Chapter Introduction CHAPTER INTRODUCTION 1.1 Spintronics In the last several decades, the number of transistors per unit area of an integrated circuit (IC) has doubled approximately every 18 months, following a trend known as Moore’s law.1 If this trend continues, the Si-based technology will soon face some physical limits which demand for alternative technologies. To this end, intensive research work are being carried out, in both academia and industries, to look for alternative technologies. Among them, spintronics, also known as spin electronics, has the potential to create devices with superior performances due to the utilization of both the charge and spin degree of freedoms of electrons.2 Electrons possess two important properties: charge and spin. The ability to generate, control and detect the motion of charges in either the free space (vacuum tubes) or in solid state (Si-based electronic devices) forms the basis of modern electronics. The controlled transition/recombination of electrons between different energy levels leads to the absorption / generation of photons in the different range of the electromagnetic spectrum, which forms the basis of optoelectronics (light-emitting diodes and laser diodes). Compared to the charge and the photon, it is rather difficult to generate, control and detect the spin of an electron. In fact, most of the spin-related materials and devices to date still rely primarily on the spontaneous ordering of spins, in the form of different types of magnetic materials. In information processing and storage, charge-based devices are dominant. In information transmission, storage, and display photonic devices are used. In contrast to the wide range of applications of electronic and optoelectronic devices, the applications of spin-based materials and Chapter Introduction devices, or spintronics, are still limited to information storage (e.g., hard disk drives).3 From a long term perspective, spintronics has the potential of creating devices with less or no moving charges, which may lead to devices and systems with faster response and low power consumption.4 It also promises a greater integration between the logic and storage devices. The excellent temporal and spatial coherence of spins will also make spintronic devices more suitable than their charge-based equivalents for quantum information processing.5 As illustrated in Fig.1-1, spintronics can be roughly divided into the following categories: (i) metal-based, (ii) dilute magnetic semiconductor (DMS)-based, (iii) pure semiconductor-based systems, (iv) hybrid devices and (v) other technologies. Spintronics MetalBased Dilute Magnetic Semiconductors II-VI III-V ZnSe:Mn ZnTe:Cr CdTe:Mn CdSe:Mn GaAs:Mn GaN:Mn GaP:Mn InAs:Mn Hybrid devices Semiconductors IV-VI IV PbTe:Mn PbSe:Mn PbTe:Gd Others Oxide-Based TiO2:Co Si:Mn Pb Ge:Mn ZnO:Co ZnO:Mn Figure 1-1 Schematic illustration of different areas of spintronics. The metal-based spintronics has its origin in the giant magnetoresistance (GMR) phenomenon, which has already been evolved into useful devices being used in hard disk drives. Discovered by Fert6 and Grünberg7, a typical GMR structure consists of two ferromagnetic (FM) layers separated by a non-magnetic (NM) spacer (or the repetition Chapter Introduction of this basic unit to form multilayers), as shown in Fig. 1-2. The resistivity of the trilayer is strongly influenced by the relative orientation of the magnetization between the two magnetic layers, due to dominantly spin-dependent interfacial scattering.8 The resistance of the trilayer is low when the magnetizations of the two layers are aligned in parallel and is high when they are in anti-parallel configuration. The GMR is then calculated as the ratio of resistance difference between the anti-parallel and parallel configurations to the resistance in parallel configuration. Although the GMR is high in a strongly coupled multilayer structure, it could not be used as it was in hard disk drives because the external magnetic field that is required to switch the magnetizations is too high. This has prompted IBM to invent a more practical structure for read sensors which is called the spin-valve.9 The state-of-the-art spin-valve consists of a dozen of thin layers; the heart of which is a trilayer structure consisting of two ferromagnetic layers separated by a nonmagnetic spacer, which is usually copper. The primary difference between the original GMR structure and spin-valve is that, in the latter, the thickness of the spacer is chosen such that the coupling between the two ferromagnetic layers is minimized. This makes it possible to use the spin-valve to detect rather weak magnetic field. The signal detection principle is the same as that of the GMR structure, i.e., the resistance is high when the magnetizations of the two layers are in opposite directions and low when they are in the same direction. To have a linear response from the sensor, the angle between the two magnetizations is normally set at 90o at zero-field with one of them “pinned” at a direction perpendicular to the media surface through exchange-coupling with an antiferromagnet and the other free to rotate in response to the fringe field of magnetic transitions recorded on the magnetic media, also known as the free layer.10 Chapter Introduction FM1 NM FM2 FM1 NM FM2 Parallel configuration Anti-Parallel configuration (Low resistance) (High resistance) Figure 1-2 Schematic illustration of a GMR trilayer structure, with two ferromagnetic layers separated by a non-magnetic spacer. The resistance is low when the magnetizations are parallel and high when they are in anti-parallel configuration. Ferromagnetic layers Spacer Low Resistance High Resistance Bit lines Word lines Figure 1-3 Schematic diagram of a MRAM cell. There are two different forms of spin-valve sensors, depending on whether the current flows in the plane of the stack of layers or perpendicular to