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Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96 www.elsevier.com/locate/jmmm
Letter to the Editor
Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form Sayak Ghoshal, P.S. Anil Kumar Department of Physics, Indian Institute of Science, Bangalore 560012, India Received 23 August 2007 Available online 5 March 2008
Abstract Structural, optical and magnetic studies of Co-doped ZnO have been carried out for bulk as well as thin films. The magnetic studies revealed the superparamagnetic nature for low-temperature synthesized samples, indicating the presence of cobalt metallic clusters, and this is supported by the optical studies. For the high-temperature sintered samples one obtains paramagnetism. The optical studies reveal the presence of Co2+ ions in the tetrahedral sites indicating proper doping. Interestingly, the films deposited by laser ablation from the paramagnetic target showed room temperature ferromagnetism. It appears that the magnetic nature of this system is process dependent. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.y; 74.25.Ha; 75.70.i Keywords: Ferromagnetism; DMS; ZnO; Superparamagnetism
1. Introduction Intense research activity is being pursued on spintronic materials and devices like spin transistors, spin lightemitting diodes, non-volatile memory, logic devices, etc. [1–7] due to their potential advantages over charge-based electronics viz. higher speed, greater efficiency and nonvolatility of memory. Semiconductor spintronics can give the added advantage of having logic, memory and communication on a single chip [8]. However, in order to facilitate this, we need to inject spin polarized electrons into a semiconductor, from a source having substantial spin polarization. So far the efficiency of spin injection from a ferromagnetic metal to an inorganic semiconductor at room temperature has been rather poor [9] and this has been mainly attributed to the conductivity mismatch at the interface between ferromagnet and the semiconductor [10,11]. In order to overcome this practical difficulty one needs a semiconductor which is ferromagnetic as a spin polarized electron source [12]. It is predicted that diluted Corresponding author.
E-mail address: [email protected] (P.S. Anil Kumar). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.174
magnetic semiconductors (DMS) can exhibit ferromagnetic properties when magnetic atoms are homogeneously dispersed in a suitable semiconductor matrix. This concept has been proved for Mn-doped GaAs, but till now the TC of this material is well below room temperature (170 K) [13] and hence it is not feasible to use this material in room temperature operating devices. Dietl et al. have shown that few DMS compounds can have TC above RT, using Zener model, in which the magnetic interaction between the local spins is assumed to be Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction, provided the hole density is sufficiently high [14]. Subsequently, research activities involving transition metal doped ZnO, TiO2, SnO2, etc. have intensified. Among them the Co-doped ZnO is one of the most investigated systems. Rao et al. [15] and Lawes et al. [16] observed absence of ferromagnetism in polycrystalline transition metal doped ZnO, whereas Sati et al. [17] and Venkatesan et al. [18] showed anisotropy as the signature of intrinsic ferromagnetism. Antiferromagnetic ordering is also reported for the same compound by a few groups [19], even from the same group who has reported ferromagnetic ordering in these samples earlier [20]. It appears that a consensus in results from these studies is lacking and
ARTICLE IN PRESS L94
S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
different groups report different properties and till now it is under debate. So we have undertaken a detailed study of Co-doped ZnO in the bulk as well as thin film form. In this letter we show that the property of this compound is purely process dependent and the diversity in the results obtained from different groups can be brought to a common platform. 2. Sample preparation Samples are synthesized by room temperature soft chemical co-precipitation and post thermal decomposition method. We doped 5% Co in ZnO, which is well below the solubility limit [21]. Stoichiometric proportions of the nitrate solutions of Zn(NO3)2 6H2O and Co(NO3)2 6H2O are mixed and co-precipitated using NaOH solution. The hydroxide precipitate has been washed several times and decomposed into oxide at 100 1C (un-sintered sample) and then this is sintered at 1000 1C (sintered sample). We assume that all the Co atoms are substituting the Zn sites homogeneously. But ideally this is not the case and that is why these compounds show different behaviors, and also in different preparation procedures sometimes the properties are completely different. Thin films are grown on sapphire substrate (R-plane) by pulsed laser deposition (PLD) using the sintered sample as the target material. Deposition is done at 0.2 mbar oxygen pressure. Laser energy density at the target material is 5 J/ cm2 with 500 1C substrate temperature during deposition. The estimated film thickness is 30 nm. After deposition the films were annealed at different annealing conditions starting from vacuum annealing for 30 min to 1 h oxygen annealing at 500 mbar pressure.
