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Co-doped In2O3 thin films: Room temperature ferromagnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 302 (2006) 228–231 www.elsevier.com/locate/jmmm
Co-doped In2O3 thin films: Room temperature ferromagnets Nguyen Hoa Honga,, Joe Sakaib, Ngo Thu Huongb, Virginie Brize´a a
Laboratoire LEMA, UMR 6157 CNRS-CEA, Universite´ F. Rabelais, Parc de Grandmont, 37200 Tours, France b School of Materials Science, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan Received 18 July 2005; received in revised form 5 September 2005 Available online 30 September 2005
Abstract Laser-ablated Co-doped In2O3 thin films were fabricated under various growth conditions on R-cut Al2O3 and MgO substrates. All Co:In2O3 films are well-crystallized, single phase, and room temperature ferromagnetic. Co atoms were well substituted for In atoms, and their distribution is greatly uniform over the whole thickness of the films. Films grown at 550 1C showed the largest magnetic moment of about 0.5 mB/Co, while films grown at higher temperatures have magnetic moments of one order smaller. The observed ferromagnetism above room temperature in Co:In2O3 thin films has confirmed that doping few percent of magnetic elements such as Co into In2O3 could result in a promising magnetic material. r 2005 Elsevier B.V. All rights reserved. Keywords: Ferromagnetic; Semiconductors; Thin films
Diluted magnetic semiconductors have attracted many research groups due to their great potential for applications in spintronics. Many compounds have been discovered to show room temperature ferromagnetism (FM). Among those, transition metal (TM)-doped semiconductor oxide thin films appear to be rather promising candidates. Experimental work on TM-doped-TiO2, ZnO, as well as SnO2 has suggested that TMs certainly could be used to dope in host semiconductor oxides to result in room temperature FM [1–7]. In2O3 is a transparent, wide-bandgap semiconductor with a bixbyite cubic structure that contains 80 atoms in a unit cell. Thus, compared to other host matrices that we have dealt with so far, this compound seems to be more complicated and somehow different [8]. However, since In2O3 has been widely used in semiconductor industries [9], it should be useful if we can achieve FM in this special type of materials. On the other hand, from the practical point of view, its cubic structure may allow us to use some types of low-cost substrates such as MgO, which might be interesting to applications. So far, there has been only one report on the FM in Fe-doped Corresponding author. Tel.: +33 2 47 36 73 75; fax: +33 2 47 36 71 21.
E-mail addresses: [email protected], nguyen.hoahong @univ-tours.fr (N.H. Hong). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.09.010
In2O3 thin films [9]. In this work, we undertook an investigation on magnetism of Co-doped In2O3 thin films. Co-doped In2O3 films were grown on (0 0 1) MgO and Rcut Al2O3 substrates by using the pulsed laser deposition (PLD) technique (KrF laser with l ¼ 248 nm) from an In1.99Co0.01O3 ceramic target made by the solid-state reaction method. The repetition rate was 10 Hz and the energy density was 2.5 J/cm2. The substrate temperature was kept constant at 650, 600 or 550 1C. During deposition, the oxygen partial pressure ðPO2 Þ was 106 Torr, and after deposition, films were slowly cooled down to room temperature under a PO2 of 20 mTorr. The thickness of the films is in the range of 350–600 nm depending on different conditions. Structural study was done by X-ray diffraction (XRD). The chemical compositions were determined by Rutherford backscattering spectroscopy (RBS) measurements. The magnetization measurements were performed by a Quantum Design superconducting quantum interference device (SQUID) system from 0 up to 0.5 T under a range of temperatures from 400 down to 5 K. Co:In2O3 films on both types of substrate are colourless and very transparent. Films were well formed as pure In2O3 structure with no peak of any secondary phase (see XRD patterns in Fig. 1 showing very sharp, and strongintensity peaks). Films on MgO were grown along both
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