them. The former is called a current-in-plane, or CIP, spin-valve sensor and the latter a currentperpendicular-to-plane, or CPP, sensor. So far, CIP is dominant, but CPP is expected to play an important role in future terabit recording systems. An alternative of the CPP spin-valve is the magnetic tunnel junction,11,12 or MTJ, in which the current also flows Chapter Introduction perpendicular to the plane. The major difference between the CPP spin-valve and MTJ is that the latter is composed of two ferromagnetic layers separated by an insulator instead of a metal. Therefore, the electrical conduction in MTJ is based on quantum mechanical tunneling. The recent rapid improvement of MTJ devices using crystalline MgO barrier has paved the way for commercialization of MTJs in both hard disk drives and magnetic random access memories (MRAMs) (see Fig. 1-3).13 Although the metal-based spintronics has achieved unparalleled success in both fundamental research and practical applications, the lack of capability in charge modulation greatly limits its applications to information processing. Therefore, one of the hottest issues in spintronics is how to create a stable source of spin-polarized carriers in semiconductors, which allows not only the modulation of charges but also manipulation of spins.14-16 So far, several approaches have been proposed to solve this problem, 17 - 20 including the injection of spins from a ferromagnetic metal into a semiconductor via either a Schottky 21,22 or tunneling barrier, 23-26 generation of spin polarized current using either a ferromagnetic semiconductor 27 or a non-magnetic semiconductors based on spin-orbit interaction,28 spin-dependent resonant tunneling,29,30 Zeeman effect31-33 and optical orientation.34 In addition to the generation of spins, some of the aforementioned effects such as spin-orbit interaction also provide a convenient way to control the spins using an electrical field.35 In spite of the significant progress made recently in pure semiconductor or hybrid spintronic structures, the true era of spintronics will probably only be materialized after room temperature magnetic semiconductors become available. Chapter Introduction 1.2 Diluted magnetic semiconductors 1.2.1 Different types of DMSs Magnetism and semiconducting properties are known to coexist in some materials, such as europium chalcogenides 36 , 37 and ferrimagnetic or ferromagnetic semiconducting spinels.38 These materials have been extensively studied since 1960’s, because of their peculiar properties resulting from the exchange interaction between itinerant electrons and localized magnetic spins. These interactions lead to a rich variety of optical and transport phenomena, which are strongly affected by the magnetic field and temperature. However, the low Curie temperature, Tc, and difficulties in material preparation make this family of compounds less attractive from the application point of view. In addition to these “concentrated” magnetic semiconductors, there were also intensive researches on diluted magnetic semiconductors which are obtained by doping them with a few percent of magnetic ions.39 Most initial work had been centered on IIVI semiconductors (A,X)B where A = Zn, Cd, Hg, X = Fe, Mn, Co, Ni, Cr and B = S, Se, Te, and in most cases the valence of group II cations is identical to that of most magnetic transition metals. Although these materials are relatively easy to prepare, most of them are random antiferromagnets or spin-glasses, which makes the II–VI DMS unattractive for electronic applications. Nevertheless, the presence of sp-d exchange interactions between d electrons of magnetic ions and electrons and holes of the host semiconductor does make some of these materials very useful for magneto-optical applications. 40 The ease of bandgap engineering as well as the preparation of heterostructures in these materials also makes them excellent candidates for studying spin-charge interaction and the corresponding dynamics in a variety of quantum structures. Chapter Introduction 1.2.2 (Ga, Mn)As: A Model DMS System Among all types of DMS materials studied so far, (Ga,Mn)As emerges as one of the best understood model ferromagnet which exhibits not only the properties of a metallic ferromagnet but also new functions such as electrical gating and strain modulation of the magnetic properties. 41 The early work on III-V magnetic semiconductors faced the difficulties of low solubility of transition metals in III-V semiconductors, which made it difficult to achieve uniform doping of magnetic impurities in these materials. The breakthrough came when attempts were made to grow (In, Mn)As42,43 and (Ga, Mn)As44 using non-equilibrium molecular beam epitaxy at low temperature, which allows the incorporation of Mn up to a few percent, with a substantially suppressed formation of secondary phase. This has led to the discovery of hole-induced ferromagnetic ordering in Mn-doped III-V semiconductors. In contrast to Mn-doped II-VI DMS, where the Mn only contributes spins, when Mn is substituted for gallium in GaAs or indium in InAs, it acts as both an acceptor, which provides holes and also localized spins associated with d electrons of Mn2+ ions.38, 41 The former mediates a ferromagnetic interaction among the localized spins of the opened d-shells of the Mn atoms. The Tc of (Ga, Mn)As is found to be dependent strongly on the hole concentration, and the collective ferromagnetic behavior of the local spins requires a minimum doping concentration of 2%. The Tc was found to be almost a linear function of the Mn composition up to about 5% beyond which, however, a further increase of Mn concentration will cause a decreases of Tc.45 Annealing at low temperature was found to greatly enhance the hole density and consequently the Tc and saturation magnetization (MS).46-51 A Tc up to 173 K has been obtained in annealed samples for a Mn concentration of 6.4%. 