CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM CHAPTER STUDIES ON THE MAGNETIC MECHANISM 7.1 Introduction For the purpose of fabrication of room temperature DMSs, Co-doped ZnO materials have been widely studies, and a considerable amount of experimental data have been already accumulated. However, many discrepancies on magnetic behaviors were involved. Even for those that were reported to be ferromagnetic, the origin of ferromagnetism is often questioned. The origin of ferromagnetism in Co-doped ZnO remains an issue of debates. Till now, some investigations of the origin of magnetism in Co-doped ZnO were reported. However, they studied the origin of ferromagnetism of Zn1-xCoxO thin films either with Co clusters [1], or precluding the Co clusters by controling processing parameters [2]. It seems that the origin of magnetic behaviors for Zn1-xCoxO thin films is still not well understood, especially when the Co concentration is low (x < 0.1). Since it is easier to understand that Co clusters may occur if more Co are incorporated, which will definitely contribute to the ferromagnetic behavior of the Zn1-xCoxO thin films. To be a candidate material to develop device in spintronics, it is essential to ensure that the magnetism does not originate from the second phases. Thus, it is significant to understand the origin of M-H hysteresis loops of Zn1-xCoxO thin films with low Co concentrations (no apparent Co clusters). In this way, it is helpful to clear National University of Singapore 121 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM the origins of ferromagnetism for Zn1-xCoxO thin films [3]. Several theories can be used to explain the magnetism in the absence of Co clusters [4]. One is that ferromagnetism itself can be understood as being carrier induced, in a similar fashion as the ferromagnetic state in the double-exchange model for manganites at intermediate doping [5]. Another is related to Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between the impurity spins. For the former, carrier density is the crucial parameter determining the magnetization behavior [6]. And, for the latter, RKKY might lead to spin glass behaviors in certain spin lattices [7]. One important criteria for DMSs to be intrinsic has been suggested to be the observation of the anomalous Hall effect (AHE) in the thin films [8]. Though the criteria was questioned when both superparamagnetism and AHE were observed to co-exist in highly reduced Co-doped Rutile TiO2-δ films [9]. However, AHE testing is still a tool to show the spin-orbital interactions in the materials. In this case, the interactions need to be analyzed further. Photo-induced phenomena in diluted magnetic semiconductors have attracted much attention due to the possible interpretations of the origin of magnetic behaviors, in terms of the exchange interaction between the photo-generated carriers and magnetic ions, if the magnetization could be manipulated by light irradiation. When studying magnetic phenomena in ferromagnetism, behaviors related to ferromagnetic order, such as anisotropy, origin of domain are generally investigated. To study a DMS system, the understanding of magnetic anisotropy is also important National University of Singapore 122 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM [10]. Ref [11] studied the influence of magnetic anisotropy to indicate the role of the hole concentration in the DMS system of (Ga,Mn)As in order to unveil the carrier induced magnetism. In the first part of this chapter, we will investigate whether magneto-crystalline anisotropy exists or not. It is known that a magnetic material is said to possess magnetic anisotropy if its internal energy depends on the direction of its spontaneous magnetization with respect to the crystallographic axies [12]. Experimental values of anisotropy fields are commonly obtained by measuring magnetic polarization curves with the field applied parallel and perpendicular to the easy magnetization direction [12,13]. It is also noted that anisotropy energy is also produced by magnetostatic energy due to magnetic free poles appearing on the outside surface or internal surfaces of an inhomogeneous magnetic material. To proceed, we fabricated the film with c-axis perpendicular and parallel to the substrate surfaces, and compared the magnetic behaviors of these two kinds of thin films, hence to determine whether magneto-crystalline anisotropy exists or not. To fabricate the Zn1-xCoxO thin films with c-axis perpendicular and parallel to the substrate surfaces, we used different sapphire substrates for matching of crystal lattice between the