[1 1 1] and [1 0 0] directions. Differently, Co:In2O3 films on Al2O3 are highly oriented along only [1 1 1] direction (see in Fig. 1(b) that only (2 2 2) and (4 4 4) peaks appeared). It is very similar to the case of Fe:In2O3 films on Al2O3 that He et al. reported in Ref. [9]. It is found that lattice parameters are a bit deviated from that of the non-doped In2O3 ˚ however, they do not change much as the (a ¼ 10:11 A); growth conditions changed: a is in the range between 10.13 and 10.20 A˚. Among all, the Co:In2O3 films, which were grown at 550 1C on Al2O3, have the lattice parameter closest to that of the non-doped In2O3. Co content was determined by RBS measurements to be 9.3%, 7.5% and 6.8% for films deposited on MgO at 650, 600 and 550 1C, and 8.5%, 7.3% and 8% for films grown on Al2O3 at 650, 600 and 550 1C, respectively. The dopant content in the Co:In2O3 films is deviated rather much from
that of the synthesized target while in the Ni:In2O3 films fabricated under the same growth conditions, it remains exactly the same [10]. Thus, we must say that due to the difference in mobility of each element, in order to control the dopant concentration to be exact as the expected one, growth conditions must be chosen differently and appropriately for each type of dopant. RBS spectra showed that in the films, the distributions of both In and Co atoms are greatly uniform. From Fig. 2, one can see clearly that In and Co peaks are very well separated, indicating that the determination of In and Co contents in the films could be very precise (on the contrary, if there is some overlapping, it should be impossible to say definitely about the number of atoms of each element). The Co peak has a perfect rectangular shape, which is very similar to that of the In peak (also see the inset showing a zoom-up picture in log-scale), indicating that the Co distribution is very uniform over the whole thickness of the films. It is completely different from the cases of Co/Fe/ Ni-doped TiO2 films [11,12] and Co-doped ZnO films [13], where the dopant atoms were mostly localized in the layer of 40 nm taken from the surface. While Jin et al. [14] reported that Cr, Mn, Fe and Co are the most soluble among all the elements of the TM group, our case of Co:In2O3 films showed that to have an uniform distribution of dopant in the films, it depends not only on the nature of dopant, but also on the nature of the host matrix as well as the growth conditions, which should influence very much the ordering in the host lattice. Magnetization data for films on MgO are shown in Fig. 3. The well-defined hysteresis loops, which can be seen from the M–H curves taken at 300 K, show that films grown at different temperatures all are ferromagnetic at room temperature. One can see from a typical M–T curve that was shown in the inset of Fig. 3(a) that Co:In2O3 films
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(222) In2O3
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50 60 2 − Theta (deg.)
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Fig. 1. XRD patterns for Co:In2O3 films: (a) deposited at 550 1C on MgO substrates and (b) deposited at 650 1C on Al2O3 substrates.
Co:In2O3/MgO 650C 1000 900 1000
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O
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100 0 100
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Fig. 2. RBS data for a Co:In2O3 film deposited on MgO substrates at 650 1C. The inset shows a zoom-up for the Co and In peaks.
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
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Fig. 3. Magnetization versus magnetic field at 300 K for the Co:In2O3 films fabricated on MgO substrates: (a) for the film grown at 550 1C and (b) for the films grown at 600 and 650 1C. The inset shows the M–T curve taken at 0.5 T for the film fabricated at 550 1C.
on MgO have Curie temperature (T C ) well above 400 K (see Fig. (3a)). However, there is an important point to be noticed in here is that, the films grown at a lower temperature could result in much larger magnetic moments (compare the magnitudes of magnetization of films grown at 550 1C in Fig. 3(a) and of films grown at higher temperatures in Fig. 3(b)). Films deposited at 550 1C on MgO have M s as of 0.5 mB/Co while films deposited at 600 as well as 650 1C are very weak ferromagnetic (M s ¼ 0:04 mB =Co). Note that to calculate the values of magnetization from magnetic moments, we used the numbers of atoms that were determined precisely from RBS data, by supposing that all the Co atoms contributed to the magnetism of the films. As for the case of films deposited at higher than 550 1C, it seems that some precipitations such as CoO might exist, so that they contribute as paramagnetic component in the total magnetization of the films (see the M–H curves in Fig. 3(b)).
(b)
-2000
0
2000
4000
Field (Oe)
Fig. 4. Magnetization of the Co:In2O3 film fabricated on Al2O3 substrates at 550 1C: (a) versus temperature at 0.5 T, and (b) versus magnetic field at 300 K.