52 It was also shown recently that the Tc can be further increased to about 250 K in heterostructures consisting of Mn δ-doped GaAs and p-type Chapter Introduction AlGaAs layers by varying the growth sequence of the structures followed by lowtemperature annealing.53 It is still not certain when and whether the Tc can be pushed up to above room temperature in the (Ga, Mn)As system.54 In order to make DMS a real technology, however, one needs to find DMS materials with a Tc higher than room temperature.55-60 Theoretically, the mean-field Zener model predicts that DMSs with a TC above room temperature are obtainable if the combination of host material, carrier concentration, and magnetic impurity (type and density) is right.61 In particular, if one could introduce 5% of Mn and 3.5×1020 cm-3 of holes into wide-gap semiconductors, such as GaN and ZnO, these materials should be ferromagnetic at room temperature. In addition, the first-principles calculations also predict a rather stable ferromagnetism for these materials. 62 , 63 Stimulated by these predications, intensive research has been carried out to explore high TC diluted magnetic semiconductors, particularly oxide and nitride based DMSs.56-60 1.2.3 ZnO-based DMS Systems Among all different types of materials that have been investigated, oxide-based DMS systems have attracted special attention, in particular TiO2 and ZnO based materials because of their attractive properties and a wide range of applications. In this proposal, the focus is on ZnO-based DMS. As mentioned above, theoretical studies predicted that V, Cr, Fe, Co, or Ni doped ZnO is a half-metallic double-exchange ferromagnet, whereas Ti or Cu doped ZnO remains paramagnetic; Mn doped ZnO is an antiferromagnetic insulator which changes to a ferromagnet by additional doping of holes, and ZnO doped with 5% Mn and 3.5×1020 cm-3 hole concentration has a Tc above room temperature.61-63 It was also shown that electron doping stabilizes the ferromagnetic ordering of Fe, Co, or Ni doped ZnO.63 The first-principles spin-density Chapter Introduction functional calculations by Lee and Chang predicted that heavy electron doping and high Co concentration are required for obtaining ferromagnetism in cobalt doped zinc oxide (ZnO:Co). 64 On the other hand, Spaldin argued theoretically that only hole doping promotes ferromagnetism in both ZnO:Co and ZnO:Mn.65 Till recently, Sluiter et al. predicted that both hole doping and electron doping promote ferromagnetic ordering in ZnO:Co and ZnO:Mn.66 Hydrogen-mediated spin-spin interaction was also predicted to be able to induce high temperature ferromagnetism in ZnO:Co.72 Similar to theoretical work, experimental investigations to date have also produced widely diverging results, ranging from non-ferromagnetic to ferromagnetic with extrinsic origins to intrinsic ferromagnetism with various TC.56,57,59 Ferromagnetism up to room temperature was first observed on 15 % Co-doped ZnO thin film. 67 The average magnetic moment per Co atom was found to be µB. It was suggested that there are three possible mechanisms responsible for the ferromagnetism: (1) carrier-mediated ferromagnetic coupling between Co atoms; (2) weak magnetism of CoO phase and (3) Co clusters. The mechanisms (2) and (3) were excluded from the strength of the magnetic properties, Co composition dependence of lattice constant and absence of Co cluster peaks in x-ray diffraction (XRD) peaks. Since this first report, there have been over 200 journal papers published on ZnO-based DMSs. If the publications on ZnO-based DMS in the last years64,68-165 has been divided into four categories, based on their magnetic character: homogeneous DMS, extrinsic ferromagnet, paramagnet, and others, the percentages of papers in the four categories were 55%, 9%, 12% and 24%, respectively. This material still requires much work before a consensus can be reached on its nature of magnetism. The large disparity in experimental results is partially caused by the fact that the properties of ZnO:Co are very sensitive to the structure and chemistry at the nanoscale regime, which Chapter Introduction strongly depends on processing conditions. It is also caused by the lack of systematic studies, particularly on the same sample. Most of the characterization techniques only probe a certain portion or aspect of the sample in either the spatial or energy domain. For samples with a nanoscale inhomogeneity, the results are meaningful only when the relationships between the structures and properties at the nanometer scale are well understood. No. of publications 100 80 60 40 20 M-T Susc - T M-H dI/dV R-T DLTS AHE MR Hall EELS MCD Optical EPM XAS EPR Mossbauer NMR RBS XPS EDX Raman Neutron TEM XRD SEM Figure 1-4 Number of papers versus characterization techniques for ZnO-based DMS. (a) Paramagnetic (b) RKKY (c) Magnetic polaron (d) Cluster (e) Secondary phase Figure 1-5 Possible magnetic phases of ZnO-based DMSs. Figure 1-4 summarizes the techniques that have been used to characterize ZnObased DMSs, which can be divided into five main groups: structural, chemical, optical, electrical and magnetic characterization techniques. Similar statistics has been obtained for TiO2-based DMS system. In order to understand how effective these techniques are in characterizing the DMS, the different types of possible magnetism phases in magnetically doped oxides is schematically shown in Figure 1-5. At very low doping level, there is no or very little interaction among the magnetic dopants; therefore, the system can be considered as either a paramagnet or weak magnet. In the latter case, a 10 Chapter Introduction 33 R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, and L. W. Molenkamp, “Injection and detection of a spinpolarized current in a light- emitting diode”, Nature 402, 787 (1999). 34 F. Meier and B. P. Zakharchenya (Eds.), Optical Orientation, (North-Holland, New York, 1984). 