film and the substrate. It is well known that substrates strongly affect crystal growth behaviors of films [14]. In the second part of this chapter, we report on the relationship between the magnetism and carrier density by comparing M-H loops with different carrier density in the films. From our experimental results, we found no magneto-crystalline anisotropy in Zn1-xCoxO thin films. There is no correlation between the origin of domain and carriers National University of Singapore 123 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM in the Zn1-xCoxO thin films. Lastly, possible mechanisms for the magnetism of Zn1-xCoxO thin films were proposed based on a picture of spins in a carrier sea. 7.2 Magnetic Anisotropy Study Zn1-xCoxO with x = 0.05 precipitate-free single crystal thin films have been fabricated by a DBPLD method under the optimum experimental condition. The films were grown on (0001) sapphire (c-plane) and (11 20) sapphire substrates (r-plane) using the DBPLD method. The room temperature M-H curves were obtained by an AGM with the magnetic field applied parallel to and perpendicular to the film planes at room temperature. A Hall effect system was employed to measure the film conductivity and carrier density with a Van der Pauw configuration at room temperature. The sample was analyzed by X-ray diffraction (Philips, X’PERT-MRD) to identify different crystal planes in the film. Detailed lattice structure and possible precipitates were obtained by a HRTEM. 7.2.1 Structures of Zn1-xCoxO Thin Films Grown on c- and r- Sapphire Substrates XRD images of the Zn0.95Co0.05O thin films are shown in Fig. 7-1. The film grown on c-plane sapphire substrate is a single crystal with a wurtzite structure whose c-axis is parallel to that of the substrate [Fig. 7-1(a)]. The lattice parameters of the film was determined to be c = 0.5207 nm, which is a little larger than the reported values for ZnO [15]. Figure 7-1(b) gives the XRD pattern of the Zn1-xCoxO (x = 0.05) film grown on r-plane substrate. Diffraction peaks corresponding to (11 20) plane of Zn1-xCoxO thin film, (10 2) , (202 4) and (30 ) planes of Al2O3 were clearly National University of Singapore 124 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM observed. No peak of other planes was detected by XRD. It is also wurtzite structure with the film (11 0) plane aligned with the substrate (10 2) planes. The (11 20) peak of the Zn1-xCoxO is located at 56.57 °. Insets of Fig. 7-1 show the Co 2p3/2 XPS spectra for the films grown on c- and r-plane sapphire substrates. From the shape of the spectra and position of the peaks, there is only Co – O bonding existed in the films in both cases. From these results we referred that Co had substituted Zn-site in ZnO and Co – O bonding exists in the host lattice. Based on the results of XPS and XRD, we conclude that this material is a single compound with the replacement of Zn by Co. Figure 7-2 gives X-ray rocking curves of thin films. The curves were taken from the diffraction peaks at 34.44° for c-plane and 56.57° for r-plane substrates, respectively. The FWHM is 0.18° for c-plane and 0.37° for r-plane sapphire substrates. This indicates that the film grown on c-plane substrate has a more concentrated crystalline orientation. In our view it is due to the higher surface energy of {0001} facet of ZnO grown on sapphire substrate. National University of Singapore 125 Intensity (a.u.) CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM 30 Co2P3/2 780 770 Zn0.95Co0.05O (0004) 790 C-Al2O3 (0006) Zn0.95Co0.05O (0002) Intensity (a.u.) (a) BE (eV) 40 50 60 70 Co2P3/2 780 BE (eV) -6 ) -2 -3 (1 40 50 60 (3 l O R -A 0. 05 o C 30 0. 95 Zn 20 770 0) 790 O R-Al2O3 (2 -2 -4) R-Al2O3 (1 -1 -2) Intensity (a.u.) (b) Intensity (a.u.) 2θ (Degree) 70 80 90 2θ (Degree) Fig. 7-1 XRD images of Zn0.95Co0.05O thin films grown on (a) c-plane substrate and (b) r-plane substrate, respectively. Insets show the corresponding XPS spectra of Co 2p3/2. National University of Singapore 126 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM (a) o FWHM 0.18 Intensity (a.u.) c-plane substrate 16.4 16.6 16.8 17.0 17.2 17.4 θ (Degree) Intensity (a.u.) (b) 27.6 o FWHM 0.37 r-plane substrate 27.8 28.0 28.2 28.4 28.6 28.8 29.0 29.2 θ (Degree) Fig. 7-2 X-ray rocking curves of Zn0.95Co0.05O thin films grown on (a) c-plane substrate and (b) r-plane substrate. National University of Singapore 127 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM (a) (b) Sapphire (11 0) (0006) (0002) f (10 0) (0002) (11 0) s (10 0) f (0006) s Film (c) Film (11 0) (d) (11 0) f (0002) ( 20 ) s (10 4) s (0002 ) f ( 20 ) (10 