Fig. 1. XRD data for the 5% Co-doped ZnO powder sample (a) before sintering and (b) after sintering.
3. Results and discussion Structural, spectroscopic and magnetic properties are studied for the bulk as well as thin film samples. Crystal structures of the sintered and un-sintered samples were investigated by XRD. Fig. 1 shows the XRD pattern for both samples, and it is seen from the figure that the pattern consists of peaks corresponding to ZnO, with the absence of any secondary phase or impurity phase. Particle size analysis using SEM showed that the particle size for the un-sintered sample is 20–30 nm, whereas after sintering this has grown to 5–8 mm. EDAX analysis confirmed that there is no impurity within its detection limit. UV–visible spectroscopic measurements at room temperature were carried out to study the effect of sintering on the band gap of ZnO:Co in the range 200–800 nm. Fig. 2 shows the absorption spectra of ZnO:Co samples (a) un-sintered, (b) sintered and (c) thin film on R-plane sapphire. It is seen from the figure that for the un-sintered material the absorption spectrum is almost the same as the pure ZnO spectra without any additional absorption peak. Band gap of this material is 3.41 eV (363 nm), which matches with the band gap of pure ZnO prepared by same method. This
Fig. 2. (Color online) UV–visible absorption spectra for (a) un-sintered bulk sample, (b) sintered bulk sample and (c) thin film on R-plane sapphire (after subtracting the substrate contribution). Inset shows the enlarged view of the transitions due to crystal field splitting for (b) and (c).
indicates that Co is not properly introduced into the Zn lattice site. But for the sintered sample no sharp absorption peak near to the ZnO band gap is observed. Instead we see some intermediate bands, which include three characteristic bands in the 550–700 nm region. These three bands are due to the transitions 4A2(F)-2E(G) (659 nm), 4 A2(F)-4T1(G) (615 nm), 4A2(F)-2A1(G) (568 nm) between the bands formed due to crystal field splitting [22] [15]. In the literature these absorption edges are correlated with the d–d transitions of the tetrahedrally coordinated Co2+ ions and attributed to the above-mentioned transitions. The inset shows an enlarged view of the transitions due to crystal field splitting in the sintered sample as well as in the film. Since the spectral data show the characteristic transitions of the tetrahedrally coordinated Co2+ ions, it
ARTICLE IN PRESS S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
can be concluded that Co is substituting the Zn2+ ions. This implies that before sintering Co ions are not properly substituting Zn ions. This is an important observation as this gives directly the nature of bonding of Co in the doped compound. In the films, after subtracting the substrate contribution presence of Co2+ ion peaks is observed, but peaks are slightly shifted from the bulk peak values. Magnetic properties are investigated using a Quantum Design SQUID magnetometer. Fig. 3 shows the M–H plot of the un-sintered and the sintered sample. The inset in Fig. 3 shows w vs. T plot for both the samples. For the sintered sample the M–H plot is nearly linear and the 1/w vs. T plot is a straight line passing through origin, indicating the paramagnetic behavior. But for the unsintered sample the M–H curve is like ‘S’ shape, which is believed to be the signature of superparamagnetism [23], without having hysteresis and saturation even at 5 T. In the superparamagnetic state, the magnetization obeys Langevin function: M ¼ M 0 ½cothðmH=kB TÞ 1=ðmH=kB TÞ,
assumption that for the un-sintered sample the doped Co exists only as Co clusters. From the obtained particle size one can estimate the blocking temperature (TB) by using the relation KAV ¼ 25kBTB (where KA is the anisotropy energy of the Co, 4.5 106 erg/cm3) [24]. This gives TB E0.1 K, which is well below our measurement temperature. It has to be noted that we have done FC and ZFC measurements down to 10 K at an applied magnetic field of 100 Oe and have not seen any signs of blocking temperature down to 10 K. But for the sintered sample, number of Co particles has increased by a factor of two and moment per Co is decreased by almost the same factor. This indicates that there is no cluster formation in case of sintered sample and it is supported by the UV–visible spectroscopy data, which shows characteristic transitions of Co2+ ions. It has to be noted that for the un-sintered sample we have not observed any transitions characteristic of Co2+ in the tetrahedral site. So it can be assumed that the doped Co is not replacing Zn and stays as a separate phase of Co in the form of nanosized Co metallic clusters. The absence of impurity peaks in XRD indicates that these impurity phases are not forming well-defined crystallites. Recent studies show that some antiferromagnetic oxides below certain critical sizes can exhibit ferromagnetism [25]. This nanophased Co metallic cluster or the oxide of Co could lead to superparamagnetism in the un-sintered sample. However, we have not seen any indications of Co being in the oxide state for the un-sintered sample. But, surprisingly, the films deposited on the sapphire substrate at 500 1C showed hysteresis loops resembling ferromagnetic behavior as shown in Fig. 4, although the target is made from the same sintered paramagnetic sample. The hysteresis loops at 100 K and 300 K, after subtraction of linear diamagnetic contribution of the substrate, show that the film is having ferromagnetic behavior even at RT with a saturation value of 0.5mB per Co atom. All the films
Moment in Bohr magneton per Co atom
where M0 ¼ Nm (N is the total number of particles). m ¼ Ms /VS, Ms is saturation moment of Co atom and /VS is the average volume of the particle. The Langevin function is fitted for both M–T and M–H plots for unsintered and sintered samples, which are shown in Fig. 3 by a continuous line. From the fitted data we get the number of Co clusters for the un-sintered sample from the M–H plot and from the M–T plot as 5.3 1019 and 7.5 1019, respectively, whereas the moment of each Co particle is 10.4mB and 8.6mB, respectively. From the m value we can estimate the particle size as 5 A˚ and from the doping concentration as well one can estimate that there are on an average six Co atoms in each cluster. This is with the
0.6
100 K 300 K
0.3
0.0
-0.3
-0.6 -30000 -20000 -10000
Fig. 3. (Color online) M–H plot for the sintered and the un-sintered sample showing paramagnetic and superparamagnetic behavior, respectively. Inset shows the field-cooled w vs T plot. Points in the plots are the experimental data points and continuous lines are the Langevin function fitting of the plots.
L95
0 10000 Field in Oe
20000
30000
Fig. 4. (Color online) Hysteresis plot (after subtracting linear diamagnetic contribution) of the 5% Co-doped ZnO film. Inset shows enlarged view of near zero field value.
ARTICLE IN PRESS L96
S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
annealed at different conditions show ferromagnetism even at room temperature. One can speculate that during deposition some ferromagnetic Co clusters which are large in size are formed and are responsible for the hysteresis loop in these films. On the other hand, it is interesting to note that the spectroscopic data showed transitions corresponding to Co2+ in the tetrahedrally coordinated sites in these films, which showed hysteresis loop characteristic of ferromagnetic samples. The most striking feature is the appearance of ferromagnetism in films that have been prepared from a paramagnetic target. Hence we believe that doping of Co in ZnO matrix induces ferromagnetism. Again, oxygen vacancies in the film may enhance ferromagnetism [26]. 4. Conclusion We have studied the properties of 5% Co-doped ZnO bulk as well as thin film samples having different preparation and processing conditions and conclude that the properties are highly sensitive to the processing condition. For the un-sintered sample the UV–visible data, XRD and magnetization measurements suggest the possibility of superparamagnetic behavior arising due to nanophased Co metallic clusters. For the sintered sample these studies indicate the paramagnetic nature of the sample and the substitution of Co in ZnO matrix. For the films that were prepared from the paramagnetic target the behavior is quite distinct and we observed ferromagnetism. In our study we have shown that it is possible to get superparamagnetism, paramagnetism and also ferromagnetism from the same sample depending on the processing technique. Acknowledgment The authors are thankful to Prof. M.S. Hegde for valuable support. References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [2] Gary A. Prinz, Science 282 (1998) 1660. [3] I. Zutic, J. Fabian, S.D. Sarma, Rev. Mod. Phys. 76 (2004) 323.