35 E. I. Rashba and Al. L. Efros, “Orbital mechanisms of electron-spin manipulation by an electric field”, Phys. Rev. Lett. 91, 126405 (2003). 36 T. Kasuya and A. Yanase, “Anomalous transport phenomena in Eu-chalcogenide alloys”, Rev. Mod. Phys. 40, 684 (1968). 37 S. Methfessel, F. Holtzberg, and T. McGuire, “Optical absorption and ferromagnetic exchange in Eu chalcogenides”, IEEE Trans. Magn. 2, 305 (1966). 38 T. Dietl, “Ferromagnetic semiconductors”, Semicond. Sci. Technol. 17, 377 (2002). 39 J. K. Furdyna, “Diluted magnetic semiconductors”, J. Appl. Phys. 64, R29 (1988). 40 A. E. Turner, R. L. Gunshor, and S. Datta, “New class of materials for optical isolators”, Appl. Opt. 22, 3152 (1983). 41 H. Ohno, “Making nonmagnetic semiconductors ferromagnetic, Science 281, 951 (1998). 42 H. Munekata, H. Ohno, S. Von Molnar, A. Segmüller, L. L. Chang, and L. Esaki, “Diluted magnetic III-V semiconductors”, Phys. Rev. Lett. 63, 1849 (1989). 43 H. Ohno, H. Munekata, T. Penney, S. von Molnár, and L. L. Chang, “Magnetotransport properties of p-type (In,Mn)As diluted magnetic III-V semiconductors”, Phys. Rev. Lett. 68, 2664 (1992). 44 H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y. Iye, “(Ga,Mn)As: A new diluted magnetic semiconductor based on GaAs”, Appl. Phys. Lett. 69, 363 (1996). 20 Chapter Introduction 45 F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, “Transport properties and origin of ferromagnetism in (Ga,Mn)As”, Phys. Rev. B 57, R2037 (1998). 46 T. Hayashi, Y. Hashimoto, S. Katsumoto, and Y. Iye, “Effect of low-temperature annealing on transport and magnetism of diluted magnetic semiconductor (Ga, Mn)As”, Appl. Phys. Lett. 78, 1691 (2001). 47 S. J. Potashnik, K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, and P. Schiffer, “Effects of annealing time on defect-controlled ferromagnetism in Ga1–xMnxAs”, Appl. Phys. Lett. 79, 1495 (2001). 48 K. W. Edmonds, K. Y. Wang, R. P. Campion, A. C. Neumann, N. R. S. Farley, B. L. Gallagher, and C. T. Foxon, “High-Curie-temperature Ga1–xMnxAs obtained by resistance-monitored annealing:, Appl. Phys. Lett. 81, 4991 (2002). 49 K. C. Ku, S. J. Potashnik, R. F. Wang, S. H. Chun, P. Schiffer, N. Samarth, M. J. Seong, A. Mascarenhas, E. Johnston-Halperin, R. C. Myers, A. C. Gossard, and D. D. Awschalom, “Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers”, Appl. Phys. Lett. 82, 2302 (2003). 50 K. W. Edmonds, P. Bogusawski, K. Y. Wang, R. P. Campion, S. N. Novikov, N. R. S. Farley, B. L. Gallagher, C. T. Foxon, M. Sawicki, T. Dietl, M. B. Nardelli, and J. Bernholc, “Mn interstitial diffusion in (Ga,Mn)As”, Phys. Rev. Lett. 92, 037201 (2004). 51 D. Chiba, K. Takamura, F. Matsukura, and H. Ohno, “Effect of low-temperature annealing on (Ga,Mn)As trilayer structures”, Appl. Phys. Lett. 82, 3020 (2003). 52 K. Y. Wang, R. P. Campion, K. W. Edmonds, M. Sawicki, T. Dietl, C. T. Foxon, and B. L. Gallagher, “Magnetism in (Ga,Mn)As thin films with TC Up To 173K”, AIP Conf. Proc. 772, 333 (2005). 21 Chapter Introduction 53 A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, “High temperature ferromagnetism in GaAs-based heterostructures with Mn δ doping”, Phys. Rev. Lett. 95, 017201 (2005). 54 A. H. MacDonald, P. Schiffer, and N. Samarth, “Ferromagnetic semiconductors: moving beyond (Ga,Mn)As”, Nat. Mater. 4, 195 (2005). 55 H. Saito, V. Zayets, S. Yamagata, and K. Ando, “Room-temperature ferromagnetism in a II−VI diluted magnetic semiconductor Zn1-xCrxTe”, Phys. Rev. Lett. 90, 207202 (2003). 56 S. J. Pearton, W. H. Heo, M. Ivill, D. P. Norton, and T. Steiner, “Dilute magnetic semiconducting oxides”, Semicond. Sci. Technol. 19, R59 (2004). 57 T. Fukumura, H. Toyosaki, and Y. Yamada, “Magnetic oxide semiconductors”, Semicond. Sci. Technol. 20, S103 (2005). 58 S. J. Pearton, C. R. Abernathy, G. T. Thaler, R. M. Frazier, D. P. Norton, F. Ren, Y. D. Park, J. M. Zavada, I. A. Buyanova, W. M. Chen, and A. F. Hebard, “Wide bandgap GaN-based semiconductors for spintronics”, J. Phys.: Condens. Matter. 16, R209 (2004). 59 R. Janisch, P. Gopal, and N. A Spaldin, “Transition metal-doped TiO2 and ZnO— present status of the field”, J. Phys.: Condens. Matter 17, R657 (2005). 60 Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Y. Koshihara, and H. Koinuma, “Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide”, Science 291, 854 (2001). 61 T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, “Zener model description of ferromagnetism in zinc-blende magnetic semiconductors”, Science 287, 1019 (2000). 62 K. Sato and H. K. Yoshida, “Material design for transparent ferromagnets with ZnO- based magnetic semiconductors”, Jpn. J. Appl. Phys. 39, L555, (2000). 22 Chapter Introduction 63 K. Sato and H. K. Yoshida, “Stabilization of ferromagnetic states by electron doping in Fe-, Co- or Ni-Doped ZnO”, Jpn. J. Appl. Phys. 40, L334 (2001). 64 E. C. Lee and K. J. Chang, “Ferromagnetic versus antiferromagnetic interaction in Co-doped ZnO”, Phys. Rev. B 69, 085205 (2004). 65 N. A. Spaldin, “Search for ferromagnetism in transition-metal-doped piezoelectric ZnO”, Phys. Rev. B 69, 125201 (2004). 66 M. H. F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A. R. Raju, C. Rout, and U. V. Waghmare, “First principles based design and experimental evidence for a ZnO-based ferromagnet at room temperature”, Phys. Rev. Lett. 94, 187204 (2005). 67 K. Ueda, H. Tabata, and T. Kawai, “Magnetic and electric properties of transition- metal-doped ZnO films”, Appl. Phys. Lett. 79, 988 (2001). 68 M. Bouloudenine, N. Viart, S. Colis, J. Kortus, and A. Dinia, “Antiferromagnetism in bulk Zn1–xCoxO magnetic semiconductors prepared by the coprecipitation technique”, Appl. Phys. Lett. 87, 052501 (2005). 69 J. Antony, S. Pendyala, A. Sharma, X. B. Chen, J. Morrison, L. Bergman, and Y. Qiang, “Room temperature ferromagnetic and ultraviolet optical properties of Co-doped ZnO nanocluster films”, J. Appl. Phys. 97, 10D307 (2005). 