However, the contribution of such a secondary phase must be very small, so that they could not be detected by XRD whose the detection limit is of less than 5%. Films grown on Al2O3 substrates at various conditions are also ferromagnetic above room temperature (see Figs. 4 and 5). A typical M–H curve taken at 300 K is shown in Fig. 4(b) demonstrating that Co:In2O3 films on Al2O3 are certainly ferromagnets at room temperature. As seen clearly in Fig. 4(a), similar to films grown on MgO, Co:In2O3 films grown on Al2O3 also have a high T C (beyond 400 K), M s in the range between 0.03 and 0.5 mB/ Co, and among all, films grown at 550 1C seem to have the largest magnetic moment (compare data in Figs. 4 and 5). However, these values are much smaller that what was reported for Fe:In2O3 films on the same type of substrates (Ref. [9] gave the M s of Fe:In2O3 films on C-Al2O3 as of 1.45 mB/Fe). From XRD data, no remarkable difference in parameters could be observed to be able to explain the smallest magnetic moment in the film deposited at 600 1C from the structural viewpoint. However, from the RBS
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
0.06
Magnetization (µB/Co)
0.04
Ts600C Ts650C
0.02 0.00 -0.02 -0.04 -0.06 -4000
-2000
0
2000
4000
Field (Oe) Fig. 5. Magnetization versus magnetic field at 300 K for the Co:In2O3 films fabricated at 600 and 650 1C on Al2O3 substrates.
profile, it is found that this film has an oxygen deficiency (as regards to the composition of (Co, In)2O2.6 instead of the nominal one as of (Co, In)2O3). It is hard to attribute the reduction in magnetic moment to such a deficiency, because other observations in other systems of DMS oxides have revealed that filling up oxygen vacancy could cause certain degradations to the ferromagnetic ordering in the films [15]. Only one certain thing that could be confirmed in here is that, only changing the substrate temperature by 50 1C could decrease 1 order of magnitude of magnetization in the grown film. It is very similar to what we observed in V-doped ZnO films [16]. Thus, it seems that the growth conditions indeed play a crucial role in tuning FM in DMS films. This remark is in accord with a recent report on Mn-doped ZnO films, which showed that an appropriate choice of combination of oxygen pressure and growth temperature could also help a compound, which was supposed to be non-ferromagnetic, to become ferromagnetic. Or in other words, growth conditions could indeed support a certain change in magnetic ordering in the host lattice [17]. The modest magnitude of saturated magnetization of equal or less than 0.5 mB/Co in Co:In2O3 films is one indirect evidence to rule out the assumption for some existence of Co metal clusters in the Co:In2O3 films, since Co metal is known to have M s as of 1.7 mB/Co. This is also consistent with the fact that all of our Co:In2O3 thin films are semiconductors with the typical resistivity at room temperature as of about 102 Ocm, and it rises up as the temperature decreases. However, in order to have more direct evidences for cluster-free in the films, magnetic force microscopy (MFM) and high-resolution transmission electron microscopy (HR-TEM) measurements are expected to be done in the near future.
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In conclusion, room temperature ferromagnetism could be well obtained in Co-doped In2O3 thin films grown by PLD on sapphire and MgO substrates. Under various conditions, ferromagnetic Co:In2O3 films were well crystallized as In2O3 structure and no trace of any secondary phase could be seen in the spectra. Co atoms were perfectly substituted for In atoms, and their distribution in the In2O3 host matrix is greatly uniform. Films grown at a rather low temperature as of 550 1C have the largest magnetic moment of about 0.5 mB/Co. A change of the substrate temperature of 50 1C could change one order of the magnitude of magnetization. It is obvious that growth conditions might play an important role in tuning ferromagnetism in the diluted magnetic oxide thin films. Finding the induced room temperature ferromagnetism in the wide-band-gap semiconductor In2O3 by doping magnetic ions such as Co is another strong proof for the common feature that have been discovered so far in TM-doped semiconductor thin films.
