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Mn-doped ZnO nanocrystalline thin films prepared by ultrasonic spray pyrolysis
Journal of Alloys and Compounds 471 (2009) 11–15
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mn-doped ZnO nanocrystalline thin films prepared by ultrasonic spray pyrolysis Preetam Singh, Ajay Kaushal, Davinder Kaur ∗ Department of Physics & Center of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 23 December 2007 Received in revised form 16 March 2008 Accepted 19 March 2008 Available online 8 May 2008 Keywords: Ultrsonic spray pyrolysis ZnO Thin films
a b s t r a c t Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films were synthesized by low cost ultrasonic spray pyrolysis technique on simple glass substrate at low substrate temperature. The films were characterized by X-ray diffraction (XRD), UV–vis spectrometer and superconducting quantum interference device magnetometer (SQUID). The influence of doping concentration on structural, optical and magnetic properties of the films was studied in detail. Structural and optical properties of the films elucidated that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. No impurity phase was observed in XRD pattern even after doping 10 at.% of Mn. The value of band gap was found to decrease from 3.24 eV to 3.14 eV with corresponding increase in Mn concentration from x = 0.03 to 0.10. The observed decrease in the band gap with increase in doping concentration was explained in terms of a sp–d exchange interaction. A non-linear M–H behavior observed at 5 K indicate weak ferrimagnetic behavior at low temperature. The value of coercivity and the remanant magnetization at 5 K for 7 at.% Mn-doped films was observed to be 147 Oe, and 3.77 × 10−5 emu, respectively. However there was no indication of room temperature ferromagnetism in these films. The concave nature of M(T) behavior with steep rise of magnetization was observed at low temperature and was explained in terms of polaron-percolation-theory. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, extensive studies have been carried out to modify the properties of zinc oxide for different applications [1,2]. Doping with transition metal elements leads to many interesting properties of ZnO. In 2000, Dietl et al. [3] used a simple theory to estimate the Tc of ferromagnetic semiconductors, and they predicted that room temperature ferromagnetic semiconductors might be created by substituting manganese ions in wide-band gap semiconductors such as GaN and ZnO. Reports of ZnO-based room temperature ferromagnetic semiconductors [4,5] soon followed. Currently, much experimental and theoretical research is focused on the dilute magnetic semiconductors (DMS) based on ZnO doped with transition metal ions such as Mn and Co, since the predicted room temperature ferromagnetism in DMS may be useful in spintronics [2]. The mechanism leading to room temperature ferromagnetism in Mn- and Co-doped ZnO is not fully established and there are many contradictory experimental observations. Even more startling are claims that undoped films of some of these oxides are ferromagnetic, or that they can become magnetic when doped with non-magnetic cations. Examples are HfO2 [6] and ZnO
∗ Corresponding author. Tel.: +91 1332 285407; fax: +91 1332 273560. E-mail address: [email protected] (D. Kaur). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.123
doped with Sc [7]. The term ‘d0 ferromagnetism’ has been suggested for these cases. Some research groups describe the observed FM ordering as an intrinsic effect [8,9], while others describe it as an extrinsic effect [10–12]. Studies on ZnMnO thin films and bulk samples by different research groups revealed paramagnetic [13,14], ferromagnetic [15] and antiferromagnetic behavior [16]. These studies indicate that the magnetic properties of Mn-doped ZnO are highly sensitive to the preparation methods and conditions. This controversy between research teams may result from the growth method used and/or from the growth conditions. In fact, depending on the different growth modes and mechanisms, microstructures in these systems such as distribution of Mn ion in ZnO crystal lattice and the local environment around Mn ions are very different, which considerably affects the magnetic properties. In present work, Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films were grown on commercial glass substrates by simple and low cost ultrasonic spray pyrolysis technique. The doping concentration was chosen below the equilibrium solubility limit of Mn2+ ions in ZnO. Spray pyrolysis technique [17] was adopted for the synthesis of Mn-doped ZnO thin films, because the process has many advantages such as better stoichiometry control, better homogeneity, low processing temperature, lower cost, easier fabrication of large area films, possibility of using high-purity starting materials and having an easy coating process of large sub-
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P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
strates. Earlier we have reported the successful fabrication of pure nanocrystalline ZnO thin films and ZnO nanopowder by the same technique [18]. In the present study we have tried to investigate the effect of doping concentration on the structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by ultrasonic spray pyrolysis technique. The aim was to compare the results obtained in here to samples prepared by other deposition methods. The structural and optical properties of these films reflect that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. The magnetic studies reveal the absence of room temperature ferromagnetic behavior in these films. 2. Experimental The Zn1−x Mnx O thin films were prepared from aqueous solution of zinc nitrate (Zn(NO3 )2 ·6H2 O) and manganese (Mn(NO3 )2 ·6H2 O) dissolved in distilled water. For each doping concentration (x), a separate solution was made. The substrates were first ultrasonically degreased with acetone, ethanol and deionized water. Various parameters such as nozzle to substrate distance, deposition rate and flow rate of air (carrier gas), deposition temperature and concentration were optimized to get good quality films as shown in Table 1. Specially designed digital substrate heater with temperature controller from excel instruments was used to heat the substrate. The rate of deposition was controlled by the carrier gas flow rate, substrate temperature as well as precursor concentration. The films of thickness 450 nm were deposited on glass (components, CaO:NaO:6SiO2 ) substrate at a fixed substrate temperature of 400 ◦ C. The chemical solution was atomized into the stream of the fine droplets via ultrasonic nebulizer operated to an atomizing frequency of 1.7 MHz. Nitrates precursor solution was poured in to the vessel from inlet side. The aerosol was generated from the vibration of the transducer. The nebulized spray, which goes up in the column, was deposited on a hot substrate. The orientation and crystallinity of these films were studied using Bruker AXS C-8 advanced diffractrometer in –2 geometry. Energy dispersive X-ray analysis (EDAX) was carried out on various samples by comparing the peaks and it confirms the concentrations of Mn to be close to those in the stated composition. PerkinElmer Lambda 25 UV–vis spectrometer was used to study the optical properties of films. The magnetic properties were investigated by superconducting quantum interference device (SQUID) magnetometer (Quantum design, MPMS, XL).
