Growth and characterisation of cobalt doped zinc oxide 5

Chapter Magnetic properties and its origins CHAPTER MAGNETIC PROPERTIES OF Co-DOPED ZnO 5.1 Introduction There are many ways to characterize the magnetic properties of a DMS. Among them the most direct way is to measure the magnetic moment as a function of an applied field using a magnetometer. The M-H loops obtained would indicate whether the material under investigation is ferromagnetic, paramagnetic or anti-ferromagnetic. In addition to M-H loops, the FC and ZFC magnetic moment versus temperature curves are also often used to characterize the homogeneity, blocking temperature and Tc of the material. In addition to magnetometer based characterizations, the magnetic properties of a DMS material can also be evaluated using techniques such as MCD and AHE. In this chapter, the results obtained by SQUID and MCD are presented. The results obtained by AHE will be discussed in the next chapter together with the transport measurement results. 5.2 Characterization by SQUID The M-H loops had been measured using SQUID for most of the samples in the temperature range of 10 to 300K. For the Co3W sample, measurements had also been performed at K and K. All the samples measured showed the presence of ferromagnetism, varying from weak ferromagnetism embedded in a strong paramagnetic phase to ferromagnetism with clear hysteresis loop as the Co content increased. Note that the strong diamagnetism from the substrate has been subtracted out from the raw data for all results unless it is indicated otherwise. Also note that, 126 Chapter Magnetic properties and its origins throughout this thesis, “ferromagnetism” and “presence of a clear hysteresis in the M-H loop” were used inter-changeably. Figure 5-1 M-H curves (in-plane) for co-doped sample Co3W (Zn0.95Co0.05O) at 2, 5, 100, 200, 300 and 400 K (a) as determined, (b) corrected for substrate effect. Before proceeding to discuss the magnetic properties of samples with different Co compositions, discussions would start with the Co3W (x = 0.05) sample. Shown above in Fig. 5-1 are both (a) the raw magnetic moment measured by SQUID and (b) the corrected data after the diamagnetism from the substrate had been removed. As seen from the raw data curves, the slopes of the curve at high fields were slightly dependent 127 Chapter Magnetic properties and its origins on the temperature. As the diamagnetism was almost independent of temperature, all the data had been corrected by subtracting the diamagnetism at 300 K. As it is shown in Fig. 5-1 (b), the paramagnetic signal increased with decreasing the temperature. The paramagnetism might have originated from other Co atoms distributed in the host matrix or from the defects. The in-plane and out-of-plane initial M-H curves for Co3W sample at different temperatures are shown in Fig. 5-2. From the initial curves, it was observed that the inplane anisotropy was more pronounced as temperature increased from K to 50 K. In the out-of-plane initial M-H curves (Fig. 5-2 (g) and (h)), also observed as temperature increased, there was a change in initial M-H shapes when moving from K to 50 K. The changes in M-H curve shapes could be an indication of different origins of ferromagnetism, or multi-phase present in this material, which would be further discussed in transport studies. In Fig. 5-2 (b) and (d), the in-plane anisotropy observed at and 50 K agreed well with the observation by Sati et. al.3, who had observed a strong anisotropy for Co2+ ions in the ZnO host matrix for very low Co concentration of x = 0.003 – 0.005. Sati interpreted it as a signature of intrinsic ferromagnetism, where Co2+ ions in the ZnO lattice were coupled ferromagnetically. However, for the lightly doped samples in this thesis, it was still not possible to pinpoint the origin of ferromagnetism from M-H curves alone. 128 Chapter Magnetic properties and its origins Figure 5-2 As determined M-H curves for co-doped sample Co3W (Zn0.95Co0.05O) at (a) K, (c) 50 K, (e) 300 K for in-plane and out-of-plane configuration and corrected M-H curves (b), (d) and (f) respectively. Out-of-plane M-H curves for co-doped sample Co3W (Zn0.95Co0.05O) at 2, 50 and 300 K (g) as determined and (c) corrected for substrate effect. 