70 B. Martínez, F. Sandiumenge, Ll. Balcells, J. Fontcuberta, F. Sibieude, and C. Monty, “Magnetic properties of Co-doped ZnO nanoparticles prepared by vaporizationcondensation in a solar reactor”, J. Appl. Phys. 97, 10D311 (2005). 71 A. Dinia, G. Schmerber, C. Mény, V. Pierron-Bohnes, and E. Beaurepaire, “Room- temperature ferromagnetism in Zn1–xCoxO magnetic semiconductors prepared by sputtering”, J. Appl. Phys. 97, 123908 (2005). 72 C. H. Park and D. J. Chadi, “Hydrogen-mediated spin-spin interaction in ZnCoO”, Phys. Rev. Lett. 94, 127204 (2005). 23 Chapter Introduction 73 K. R. Kittilstved, N. S. Norberg, and D. R. Gamelin, “Chemical manipulation of high- TC ferromagnetism in ZnO diluted magnetic semiconductors”, Phys. Rev. Lett. 94, 147209 (2005). 74 B. Martínez, F. Sandiumenge, Ll. Balcells, J. Arbiol, F. Sibieude, and C. Monty, “Role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles”, Appl. Phys. Lett. 86, 103113 (2005). 75 J. S. Hong and R. Q. Wu, “Magnetic ordering and x-ray magnetic circular dichroism of Co doped ZnO”, J. Appl. Phys. 97, 063911 (2005). 76 G. Lawes, A. S. Risbud, A. P. Ramirez, and Ram Seshadri, “Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO”, Phys. Rev. B 71, 045201 (2005). 77 W. Chen, L. F. Zhao, Y. Q. Wang, J. H. Miao, S. Liu, Z. C. Xia, and S. L. Yuan, “Effects of temperature and atmosphere on the magnetism properties of Mn-doped ZnO”, Appl. Phys. Lett. 87, 042507 (2005). 78 J. Zhang, R. Skomski, and D. J. Sellmyer, “Sample preparation and annealing effects on the ferromagnetism in Mn-doped ZnO”, J. Appl. Phys. 97, 10D303 (2005). 79 M. A. García, M. L. Ruiz-González, A. Quesada, J. L. Costa-Krämer, J. F. Fernández, S. J. Khatib, A. Wennberg, A. C. Caballero, M. S. Martín-González, M. Villegas, F. Briones, J. M. González-Calbet, and A. Hernando, “Interface double-exchange ferromagnetism in the Mn-Zn-O system: New class of biphase magnetism”, Phys. Rev. Lett. 94, 217206 (2005). 80 A. K. Pradhan, Kai Zhang, S. Mohanty, J. B. Dadson, D. Hunter, Jun Zhang, D. J. Sellmyer, U. N. Roy, Y. Cui, A. Burger, S. Mathews, B. Joseph, B. R. Sekhar, and B. K. Roul, “High-temperature ferromagnetism in pulsed-laser deposited epitaxial (Zn,Mn)O thin films: Effects of substrate temperature”, Appl. Phys. Lett. 86, 152511 (2005). 24 Chapter Introduction 81 J. Luo, J. K. Liang, Q. L. Liu, F. S. Liu, Y. Zhang, B. J. Sun, and G. H. Rao, “Structure and magnetic properties of Mn-doped ZnO nanoparticles”, J. Appl. Phys. 97, 086106 (2005). 82 M. Ivill, S. J. Pearton, D. P. Norton, J. Kelly, and A. F. Hebard, “Magnetization dependence on electron density in epitaxial ZnO thin films codoped with Mn and Sn”, J. Appl. Phys. 97, 053904 (2005). 83 N. H. Hong, V. BrizŽ, and J. Sakai, “Mn-doped ZnO and (Mn, Cu)-doped ZnO thin films: Does the Cu doping indeed play a key role in tuning the ferromagnetism?”, Appl. Phys. Lett. 86, 082505 (2005). 84 B. K. Roberts, A. B. Pakhomov, V. S. Shutthanandan, and K. M. Krishnan, “Ferromagnetic Cr-doped ZnO for spin electronics via magnetron sputtering”, J. Appl. Phys. 97, 10D310 (2005). 85 N. H. Hong, J. Sakai, and A. Hassini, “Magnetic properties of V-doped ZnO thin films”, J. Appl. Phys. 97, 10D312 (2005). 86 L. S. Dorneles, D. O’Mahony, C. B. Fitzgerald, F. McGee, M. Venkatesan, I. Stanca, J. G. Lunney, and J. M. D. Coey, “Structural and compositional analysis of transitionmetal-doped ZnO and GaN PLD thin films”, Appl. Surf. Sci. 248, 406 (2005). 87 C. B. Fitzgerald, M. Venkatesan, J. G. Lunney, L. S. Dorneles, and J. M. D. Coey, “Cobalt-doped ZnO – a room temperature dilute magnetic semiconductor”, Appl. Surf. Sci. 247, 493 (2005). 88 W. Chen, L. F. Zhao, Y. Q. Wang, J. H. Miao, S. Liu, Z. C. Xia, and S. L. Yuan, “Magnetism in Mn-doped ZnO bulk samples”, Solid State Comm. 134, 827 (2005). 89 Z. G. Yin, N. F. Chen, F. Yang, S. L. Song, C. L. Chai, J. Zhong, H. J. Qian, and K. Ibrahim, “Structural, magnetic properties and photoemission study of Ni-doped ZnO”, Solid State Comm. 135, 430 (2005). 25 Chapter Introduction 90 J. H. Shim, T. Hwang, S. Lee, J. H. Park, S. J. Han, and Y. H. Jeong, “Origin of ferromagnetism in Fe- and Cu- codoped ZnO”, Appl. Phys. Lett. 86, 082503 (2005). 91 N. H. Hong, J. Sakai, N. T. Huong, N. Poirot, and A. Ruyter, “Role of defects in tuning ferromagnetism in diluted magnetic oxide thin films”, Phys. Rev. B 72, 045336 (2005). 92 D. Ferrand, S. Marcet, W. Pacuski, E. Gheeraert, P. Kossacki, J. A. Gaj, J. Cibert, C. Deparis, H. Mariette, and C. Morhain, “Spin carrier exchange interactions in (Ga,Mn)N and (Zn,Co)O wide band gap diluted magnetic semiconductor epilayers”, J. of Superconduct. 18, 15 (2005). 93 Y. Z. Peng, T. Liew, W. D. Song, C. W. An, K. L. Teo, and T. C. Chong, “Structural and optical properties of Co-doped ZnO thin films”, J. of Superconduct. 18, 97 (2005). 94 C. N. R. Rao and F. L. Deepak, “Absence of ferromagnetism in Mn- and Co-doped ZnO”, J. Mater. Chem. 15, 573 (2005). 95 R. C. Budhani, P. Pant, R. K. Rakshit, K. Senapati, S. Mandal, N. K. Pandey, and J. Kumar, “Magnetotransport in epitaxial films of the degenerate semiconductor Zn1−xCoxO”, J. Phys.: Condens. Matter 17, 75 (2005). 96 M. Diaconua, H. Schmidta, H. Hochmutha, M. Lorenza, G. Benndorfa, J. Lenznera, D. Spemanna, A. Setzera, K. W. Nielsenb, P. Esquinazia, and M. Grundmanna, “UV optical properties of ferromagnetic Mn-doped ZnO thin films grown by PLD”, Thin Solid Films 486, 117 (2005). 97 D. P. Joseph, G. S. Kumar, and C. Venkateswaran, “Structural, magnetic and optical studies of Zn0.95Mn0.05O DMS”, Mater. Lett. 59, 2720 (2005). 98 E. Chikoidze, H. J. Von Bardeleben, Y. Dumont, P. Galtier, and J. L. Cantin, “Magnetic interactions in Zn1− xMnxO studied by electron paramagnetic resonance spectroscopy”, J. Appl. Phys. 97, 10D316 (2005). 26 Chapter Introduction 99 E. Chikoidze, Y. Dumont, F. Jomard, D. Ballutaud, P. Galtier, O. Gorochov, and D. Ferrand, “Semiconducting and magnetic properties of Zn1− xMnxO films grown by metalorganic chemical vapor deposition”, J. Appl. Phys. 97, 10D327 (2005). 