3. Results and discussion 3.1. Structural properties X-ray diffraction (XRD) pattern of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) films deposited on glass substrate at 400 ◦ C are shown in Fig. 1. All the films were single phase and had hexagonal wurtzite structure with c-axis preferred orientation. No evidence of any other secondary phases and impurities was detected in the XRD pattern. However this does not preclude the presence of secondary phases, since a small volume fraction of a randomly oriented impurity phase would be difficult to detect. Increasing Mn concentration caused the position shift of (0 0 2) peak of the Zn1−x Mnx O towards lower angles. Correspondingly, the c-axis lattice constant of Zn1−x Mnx O films increased from 5.2150 A˚ (x = 0.03), 5.2224 A˚ (x = 0.05), 5.2284 A˚ (x = 0.07) to 5.2372 A˚ (x = 0.10). The
Fig. 1. XRD pattern of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin film deposited on glass at 400 ◦ C.
similar dependence of the lattice constant on Mn concentration is usually observed in reported results on Zn1−x Mnx O films [19]. ˚ is smaller than that of Mn2+ Since the ionic radius of Zn2+ (0.74 A) 3+ ˚ ˚ the linear increase of (0.80 A) and larger than that of Mn (0.66 A), the c-axis lattice constant with Mn concentration indicated that the divalent Mn2+ ions substituted for Zn2+ ions in ZnO crystal lattice. Moreover, as Mn concentration increased in Zn1−x Mnx O films, peak width of (0 0 2) diffraction peak was found to be increased, which could be due to lattice disorder and strain induced by Mn2+ ions substitution. The observed increase in value of the FWHM of (0 0 2) reflection of Zn1−x Mnx O was found to be 0.183 A˚ (x = 0.03), 0.201 A˚ (x = 0 .05), 0.334 A˚ (x = 0.07) and 0.44 A˚ (x = 0.10), respectively. Further the crystallite size was calculated from XRD data and was observed to be 44.92 nm (x = 0.02), 40.89 nm (x = 0.05), 24.61 nm (x = 0.07) and 18.67 nm (x = 0.10). We have also calculated the grain size from the AFM and FESEM data (figures are not included in this paper) which was observed to be less than 100 nm in case of all the films.
Table 1 Optimized spray parameters for Zn1−x Mnx O thin films Spray mode Air blast Ultrasonic frequency (MHz) Droplet size (m) Solution flow rate (ml/h) Distance from heater to substrate (cm) Solvent Precursor Concentration (mol/l) Doping concentration (at.%) Deposition temperature ( ◦ C) Substrate
Ultrasonic nebulizer Atomizer 1.7 2.8 10 5 Distilled water and methanol Zinc nitrate, and manganese nitrate 0.1 3, 5, 7 and 10 400 Glass
Fig. 2. Optical absorbance spectra of Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films deposited on glass at 400 ◦ C.
P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
13
3.2. Optical properties Fig. 2 shows the optical absorption spectra of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films. The absorption edge of the samples was found to shift towards the longer wavelength side which may be due to the fact that absorption higher energy activation of sp–d exchange interactions. The direct band gap of the films was calculated using the Tauc relationship as follows: ˛h = A(h − Eg )
n
where ˛ is the absorption coefficient, A is a constant, h is Planck’s constant, is the photon frequency, Eg is the energy band gap and n is 1/2 for direct semiconductor. An extrapolation of the linear region of a plot of (˛h)1/n on the y-axis versus photon energy (h) on the x-axis gave the value of the energy band gap (Eg ). Since Eg = h when (˛h)1/n = 0. Here the direct band gap of the Mn-doped ZnO films was evaluated by extrapolating the straightline part of the curves (˛h)2 = 0 as shown in Fig. 3. The value of band gap was found to decrease from 3.24 eV to 3.14 eV with corresponding increase in Mn concentration from x = 0.03 to 0.10, respectively as shown in inset of figure and is also in consistent with the reported results [20]. The observed decrease in the band gap could be explained because of a sp–d exchange interaction, i.e. manifestation of strong exchange interaction present between d electron of Mn, and the s and p electrons of host matrix. The main d–d transitions occur at 6 A1 → 4 T1 , 4 T2 , 4 A1 , 4 E energy levels of Mn2+ ion in presence of tetrahedral crystal field interaction. Although exchange interaction is especially important in the presence of an external magnetic field H, in certain cases it can manifest at H = 0. However Fukumura et al. [21] has reported an overall blue shift in the band gap with increase in Mn concentration in thin epitaxial films of Mn-doped ZnO and the reason for blue shift was attributed to the higher band gap energy of MnO (4.2 eV). 3.3. Magnetic properties Magnetization of the Zn1−x Mnx O films of dimension 0.5 cm × 0.5 cm was measured as function of magnetic field (M–H) and temperature (M–T) in the magnetic field range of ±4 T and in the temperature range of 5–300 K. The data have been corrected for the diamagnetic contribution due to the background
Fig. 3. (˛h)2 vs. h plots of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films deposited on glass at 400 ◦ C. The inset shows the variation of band gap with Mn concentration (%).