[4] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790. [5] G. Kioseoglou, A. Hanbicki, J.M. Sullivan, O.M.J. Van’t Erve, C.H. Li, S.C. Erwin, R. Mallory, M. Yasar, A. Petrou, B.T. Jonker, Nature 3 (2004) 799. [6] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [7] S. Bandyopadhyay, B. Das, A.E. Miller, Nanotechnology 5 (1994) 113. [8] D.D. Awschalom, M.E. Flatte, N. Samarth, Sci. Am. Inc. 286 (2002) 67. [9] A.T. Hanbicki, B.T. Jonker, G. Itskos, G. Kioseoglou, A. Petrou, Appl. Phys. Lett. 80 (2002) 1240. [10] Schmidt, D. Ferrand, L.W. Molenkamp, A.T. Filip, B.J. van Wees, Phys. Rev. B 62 (R) (2000) R4790. [11] P.C. van Son, H. van Kempen, P. Wyder, Phys. Rev. Lett. 58 (1987) 2271. [12] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L.W. Molenkamp, Nature 402 (1999) 787. [13] T. Jungwirth, K.Y. Wang, J. Masˇ ek, K.W. Edmonds, Ju¨rgen Ko¨nig, Jairo Sinova, M. Polini, N.A. Goncharuk, A.H. MacDonald, M. Sawicki, A.W. Rushforth, R.P. Campion, L.X. Zhao, C.T. Foxon, B.L. Gallagher, Phys. Rev. B 72 (2005) 165204. [14] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [15] C.N.R. Rao, F.L. Deepak, J. Mater. Chem. 15 (2005) 573. [16] G. Lawes, A.S. Risbud, A.P. Ramirez, Ram. Seshadri, Phys. Rev. B 71 (2005) 045201. [17] 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. [18] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [19] M. Bouloudenine, N. Viart, S. Colis, J. Kortus, A. Dinia, Appl. Phys. Lett. 87 (2005) 052501. [20] P. Sati, C. Deparis, C. Morhain, S. Schafer, A. Stepanov, Phys. Rev. Lett. 98 (2007) 137204. [21] S.K. Mandal, A.K. Das, T.K. Nath, D. Karmakar, Appl. Phys. Lett. 89 (2006) 144105. [22] F.A. Cotton, D.M.L. Goodgame, M. Goodgame, J. Am. Chem. Soc. 83 (1961) 4690. [23] T.J. Jackson, E.M. Forgan, T.M. Riseman, H. Gluckler, E. Morenzoni, T. Prokscha, H.P. Weber, CH. Niedermayer, M. Pleines, G. Schatz, J. Litterst, H. Luetkens, H. Keller, R. Khasonov, T.S. Rong, C. Binns, Hyperfine Interact. 136–137 (2001) 403. [24] S.R. Shindhe, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkateshan, Phys. Rev. Lett. 92 (2004) 166601. [25] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393. [26] H.S. Hsu, J.C.A. Huang, Y.H. Huang, Y.F. Liao, M.Z. Lin, C.H. Lee, J.F. Lee, S.F. Chen, L.Y. Lai, C.P. Liu, Appl. Phys. Lett. 88 (2006) 242507.
Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96 www.elsevier.com/locate/jmmm
Letter to the Editor
Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form Sayak Ghoshal, P.S. Anil Kumar Department of Physics, Indian Institute of Science, Bangalore 560012, India Received 23 August 2007 Available online 5 March 2008
Abstract Structural, optical and magnetic studies of Co-doped ZnO have been carried out for bulk as well as thin films. The magnetic studies revealed the superparamagnetic nature for low-temperature synthesized samples, indicating the presence of cobalt metallic clusters, and this is supported by the optical studies. For the high-temperature sintered samples one obtains paramagnetism. The optical studies reveal the presence of Co2+ ions in the tetrahedral sites indicating proper doping. Interestingly, the films deposited by laser ablation from the paramagnetic target showed room temperature ferromagnetism. It appears that the magnetic nature of this system is process dependent. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.y; 74.25.Ha; 75.70.i Keywords: Ferromagnetism; DMS; ZnO; Superparamagnetism
1. Introduction Intense research activity is being pursued on spintronic materials and devices like spin transistors, spin lightemitting diodes, non-volatile memory, logic devices, etc. [1–7] due to their potential advantages over charge-based electronics viz. higher speed, greater efficiency and nonvolatility of memory. Semiconductor spintronics can give the added advantage of having logic, memory and communication on a single chip [8]. However, in order to facilitate this, we need to inject spin polarized electrons into a semiconductor, from a source having substantial spin polarization. So far the efficiency of spin injection from a ferromagnetic metal to an inorganic semiconductor at room temperature has been rather poor [9] and this has been mainly attributed to the conductivity mismatch at the interface between ferromagnet and the semiconductor [10,11]. In order to overcome this practical difficulty one needs a semiconductor which is ferromagnetic as a spin polarized electron source [12]. It is predicted that diluted Corresponding author.
E-mail address: [email protected] (P.S. Anil Kumar). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.174
magnetic semiconductors (DMS) can exhibit ferromagnetic properties when magnetic atoms are homogeneously dispersed in a suitable semiconductor matrix. This concept has been proved for Mn-doped GaAs, but till now the TC of this material is well below room temperature (170 K) [13] and hence it is not feasible to use this material in room temperature operating devices. Dietl et al. have shown that few DMS compounds can have TC above RT, using Zener model, in which the magnetic interaction between the local spins is assumed to be Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction, provided the hole density is sufficiently high [14]. Subsequently, research activities involving transition metal doped ZnO, TiO2, SnO2, etc. have intensified. Among them the Co-doped ZnO is one of the most investigated systems. Rao et al. [15] and Lawes et al. [16] observed absence of ferromagnetism in polycrystalline transition metal doped ZnO, whereas Sati et al. [17] and Venkatesan et al. [18] showed anisotropy as the signature of intrinsic ferromagnetism. Antiferromagnetic ordering is also reported for the same compound by a few groups [19], even from the same group who has reported ferromagnetic ordering in these samples earlier [20]. It appears that a consensus in results from these studies is lacking and
ARTICLE IN PRESS L94
S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
different groups report different properties and till now it is under debate. So we have undertaken a detailed study of Co-doped ZnO in the bulk as well as thin film form. In this letter we show that the property of this compound is purely process dependent and the diversity in the results obtained from different groups can be brought to a common platform. 2. Sample preparation Samples are synthesized by room temperature soft chemical co-precipitation and post thermal decomposition method. We doped 5% Co in ZnO, which is well below the solubility limit [21]. Stoichiometric proportions of the nitrate solutions of Zn(NO3)2 6H2O and Co(NO3)2 6H2O are mixed and co-precipitated using NaOH solution. The hydroxide precipitate has been washed several times and decomposed into oxide at 100 1C (un-sintered sample) and then this is sintered at 1000 1C (sintered sample). We assume that all the Co atoms are substituting the Zn sites homogeneously. But ideally this is not the case and that is why these compounds show different behaviors, and also in different preparation procedures sometimes the properties are completely different. Thin films are grown on sapphire substrate (R-plane) by pulsed laser deposition (PLD) using the sintered sample as the target material. Deposition is done at 0.2 mbar oxygen pressure. Laser energy density at the target material is 5 J/ cm2 with 500 1C substrate temperature during deposition. The estimated film thickness is 30 nm. After deposition the films were annealed at different annealing conditions starting from vacuum annealing for 30 min to 1 h oxygen annealing at 500 mbar pressure.