100 C. J. Cong, L. Liao, J. C. Li, L. X. Fan, and K. L. Zhang, “Synthesis, structure and ferromagnetic properties of Mn-doped ZnO nanoparticles”, Nanotechnology 16, 981 (2005). 101 K. W. Nielsen, J. B. Philipp, M. Opel, A. Erb, J. Simon,G L. Alff, and R. Gross, “Ferromagnetism in Mn-doped ZnO due to impurity bands”, Superlattices and Microstructures 37, 327 (2005). 102 Y. Q. Chang, X. Y. Xu, X. H . Luo, Y. Long, and R. C. Ye, “Magnetic properties of diluted magnetic semiconductor Zn1−xMnxO nanowires”, Chin. Phys. Lett. 22, 991 (2005). 103 S. Kolesnik, B. Dabrowski, and J. Mais, “Structural and magnetic properties of transition metal substituted ZnO”, J. Appl. Phys. 95, 2582 (2004). 104 Z. G. Yin, N. F. Chen, C. L. Chai, and F. Yang, “Structural and magnetic properties of insulating Zn1–xCoxO thin films”, J. Appl. Phys. 96, 5093 (2004). 105 T. Shinagawa, M. Izaki, Y. Matsumura, K. Murase, and Y. Awakura, “Transparent ferromagnetic semiconductor Fe-Zn-O heterogranular films”, Electrochemical and Solid-State Lett. 7, G235 (2004). 106 C. J. Kao, Y. W. Kwon, Y. W. Heo, D. P. Norton, S. J. Pearton, F. Ren, and G. C. Chi, “Comparison of ZnO metal–oxide–semiconductor field effect transistor and metal– semiconductor field effect transistor structures grown on sapphire by pulsed laser deposition”, J. Vac. Sci. Technol. B 23, 1024 (2005). 107 J.C. Ronfard-Haret, “Influence of the sintering temperature on the electrical and luminescent properties of Mn-doped ZnO”, Solid State Ionics 167, 355 (2004). 27 Chapter Introduction 108 S. J. Lee, H. S. Lee, D. Y. Kim, and T. W. Kim, “Microstructural, optical, and magnetic properties of (Zn1-xMnx)O thin films grown on (0001) Al2O3 substrates”, J. Crystal Growth 276, 121 (2004). 109 J. C. A. Huang, H. S. Hsu, Y. M. Hu, C. H. Lee, Y. H. Huang, and M. Z. Lin, “Origin of ferromagnetism in ZnO/CoFe multilayers: Diluted magnetic semiconductor or clustering effect?”, Appl. Phys. Lett . 85, 3815 (2004). 110 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). 111 J. J. Wu, S. C. Liu, and M. H. Yang, “Room-temperature ferromagnetism in well- aligned Zn1–xCoxO nanorods”, Appl. Phys. Lett. 85, 1027 (2004). 112 H. T. Lin, T. S. Chin, J. C. Shih, S. H. Lin, T. M. Hong, R. T. Huang, F. R. Chen, and J. J. Kai, “Enhancement of ferromagnetic properties in Zn1–xCoxO by additional Cu doping”, Appl. Phys. Lett. 85, 621 (2004). 113 S. Ramachandran, A. Tiwari, and J. Narayan, “Zn0.9Co0.1O-based diluted magnetic semiconducting thin films”, Appl. Phys. Lett. 84, 5255 (2004). 114 L. Yan, C. K. Ong, and X. S. Rao, “Magnetic order in Co-doped and (Mn, Co) codoped ZnO thin films by pulsed laser deposition”, J. Appl. Phys. 96, 508 (2004). 115 S. C. Wi, J. S. Kang, J. H. Kim, S. B. Cho, B. J. Kim, S. Yoon, B. J. Suh, S. W. Han, K. H. Kim, K. J. Kim, B. S. Kim, H. J. Song, H. J. Shin, J. H. Shim, and B. I. Min, “Electronic structure of Zn1–xCoxO using photoemission and x-ray absorption spectroscopy”, Appl. Phys. Lett. 84, 4233 (2004). 28 Chapter Introduction 116 S. S. Yan, C. Ren, X. Wang, Y. Xin, Z. X. Zhou, L. M. Mei, M. J. Ren, Y. X. Chen, and Y. H. Liu, “Ferromagnetism and magnetoresistance of Co–ZnO inhomogeneous magnetic semiconductors”, Appl. Phys. Lett. 84, 2376 (2004). 117 B. I. Min, M. S. Park, and J. H. Park, “The search for new spintronic materials: half- metallic antiferromagnets and diluted magnetic semiconductors”, J. Phys.: Condens. Matter 16, S5509 (2004). 118 C. H. Chien, S. H. Chiou, G.Y. Guo, and Y. D. Yao, “Electronic structure and magnetic moments of 3d transition metal-doped ZnO”, J. Magn. Magn. Mater. 282, 275 (2004). 119 J. M. D. Coey and S. Sanvito, “Magnetic semiconductors and half-metals”, J. Phys. D: Appl. Phys. 37, 988 (2004). 120 D. A. Schwartz and D. R. Gamelin, “Reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor”, Adv. Mater. 16, 2115 (2004). 121 S. Ramachandran, A. Tiwari, and J. Narayani, “Origin of room-temperature ferromagnetism in cobalt-doped ZnO”, J. Electronic Mater. 33, 1298 (2004). 122 H. J. Lee, G. H. Ryu, S. K. Kim, S. A. Kim, C. H. Lee, S. Y. Jeong, and C. R. Cho, “A study of magnetic clusters in Co-doped ZnO using neutron scattering”, Phys. stat. sol. (b) 241, 2858 (2004). 123 M. Bouloudenine, N. Viart, S. Colis, and A. Dinia, “Bulk Zn1-xCoxO magnetic semiconductors prepared by hydrothermal technique”, Chem. Phys. Lett. 397, 73 (2004). 124 S. C. Wi, J. S. Kang, J. H. Kim, S. S. Lee, S. B. Cho, B. J. Kim, S. Yoon, B. J. Suh, S. W. Han, K. H. Kim, K. J. Kim, B. S. Kim, H. J. Song, H. J. Shin, J. H. Shim, and B. I. Min, “Photoemission study of Zn1–xCoxO as a possible DMS”, Phys. stat. sol. (b) 241, 1529 (2004). 29 Chapter Introduction 125 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. and Eng. B 109, 192 (2004). 126 J. H. Kim, H. Kim, D. Kim, Y. Ihm, and W. K. Choo, “The origin of room temperature ferromagnetism in cobalt-doped zinc oxide thin films fabricated by PLD”, J. Euro. Ceramic Soc. 24, 1847 (2004). 127 A. I. Savchuk , P. N. Gorley, V. V. Khomyak, K. S. Ulyanytsky,G S. V. Bilichuk, A. Perrone, and P. I. Nikitin, “ZnO-based semimagnetic semiconductors: growth and magnetism aspects”, Mater. Sci. and Eng. B 109, 196 (2004). 128 T. Dietl, H. Ohno, and F. Matsukura, “Hole-mediated ferromagnetism in tetrahedrally coordinated semiconductors”, Phys. Rev. B 63, 195205 (2001). 129 C. R. Cho, J. Y. Hwang, J. P. Kim, S. Y. Jeong, M. S. Jang, W. J. Lee, and D. H. Kim, “Ferromagnetism of heteroepitaxial Zn1-xCuxO Films grown on n-GaN substrates”, Jpn. J. Appl. Phys. 43, L1383 (2004). 130 A. Y. Polyakov, A. V. Govorkov, N. B. Smirnov, N.V. Pashkov, S. J. Pearton, K. Ip, R. M. Frazier, C. R. Abernathy, D. P. Norton, J. M. Zavada, and R. G. Wilson, “Optical and magnetic properties of ZnO bulkcrystals implanted with Cr and Fe”, Mater. Sci. in Semicon. Processing 7, 77 (2004). 