Fig. 4. Magnetization vs. magnetic field of Zn0.95 Mn0.05 O thin film at 5 K and 300 K. The inset shows the data for the low field regions at 5 K.
signal from glass substrate as given by
M
MC = MF+S − −
H
S
H
where MC is the magnetic moment after subtracting the substrate contribution, MF+S is the total magnetic moment of film and substrate and (−M/H)S . H is the diamagnetic contribution due to the substrate. Figs. 4 and 5 show the magnetization (M) versus magnetic field (H) curve of Zn1−x Mnx O films with x = 0.05 and 0.07, respectively at 5 K and 300 K. Non-linear behavior of the magnetic moment was observed at both 5 K and 300 K temperatures. The M(H) curves do not show clear hysteresis loops in our case for both 5 at.% and 7 at.% Mn-doped ZnO films at 300 K which was in consistent with other reports on transition metaldoped DMS films prepared by spray pyrolysis [22]. However at 5 K, there was very weak signature of hysteresis. We have plotted the low field region of the loop as inset in Figs. 4 and 5 showing coericivity HC ∼85 Oe and remanant magnetization (Mr ) of 4.4 × 10−5 emu for 5 at.% Mn-doped ZnO films at 5 K. The value of coercivity HC ∼147 Oe and the remanant magnetization Mr = 3.77 × 10−5 emu was observed at 5 K for 7 at.% Mn-doped films.
Fig. 5. Magnetization vs. magnetic field of Zn0.93 Mn0.07 O thin film at 5 K and 300 K. The inset shows the data for the low field regions at 5 K.
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P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
Fig. 6. Temperature dependence of magnetization at a field of 1 kOe of Zn0.95 Mn0.05 O thin film.
We further investigated the temperature dependent magnetization of these films at constant magnetic field of 1 kOe and temperature range of 5–300 K (Figs. 6 and 7). In both cases of Zn1−x Mnx O films with x = 0.05 and x = 0.07, the trend of curve was similar. The M–T curve at field cooled (FC) and zero field cooled (ZFC) conditions at field of H = 1 kOe is shown in the inset of Fig. 6. The non-zero value of M was observed to persist at least up to temperature of 200 K. The value of magnetization was found to increase slowly with decreasing temperature from 300 K to 45 K, and afterwards there was a steep rise of magnetization with very strong concave curvature, which was different from the Weiss mean field prediction. Fig. 8 shows the temperature dependent inverse of dc-magnetic susceptibility () plot of the Zn0.95 Mn0.05 O film at 1 kOe field. Inset of Fig. 8 is the fit with mean-field Curie–Weiss law (−1 = (T − )/C) in the temperature range of 5–300 K, where C is the Curie constant and is the Curie–Weiss temperature. From fitted parameters the magnitude of calculated value of was found to be very large and negative (∼−280 K). The strongly concave M(T) behavior is often observed in a carrier-localized regime, which can be understood in the scheme of the polaronpercolation-theory as reported by Sarma et al. [23]. The problem of ferromagnetic transition in a system of magnetic polarons can be considered as equivalent to a problem of overlapping spheres studied in the percolation theory. According to the theory, the
Fig. 7. Temperature dependence of magnetization at a field of 1 kOe of Zn0.93 Mn0.07 O thin film.
sphere of radius in space are overlapped or collapsed and they can form “clusters”. Each sphere of overlapping clusters corresponds to the bound magnetic polarons. The polarons contain many impurity spins and one localized hole. The hole localized wave function decay exponentially with distance from a localized center where ad is the decay length from the localized center of the exponential function (radius of the localized hole). In the limit of low carrier density regime (strong carrier localization), ad 3 nh is very small (ad 3 nh 1). In a system of magnetic polarons this condition satisfies ferromagnetic ordering with highly concave nature in the M(T) curve at lower temperatures, where nh is the finite density of hole in host semiconductor. This generally happens in the case of a strongly localized insulating DMS. The effective radius of the polaron increases logarithmically with inverse temperature. At a very low temperature the effective radius of polarons are comparable to the size of the samples, i.e. the formation of infinite cluster corresponds the long range ordering of ferromagnetic regime when many spins of magnetic impurity are polarized by kinetic exchange interaction of hole spin. Moreover, at enough low temperatures the neighboring polarons are overlapped and the spins of polarons are aligned by the interaction of polarons through the magnetic impurities between them. In the presence of disorder state, various type of spin-glass ground state may compete with the ferromagnetic ground state [24]. There have been several reports on measurements in different TM-doped DMS thin films where all of them have observed a concave nature in the M(T) curve with steep rise of magnetization at lower temperatures without any magnetic transition down to the lowest attainable temperature [25]. However, there exist prominent nonlinear hysteresis loop in the M(H) curve for all those TM-doped DMS thin films at lower temperatures. In our case of Mn-doped ZnO thin films, we have observed concave nature in M(T) curve with steep rise of magnetization at lower temperature, but we have not observed any prominent non-linear hysteresis loop in M(H) curve. The reported experimental results on the magnetic properties of Mn-doped ZnO are very different in thin films as well as in bulk system. Both ferromagnetism and absence of ferromagnetism were observed in doped ZnO thin films grown by pulsed laser deposition (PLD) [5,26] and magnetron sputtering [27]. Films grown by molecular beam epitaxy (MBE) [28] exhibit ferromagnetism below 45 K. In bulk system the same diversity is observed. Although Mn implanted ZnO single crystals are ferromagnetic up to 250 K [29], weak ferromagnetism has been reported in bulk Zn0.98 Mn0.02 O pre-
Fig. 8. Temperature dependent susceptibility () of Zn0.95 Mn0.05 O thin film measured at field of 1 kOe.The inset shows the temperature dependent −1 curve.