References [1] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [2] Z. Wang, J. Tang, L.D. Tung, W. Zhou, L. Spinu, J. Appl. Phys. 93 (2003) 7870. [3] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, A. Ruyter, F. Gervais, Phys. Rev. B 70 (2004) 195204. [4] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [5] S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 91 (2003) 77205. [6] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Appl. Phys. Lett. 84 (2004) 1332. [7] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, J. Phys.: Condens. Matter 17 (2005) 1697. [8] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, Y.D. Park, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [9] J. He, S. Xu, Y.K. Yoo, Q. Xue, H.C. Lee, S. Cheng, X.-D. Xiang, G. Dionne, I. Takeuchi, Appl. Phys. Lett. 86 (2005) 052503. [10] N.H. Hong, J. Sakai, N.T. Huong, V. Brize´, Appl. Phys. Lett. 87 (2005) 102505. [11] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, Appl. Phys. Lett. 83 (2003) 3129. [12] N.H. Hong, J. Sakai, W. Prellier, J. Magn. Magn. Mater. 281 (2004) 347. [13] A. Fouchet, W. Prellier, P. Padhan, Ch. Simon, B. Mercey, V.N. Kulkarni, T. Venkatesan, J. Appl. Phys. 95 (2004) 7187. [14] Z. Jin, M. Murakami, T. Fukumura, Y. Matsumoto, A. Ohtomo, M. Kawasaki, H. Koinuma, J. Cryst. Growth 55 (2000) 214. [15] N.H. Hong, J. Sakai, N.T. Huong, N. Poirot, A. Ruyter, Phys. Rev. B 72 (2005) 45336. [16] N.H. Hong, J. Sakai, A. Hassini, J. Phys.: Condens. Matter 17 (2005) 199. [17] N.H. Hong, V. Brize´, J. Sakai, Appl. Phys. Lett. 86 (2005) 82505.
Journal of Magnetism and Magnetic Materials 302 (2006) 228–231 www.elsevier.com/locate/jmmm
Co-doped In2O3 thin films: Room temperature ferromagnets Nguyen Hoa Honga,, Joe Sakaib, Ngo Thu Huongb, Virginie Brize´a a
Laboratoire LEMA, UMR 6157 CNRS-CEA, Universite´ F. Rabelais, Parc de Grandmont, 37200 Tours, France b School of Materials Science, JAIST, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan Received 18 July 2005; received in revised form 5 September 2005 Available online 30 September 2005
Abstract Laser-ablated Co-doped In2O3 thin films were fabricated under various growth conditions on R-cut Al2O3 and MgO substrates. All Co:In2O3 films are well-crystallized, single phase, and room temperature ferromagnetic. Co atoms were well substituted for In atoms, and their distribution is greatly uniform over the whole thickness of the films. Films grown at 550 1C showed the largest magnetic moment of about 0.5 mB/Co, while films grown at higher temperatures have magnetic moments of one order smaller. The observed ferromagnetism above room temperature in Co:In2O3 thin films has confirmed that doping few percent of magnetic elements such as Co into In2O3 could result in a promising magnetic material. r 2005 Elsevier B.V. All rights reserved. Keywords: Ferromagnetic; Semiconductors; Thin films
Diluted magnetic semiconductors have attracted many research groups due to their great potential for applications in spintronics. Many compounds have been discovered to show room temperature ferromagnetism (FM). Among those, transition metal (TM)-doped semiconductor oxide thin films appear to be rather promising candidates. Experimental work on TM-doped-TiO2, ZnO, as well as SnO2 has suggested that TMs certainly could be used to dope in host semiconductor oxides to result in room temperature FM [1–7]. In2O3 is a transparent, wide-bandgap semiconductor with a bixbyite cubic structure that contains 80 atoms in a unit cell. Thus, compared to other host matrices that we have dealt with so far, this compound seems to be more complicated and somehow different [8]. However, since In2O3 has been widely used in semiconductor industries [9], it should be useful if we can achieve FM in this special type of materials. On the other hand, from the practical point of view, its cubic structure may allow us to use some types of low-cost substrates such as MgO, which might be interesting to applications. So far, there has been only one report on the FM in Fe-doped Corresponding author. Tel.: +33 2 47 36 73 75; fax: +33 2 47 36 71 21.