4) Sapphire Fig. 7-3 HRTEM images for Zn0.95Co0.05O films on c-plane substrate (a) film, substrate and interface, (b) electron diffraction pattern taken on the interface; r-plane substrate (c) film, substrate and interface, (d) electron diffraction pattern taken on the interface. HRTEM images of Zn0.95Co0.05O thin film-substrate interface are shown in Fig. 7-3. In Fig. 7-3(a), the interface between the film and c-plane substrate was found to be smooth. The micrographs reveal well ordered lattice planes with few defects. No precipitate was observed. From these images, the epitaxial relationship between the film (f) and substrate (s), expressed by Exp. (5-2), agrees well with the XRD results. National University of Singapore 128 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM In Fig. 7-3(c), the micrographs for those grown on r-plane sapphire substrate also showed well ordered lattice planes with few defects. No precipitates was observed. Although the interface between the Zn0.95Co0.05O thin film and r-plane sapphire substrate was found to be clear, and the (112 0) plane of the film parallel to the (10 2) plane of the substrate, the lattice (0002) plane of the film was observed to be tilt with respect to the substrate (10 4) plane. This tilt can be observed more clearly in Fig. 7-3(d). The tilt angle ϕ1 is 6°. From the HRTEM diffractions and imaging studies, the relationship between the film (f) and substrate (s), was determined to be as follows: (11 0) f //(10 ) s , (0002) f and (10 4) s with an angle ϕ1 of 6° between the (0002) f and (10 4) s . To determine the direction of c-axis of the Zn0.95Co0.05O relative to the r-plane substrate, we can calculate the angle ϕ2 using crystallographic relationship, cos ϕ = G 202 • G10 G 202 G10 , (7-1) where G is reciprocal lattice vector, and ϕ2 the angle between G202 and G10 . Substituting the lattice parameters of film and substrate, we obtained ϕ = 84° . This verifies that the relationship ϕ1 + ϕ = 90° holds. The c-axis of the film lies in the surface plane. The tilt angle between the (0002) f and (10 4) s results in larger FWHM of the rocking curves for the Zn0.95Co0.05O thin film grown on r-plane substrate. From the lattice mismatching geometry shown in Fig. 7-3, we calculated and obtained 18% for the lattice mismatch of the Zn0.95Co0.05O thin films grown on c-plane substrate, and 5% on r-plane substrate. This leads to a large compressive strain in the Zn0.95Co0.05O thin films on the c-plane substrate. National University of Singapore 129 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM 7.2.2 Magnetic Properties of Zn1-xCoxO Thin Films Grown on c- and r- Sapphire Substrates The curves of Figure 7-4 show the M-H loops of the Zn1-xCoxO (x = 0.05) thin films grown on c- and r-plane sapphire substrates. The coercivity Hc is about 100 Oe. This result indicates in both cases, they are magnetic at room temperature. We emphasize that different M-H loops can be obtained under different experimental conditions, as shown in Fig. 7-4. From this figure, the film grown on the c-plane sapphire substrate has a remanent squareness S of 0.056 when the applied magnetic field H is perpendicular to c-axis of the film, and 0.041 when H is parallel to c-axis. For the r-plane substrate, S is about 0.107 when H is perpendicular to [1120] of the film and 0.066 in the case of H perpendicular to c-axis. S is small for both r-plane and c-plane substrates. 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Kasuga, and A. Shimizu, Growth of p-type Zinc oxide films by chemical vapor deposition, Jpn. J. Appl. Phys. 36, pp. L1453-1455, 1997. 2. H. Matsui, H. Saeki, T. Kawai, H. Tabata, and B. Mizobuchi, N doping using N2O and NO sources: From the viewpoint of ZnO, J. Appl. Phys. 95, pp. 5882-5888, 2004. 3. F. Zhuge, L. P. Zhu, Z. Z. Ye, D. W. Ma, J. G. Lu, J. Y. Huang, F. Z. Wang, and Z. G. Ji and S. B. Zhang, ZnO p-n homojunctions and ohmic contacts to Al–N-co-doped p-type ZnO, Appl. Phys. Lett. 87, pp. 092103-1~3, 2005. 4. Magnetic Circular Dichroism (MCD) spectroscopy: be available from http://www.uga.edu/cms/MCD.html National University of Singapore 185 APPENDIX 1: APPENDIX FIGURE APPENDIX APPENDIX FIGURE The energy level scheme for the lower-states of Co2+ (deriving from the free-ion ground state F ) is shown in Fig. A-1 Figure A-1. Low-energy levels of Co2+ in tetrahedral and trigaonal symmetry (not to scale) [5-12] National University of Singapore 186 [...]... 