P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
pared by solid-state reaction [30] and no ferromagnetism at all was observed in samples prepared by co-precipitation method [31]. A careful examination of the reported results indicate that where ferromagnetism has been found, the samples were heated to relatively high temperatures which could give rise to spinel impurity phases. Even where the temperature of synthesis is relatively low, some of the synthetic procedures are not convincing as to whether the dopant has substituted the Zn site. Furthermore, the magnetization values reported by many workers is very low and can arise from the presence of magnetic impurities which cannot be detected by X-ray diffraction. It has also been shown that the secondary phase Mn2−x Znx O3− , or an extrinsic source, is responsible for ferromagnetism in Mn-doped ZnO which was initially thought to be intrinsic. In our case of Zn1−x Mnx O thin films synthesized by spray pyrolysis technique, the constituent elements were mixed at molecular level ensuring dopant atoms are present at substitutional sites as confirmed from XRD and optical properties of these films. Moreover the deposition temperature was kept relatively low to avoid any possible impurity phase segregation even in relatively higher doping content. The absence of room temperature ferromagnetism in these films could possibly be due to neighboring antiferromagnetic interactions which suppress the ferromagnetic coupling or due to lack of free carriers in these films, which is in confirmation with the reports of Spaldin [32], that showed that robust ferromagnetism cannot occur in Mn- and Co-doped ZnO. If at all, it may occur if additional charge carriers are present. 4. Conclusion In summary, we have successfully prepared the Zn1−x Mnx O thin films with different doping concentration (x = 0.03, 0.05, 0.07 and 0.10) by ultrasonic spray pyrolysis at relatively low temperature (400 ◦ C). The structural and optical properties of these films reflect that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. The observed decrease in the band gap with increase in doping concentration of Mn-doped ZnO films was explained in terms of a sp–d exchange interaction. The concave nature of M(T) behavior with steep rise of magnetization was observed at low temperature and was explained in terms of polaron-percolation-theory. Non-linear behavior of the magnetic moment was observed at both 5 K and 300 K temperatures. The value of coercivity HC ∼147 Oe and the remanant magnetization Mr = 3.77 × 10−5 emu was observed at 5 K for 7 at.% Mn-doped films. We have not observed any impurity phases of manganese oxides in XRD curve, even after doping the 10 at.% of Mn in these doped ZnO thin films. But we also have not observed the room temperature ferromagnetism in Zn1−x Mnx O thin films.
15
Acknowledgements The financial support provided by DST, India, under the scheme Nanoscience and Technology Initiatives (NSTI) with Reference No. DST SR/S5/NM-32/2005 is highly acknowledged. The author Preetam Singh is thankful to CSIR for award of senior research fellowship. References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34 (2003) 3. [2] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] S. Sonada, S. Shimizu, T. Sasaki, Y. Yamamoto, H. Hori, J. Cryst. Growth 237–239 (2002) 1358. [5] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [6] N.H. Hong, J. Sakai, N. Poirot, V. Brize, Phys. Rev. B 73 (2006) 132404. [7] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [8] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gamelin, J. Am. Chem. Soc. 125 (2003) 13205. [9] P. Sati, R. Hayn, R. Kuzian, S. Regnier, S. Schafer, A. Stepanov, C. Morhain, C. Deparis, M. Laugt, M. Goiran, Z. Golacki, Phys. Rev. Lett. 96 (2006) 017203. [10] D.P. Norton, M.E. Overberg, S.J. Pearton, K. Pruessner, J.D. Budai, L.A. Boatner, M.F. Chisolm, J.S. Lee, Z.G. Khim, Y.D. Park, R.G. Wilson, Appl. Phys. Lett. 83 (2003) 5488. [11] B.S. Jeong, Y.W. Heo, D.P. Norton, J.G. Kelly, R. Rairigh, A.F. Hebard, J.D. Budai, Y.D. Park, Appl. Phys. Lett. 84 (2004) 2608. [12] K.P. Bhatti, S. Chaudhary, D.K. Pandya, S.C. Kashyap, J. Appl. Phys. 101 (2007) 103919. [13] S.S. Kim, J.H. Moon, B.T. Lee, O.S. Song, J.H. Je, J. Appl. Phys. 95 (2004) 454. [14] A. Tiwari, C. Jin, A. Kvit, D. Kumar, J.F. Muth, J. Narayan, Solid State Commun. 121 (2002) 371. [15] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M.O. Gillen, B. Johasson, G.A. Gehring, Nat. Mater. 2 (2003) 673. [16] C.N.R. Rao, F.L. Deepak, J. Mater. Chem. 15 (2005) 573. [17] D. Kaur, A.K. Gupta, J. Phys. D: Appl. Phys. 35 (2002) 729. [18] P. Singh, A. Kumar, Deepak, D. Kaur, J. Cryst. Growth 306 (2007) 303. [19] H.Y. Xu, Y.C. Liu, C.S. Xu, Y.X. Liu, C.L. Shao, R. Mu, J. Chem. Phys. 124 (2006) 074707. [20] V.S. Bhat, F.L. Deepak, Solid State Commun. 135 (2005) 345. [21] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 3366. [22] A. Manivannan, M.S. Seera, S.B. Majumdar, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 111. [23] D.D. Sarma, E.H. Hwang, A. Kaminski, Phys. Rev. B 67 (2003) 155201. [24] C. Timm, F. Schafer, F.V. Oppen, Phys. Rev. Lett. 89 (2002) 137201. [25] Z.B. Gu, C.S. Yuan, M.H. Lu, J. Wang, D. Wu, S.T. Zhang, S.N. Zhu, Y.Y. Zhu, Y.F. Chen, J. Appl. Phys. 98 (2005) 053908. [26] K.W. Nielsen, J.B. Philipp, M. Opel, A. Erb, J. Simon, L. Alff, R. Gross, Superlattices Microstruct. (2005) 327. [27] X.M. Cheng, C.L. Chien, J. Appl. Phys. 93 (2003) 7876. [28] S.W. Jung, S.J. An, G.C. Yi, C.U. Jung, S.I. Lee, S. Cho, Appl. Phys. Lett. 80 (2002) 4561. [29] D.P. Norton, S.J. Pearton, A.F. Hebard, N. Theodoropoulou, L.A. Boatner, R.G. Wilson, Appl. Phys. Lett. 82 (2003) 239. [30] W. Chen, L.F. Zhao, Y.Q. Wang, J.H. Miao, S. Liu, Z.C. Xia, S.L. Yuan, Solid State Commun. 34 (2005) 827. [31] J. Alaria, Chem. Phys. Lett. 415 (2005) 337–341. [32] N.A. Spaldin, Phys. Rev. B 69 (2004) 125201-1–125201-7.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mn-doped ZnO nanocrystalline thin films prepared by ultrasonic spray pyrolysis Preetam Singh, Ajay Kaushal, Davinder Kaur ∗ Department of Physics & Center of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 23 December 2007 Received in revised form 16 March 2008 Accepted 19 March 2008 Available online 8 May 2008 Keywords: Ultrsonic spray pyrolysis ZnO Thin films