E-mail addresses: [email protected], nguyen.hoahong @univ-tours.fr (N.H. Hong). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.09.010
In2O3 thin films [9]. In this work, we undertook an investigation on magnetism of Co-doped In2O3 thin films. Co-doped In2O3 films were grown on (0 0 1) MgO and Rcut Al2O3 substrates by using the pulsed laser deposition (PLD) technique (KrF laser with l ¼ 248 nm) from an In1.99Co0.01O3 ceramic target made by the solid-state reaction method. The repetition rate was 10 Hz and the energy density was 2.5 J/cm2. The substrate temperature was kept constant at 650, 600 or 550 1C. During deposition, the oxygen partial pressure ðPO2 Þ was 106 Torr, and after deposition, films were slowly cooled down to room temperature under a PO2 of 20 mTorr. The thickness of the films is in the range of 350–600 nm depending on different conditions. Structural study was done by X-ray diffraction (XRD). The chemical compositions were determined by Rutherford backscattering spectroscopy (RBS) measurements. The magnetization measurements were performed by a Quantum Design superconducting quantum interference device (SQUID) system from 0 up to 0.5 T under a range of temperatures from 400 down to 5 K. Co:In2O3 films on both types of substrate are colourless and very transparent. Films were well formed as pure In2O3 structure with no peak of any secondary phase (see XRD patterns in Fig. 1 showing very sharp, and strongintensity peaks). Films on MgO were grown along both
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
[1 1 1] and [1 0 0] directions. Differently, Co:In2O3 films on Al2O3 are highly oriented along only [1 1 1] direction (see in Fig. 1(b) that only (2 2 2) and (4 4 4) peaks appeared). It is very similar to the case of Fe:In2O3 films on Al2O3 that He et al. reported in Ref. [9]. It is found that lattice parameters are a bit deviated from that of the non-doped In2O3 ˚ however, they do not change much as the (a ¼ 10:11 A); growth conditions changed: a is in the range between 10.13 and 10.20 A˚. Among all, the Co:In2O3 films, which were grown at 550 1C on Al2O3, have the lattice parameter closest to that of the non-doped In2O3. Co content was determined by RBS measurements to be 9.3%, 7.5% and 6.8% for films deposited on MgO at 650, 600 and 550 1C, and 8.5%, 7.3% and 8% for films grown on Al2O3 at 650, 600 and 550 1C, respectively. The dopant content in the Co:In2O3 films is deviated rather much from
that of the synthesized target while in the Ni:In2O3 films fabricated under the same growth conditions, it remains exactly the same [10]. Thus, we must say that due to the difference in mobility of each element, in order to control the dopant concentration to be exact as the expected one, growth conditions must be chosen differently and appropriately for each type of dopant. RBS spectra showed that in the films, the distributions of both In and Co atoms are greatly uniform. From Fig. 2, one can see clearly that In and Co peaks are very well separated, indicating that the determination of In and Co contents in the films could be very precise (on the contrary, if there is some overlapping, it should be impossible to say definitely about the number of atoms of each element). The Co peak has a perfect rectangular shape, which is very similar to that of the In peak (also see the inset showing a zoom-up picture in log-scale), indicating that the Co distribution is very uniform over the whole thickness of the films. It is completely different from the cases of Co/Fe/ Ni-doped TiO2 films [11,12] and Co-doped ZnO films [13], where the dopant atoms were mostly localized in the layer of 40 nm taken from the surface. While Jin et al. [14] reported that Cr, Mn, Fe and Co are the most soluble among all the elements of the TM group, our case of Co:In2O3 films showed that to have an uniform distribution of dopant in the films, it depends not only on the nature of dopant, but also on the nature of the host matrix as well as the growth conditions, which should influence very much the ordering in the host lattice. Magnetization data for films on MgO are shown in Fig. 3. The well-defined hysteresis loops, which can be seen from the M–H curves taken at 300 K, show that films grown at different temperatures all are ferromagnetic at room temperature. One can see from a typical M–T curve that was shown in the inset of Fig. 3(a) that Co:In2O3 films
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Fig. 1. XRD patterns for Co:In2O3 films: (a) deposited at 550 1C on MgO substrates and (b) deposited at 650 1C on Al2O3 substrates.
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Fig. 2. RBS data for a Co:In2O3 film deposited on MgO substrates at 650 1C. The inset shows a zoom-up for the Co and In peaks.
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
230
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Fig. 3. Magnetization versus magnetic field at 300 K for the Co:In2O3 films fabricated on MgO substrates: (a) for the film grown at 550 1C and (b) for the films grown at 600 and 650 1C. The inset shows the M–T curve taken at 0.5 T for the film fabricated at 550 1C.
on MgO have Curie temperature (T C ) well above 400 K (see Fig. (3a)). However, there is an important point to be noticed in here is that, the films grown at a lower temperature could result in much larger magnetic moments (compare the magnitudes of magnetization of films grown at 550 1C in Fig. 3(a) and of films grown at higher temperatures in Fig. 3(b)). Films deposited at 550 1C on MgO have M s as of 0.5 mB/Co while films deposited at 600 as well as 650 1C are very weak ferromagnetic (M s ¼ 0:04 mB =Co). Note that to calculate the values of magnetization from magnetic moments, we used the numbers of atoms that were determined precisely from RBS data, by supposing that all the Co atoms contributed to the magnetism of the films. As for the case of films deposited at higher than 550 1C, it seems that some precipitations such as CoO might exist, so that they contribute as paramagnetic component in the total magnetization of the films (see the M–H curves in Fig. 3(b)).
