt-journal.org © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

reported RTFM in nanocrystalline ZnO, HfO2, TiO2 and In2O3 without introducing any magnetic ions and they thought that defects or oxygen vacancies are responsible for the observed ferromagnetism. These results further raise the controversy about the mechanism of origin of ferromagnetism in DMS, i.e., whether it is intrinsic to the materials or due to some secondary phases present in the samples.

In the present work, bulk samples of Zn1-xCoxO for x = 0.05, 0.10 and 0.15 have been synthesized via sol-gel route. The effect of annealing in Ar/H2 atmosphere on the structural and magnetic properties of the as synthesized materials has been explored. RTFM has been observed in all the Ar/H2 annealed samples and the evidence of formation of Co metal nanoclusters has been found in these samples.

2 Experimental

Bulk polycrystalline samples of Zn1-xCoxO for x = 0.05, 0.10 and 0.15 were synthesized via sol-gel route. Stoichiometric amount of Zinc nitrate [(NO3)2Zn.6H2O] and Cobalt acetate [(CH3COO)2Co.4H2O] were dissolved in N, N-Dimethylformamide using magnetic stirrer for ~ 10 h at ~ 50°C. The resulting gel was heated at 150°C for 10 h in an oven. The resulting powder was heated at 450°C for 15 h in air followed by furnace cooling. After this, the resulting material was reground and pelletized. Finally, the pellets were sintered in air for 12 h at 1100°C followed by furnace cooling. Some of the air sintered samples were further annealed at 800°C in a mixture of argon and hydrogen gas (Ar 90% + H2 10%) for 5 h. The crystal structure of the samples was studied using an x-ray diffractometer (XRD - BRUKER D8 ADVANCE) employing Cu-Kα (1.54Å) radiation. The temperature dependence of the magnetization (M-T) and magnetic hysteresis (M-H) loops were measured using vibrating sample magnetometer (VSM - Model 155 Princeton Applied Research). Microstructural and chemical analysis of the samples was done using field emission scanning electron microscope (FESEM - FEI Quanta 200 ESEM FEG) equipped with an Oxford Inca energy dispersive x-ray (EDAX) detector.

Fig 1 X-ray diffraction patterns (intensity on

logarithmic scale) of Zn1-xCoxO samples.

Fig 2 M-H curves of Zn1-xCoxO samples sintered in air.

(Online color at www.crt-journal.org)

3 Results and discussion

The X-ray diffraction patterns (intensity plotted on logarithmic scale) of air sintered and Ar/H2 annealed samples of Zn1-xCoxO (x = 0.05, 0.10 and 0.15) are shown in figure 1. In the air sintered samples all the diffraction peaks correspond to ZnO wurtzite structure except a peak of Co3O4 impurity phase and intensity of this impurity peak increases with the concentration of cobalt (x). On the other hand, XRD results of the Ar/H2 annealed samples show presence of Co metal together with the majority wurtzite structural phase of ZnO. The intensity of Co metal peaks in the XRD patterns of the Ar/H2 annealed samples increases with x. The Co metal peaks observed in all the Ar/H2 annealed samples are indexed as 111(hkl) and 200(hkl) [27]. From these results

1048 V. K. Sharma and G. D. Varma: Zn1-xCoxO nanoclusters

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

it can be concluded that the Co3O4 impurity phase of the air sintered samples upon annealing in Ar/H2 atmosphere reduces into Co. The average size of Co clusters; estimated from the Scherer’s formula, lie in the range of 36 - 42 nm. The lattice parameters (a, c) are calculated from the peaks of higher angles for accuracy and it is observed that with the increase in x, the lattice parameter a decreases marginally, whereas, the lattice parameter c increases by small value. These changes leave the volume of the unit cell practically unchanged. This is because ionic radii of the Co2+ (0.058 Å) and Zn2+ (0.060 Å) in the tetrahedral coordination are nearly same.

Fig. 3 M-H curves of Zn1-xCoxO samples annealed in

Ar/H2 atmosphere. Inset shows the M-H curve expanded

near origin for x = 0.05. (Online color at www.crt-

journal.org)

Fig. 4 M-H curves of pure Co3O4 and the Co3O4

sample annealed in Ar/H2 at 800oC for 5 h. (Online

color at www.crt-journal.org)

The magnetization versus magnetic field (M-H) curves, recorded at room temperature (300 K), of the air sintered samples of Zn1-xCoxO for x = 0.05, 0.10 and 0.15 are shown in figure 2. For the M-H measurements the samples have been taken in powder form and the diamagnetic contribution from the sample holder has been subtracted. The M-H curves of the air sintered samples show linear variation of magnetization with magnetic field, suggesting paramagnetic behavior of the sample. For x > 0.05 a small deviation in the linearity has been observed in the M-H curves near H = 0, indicating weak ferromagnetism in the samples. The value of magnetization increases with increase in the value of x. On the other hand, the M-H curves of the sample of same composition annealed in Ar/H2 show presence of hysteresis loops (see Fig. 3). Saturation magnetization (Ms) increases with the increase in the Co content in the samples. From the XRD results we have seen that the intensity of Co metal peaks increases as Co content (x) in the sample increases. Thus increase in the saturation magnetization values with x can be related with increase in the Co metal clusters in the Ar/H2 annealed samples. The M-H curve expanded near H = 0 for Zn0.95Co0.05O is shown in the inset of figure 3. The values of coercive field (Hc) for all the Ar/H2 annealed samples lie in the range of 100 - 200 Oe. To check the role of Co3O4 impurity phase in inducing ferromagnetism in Zn1-xCoxO, we made the pellets of pure Co3O4 and annealed it at 800°C in Ar/H2 atmosphere for 5 h. We found that air sintered samples are paramagnetic whereas the Ar/H2 annealed samples are ferromagnetic with a strong magnetic moment (see Fig. 4) i.e., comparable to magnetic moment of pure Co (~162.5 emu/g) [28]. From XRD we found that after hydrogenation Co3O4 gets reduced to Co. The reduction process is a two stage process [29] as given below:

Co3O4 → CoO → Co

Microstructural characteristics and chemical compositions of all the samples have been studied using FESEM-EDAX technique. Figures 5 shows the FESEM micrographs of the air sintered and Ar/H2 annealed samples. The FESEM micrographs of the air sintered samples reveal the presence of crystallites of varying sizes for each value of x, and on an average the crystallite sizes increase with increase in the Co concentration (x) in the samples. From the FESEM micrographs of Ar/H2 annealed samples we see curious microstructural changes in the samples after annealing. Several circular particles lying over the usual grains could be seen in the FESEM

Cryst. Res. Technol. 43, No. 10 (2008) 1049

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micrographs of the Ar/H2 annealed samples (see Fig. 5). The number and size of these circular particles increase as Co concentration (x) in the samples increases. In order to compare the chemical composition of the circular particles with that of the main grains we have recorded the EDAX spectra from various regions of the samples as seen in the FESEM micrographs.

Fig 5 FESEM micrographs of air sintered samples (a, c

and e) and Ar/H2 annealed samples (b, d and f) of

Zn1-xCoxO for x = 0.05, 0.10 and 0.15.

Fig 6 (a) FESEM micrograph of the

Ar/H2 annealed Zn0.90Co0.10O sample.

(b) & (c) EDAX patterns recorded,

respectively, from the points lying on

the grain (point 1) and circular particle

(point 4). (Online color at www.crt-

journal.org) Figure 6 shows the EDAX patterns of a Ar/H2 annealed Zn0.90Co0.10O sample recorded from the points lying on the grain (point 1) and on the circular particle (point 4) revealing the higher at % of Co in later region. EDAX patterns recorded from all the points on the grains (point 1, 2) (see Fig. 6b) are almost similar. Similarly, EDAX patterns recorded from all the circular particles (point 3, 4, 5 and 6) (see Fig. 6c) are nearly same. Thus, from the EDAX results it has been found that the circular particles have greater atomic percentage (at %) of Co as compared to usual grains. Au peak in the EDAX result is coming since gold coating was done on the samples to make them conducting. Thus, we come to a conclusion that the circular particles do posses higher concentration of cobalt. We further did the elemental mapping of the air sintered and Ar/H2 annealed samples. Figure 7 shows elemental mapping of Zn0.85Co0.15O sample, where blue, green and red color represents elements Zn, Co and O, respectively. Figure 7a represents the elemental mapping of air sintered sample of Zn0.85Co0.15O. This figure shows that all the constituent elements are uniformly distributed throughout the sample. But the elemental mapping of the Ar/H2 annealed samples clearly reveals inhomogeneous distribution of constituent elements (see Fig. 7b). From figure 7b we can see the presence of clusters of green dots which represent Co and also decrease in the density of red dots that represent the oxygen. Average diameter of the green dots in figure 7b is ~ 200 nm. Thus elemental mapping confirms the formation of Co clusters and

1050 V. K. Sharma and G. D. Varma: Zn1-xCoxO nanoclusters

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

reduction in oxygen content in the Ar/H2 annealed samples. Therefore, based on the XRD and FESEM-EDAX results it is argued that in the Ar/H2 annealed samples the RTFM is due to formation of Co metal clusters. The increase in the saturation magnetization with x in the Ar/H2 annealed samples can be correlated with the increase in the density of circular particles with x. Size of these circular particles also increases with x.

Fig 7 Elemental mapping, recorded through FESEM-EDAX, of the air sintered (a) and Ar/H2 annealed

samples (b), respectively, of the composition Zn0.85Co0.15O. (Online color at www.crt-journal.org)

From the FESEM results we find that the average size of circular particles increases from 140 to 230 nanometers as x increases from 0.05 to 0.15. From XRD results the average size of Co cluster estimated from the broadening of the Co peaks using Scherer’s formula is found to be ~ 40 nm. So, there lies a possibility of finding Co nanoclusters of this order in the circular particles seen in the FESEM micrographs of the Ar/H2 annealed samples. Further, TEM investigations to find the exact cluster sizes are in progress. The recent experimental [15,30,31] studies on ZnO based DMSs have shown that oxygen vacancies play an important role in introducing RTFM. In our samples we have found decrease in the O at % in the Ar/H2 annealed samples through FESEM-EDAX. Therefore, it is expected that there may be some contribution of oxygen vacancies in the observed RTFM. Recently, Lee et al. [32] showed that ZnCo(9.1%)O and ZnCo(5%)O thin films become ferromagnetic when annealed in Ar/H2 atmosphere, due to the enhanced ferromagnetic spin-spin interaction in the H-Co coupling. Therefore, in our case also the role of these interactions can not be ruled out. Further investigations are under progress to establish this fact.

4 Conclusions

In summary, we observed RTFM in the Ar/H2 annealed samples of Zn1-xCoxO (x = 0.05, 0.10 and 0.15) synthesized by sol-gel route. Based on XRD and FESEM-EDAX results it has been concluded that observed RTFM in the Ar/H2 annealed samples is mainly due to formation of Co nanoclusters.

Acknowledgments One of the authors (V. K. Sharma) is grateful to C.S.I.R., Government of India, for a research

fellowship.

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