a b s t r a c t Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films were synthesized by low cost ultrasonic spray pyrolysis technique on simple glass substrate at low substrate temperature. The films were characterized by X-ray diffraction (XRD), UV–vis spectrometer and superconducting quantum interference device magnetometer (SQUID). The influence of doping concentration on structural, optical and magnetic properties of the films was studied in detail. Structural and optical properties of the films elucidated that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. No impurity phase was observed in XRD pattern even after doping 10 at.% of Mn. The value of band gap was found to decrease from 3.24 eV to 3.14 eV with corresponding increase in Mn concentration from x = 0.03 to 0.10. The observed decrease in the band gap with increase in doping concentration was explained in terms of a sp–d exchange interaction. A non-linear M–H behavior observed at 5 K indicate weak ferrimagnetic behavior at low temperature. The value of coercivity and the remanant magnetization at 5 K for 7 at.% Mn-doped films was observed to be 147 Oe, and 3.77 × 10−5 emu, respectively. However there was no indication of room temperature ferromagnetism in these films. The concave nature of M(T) behavior with steep rise of magnetization was observed at low temperature and was explained in terms of polaron-percolation-theory. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, extensive studies have been carried out to modify the properties of zinc oxide for different applications [1,2]. Doping with transition metal elements leads to many interesting properties of ZnO. In 2000, Dietl et al. [3] used a simple theory to estimate the Tc of ferromagnetic semiconductors, and they predicted that room temperature ferromagnetic semiconductors might be created by substituting manganese ions in wide-band gap semiconductors such as GaN and ZnO. Reports of ZnO-based room temperature ferromagnetic semiconductors [4,5] soon followed. Currently, much experimental and theoretical research is focused on the dilute magnetic semiconductors (DMS) based on ZnO doped with transition metal ions such as Mn and Co, since the predicted room temperature ferromagnetism in DMS may be useful in spintronics [2]. The mechanism leading to room temperature ferromagnetism in Mn- and Co-doped ZnO is not fully established and there are many contradictory experimental observations. Even more startling are claims that undoped films of some of these oxides are ferromagnetic, or that they can become magnetic when doped with non-magnetic cations. Examples are HfO2 [6] and ZnO
∗ Corresponding author. Tel.: +91 1332 285407; fax: +91 1332 273560. E-mail address: [email protected] (D. Kaur). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.123
doped with Sc [7]. The term ‘d0 ferromagnetism’ has been suggested for these cases. Some research groups describe the observed FM ordering as an intrinsic effect [8,9], while others describe it as an extrinsic effect [10–12]. Studies on ZnMnO thin films and bulk samples by different research groups revealed paramagnetic [13,14], ferromagnetic [15] and antiferromagnetic behavior [16]. These studies indicate that the magnetic properties of Mn-doped ZnO are highly sensitive to the preparation methods and conditions. This controversy between research teams may result from the growth method used and/or from the growth conditions. In fact, depending on the different growth modes and mechanisms, microstructures in these systems such as distribution of Mn ion in ZnO crystal lattice and the local environment around Mn ions are very different, which considerably affects the magnetic properties. In present work, Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films were grown on commercial glass substrates by simple and low cost ultrasonic spray pyrolysis technique. The doping concentration was chosen below the equilibrium solubility limit of Mn2+ ions in ZnO. Spray pyrolysis technique [17] was adopted for the synthesis of Mn-doped ZnO thin films, because the process has many advantages such as better stoichiometry control, better homogeneity, low processing temperature, lower cost, easier fabrication of large area films, possibility of using high-purity starting materials and having an easy coating process of large sub-
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P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
strates. Earlier we have reported the successful fabrication of pure nanocrystalline ZnO thin films and ZnO nanopowder by the same technique [18]. In the present study we have tried to investigate the effect of doping concentration on the structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by ultrasonic spray pyrolysis technique. The aim was to compare the results obtained in here to samples prepared by other deposition methods. The structural and optical properties of these films reflect that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. The magnetic studies reveal the absence of room temperature ferromagnetic behavior in these films. 2. Experimental The Zn1−x Mnx O thin films were prepared from aqueous solution of zinc nitrate (Zn(NO3 )2 ·6H2 O) and manganese (Mn(NO3 )2 ·6H2 O) dissolved in distilled water. For each doping concentration (x), a separate solution was made. The substrates were first ultrasonically degreased with acetone, ethanol and deionized water. Various parameters such as nozzle to substrate distance, deposition rate and flow rate of air (carrier gas), deposition temperature and concentration were optimized to get good quality films as shown in Table 1. Specially designed digital substrate heater with temperature controller from excel instruments was used to heat the substrate. The rate of deposition was controlled by the carrier gas flow rate, substrate temperature as well as precursor concentration. The films of thickness 450 nm were deposited on glass (components, CaO:NaO:6SiO2 ) substrate at a fixed substrate temperature of 400 ◦ C. The chemical solution was atomized into the stream of the fine droplets via ultrasonic nebulizer operated to an atomizing frequency of 1.7 MHz. Nitrates precursor solution was poured in to the vessel from inlet side. The aerosol was generated from the vibration of the transducer. The nebulized spray, which goes up in the column, was deposited on a hot substrate. The orientation and crystallinity of these films were studied using Bruker AXS C-8 advanced diffractrometer in –2 geometry. Energy dispersive X-ray analysis (EDAX) was carried out on various samples by comparing the peaks and it confirms the concentrations of Mn to be close to those in the stated composition. PerkinElmer Lambda 25 UV–vis spectrometer was used to study the optical properties of films. The magnetic properties were investigated by superconducting quantum interference device (SQUID) magnetometer (Quantum design, MPMS, XL).