(b)
-2000
0
2000
4000
Field (Oe)
Fig. 4. Magnetization of the Co:In2O3 film fabricated on Al2O3 substrates at 550 1C: (a) versus temperature at 0.5 T, and (b) versus magnetic field at 300 K.
However, the contribution of such a secondary phase must be very small, so that they could not be detected by XRD whose the detection limit is of less than 5%. Films grown on Al2O3 substrates at various conditions are also ferromagnetic above room temperature (see Figs. 4 and 5). A typical M–H curve taken at 300 K is shown in Fig. 4(b) demonstrating that Co:In2O3 films on Al2O3 are certainly ferromagnets at room temperature. As seen clearly in Fig. 4(a), similar to films grown on MgO, Co:In2O3 films grown on Al2O3 also have a high T C (beyond 400 K), M s in the range between 0.03 and 0.5 mB/ Co, and among all, films grown at 550 1C seem to have the largest magnetic moment (compare data in Figs. 4 and 5). However, these values are much smaller that what was reported for Fe:In2O3 films on the same type of substrates (Ref. [9] gave the M s of Fe:In2O3 films on C-Al2O3 as of 1.45 mB/Fe). From XRD data, no remarkable difference in parameters could be observed to be able to explain the smallest magnetic moment in the film deposited at 600 1C from the structural viewpoint. However, from the RBS
ARTICLE IN PRESS N.H. Hong et al. / Journal of Magnetism and Magnetic Materials 302 (2006) 228–231
0.06
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4000
Field (Oe) Fig. 5. Magnetization versus magnetic field at 300 K for the Co:In2O3 films fabricated at 600 and 650 1C on Al2O3 substrates.
profile, it is found that this film has an oxygen deficiency (as regards to the composition of (Co, In)2O2.6 instead of the nominal one as of (Co, In)2O3). It is hard to attribute the reduction in magnetic moment to such a deficiency, because other observations in other systems of DMS oxides have revealed that filling up oxygen vacancy could cause certain degradations to the ferromagnetic ordering in the films [15]. Only one certain thing that could be confirmed in here is that, only changing the substrate temperature by 50 1C could decrease 1 order of magnitude of magnetization in the grown film. It is very similar to what we observed in V-doped ZnO films [16]. Thus, it seems that the growth conditions indeed play a crucial role in tuning FM in DMS films. This remark is in accord with a recent report on Mn-doped ZnO films, which showed that an appropriate choice of combination of oxygen pressure and growth temperature could also help a compound, which was supposed to be non-ferromagnetic, to become ferromagnetic. Or in other words, growth conditions could indeed support a certain change in magnetic ordering in the host lattice [17]. The modest magnitude of saturated magnetization of equal or less than 0.5 mB/Co in Co:In2O3 films is one indirect evidence to rule out the assumption for some existence of Co metal clusters in the Co:In2O3 films, since Co metal is known to have M s as of 1.7 mB/Co. This is also consistent with the fact that all of our Co:In2O3 thin films are semiconductors with the typical resistivity at room temperature as of about 102 Ocm, and it rises up as the temperature decreases. However, in order to have more direct evidences for cluster-free in the films, magnetic force microscopy (MFM) and high-resolution transmission electron microscopy (HR-TEM) measurements are expected to be done in the near future.
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In conclusion, room temperature ferromagnetism could be well obtained in Co-doped In2O3 thin films grown by PLD on sapphire and MgO substrates. Under various conditions, ferromagnetic Co:In2O3 films were well crystallized as In2O3 structure and no trace of any secondary phase could be seen in the spectra. Co atoms were perfectly substituted for In atoms, and their distribution in the In2O3 host matrix is greatly uniform. Films grown at a rather low temperature as of 550 1C have the largest magnetic moment of about 0.5 mB/Co. A change of the substrate temperature of 50 1C could change one order of the magnitude of magnetization. It is obvious that growth conditions might play an important role in tuning ferromagnetism in the diluted magnetic oxide thin films. Finding the induced room temperature ferromagnetism in the wide-band-gap semiconductor In2O3 by doping magnetic ions such as Co is another strong proof for the common feature that have been discovered so far in TM-doped semiconductor thin films.
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