Fig. 1. XRD data for the 5% Co-doped ZnO powder sample (a) before sintering and (b) after sintering.
3. Results and discussion Structural, spectroscopic and magnetic properties are studied for the bulk as well as thin film samples. Crystal structures of the sintered and un-sintered samples were investigated by XRD. Fig. 1 shows the XRD pattern for both samples, and it is seen from the figure that the pattern consists of peaks corresponding to ZnO, with the absence of any secondary phase or impurity phase. Particle size analysis using SEM showed that the particle size for the un-sintered sample is 20–30 nm, whereas after sintering this has grown to 5–8 mm. EDAX analysis confirmed that there is no impurity within its detection limit. UV–visible spectroscopic measurements at room temperature were carried out to study the effect of sintering on the band gap of ZnO:Co in the range 200–800 nm. Fig. 2 shows the absorption spectra of ZnO:Co samples (a) un-sintered, (b) sintered and (c) thin film on R-plane sapphire. It is seen from the figure that for the un-sintered material the absorption spectrum is almost the same as the pure ZnO spectra without any additional absorption peak. Band gap of this material is 3.41 eV (363 nm), which matches with the band gap of pure ZnO prepared by same method. This
Fig. 2. (Color online) UV–visible absorption spectra for (a) un-sintered bulk sample, (b) sintered bulk sample and (c) thin film on R-plane sapphire (after subtracting the substrate contribution). Inset shows the enlarged view of the transitions due to crystal field splitting for (b) and (c).
indicates that Co is not properly introduced into the Zn lattice site. But for the sintered sample no sharp absorption peak near to the ZnO band gap is observed. Instead we see some intermediate bands, which include three characteristic bands in the 550–700 nm region. These three bands are due to the transitions 4A2(F)-2E(G) (659 nm), 4 A2(F)-4T1(G) (615 nm), 4A2(F)-2A1(G) (568 nm) between the bands formed due to crystal field splitting [22] [15]. In the literature these absorption edges are correlated with the d–d transitions of the tetrahedrally coordinated Co2+ ions and attributed to the above-mentioned transitions. The inset shows an enlarged view of the transitions due to crystal field splitting in the sintered sample as well as in the film. Since the spectral data show the characteristic transitions of the tetrahedrally coordinated Co2+ ions, it
ARTICLE IN PRESS S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
can be concluded that Co is substituting the Zn2+ ions. This implies that before sintering Co ions are not properly substituting Zn ions. This is an important observation as this gives directly the nature of bonding of Co in the doped compound. In the films, after subtracting the substrate contribution presence of Co2+ ion peaks is observed, but peaks are slightly shifted from the bulk peak values. Magnetic properties are investigated using a Quantum Design SQUID magnetometer. Fig. 3 shows the M–H plot of the un-sintered and the sintered sample. The inset in Fig. 3 shows w vs. T plot for both the samples. For the sintered sample the M–H plot is nearly linear and the 1/w vs. T plot is a straight line passing through origin, indicating the paramagnetic behavior. But for the unsintered sample the M–H curve is like ‘S’ shape, which is believed to be the signature of superparamagnetism [23], without having hysteresis and saturation even at 5 T. In the superparamagnetic state, the magnetization obeys Langevin function: M ¼ M 0 ½cothðmH=kB TÞ 1=ðmH=kB TÞ,
assumption that for the un-sintered sample the doped Co exists only as Co clusters. From the obtained particle size one can estimate the blocking temperature (TB) by using the relation KAV ¼ 25kBTB (where KA is the anisotropy energy of the Co, 4.5 106 erg/cm3) [24]. This gives TB E0.1 K, which is well below our measurement temperature. It has to be noted that we have done FC and ZFC measurements down to 10 K at an applied magnetic field of 100 Oe and have not seen any signs of blocking temperature down to 10 K. But for the sintered sample, number of Co particles has increased by a factor of two and moment per Co is decreased by almost the same factor. This indicates that there is no cluster formation in case of sintered sample and it is supported by the UV–visible spectroscopy data, which shows characteristic transitions of Co2+ ions. It has to be