129 Chapter Magnetic properties and its origins Increasing the Co content near the onset of secondary phase formation, x = 0.24 (Fig. 5-3 (a)), a switch from out-of-plane anisotropy to in-plane anisotropy as temperature decreased from 300 K to K was observed. As the temperature was decreased to K, not much change was observed in the M-H curves in-plane, but a sudden increase in coercivity was observed in the out-of-plane curve. This increase was much larger in magnitude as compared to the Co3W sample, but their ferromagnetic origin up to this point was still not clear and would be further explored using other characterization methods. The M-H curves for samples with x > 0.25, Fig. 5-3 (b) and (c), showed a well defined perpendicular anisotropy similar to those observed by Dinia and Rode. In both Dinia’s and Rode’s Zn0.75Co0.25O samples, the presence of Co clusters had been eliminated via XRD studies and the magnetic anisotropy was attributed to intrinsic properties of Co2+. Although the results obtained were similar to those obtained by Dinia’s and Rode’s, whom believe that the ferromagnetism was intrinsic, the origin of the ferromagnetism of the films studied in this study was not intrinsic in nature. This was probably due to the presence of secondary phases as observed from the structural studies and this would be further characterized by transport studies. 130 Chapter Magnetic properties and its origins (a) (b) (c) Figure 5-3 M-H curves (in-plane and out-of-plane) for co-doped samples (a) Co20W (Zn0.76Co0.24O), (b) Co 32W (Zn0.71Co0.29O), (c) Co45W (Zn0.70Co0.30O) at 10 and 300 K. 131 Chapter Magnetic properties and its origins Fig. 5-4 (a) compared the in-plane M-H loops for samples A, B, D, and H at room temperature. According to theoretical studies, it had been predicted that the magnetism of the lightly co-doped ZnO samples should be weak but yet ferromagnetic at room temperature. Experimental results from various groups had also made use of this weak magnetism at room temperature, coupled with good structural properties without secondary phases, as an indication of DMS material. However, more studies need to be carried out, particularly transport studies to confirm their true origin. As the Co composition was increased, especially passes the onset of secondary phase formation, the shapes of the M-H curves evolved from a weakly ferromagnetic character to one with a 100 fold increase in magnitude of magnetization and well defined coercivity. Caution should again be stressed that presence of ferromagnetism in any film should not be used to conclude that the material was an intrinsic DMS. In fact, the magnetic properties of the heavier co-doped samples were dominated by secondary phases, as confirmed from structural studies, rather than behaving as an intrinsic DMS phase. Before reaching the onset of secondary phase or cluster formation, the saturation magnetization of the sample was ~ 0.01 µB/Co atom. However, even after passing the onset composition, a further increase of Co composition would not lead to a constant increase of saturation magnetization, which was always below µB/Co atom. The rather low value of saturation magnetization suggested that the Co atoms incorporated in the host matrix are not all “magnetically active”, due to presumably the formation of antiferromagnetic phases. The coercivity, however, was considerably larger than those reported by other groups (typically around 50-200Oe1,10,11), after the Co composition exceeded the onset value of secondary phase formation. This agreed well with the 132 Chapter Magnetic properties and its origins observation in XRD studies that secondary phases and Co clusters started to from at x > 0.25. Looking at the overall trend of magnetic moment and coercivity with Co composition, as shown in Fig. 5-4 (b) and (c), it was interesting to observe that both the magnetic moment and coercivity initially increased slowly and when it reached