3. Results and discussion 3.1. Structural properties X-ray diffraction (XRD) pattern of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) films deposited on glass substrate at 400 ◦ C are shown in Fig. 1. All the films were single phase and had hexagonal wurtzite structure with c-axis preferred orientation. No evidence of any other secondary phases and impurities was detected in the XRD pattern. However this does not preclude the presence of secondary phases, since a small volume fraction of a randomly oriented impurity phase would be difficult to detect. Increasing Mn concentration caused the position shift of (0 0 2) peak of the Zn1−x Mnx O towards lower angles. Correspondingly, the c-axis lattice constant of Zn1−x Mnx O films increased from 5.2150 A˚ (x = 0.03), 5.2224 A˚ (x = 0.05), 5.2284 A˚ (x = 0.07) to 5.2372 A˚ (x = 0.10). The
Fig. 1. XRD pattern of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin film deposited on glass at 400 ◦ C.
similar dependence of the lattice constant on Mn concentration is usually observed in reported results on Zn1−x Mnx O films [19]. ˚ is smaller than that of Mn2+ Since the ionic radius of Zn2+ (0.74 A) 3+ ˚ ˚ the linear increase of (0.80 A) and larger than that of Mn (0.66 A), the c-axis lattice constant with Mn concentration indicated that the divalent Mn2+ ions substituted for Zn2+ ions in ZnO crystal lattice. Moreover, as Mn concentration increased in Zn1−x Mnx O films, peak width of (0 0 2) diffraction peak was found to be increased, which could be due to lattice disorder and strain induced by Mn2+ ions substitution. The observed increase in value of the FWHM of (0 0 2) reflection of Zn1−x Mnx O was found to be 0.183 A˚ (x = 0.03), 0.201 A˚ (x = 0 .05), 0.334 A˚ (x = 0.07) and 0.44 A˚ (x = 0.10), respectively. Further the crystallite size was calculated from XRD data and was observed to be 44.92 nm (x = 0.02), 40.89 nm (x = 0.05), 24.61 nm (x = 0.07) and 18.67 nm (x = 0.10). We have also calculated the grain size from the AFM and FESEM data (figures are not included in this paper) which was observed to be less than 100 nm in case of all the films.
Table 1 Optimized spray parameters for Zn1−x Mnx O thin films Spray mode Air blast Ultrasonic frequency (MHz) Droplet size (m) Solution flow rate (ml/h) Distance from heater to substrate (cm) Solvent Precursor Concentration (mol/l) Doping concentration (at.%) Deposition temperature ( ◦ C) Substrate
Ultrasonic nebulizer Atomizer 1.7 2.8 10 5 Distilled water and methanol Zinc nitrate, and manganese nitrate 0.1 3, 5, 7 and 10 400 Glass
Fig. 2. Optical absorbance spectra of Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films deposited on glass at 400 ◦ C.
P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
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3.2. Optical properties Fig. 2 shows the optical absorption spectra of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films. The absorption edge of the samples was found to shift towards the longer wavelength side which may be due to the fact that absorption higher energy activation of sp–d exchange interactions. The direct band gap of the films was calculated using the Tauc relationship as follows: ˛h = A(h − Eg )
n
where ˛ is the absorption coefficient, A is a constant, h is Planck’s constant, is the photon frequency, Eg is the energy band gap and n is 1/2 for direct semiconductor. An extrapolation of the linear region of a plot of (˛h)1/n on the y-axis versus photon energy (h) on the x-axis gave the value of the energy band gap (Eg ). Since Eg = h when (˛h)1/n = 0. Here the direct band gap of the Mn-doped ZnO films was evaluated by extrapolating the straightline part of the curves (˛h)2 = 0 as shown in Fig. 3. The value of band gap was found to decrease from 3.24 eV to 3.14 eV with corresponding increase in Mn concentration from x = 0.03 to 0.10, respectively as shown in inset of figure and is also in consistent with the reported results [20]. The observed decrease in the band gap could be explained because of a sp–d exchange interaction, i.e. manifestation of strong exchange interaction present between d electron of Mn, and the s and p electrons of host matrix. The main d–d transitions occur at 6 A1 → 4 T1 , 4 T2 , 4 A1 , 4 E energy levels of Mn2+ ion in presence of tetrahedral crystal field interaction. Although exchange interaction is especially important in the presence of an external magnetic field H, in certain cases it can manifest at H = 0. However Fukumura et al. [21] has reported an overall blue shift in the band gap with increase in Mn concentration in thin epitaxial films of Mn-doped ZnO and the reason for blue shift was attributed to the higher band gap energy of MnO (4.2 eV). 3.3. Magnetic properties Magnetization of the Zn1−x Mnx O films of dimension 0.5 cm × 0.5 cm was measured as function of magnetic field (M–H) and temperature (M–T) in the magnetic field range of ±4 T and in the temperature range of 5–300 K. The data have been corrected for the diamagnetic contribution due to the background
Fig. 3. (˛h)2 vs. h plots of the Zn1−x Mnx O (x = 0.03, 0.05, 0.07 and 0.10) thin films deposited on glass at 400 ◦ C. The inset shows the variation of band gap with Mn concentration (%).
Fig. 4. Magnetization vs. magnetic field of Zn0.95 Mn0.05 O thin film at 5 K and 300 K. The inset shows the data for the low field regions at 5 K.
signal from glass substrate as given by
M
MC = MF+S − −
H
S
H
where MC is the magnetic moment after subtracting the substrate contribution, MF+S is the total magnetic moment of film and substrate and (−M/H)S . H is the diamagnetic contribution due to the substrate. Figs. 4 and 5 show the magnetization (M) versus magnetic field (H) curve of Zn1−x Mnx O films with x = 0.05 and 0.07, respectively at 5 K and 300 K. Non-linear behavior of the magnetic moment was observed at both 5 K and 300 K temperatures. The M(H) curves do not show clear hysteresis loops in our case for both 5 at.% and 7 at.% Mn-doped ZnO films at 300 K which was in consistent with other reports on transition metaldoped DMS films prepared by spray pyrolysis [22]. However at 5 K, there was very weak signature of hysteresis. We have plotted the low field region of the loop as inset in Figs. 4 and 5 showing coericivity HC ∼85 Oe and remanant magnetization (Mr ) of 4.4 × 10−5 emu for 5 at.% Mn-doped ZnO films at 5 K. The value of coercivity HC ∼147 Oe and the remanant magnetization Mr = 3.77 × 10−5 emu was observed at 5 K for 7 at.% Mn-doped films.