131 I. Satoh, T. Kobayashi, K. Katayama, T. Okada, and T. Itoh, “Magneto photoluminescence of novel magnetic semiconductor Zn1−xCrxO grown by PLD method”, Appl. Phys. A 79, 1445 (2004). 132 H. J. Lee, B. S. Kim, C. R. Cho, and S. Y. Jeong, “A study of magnetic and optical properties of Cu-doped ZnO”, Phys. Stat. Sol. (b) 241, 1533 (2004). 133 K. Ando, H. Saito, V. Zayets, and M. C. Debnath, “Optical properties and functions of dilute magnetic semiconductors”, J. Phys.: Condens. Matter 16, S5541 (2004). 30 Chapter Introduction 134 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). 135 S. Kolesnik and B. Dabrowski, “Absence of room temperature ferromagnetism in bulk Mn-doped ZnO”, J. Appl. Phys. 96, 5379 (2004). 136 D. C. Kundaliya, S. B. Ogale, S. E. Lofland, S. Dhar, C. J. Metting, S. R. Shinde, Z. Ma, B. Varaughese, K. V. Ramanujachary, L. Salamanca-Riba, and T. Venkatesan, “On the origin of high-temperature ferromagnetism in the low-temperature-processed Mn−Zn−O system”, Nat. Mater. 3, 709 (2004). 137 Q. Wang, Q. Sun, B. K. Rao, and P. Jena, “Magnetism and energetics of Mn-Doped ZnO (10 0) thin films”, Phys. Rev. B 69, 233310 (2004). 138 Q. Wang and P. Jena, “First-principles study of magnetism in (11 0) Zn1–xMnxO thin film”, Appl. Phys. Lett. 84, 4170 (2004). 139 V. A. L. Roy, A. B. Djurisic, H. Liu and X. X. Zhang, Y. H. Leung, M. H. Xie, J. Gao, H. F. Lui, and C. Surya, “Magnetic properties of Mn doped ZnO tetrapod structures”, Appl. Phys. Lett. 84, 756 (2004). 140 C. Ronning, P. X. Gao, Y. Ding, Z. L. Wang, and D. Schwen, “Manganese-doped ZnO nanobelts for spintronics”, Appl. Phys. Lett. 84, 783 (2004). 141 K. J. Kim and Y. R. Park, “Optical investigation of Zn1–xFexO films grown on Al2O3 (0001) by radio-frequency sputtering”, J. Appl. Phys. 96, 4150 (2004). 142 T. Fukumura, Y. Yamada, H. Toyosaki, T. Hasegawa, H. Koinuma, and M. Kawasaki, “Exploration of oxide-based diluted magnetic semiconductors toward transparent spintronics”, Appl. Surf. Sci. 223, 62 (2004). 31 Chapter Introduction 143 J. H. Kim, H. Kim, D. Kim, S. G. Yoon, and W. K. Choo, “Optical and magnetic properties of laser-deposited Co-doped ZnO thin films”, Solid State Comm. 131, 677 (2004). 144 Y. M. Kim, M. Yoon, I. W. Park, Y. J. Park, and J. H. Lyou, “Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol–gel method”, Solid State Comm. 129, 175 (2004). 145 S. J. Han, B. Y. Lee, J. S. Ku, Y. B. Kim, and Y. H. Jeong, “Magnetic properties of Zn1−xCoxO”, J. Magn. Magn. Mater. 272, 2008 (2004). 146 H. J. Blythe, R. M. Ibrahim, G. A. Gehring, J. R. Neal, and A. M. Fox, “Mechanical alloying: a route to room-temperature ferromagnetism in bulk Zn1-xMnxO”, J. Magn. Magn. Mater. 283, 117 (2004). 147 P. Sharma, A. Gupta, F. J. Owens, A. Inoue, and K. V. Rao, “Room temperature spintronic material—Mn-doped ZnO revisited”, J. Magn. Magn. Mater. 282, 115 (2004). 148 G. Y. Ahn, S. I. Park, I. B. Shim, and C. S. Kim, “Mössbauer studies of ferromagnetism in Fe-doped ZnO magnetic semiconductor”, J. Magn. Magn. Mater. 282, 166 (2004). 149 M. S. Park and B. I. Min, “Ferromagnetism in ZnO codoped with transition metals: Zn1–x(FeCo)xO and Zn1–x(FeCu)xO”, Phys. Rev. B 68, 224436 (2003). 150 D. P. Norton, M. E. Overberg, S. J. Pearton, K. Pruessner, J. D. Budai, L. A. Boatner, M. F. Chisholm, S. Lee, Z. G. Khim, Y. D. Park, and R. G. Wilson, “Ferromagnetism in cobalt-implanted ZnO”, Appl. Phys. Lett. 83, 5488 (2003). 151 W. Prellier, A. Fouchet, B. Mercey, Ch. Simon, and B. Raveau, “Laser ablation of Co:ZnO films deposited from Zn and Co metal targets on (0001) Al2O3 substrates”, Appl. Phys. Lett . 82, 3490 (2003). 32 Chapter Introduction 152 Y. Q. Chang, D. B. Wang, X. H. Luo, X. Y. Xu, X. H. Chen, L. Li, C. P. Chen, R. M. Wang, J. Xu, and D. P. Yu, “Synthesis, optical, and magnetic properties of diluted magnetic semiconductor Zn1–xMnxO nanowires via vapor phase growth”, Appl. Phys. Lett. 83, 4020 (2003). 153 P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma, R. Ahuja, J. M. Osorio Guillen, B. Johansson, and G. A. Gehring, “Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO”, Nat. Mater. 2, 673 (2003). 154 S. J. Han, T. H. Jang, Y. B. Kim, B. G. Park, J. H. Park, and Y. H. Jeong, “Magnetism in Mn-doped ZnO bulk samples prepared by solid state reaction”, Appl. Phys. Lett. 83, 920 (2003). 155 X. M. Cheng and C. L. Chien, “Magnetic properties of epitaxial Mn-doped ZnO thin films”, J. Appl. Phys. 93, 7876 (2003). 156 S. W. Yoon, S. B. Cho, S. C. We, S. Yoon, B. J. Suh, H. K. Song, and Y. J. Shin, “Magnetic properties of ZnO-based diluted magnetic semiconductors”, J. Appl. Phys. 93, 7879 (2003). 157 D. P. Norton, S. J. Pearton, A. F. Hebard, N. Theodoropoulou, L. A. Boatner, and R. G. Wilson, “Ferromagnetism in Mn-implanted ZnO:Sn single crystals”, Appl. Phys. Lett. 82, 239 (2003). 158 I. Satoh and T. Kobayashi, “Magnetic and optical properties of novel magnetic semiconductor Cr-doped ZnO and its application to all oxide p–i–n diode”, Appl. Surf. Sci. 216, 603 (2003). 159 E. de Posada, G. Tobin, E. McGlynn, and J. G. Lunney, “Pulsed laser deposition of ZnO and Mn-doped ZnO thin films”, Appl. Surf. Sci. 208, 589 (2003). 33 Chapter Introduction 160 S. W. Lim, D. K. Hwang, and J. M. Myoung, “Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films”, Solid State Comm.125, 231 (2003). 161 Z. M. Qi, A. X. Li, F. L. Su, S. M. Zhou, Y. M. Liu, and Z. Y. Zhao, “X-ray diffraction analysis of Zn0.85Co0.15O powder and thin films”, Mater. Res. Bulletin 38, 1791 (2003). 162 N. A. Theodoropoulou, A. F. Hebard, D. P. Norton, J. D. Budai, L. A. Boatner, J. S. Lee, Z. G. Khim, Y. D. Park, M. E. Overberg, S. J. Pearton, and R. G. Wilson, “Ferromagnetism in Co- and Mn-doped ZnO”, Solid-State Electronics 47, 2231 (2003). 163 D.A. Schwartz, N.S. Norberg, Q. P. Nguyen, J.M. Parker, and D.R. Gamelin, “Magnetic quantum dots: Synthesis, spectroscopy, and magnetism of Co2+ and Ni2+ doped ZnO nanocrystals”, J. Am. Chem. Soc. 125, 13205 (2003). 