the onset of secondary phase formation, increased sharply, followed by a dip around x = 0.28. The initial magnetic character, in particular the slight increase of coercivity, should be due to isolated Co ions with large anisotropy. Weak magnetism of these films was observed as Co was soluble in the ZnO host matrix and also due to the absence of ferromagnetic nanoclusters. However, as the Co content was increased to x = 0.25, there was an onset of secondary phase formation and the magnetic moment increased sharply which was due to the presence of ferromagnetic secondary phases. In addition, the appearance of a local minimum for coercivity was due to the switching over from in-plane easy-axis anisotropy to out-of-plane easy-axis anisotropy. As the Co content was further increased, magnetism and coercivity reached a maximum and then decreased again. This could be the indication of dominance of different ferromagnetic secondary phases as Co content increased. The sudden increase of the coercivity at x > 0.25 implied that the Co particles started to form a mutually connected magnetic network due to presumably spinoidal decomposition. Of course, in addition to Co clusters; there could also be the formation of other secondary phases. 133 Chapter Magnetic properties and its origins 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Figure 5-4 (a) In-plane M-H curves of co-doped Co3W (Zn0.95Co0.05O), Co 8W(Zn0.86Co0.14O), Co 15W (Zn0.80Co0.20O) and Co 32W (Zn0.71Co0.29O) at room temperature, (b) Saturation magnetization as a function of Co composition at room temperature, (c) Coercivity as a function of Co composition at 10, 150 and 300 K. 134 Chapter Magnetic properties and its origins As it was discussed in last chapter, the secondary phases could be in the form of Co, CoO, ZnCo2O4 and/or Co3O4. Bulk CoO, in rock-salt structure, is a known antiferromagnet with a Néel temperature of 297 K, but small CoO nanoparticles can exhibit ferromagnetic behaviour due to frustrated surface spins. Another Co oxide, Co3O4 is also antiferromagnetic, but in the nanoparticle form, can be ferromagnetic below 25 K.6 In a Zn:Co system, ZnCo2O4 can exist as a paramagnetic phase when it is an n-type semiconductor and ferromagnetic when it is a p-type semiconductor. In addition, Co nanoclusters, with a size larger than the superparamagnetic limit, will exhibit ferromagnetic properties. However, the measured M-H loops alone could not tell whether the origin of the ferromagnetism was from ZnO:Co system or from these secondary phases mentioned, as the M-H loops obtained were a summation of magnetisation data from all the different phases. It is thus crucial to correlate the data obtained with those of different characterization methods. To further study the magnetic properties of the films, FC and ZFC measurements were carried out at 10 – 400 K. The curves, as shown in Fig. 5-5 (a), (b) and (c) showed that the Tc of all the samples were above 400 K, agreeing well with results obtained by other groups which are listed in Table 5-1. Different magnetic fields were applied for different samples to allow determination of curves while eliminating noise from the data. Thus, the magnitude of magnetic moment and also ZFC-FC dependence on magnetic field will not be discussed in this section. As Co clusters were not seen by TEM in the lightly doped sample, the unusually high Tc was likely due to the uncompensated surface spins of antiferromagnetic clusters because the latter has a Neel temperature far around room temperature.5 As Co content was increased, as shown in Fig. 5-5 (b), a rapid decrease of FC magnetization was 135 Chapter Magnetic properties and its origins observed, an indication of magnetic clusters.15 The hump around 200 – 300 K, observed in Fig. 5-5 (c), could be an indication of presence of antiferromagnetic CoO, which would be discussed further in the chapter. Although all the samples were ferromagnetic based on the SQUID results, there was no direct evidence to show that the ferromagnetism was of intrinsic nature, i.e., originating from carrier induced ferromagnetic interactions among the Co2+ ions which substitute Zn. The results obtained by SQUID were likely a summation of contributions from various sources including isolated Co2+ ions, ZnO:Co ferromagnetic clusters, partially compensated surface spins of antiferromagnetic clusters for lightly co-doped samples and secondary phase clusters for heavily co-doped samples. 