noted that for the un-sintered sample we have not observed any transitions characteristic of Co2+ in the tetrahedral site. So it can be assumed that the doped Co is not replacing Zn and stays as a separate phase of Co in the form of nanosized Co metallic clusters. The absence of impurity peaks in XRD indicates that these impurity phases are not forming well-defined crystallites. Recent studies show that some antiferromagnetic oxides below certain critical sizes can exhibit ferromagnetism [25]. This nanophased Co metallic cluster or the oxide of Co could lead to superparamagnetism in the un-sintered sample. However, we have not seen any indications of Co being in the oxide state for the un-sintered sample. But, surprisingly, the films deposited on the sapphire substrate at 500 1C showed hysteresis loops resembling ferromagnetic behavior as shown in Fig. 4, although the target is made from the same sintered paramagnetic sample. The hysteresis loops at 100 K and 300 K, after subtraction of linear diamagnetic contribution of the substrate, show that the film is having ferromagnetic behavior even at RT with a saturation value of 0.5mB per Co atom. All the films
Moment in Bohr magneton per Co atom
where M0 ¼ Nm (N is the total number of particles). m ¼ Ms /VS, Ms is saturation moment of Co atom and /VS is the average volume of the particle. The Langevin function is fitted for both M–T and M–H plots for unsintered and sintered samples, which are shown in Fig. 3 by a continuous line. From the fitted data we get the number of Co clusters for the un-sintered sample from the M–H plot and from the M–T plot as 5.3 1019 and 7.5 1019, respectively, whereas the moment of each Co particle is 10.4mB and 8.6mB, respectively. From the m value we can estimate the particle size as 5 A˚ and from the doping concentration as well one can estimate that there are on an average six Co atoms in each cluster. This is with the
0.6
100 K 300 K
0.3
0.0
-0.3
-0.6 -30000 -20000 -10000
Fig. 3. (Color online) M–H plot for the sintered and the un-sintered sample showing paramagnetic and superparamagnetic behavior, respectively. Inset shows the field-cooled w vs T plot. Points in the plots are the experimental data points and continuous lines are the Langevin function fitting of the plots.
L95
0 10000 Field in Oe
20000
30000
Fig. 4. (Color online) Hysteresis plot (after subtracting linear diamagnetic contribution) of the 5% Co-doped ZnO film. Inset shows enlarged view of near zero field value.
ARTICLE IN PRESS L96
S. Ghoshal, P.S. Anil Kumar / Journal of Magnetism and Magnetic Materials 320 (2008) L93–L96
annealed at different conditions show ferromagnetism even at room temperature. One can speculate that during deposition some ferromagnetic Co clusters which are large in size are formed and are responsible for the hysteresis loop in these films. On the other hand, it is interesting to note that the spectroscopic data showed transitions corresponding to Co2+ in the tetrahedrally coordinated sites in these films, which showed hysteresis loop characteristic of ferromagnetic samples. The most striking feature is the appearance of ferromagnetism in films that have been prepared from a paramagnetic target. Hence we believe that doping of Co in ZnO matrix induces ferromagnetism. Again, oxygen vacancies in the film may enhance ferromagnetism [26]. 4. Conclusion We have studied the properties of 5% Co-doped ZnO bulk as well as thin film samples having different preparation and processing conditions and conclude that the properties are highly sensitive to the processing condition. For the un-sintered sample the UV–visible data, XRD and magnetization measurements suggest the possibility of superparamagnetic behavior arising due to nanophased Co metallic clusters. For the sintered sample these studies indicate the paramagnetic nature of the sample and the substitution of Co in ZnO matrix. For the films that were prepared from the paramagnetic target the behavior is quite distinct and we observed ferromagnetism. In our study we have shown that it is possible to get superparamagnetism, paramagnetism and also ferromagnetism from the same sample depending on the processing technique. Acknowledgment The authors are thankful to Prof. M.S. Hegde for valuable support. References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [2] Gary A. Prinz, Science 282 (1998) 1660. [3] I. Zutic, J. Fabian, S.D. Sarma, Rev. Mod. Phys. 76 (2004) 323.