Fig. 5. Magnetization vs. magnetic field of Zn0.93 Mn0.07 O thin film at 5 K and 300 K. The inset shows the data for the low field regions at 5 K.
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P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
Fig. 6. Temperature dependence of magnetization at a field of 1 kOe of Zn0.95 Mn0.05 O thin film.
We further investigated the temperature dependent magnetization of these films at constant magnetic field of 1 kOe and temperature range of 5–300 K (Figs. 6 and 7). In both cases of Zn1−x Mnx O films with x = 0.05 and x = 0.07, the trend of curve was similar. The M–T curve at field cooled (FC) and zero field cooled (ZFC) conditions at field of H = 1 kOe is shown in the inset of Fig. 6. The non-zero value of M was observed to persist at least up to temperature of 200 K. The value of magnetization was found to increase slowly with decreasing temperature from 300 K to 45 K, and afterwards there was a steep rise of magnetization with very strong concave curvature, which was different from the Weiss mean field prediction. Fig. 8 shows the temperature dependent inverse of dc-magnetic susceptibility () plot of the Zn0.95 Mn0.05 O film at 1 kOe field. Inset of Fig. 8 is the fit with mean-field Curie–Weiss law (−1 = (T − )/C) in the temperature range of 5–300 K, where C is the Curie constant and is the Curie–Weiss temperature. From fitted parameters the magnitude of calculated value of was found to be very large and negative (∼−280 K). The strongly concave M(T) behavior is often observed in a carrier-localized regime, which can be understood in the scheme of the polaronpercolation-theory as reported by Sarma et al. [23]. The problem of ferromagnetic transition in a system of magnetic polarons can be considered as equivalent to a problem of overlapping spheres studied in the percolation theory. According to the theory, the
Fig. 7. Temperature dependence of magnetization at a field of 1 kOe of Zn0.93 Mn0.07 O thin film.
sphere of radius in space are overlapped or collapsed and they can form “clusters”. Each sphere of overlapping clusters corresponds to the bound magnetic polarons. The polarons contain many impurity spins and one localized hole. The hole localized wave function decay exponentially with distance from a localized center where ad is the decay length from the localized center of the exponential function (radius of the localized hole). In the limit of low carrier density regime (strong carrier localization), ad 3 nh is very small (ad 3 nh 1). In a system of magnetic polarons this condition satisfies ferromagnetic ordering with highly concave nature in the M(T) curve at lower temperatures, where nh is the finite density of hole in host semiconductor. This generally happens in the case of a strongly localized insulating DMS. The effective radius of the polaron increases logarithmically with inverse temperature. At a very low temperature the effective radius of polarons are comparable to the size of the samples, i.e. the formation of infinite cluster corresponds the long range ordering of ferromagnetic regime when many spins of magnetic impurity are polarized by kinetic exchange interaction of hole spin. Moreover, at enough low temperatures the neighboring polarons are overlapped and the spins of polarons are aligned by the interaction of polarons through the magnetic impurities between them. In the presence of disorder state, various type of spin-glass ground state may compete with the ferromagnetic ground state [24]. There have been several reports on measurements in different TM-doped DMS thin films where all of them have observed a concave nature in the M(T) curve with steep rise of magnetization at lower temperatures without any magnetic transition down to the lowest attainable temperature [25]. However, there exist prominent nonlinear hysteresis loop in the M(H) curve for all those TM-doped DMS thin films at lower temperatures. In our case of Mn-doped ZnO thin films, we have observed concave nature in M(T) curve with steep rise of magnetization at lower temperature, but we have not observed any prominent non-linear hysteresis loop in M(H) curve. The reported experimental results on the magnetic properties of Mn-doped ZnO are very different in thin films as well as in bulk system. Both ferromagnetism and absence of ferromagnetism were observed in doped ZnO thin films grown by pulsed laser deposition (PLD) [5,26] and magnetron sputtering [27]. Films grown by molecular beam epitaxy (MBE) [28] exhibit ferromagnetism below 45 K. In bulk system the same diversity is observed. Although Mn implanted ZnO single crystals are ferromagnetic up to 250 K [29], weak ferromagnetism has been reported in bulk Zn0.98 Mn0.02 O pre-
Fig. 8. Temperature dependent susceptibility () of Zn0.95 Mn0.05 O thin film measured at field of 1 kOe.The inset shows the temperature dependent −1 curve.