136 Magnetic Moment ( µB/Co) Chapter Magnetic properties and its origins 0.30 0.25 0.20 0.15 ZFC 0.10 FC (a) 0.05 0.00 100 200 300 400 Magnetic Moment ( µB/Co) Temperature (K) 0.12 0.10 ZFC 0.08 FC 0.06 (b) 0.04 0.02 0.00 100 200 300 400 300 400 Magnetic Moment ( µB/Co) Temperature (K) 0.08 0.07 0.06 0.05 0.04 ZFC 0.03 0.02 (c) FC 0.01 0.00 100 200 Temperature (K) Figure 5-5 ZFC-FC curve for codoped samples measured from 10 – 400K for (a) Co 3W (Zn0.95Co0.05O) at 2000 Oe, (b) Co20W (Zn0.76Co0.24O) at 200 Oe and (c) Co 45W (Zn0.70Co0.30O) at 50 Oe. 137 Chapter Magnetic properties and its origins Table 5-1 Tc of various ferromagnetic ZnO:Co samples. Co content (x) Growth method Tc (K) Remarks 0.25 PLD > 300 Intrinsic ferromagnetism PLD > 350 Ferromagnetic but origin x 0.25 Reference not confirmed, secondary phase when x > 0.25 0.08 PLD 300 from higher 10 Ferromagnetism Co2+ 0.05, 0.15, 0.25 PLD 280 - 300 Samples with carrier concentration are ferromagnetic 0.35 PLD > 350 Ferromagnetism from 11 Co2+, no metallic Co or CoOx particles 0.035 – 0.115 Sputtering > 350 RKKY not applicable, Co 12 cluster ruled out, indirect exchange interaction proposed 0.25 Sputtering > 300 Ferromagnetism from Co2+ 0.04 Sputtering 790 Bound magnetic polarons 13 0.03 Co implant > 300 Co nanoclusters 14 0.25 PLD > 300 Co glass nanoclusters, or spin 15 paramagnetic depending on growth 0.15, 0.25 Sol-gel > 300 Co nanoclusters 138 16 Chapter Magnetic properties and its origins 5.3 Characterization by MCD The discussion will now be focused on the results obtained by MCD. The measurements were concentrated on two co-doped samples, lightly doped sample Co 8W (x = 0.137) and heavily doped sample Co 32W (x = 0.289), and one delta-doped sample, Co 98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60). For the MCD spectra of ZnO:Co, discussions were concentrated on the absorptions at two particular wavelength regions, one at the band edge (near 3.4 eV) and the other at the d-d transition regime (near eV). As shown in Fig. 5-6 (a)-(c)), the MCD spectra for all the three samples exhibited rather broad peaks in the above regions. The broadening of the peaks suggested that the films were highly non-uniform at microscopic scale which could be seen clearly in low magnification TEM images for the heavily doped samples. For the lightly co-doped sample, paramagnetic behaviour dominated the MCD spectrum obtained. Fig. 5-7 (a) and (b), shows the temperature dependence of MCD for heavily doped samples. There seems to be no temperature dependence for these samples, as they had passed the onset of secondary phase formation, indicating that they were less influenced by sp-d interaction. As there were no noticeable structures around 2.0 and 3.4 eV, even as temperature was varied, the magnetic properties of this material had no correlation with a DMS form of ZnO:Co. This result agreed well with the structural and SQUID results as determined above. Thus, magnetic properties for these heavily doped samples were likely not due to sp-d interactions, but more likely from extrinsic origins. 139 Chapter Magnetic properties and its origins 40 (a) MCD (mdeg) 30 20 10 -10 -20 Photon Energy (eV) MCD (mdeg) 400 200 (b) -200 -400 -600 Photon Energy (eV) MCD (mdeg) 300 (c) 100 -100 -300 -500 -700 Photon Energy (eV) Figure 5-6 MCD spectra of co-doped (a) Co8W (Zn0.86Co0.14O), (b) Co32W (Zn0.71Co0.29O) and δ-doped (c) Co98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60) at K. 140 Chapter Magnetic properties and its origins 600 MCD (mdeg) (a) 300 300K -300 6K -600 1.0 2.0 3.0 4.0 5.0 Photon Energy (eV) MCD (mdeg) 700 (b) 350 300K -350 6K -700 1.0 2.0 3.0 4.0 5.0 Photon Energy (eV) Figure 5-7 MCD spectra of co-doped (a) Co32W (Zn0.71Co0.29O) and δ-doped (b) Co98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60) at and 300 K. From the hysteresis curve carried out on the lightly co-doped sample near the dd transition wavelength, Co8W was purely paramagnetic in nature. This contradicted the SQUID measurement results where the same sample was found to be weakly ferromagnetic. Again, it was difficult to have a direct comparison because the SQUID picked up contributions from all available sources, whereas the MCD only picked up the signal which was dominant at one wavelength. Also, when the signal was weak, it might be buried in the noise background. 