6000 2000 40 00 6000 Magnetic Field (Oe) Fig 7-10(a) Schematic diagram of the light response measurement for M-H curves under the light of a hand-held light source (b) Comparative M-H curves before and under that light of a hand-held light source National University of Singapore 142 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM 7 .4 Magnetic Mechanisms Proposal 7 .4. 1 Electron Configuration and Exchange... orbitals of the cation, and the valence band by the p orbitals of the anion The exchange interaction between the s electrons in the conduction band and the d National University of Singapore 145 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM electrons of the Co2+ is derived from the potential exchange exchange On the other hand, the exchange interaction between the p band in the valence band and the d electrons... results show that Zn1-xCoxO thin films have similar band structures to that of ZnO, in particular when x is small Here, it is reasonable to consider that EF locates in the center of the bandgap, and the relative position of Fermi level and the top of the valence band, EF − Ev for Zn1-xCoxO is that EF − Ev ≈ 1.6 eV, similar to that of ZnO [25] In Fig 5- 14( b), band structures presenting Zn 3d, O 2p hybridization... account of our experimental results, we conclude that there is no such correlation National University of Singapore 141 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM (a) Magnet for AGM Sample Filter Light source (b) 0.000 04 0.000 04 Magnetic moment (emu) Before light 0.00002 0.00002 0.00000 0.00000 -0.00002 -0.00002 Light irradiation -0.000 04 -0.00006 -0.000 04 -6000 -40 00 -2000 -6000 -40 00 -2000 0 0 2000 40 00... interaction is National University of Singapore 146 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM expressed by Kanamori parameters u, u′ , j, and j′ (3) The two parameters, ∆ and U are related through the relation ∆ ≡ ε d − ε p + nU , where ε d and ε p are the bare energies of the 3d transition metal and the 2p ligand orbitals, respectively (4) ∆eff and Ueff denote the charge-transfer and Coulomb interaction... Thin Films On account of our experimental results and understanding, the Zn1-xCoxO system can be regarded as the case of Co2+ (d7) in a tetrahedral field [ 24] From our absorption results, the many electron ground states are singlet A2 We observed the absorption band to the d-d transition of the Co2+ impurity in Zn1−xCoxO, indicating the band splitting, i.e., the trigonal splitting of 4T1 ( P) Our experimental... the reflection of the field dependence of M, though the magnetic field range did not reach the saturation value due to the limits of the experimental conditions National University of Singapore 136 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM Applying the Hall effect measurements results, the evaluation of ∆ρ / ρ 0 = ( ρ B − ρ 0 ) / ρ 0 (7-3) as a function of B for a given direction of I and B with respect... eg (σ *) π O 2 p6 σ Fig 7-12 A schematic energy diagram of the p - d band of interest in Zn1-xCoxO magnetic system with O 2p states by the Co 3d states (a), (b) and (c) show the atomic unpolarized level, the exchange split atomic levels and the final interacting states, respectively National University of Singapore 144 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM In summary, under our experimental conditions,... -1.0 -8000 -6000 -40 00 -2000 0 2000 40 00 6000 8000 Magnetic Field (Oe) Fig 7 -4 Hystersis loops for Zn0.95Co0.05O films grown on (a) c-plane and (b) r-plane sapphire substrate normalized to the saturation magnetization Ms The data were taken at room temperature with the external field both in plane and out of plane of the films Insets show enlarged images of the loops National University of Singapore 131... the magnetic moments of the transition-metal impurities are aligned in a strong magnetic field, the valence and conduction bands are split through the exchange interaction [27] Kinetic exchange in Zn1-xCoxO is considered as a band electron interacting with a single Co2+ having 7d electrons We cite the results of spin-dependent interaction resulting from hybridization of the Bloch function of the band-edge . relationship, 41 1 042 20 41 1 042 20 2 cos GG GG • = ϕ , (7-1) where G is reciprocal lattice vector, and ϕ 2 the angle between 42 20 G and 41 10 G . Substituting the lattice parameters of film and substrate,. signals and they are not pronounced. National University of Singapore 135 CHAPTER 7: STUDIES ON THE MAGNETIC MECHANISM -40 0 -200 0 200 40 0 -0. 04 -0.02 0.00 0.02 0. 04 Hall Resistivity (Ω cm) Magnetic. origin of domain are generally investigated. To study a DMS system, the understanding of magnetic anisotropy is also important National University of Singapore 122 CHAPTER 7: STUDIES ON THE MAGNETIC