P. Singh et al. / Journal of Alloys and Compounds 471 (2009) 11–15
pared by solid-state reaction [30] and no ferromagnetism at all was observed in samples prepared by co-precipitation method [31]. A careful examination of the reported results indicate that where ferromagnetism has been found, the samples were heated to relatively high temperatures which could give rise to spinel impurity phases. Even where the temperature of synthesis is relatively low, some of the synthetic procedures are not convincing as to whether the dopant has substituted the Zn site. Furthermore, the magnetization values reported by many workers is very low and can arise from the presence of magnetic impurities which cannot be detected by X-ray diffraction. It has also been shown that the secondary phase Mn2−x Znx O3− , or an extrinsic source, is responsible for ferromagnetism in Mn-doped ZnO which was initially thought to be intrinsic. In our case of Zn1−x Mnx O thin films synthesized by spray pyrolysis technique, the constituent elements were mixed at molecular level ensuring dopant atoms are present at substitutional sites as confirmed from XRD and optical properties of these films. Moreover the deposition temperature was kept relatively low to avoid any possible impurity phase segregation even in relatively higher doping content. The absence of room temperature ferromagnetism in these films could possibly be due to neighboring antiferromagnetic interactions which suppress the ferromagnetic coupling or due to lack of free carriers in these films, which is in confirmation with the reports of Spaldin [32], that showed that robust ferromagnetism cannot occur in Mn- and Co-doped ZnO. If at all, it may occur if additional charge carriers are present. 4. Conclusion In summary, we have successfully prepared the Zn1−x Mnx O thin films with different doping concentration (x = 0.03, 0.05, 0.07 and 0.10) by ultrasonic spray pyrolysis at relatively low temperature (400 ◦ C). The structural and optical properties of these films reflect that the Mn2+ ions have substituted the Zn2+ ion without changing the wurtzite structure of ZnO. The observed decrease in the band gap with increase in doping concentration of Mn-doped ZnO films was explained in terms of a sp–d exchange interaction. The concave nature of M(T) behavior with steep rise of magnetization was observed at low temperature and was explained in terms of polaron-percolation-theory. Non-linear behavior of the magnetic moment was observed at both 5 K and 300 K temperatures. The value of coercivity HC ∼147 Oe and the remanant magnetization Mr = 3.77 × 10−5 emu was observed at 5 K for 7 at.% Mn-doped films. We have not observed any impurity phases of manganese oxides in XRD curve, even after doping the 10 at.% of Mn in these doped ZnO thin films. But we also have not observed the room temperature ferromagnetism in Zn1−x Mnx O thin films.
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Acknowledgements The financial support provided by DST, India, under the scheme Nanoscience and Technology Initiatives (NSTI) with Reference No. DST SR/S5/NM-32/2005 is highly acknowledged. The author Preetam Singh is thankful to CSIR for award of senior research fellowship. References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34 (2003) 3. [2] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] S. Sonada, S. Shimizu, T. Sasaki, Y. Yamamoto, H. Hori, J. Cryst. Growth 237–239 (2002) 1358. [5] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [6] N.H. Hong, J. Sakai, N. Poirot, V. Brize, Phys. Rev. B 73 (2006) 132404. [7] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [8] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gamelin, J. Am. Chem. Soc. 125 (2003) 13205. [9] P. Sati, R. Hayn, R. Kuzian, S. Regnier, S. Schafer, A. Stepanov, C. Morhain, C. Deparis, M. Laugt, M. Goiran, Z. Golacki, Phys. Rev. Lett. 96 (2006) 017203. [10] D.P. Norton, M.E. Overberg, S.J. Pearton, K. Pruessner, J.D. Budai, L.A. Boatner, M.F. Chisolm, J.S. Lee, Z.G. Khim, Y.D. Park, R.G. Wilson, Appl. Phys. Lett. 83 (2003) 5488. [11] B.S. Jeong, Y.W. Heo, D.P. Norton, J.G. Kelly, R. Rairigh, A.F. Hebard, J.D. Budai, Y.D. Park, Appl. Phys. Lett. 84 (2004) 2608. [12] K.P. Bhatti, S. Chaudhary, D.K. Pandya, S.C. Kashyap, J. Appl. Phys. 101 (2007) 103919. [13] S.S. Kim, J.H. Moon, B.T. Lee, O.S. Song, J.H. Je, J. Appl. Phys. 95 (2004) 454. [14] A. Tiwari, C. Jin, A. Kvit, D. Kumar, J.F. Muth, J. Narayan, Solid State Commun. 121 (2002) 371. [15] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M.O. Gillen, B. Johasson, G.A. Gehring, Nat. Mater. 2 (2003) 673. [16] C.N.R. Rao, F.L. Deepak, J. Mater. Chem. 15 (2005) 573. [17] D. Kaur, A.K. Gupta, J. Phys. D: Appl. Phys. 35 (2002) 729. [18] P. Singh, A. Kumar, Deepak, D. Kaur, J. Cryst. Growth 306 (2007) 303. [19] H.Y. Xu, Y.C. Liu, C.S. Xu, Y.X. Liu, C.L. Shao, R. Mu, J. Chem. Phys. 124 (2006) 074707. [20] V.S. Bhat, F.L. Deepak, Solid State Commun. 135 (2005) 345. [21] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 3366. [22] A. Manivannan, M.S. Seera, S.B. Majumdar, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 111. [23] D.D. Sarma, E.H. Hwang, A. Kaminski, Phys. Rev. B 67 (2003) 155201. [24] C. Timm, F. Schafer, F.V. Oppen, Phys. Rev. Lett. 89 (2002) 137201. [25] Z.B. Gu, C.S. Yuan, M.H. Lu, J. Wang, D. Wu, S.T. Zhang, S.N. Zhu, Y.Y. Zhu, Y.F. Chen, J. Appl. Phys. 98 (2005) 053908. [26] K.W. Nielsen, J.B. Philipp, M. Opel, A. Erb, J. Simon, L. Alff, R. Gross, Superlattices Microstruct. (2005) 327. [27] X.M. Cheng, C.L. Chien, J. Appl. Phys. 93 (2003) 7876. [28] S.W. Jung, S.J. An, G.C. Yi, C.U. Jung, S.I. Lee, S. Cho, Appl. Phys. Lett. 80 (2002) 4561. [29] D.P. Norton, S.J. Pearton, A.F. Hebard, N. Theodoropoulou, L.A. Boatner, R.G. Wilson, Appl. Phys. Lett. 82 (2003) 239. [30] W. Chen, L.F. Zhao, Y.Q. Wang, J.H. Miao, S. Liu, Z.C. Xia, S.L. Yuan, Solid State Commun. 34 (2005) 827. [31] J. Alaria, Chem. Phys. Lett. 415 (2005) 337–341. [32] N.A. Spaldin, Phys. Rev. B 69 (2004) 125201-1–125201-7.