141 Chapter Magnetic properties and its origins 1.0 Intensity (a.u.) MCD (1.95eV,6K) 0.5 SQUID (10K) 0.0 -0.5 -1.0 -10 -5 10 H (T) Figure 5-8 Hysteresis curve determined by MCD and SQUID for co-doped Co8W (Zn86Co0.14O) at and 10 K respectively. For the heavily co-doped sample, sample Co 32W, the MCD showed that ferromagnetic behaviour was dominant. Figures 5-9(a) and (b) showed the MCD hysteresis curves of sample Co 32 W obtained at different photon energies, 756, 425, 343 and 325 nm (1.64, 2.92, 3.61 and 3.83 eV), which was also marked in the transmission spectra shown in Fig. 4-15 by the “*” symbol, at K and 300 K, respectively. Figure 5-9(c) and (d) showed the MCD hysteresis curve of its delta-doped counterpart. The MCD curves, for both co-doped and delta-doped samples, were strongly dependent on the photon energy, confirming the inhomogeneous nature of the samples which consisted of ferromagnetic regions of different phases. Although the origin of ferromagnetism for all the curves at different photon energies could not be pinpointed, as it would be discussed later in transport measurements, only the MCD curve taken at 2.92 eV (425 nm) for the co-doped sample agreed well with the hysteresis curves measured by SQUID and AHE and not those taken at d-d transition 142 Chapter Magnetic properties and its origins wavelengths. As the photon energy of 2.92 eV was close to the bandgap of CoO, it strongly suggested that the ferromagnetic response was due to uncompensated surface spins Co-rich ZnO:Co clusters. Figure 5-9 Hysteresis curve determined by MCD for co-doped Co32W (Zn0.71Co0.29O) measured at various wavelength at (a) and (b) 300 K respectively and for δ-doped Co98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60) measured at 425 and 756 nm at (c) and (d) 300 K respectively. 143 Chapter Magnetic properties and its origins 5.4 Summary The SQUID measurements indicate that all the samples are ferromagnetic in nature. However, this does not mean that intrinsic DMS accounts for the origin of ferromagnetism observed. It merely shows that the films contain ferromagnetic phases. Based on the M-H curves alone, it is difficult, if not impossible to differentiate the contributions from various magnetic phases. For samples with x < 0.2, the ferromagnetism could be from uncompensated spins in antiferromagentic clusters or isolated Co2+ ions. However, as the Co content exceeds 0.2, in addition to the contribution from antiferromagnetic clusters, there is also contribution from ferromagnetic cluster. The MCD results seem to contradict the SQUID results for the lightly co-doped sample. This could be due to the fact that the SQUID measurement picks up the “ferromagnetic” signal from the antiferromagentic clusters, which can not be detected by the MCD. The MCD results also show clearly that the ferromagnetic phase is inhomogeneous in heavily doped samples, which will be further confirmed by the transport measurements to be discussed in next chapter. Following is a summary of the magnetic properties of the samples that have been investigated: 1) x < 0.2, Co-doped ZnO films exhibiting weak ferromagnetism with an easyaxis in-plane anisotropy, which is presumably due to uncompensated spins of antiferromagnetic clusters or maybe isolated Co2+ ions; 2) x = 0.25, onset of secondary phase formation, as detected with a sudden increase in the coercivity and magnetization; 3) x 0.3, inhomogeneous ferromagnetism of extrinsic origin with an easy- axis out-of plane anisotropy. 