[4] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790. [5] G. Kioseoglou, A. Hanbicki, J.M. Sullivan, O.M.J. Van’t Erve, C.H. Li, S.C. Erwin, R. Mallory, M. Yasar, A. Petrou, B.T. Jonker, Nature 3 (2004) 799. [6] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [7] S. Bandyopadhyay, B. Das, A.E. Miller, Nanotechnology 5 (1994) 113. [8] D.D. Awschalom, M.E. Flatte, N. Samarth, Sci. Am. Inc. 286 (2002) 67. [9] A.T. Hanbicki, B.T. Jonker, G. Itskos, G. Kioseoglou, A. Petrou, Appl. Phys. Lett. 80 (2002) 1240. [10] Schmidt, D. Ferrand, L.W. Molenkamp, A.T. Filip, B.J. van Wees, Phys. Rev. B 62 (R) (2000) R4790. [11] P.C. van Son, H. van Kempen, P. Wyder, Phys. Rev. Lett. 58 (1987) 2271. [12] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L.W. Molenkamp, Nature 402 (1999) 787. [13] T. Jungwirth, K.Y. Wang, J. Masˇ ek, K.W. Edmonds, Ju¨rgen Ko¨nig, Jairo Sinova, M. Polini, N.A. Goncharuk, A.H. MacDonald, M. Sawicki, A.W. Rushforth, R.P. Campion, L.X. Zhao, C.T. Foxon, B.L. Gallagher, Phys. Rev. B 72 (2005) 165204. [14] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [15] C.N.R. Rao, F.L. Deepak, J. Mater. Chem. 15 (2005) 573. [16] G. Lawes, A.S. Risbud, A.P. Ramirez, Ram. Seshadri, Phys. Rev. B 71 (2005) 045201. [17] 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. [18] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [19] M. Bouloudenine, N. Viart, S. Colis, J. Kortus, A. Dinia, Appl. Phys. Lett. 87 (2005) 052501. [20] P. Sati, C. Deparis, C. Morhain, S. Schafer, A. Stepanov, Phys. Rev. Lett. 98 (2007) 137204. [21] S.K. Mandal, A.K. Das, T.K. Nath, D. Karmakar, Appl. Phys. Lett. 89 (2006) 144105. [22] F.A. Cotton, D.M.L. Goodgame, M. Goodgame, J. Am. Chem. Soc. 83 (1961) 4690. [23] T.J. Jackson, E.M. Forgan, T.M. Riseman, H. Gluckler, E. Morenzoni, T. Prokscha, H.P. Weber, CH. Niedermayer, M. Pleines, G. Schatz, J. Litterst, H. Luetkens, H. Keller, R. Khasonov, T.S. Rong, C. Binns, Hyperfine Interact. 136–137 (2001) 403. [24] S.R. Shindhe, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkateshan, Phys. Rev. Lett. 92 (2004) 166601. [25] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393. [26] H.S. Hsu, J.C.A. Huang, Y.H. Huang, Y.F. Liao, M.Z. Lin, C.H. Lee, J.F. Lee, S.F. Chen, L.Y. Lai, C.P. Liu, Appl. Phys. Lett. 88 (2006) 242507.