164 K. Ip, R. M. Frazier, Y. W. Heo, D. P. Norton, C. R. Abernathy, S. J. Pearton, J. Kelly, R. Rairigh, A. F. Hebard, J. M. Zavada, and R. G. Wilson, “Ferromagnetism in Mn- and Co-implanted ZnO nanorods”, J. Vac. Sci. Technol. B 21, 1476 (2003). 165 A. Y. Polyakov, A. V. Govorkov, N. B. Smirnov, N. V. Pashkova, S. J. Pearton, M. E. Overberg, C. R. Abernathy, D. P. Norton, J. M. Zavada, and R. G. Wilson, “Properties of Mn and Co implanted ZnO crystals”, Solid-State Electronics 47, 1523 (2003). 166 A. Kaminski and S. Das Sarma, “Polaron percolation in diluted magnetic semiconductors”, Phys. Rev. Lett. 88, 247202 (2002). 167 S. R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A. J. Millis, V.N. Kulkarni, R. Ramesh, R. L. Greene, and T. Venkatesan, “Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO2-δ films”, Phys. Rev. Lett. 92, 166601 (2004). 34 Chapter Introduction 168 A.V. Vedyaev, A.B. Granovski, A.V.Kalitsov, and F. Brouers, “Anomalous Hall effect in granular alloys”, JETP 85, 1204 (1997). 169 A.A. Burkov and L. Balents, “Anomalous Hall effect in ferromagnetic semiconductors in the hopping transport regime”, Phys. Rev. Lett. 91, 057202 (2003). 170 S. Sugano and N. Kojima (Eds.), Magneto-optics, (Springer, Berlin, 2000), pp. 211- 244. 171 J. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, “Donor impurity band exchange in dilute ferromagnetic oxides”, Nat. Mater. 4, 173 (2005). 35 [...]... interface states”, Phys Rev Lett 71, 16 41 (19 93) 9 B Dieny, V S Speriosu, S S P Parkin, B A Gurney, D R Wilhoit, and D Mauri, “Giant magnetoresistive in soft ferromagnetic multilayers”, Phys Rev B 43, 12 97 (19 91) 10 S Parkin, J Xin, C Kaiser, A Panchula, K Roche, and M Samant, “Magnetically engineered spintronic sensors and memory”, Proc IEEE 91, 6 61 (2003) 11 T Miyazaki and N Tezuka, “Giant magnetic tunneling... implanted with Cr and Fe”, Mater Sci in Semicon Processing 7, 77 (2004) 13 1 I Satoh, T Kobayashi, K Katayama, T Okada, and T Itoh, “Magneto photoluminescence of novel magnetic semiconductor Zn1−xCrxO grown by PLD method”, Appl Phys A 79, 14 45 (2004) 13 2 H J Lee, B S Kim, C R Cho, and S Y Jeong, “A study of magnetic and optical properties of Cu -doped ZnO”, Phys Stat Sol (b) 2 41, 15 33 (2004) 13 3 K Ando, H Saito,... Sci 223, 62 (2004) 31 Chapter 1 Introduction 14 3 J H Kim, H Kim, D Kim, S G Yoon, and W K Choo, “Optical and magnetic properties of laser-deposited Co -doped ZnO thin films”, Solid State Comm 13 1, 677 (2004) 14 4 Y M Kim, M Yoon, I W Park, Y J Park, and J H Lyou, “Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol–gel method”, Solid State Comm 12 9, 17 5 (2004) 14 5 S J Han, B Y Lee,... magnetic moments of 3d transition metal -doped ZnO”, J Magn Magn Mater 282, 275 (2004) 11 9 J M D Coey and S Sanvito, “Magnetic semiconductors and half-metals”, J Phys D: Appl Phys 37, 988 (2004) 12 0 D A Schwartz and D R Gamelin, “Reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor”, Adv Mater 16 , 211 5 (2004) 12 1 S Ramachandran, A Tiwari, and J Narayani, “Origin of room-temperature... H Shim, and B I Min, “Photoemission study of Zn1–xCoxO as a possible DMS”, Phys stat sol (b) 2 41, 15 29 (2004) 29 Chapter 1 Introduction 12 5 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 and Eng B 10 9, 19 2 (2004) 12 6 J H Kim, H Kim, D Kim, Y Ihm, and W K Choo, “The origin of room temperature... Rev Lett 94, 14 7209 (2005) 74 B Martínez, F Sandiumenge, Ll Balcells, J Arbiol, F Sibieude, and C Monty, “Role of the microstructure on the magnetic properties of Co -doped ZnO nanoparticles”, Appl Phys Lett 86, 10 311 3 (2005) 75 J S Hong and R Q Wu, “Magnetic ordering and x-ray magnetic circular dichroism of Co doped ZnO”, J Appl Phys 97, 063 911 (2005) 76 G Lawes, A S Risbud, A P Ramirez, and Ram Seshadri,... luminescent properties of Mn -doped ZnO”, Solid State Ionics 16 7, 355 (2004) 27 Chapter 1 Introduction 10 8 S J Lee, H S Lee, D Y Kim, and T W Kim, “Microstructural, optical, and magnetic properties of (Zn1-xMnx)O thin films grown on (00 01) Al2O3 substrates”, J Crystal Growth 276, 12 1 (2004) 10 9 J C A Huang, H S Hsu, Y M Hu, C H Lee, Y H Huang, and M Z Lin, “Origin of ferromagnetism in ZnO/CoFe multilayers: Diluted... deposition of ZnO and Mn -doped ZnO thin films”, Appl Surf Sci 208, 589 (2003) 33 Chapter 1 Introduction 16 0 S W Lim, D K Hwang, and J M Myoung, “Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films”, Solid State Comm .12 5, 2 31 (2003) 16 1 Z M Qi, A X Li, F L Su, S M Zhou, Y M Liu, and Z Y Zhao, “X-ray diffraction analysis of Zn0.85Co0 .15 O powder and. .. Appl Phys Lett 85, 10 27 (2004) 11 2 H T Lin, T S Chin, J C Shih, S H Lin, T M Hong, R T Huang, F R Chen, and J J Kai, “Enhancement of ferromagnetic properties in Zn1–xCoxO by additional Cu doping”, Appl Phys Lett 85, 6 21 (2004) 11 3 S Ramachandran, A Tiwari, and J Narayan, “Zn0.9Co0.1O-based diluted magnetic semiconducting thin films”, Appl Phys Lett 84, 5255 (2004) 11 4 L Yan, C K Ong, and X S Rao, “Magnetic... of Mn -Doped ZnO (10 1 0) thin films”, Phys Rev B 69, 233 310 (2004) 13 8 Q Wang and P Jena, “First-principles study of magnetism in (11 2 0) Zn1–xMnxO thin film”, Appl Phys Lett 84, 417 0 (2004) 13 9 V A L Roy, A B Djurisic, H Liu and X X Zhang, Y H Leung, M H Xie, J Gao, H F Lui, and C Surya, “Magnetic properties of Mn doped ZnO tetrapod structures”, Appl Phys Lett 84, 756 (2004) 14 0 C Ronning, P X Gao, . sensors and memory”, Proc. IEEE 91, 6 61 (2003). 11 T. Miyazaki and N. Tezuka, “Giant magnetic tunneling effect in Fe/Al 2 O 3 /Fe junction”, J. Magn. Magn. Mater. 13 9, L2 31 (19 95). 12 J impurity band forms in the deep gap of the host semiconductor. 17 1 The interaction of magnetic impurity and localized carriers leads to the formation of magnetic polarons. The size of Chapter 1 Introduction. combination of host material, carrier concentration, and magnetic impurity (type and density) is right. 61 In particular, if one could introduce 5% of Mn and 3.5 10 20 cm -3 of holes into