144 Chapter Magnetic properties and its origins References: A. Dinia, G. Schmerber, V. Pierron-Bohnes, C. Mény, P. Panissod, and E. Beaurepaire, “Magnetic perpendicular anisotropy in sputtered (Zn0.75Co0.25)O dilute magnetic semiconductor”, J. Magn. Magn. Mater. 286, 37 (2005). K. Rode, A. Anane, R. Mattana, J. P. Contour, O. Durand, and R. LeBourgeois, “Magnetic semiconductors based on cobalt substituted ZnO”, J. Appl. Phys. 93, 7676 (2003). P. Sati, R. Hayn, R. Kuzian, S. Regnier, S. Schafer, A. Stepanov, C. Morhain, C. Deparis, M. Laugt, M. Goiran, and Z. Golacki, “Magnetic anisotropy of Co2+ as signature of intrinsic ferromagnetism in ZnO:Co”, Phys. Rev. Lett. 96, 017203 (2006). K. Sato, H. K. Yoshida, and P. H. Dederichs, “High Curie temperature and nano-scale spinodal decomposition phase in dilute magnetic semiconductors”, Jpn. J. Appl. Phys. 44, L948 (2005). ; T. Fukushima, K. Sato, H. K. Yoshida, and P. H. Dederichs, “Spinodal decomposition under layer by layer growth condition and high Curie temperature quasi-one-dimensional nano-structure in dilute magnetic semiconductors”, Jpn. J. Appl. Phys. 45, L416 (2006). H. T. Zhang and X. H. Chen, “Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoO nanocrystals”, Nanotechnology 16, 2288 (2005). S.A. Makhlouf, “Magnetic properties of Co3O4 nanoparticles”, J. Magn. Magn. Matl. 246, 184 (2002). H. J. Kim, I. C. Song, J. H. Sim, H. Kim, D. Kim, Y. E. Ihm, and W. K. Choo, “Growth and characterization of spinel-type magnetic semiconductor ZnCo2O4 by reactive magnetron sputtering”, Phys. Stat. Sol. (b) 241, 1553 (2004). 145 Chapter Magnetic properties and its origins H. J. Lee, S. Y. Jeong, C. R. Cho, and C. H. Park, “Study of diluted magnetic semiconductor: Co-doped ZnO”, Appl. Phys. Lett. 81, 4020 (2002). W. Prellier, A. Fouchet, Ch. Simon, and B. Mercey, “Ferromagnetic Co-doped ZnO thin films grown using pulsed laser deposition from Zn and Co metallic targets”, Mater. Sci. Eng. B 109, 192 (2004). 10 K. Ueda, H. Tabata, and T. Kawai, “Magnetic and electric properties of transition- metal-doped ZnO films”, Appl. Phys. Lett. 79, 988 (2001). 11 A. C. Tuan, J. D. Bryan, A. B. Pakhomov, V. Shutthanandan, S. Thevuthasan, D. E. McCready, D. Gaspar, M. H. Engelhard, J. W. Rogers, Jr., K. Krishnan, D. R. Gamelin, and S. A. Chambers, “Epitaxial growth and properties of cobalt-doped ZnO on α-Al2O3 single-crystal substrates”, Phys. Rev. B 70, 054424 (2004). 12 S. G. Yang, A. B. Pakhomov, S. T. Hung, and C. Y. Wong, “Room temperature magnetism in sputtered (Zn,Co)O films”, IEEE. Trans. Magn. 38, 2877 (2002). 13 C. Song, K. W. Geng, F. Zeng, X. B. Wang, Y. X. Shen, F. Pan, Y. N. Xie, T. Liu, H. T. Zhou, and Z. Fan, “Giant magnetic moment in an anomalous ferromagnetic insulator: Co-doped ZnO”, Phys. Rev. B 73, 024405 (2006). 14 D. P. Norton, M. E. Overberg, S J. Pearton, K. Pruessner, J. D. Budai, L. A. Boatner, M. F. Chisholm, J. S. Lee, Z. G. Khim, Y. D. Park, and R. G. Wilson, “Ferromagnetism in cobalt-implanted ZnO”, Appl. Phys. Lett. 83, 5488 (2003). 15 J. H. Kim, H. Kim, D. Kim, Y. E. Ihm, and W. K. Choo, “Magnetic properties of epitaxially grown semiconducting Zn1–xCoxO thin films by pulsed laser deposition”, J. Appl. Phys. 92, 6066 (2002). 16 J. H. Park, M. G. Kim, H. M. Jang, S. Ryu, and Y. M. Kim, “Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films”, Appl. Phys. Lett. 84, 1338 (2004). 146 Chapter Magnetic properties and its origins 147 [...]... 0. 05 0.04 ZFC 0.03 0.02 (c) FC 0.01 0.00 0 100 200 Temperature (K) Figure 5- 5 ZFC-FC curve for codoped samples measured from 10 – 400K for (a) Co 3W (Zn0.95Co0.05O) at 2000 Oe, (b) Co20W (Zn0.76Co0.24O) at 200 Oe and (c) Co 45W (Zn0.70Co0.30O) at 50 Oe 137 Chapter 5 Magnetic properties and its origins Table 5- 1 Tc of various ferromagnetic ZnO:Co samples Co content (x) Growth method Tc (K) Remarks 0. 25. .. MCD hysteresis curves of sample Co 32 W obtained at different photon energies, 756 , 4 25, 343 and 3 25 nm (1.64, 2.92, 3.61 and 3.83 eV), which was also marked in the transmission spectra shown in Fig 4- 15 by the “*” symbol, at 6 K and 300 K, respectively Figure 5- 9(c) and (d) showed the MCD hysteresis curve of its delta -doped counterpart The MCD curves, for both co -doped and delta -doped samples, were... background 141 Chapter 5 Magnetic properties and its origins 1.0 Intensity (a.u.) MCD (1.95eV,6K) 0 .5 SQUID (10K) 0.0 -0 .5 -1.0 -10 -5 0 5 10 H (T) Figure 5- 8 Hysteresis curve determined by MCD and SQUID for co -doped Co8W (Zn86Co0.14O) at 6 and 10 K respectively For the heavily co -doped sample, sample Co 32W, the MCD showed that ferromagnetic behaviour was dominant Figures 5- 9(a) and (b) showed the MCD... Chapter 5 Magnetic properties and its origins 600 MCD (mdeg) (a) 300 0 300K -300 6K -600 1.0 2.0 3.0 4.0 5. 0 Photon Energy (eV) MCD (mdeg) 700 (b) 350 0 300K - 350 6K -700 1.0 2.0 3.0 4.0 5. 0 Photon Energy (eV) Figure 5- 7 MCD spectra of co -doped (a) Co32W (Zn0.71Co0.29O) and δ -doped (b) Co98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60) at 6 and 300 K From the hysteresis curve carried out on the lightly co -doped. .. 2288 (20 05) 6 S.A Makhlouf, “Magnetic properties of Co3O4 nanoparticles”, J Magn Magn Matl 246, 184 (2002) 7 H J Kim, I C Song, J H Sim, H Kim, D Kim, Y E Ihm, and W K Choo, Growth and characterization of spinel-type magnetic semiconductor ZnCo2O4 by reactive magnetron sputtering”, Phys Stat Sol (b) 241, 155 3 (2004) 1 45 Chapter 5 Magnetic properties and its origins 8 H J Lee, S Y Jeong, C R Cho, and C... Intrinsic ferromagnetism 2 PLD > 350 Ferromagnetic but origin 8 0 x 0. 25 Reference not confirmed, secondary phase when x > 0. 25 0.08 PLD 300 from 9 higher 10 Ferromagnetism Co2+ 0. 05, 0. 15, 0. 25 PLD 280 - 300 Samples with carrier concentration are ferromagnetic 0. 35 PLD > 350 Ferromagnetism from 11 Co2+, no metallic Co or CoOx particles 0.0 35 – 0.1 15 Sputtering > 350 RKKY not applicable, Co 12 cluster... from extrinsic origins 139 Chapter 5 Magnetic properties and its origins 40 (a) MCD (mdeg) 30 20 10 0 -10 -20 1 2 3 4 5 4 5 4 5 Photon Energy (eV) MCD (mdeg) 400 200 (b) 0 -200 -400 -600 1 2 3 Photon Energy (eV) MCD (mdeg) 300 (c) 100 -100 -300 -50 0 -700 1 2 3 Photon Energy (eV) Figure 5- 6 MCD spectra of co -doped (a) Co8W (Zn0.86Co0.14O), (b) Co32W (Zn0.71Co0.29O) and δ -doped (c) Co98s ([ZnO:Al (2.38 nm)/Co(1.0... two co -doped samples, lightly doped sample Co 8W (x = 0.137) and heavily doped sample Co 32W (x = 0.289), and one delta -doped sample, Co 98s ([ZnO:Al (2.38 nm)/Co(1.0 nm)]×60) For the MCD spectra of ZnO:Co, discussions were concentrated on the absorptions at two particular wavelength regions, one at the band edge (near 3.4 eV) and the other at the d-d transition regime (near 2 eV) As shown in Fig 5- 6... Chapter 5 Magnetic properties and its origins wavelengths As the photon energy of 2.92 eV was close to the bandgap of CoO, it strongly suggested that the ferromagnetic response was due to uncompensated surface spins Co-rich ZnO:Co clusters Figure 5- 9 Hysteresis curve determined by MCD for co -doped Co32W (Zn0.71Co0.29O) measured at various wavelength at (a) 6 and (b) 300 K respectively and for δ -doped. .. ferromagnetism of extrinsic origin with an easy- axis out -of plane anisotropy 144 Chapter 5 Magnetic properties and its origins References: 1 A Dinia, G Schmerber, V Pierron-Bohnes, C Mény, P Panissod, and E Beaurepaire, “Magnetic perpendicular anisotropy in sputtered (Zn0.75Co0. 25) O dilute magnetic semiconductor”, J Magn Magn Mater 286, 37 (20 05) 2 K Rode, A Anane, R Mattana, J P Contour, O Durand, and R LeBourgeois, . Co composition at 10, 150 and 300 K. 0 0. 05 0.10 0. 15 0.20 0. 25 0.30 0. 35 0 0. 05 0.10 0. 15 0.20 0. 25 0.30 0. 35 Chapter 5 Magnetic properties and its origins 1 35 As it was discussed in. formation of other secondary phases. Chapter 5 Magnetic properties and its origins 134 Figure 5- 4 (a) In-plane M-H curves of co -doped Co3W (Zn 0. 95 Co 0. 05 O), Co 8W(Zn 0.86 Co 0.14 O), Co 15W. (c) 50 K, (e) 300 K for in-plane and out -of- plane configuration and corrected M-H curves (b), (d) and (f) respectively. Out -of- plane M-H curves for co -doped sample Co3W (Zn 0. 95 Co 0. 05 O)