SCIENCE CHINA Physics, Mechanics & Astronomy • Review •
January 2013 Vol.56 No.1: 111–123
Progress of Projects Supported by NSFC·Spintronics
doi: 10.1007/s11433-012-4966-4
Diluted magnetic oxides LI XiaoLi, QI ShiFei, JIANG FengXian, QUAN ZhiYong & XU XiaoHong* School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and Magnetic Information Materials, Ministry of Education, Shanxi Normal University, Linfen 041004, China Received October 15, 2012; accepted November 23, 2012; published online December 26, 2012
In this review, we review the progress of research on ZnO- and In2O3-based diluted magnetic oxides (DMOs). Firstly, we present the preparation and characterization of DMOs. The former includes the preparation methods and conditions, and the latter includes the characterization techniques for measuring microstructures. Secondly, we introduce the magnetic and transport properties of DMOs, as well as the relationship between them. Thirdly, the origin and mechanism of the ferromagnetism are discussed. Fourthly, we introduce other related work, including computational work and pertinent heterogeneous structures, such as multilayers and magnetic tunnel junctions. Finally, we provide an overview and outlook for DMOs. diluted magnetic oxide, preparation, magnetism, transport PACS number(s): 85.75.-d, 75.50.Pp, 75.70.-i Citation:
Li X L, Qi S F, Jiang F X, et al. Diluted magnetic oxides. Sci China-Phys Mech Astron, 2013, 56: 111123, doi: 10.1007/s11433-012-4966-4
1 Introduction The charge and spin of electrons in the solid state lay the foundations of the information technology that we use today. Integrated circuits and high-frequency devices made of semiconductors, which are based on electron charges, are successfully used for information processing and communications. The mass storage of information, which is indispensable to information technology, is realized by magnetic recording (hard disks, magnetic tapes, and magneto-optical disks) using the spin of electrons in ferromagnetic materials. Thus, the charge and spin of electrons can be possibly used to enhance further the performance of devices [1]. The emerging field of spintronics aims at the utilization of both the spin and charge of electrons [2,3]. Both semiconducting and magnetic properties within a single material can be achieved by doping the semiconducting material with low concentrations of transition metals (TMs), producing the
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commonly known diluted magnetic semiconductors (DMSs) [4]. Typical semiconductors used for devices and integrated circuits do not contain magnetic ions and are non-magnetic (Figure 1(c)). Ferromagnetism (FM) and semiconducting properties coexist in magnetic semiconductors, which have a periodic array of magnetic elements (Figure 1(a)). DMSs are produced by doping semiconducting materials with small concentrations of TMs (Figure 1(b)). Magnetic semiconductors, such as europium chalcogenides and semiconducting spinels (Figure 1(a)), were studied extensively [5]. Unfortunately, the crystal growth of these compounds is extremely difficult. To obtain even a small, single crystal requires weeks of preparation and growth [1]. The Curie temperature (Tc) is also below room temperature (RT). To date, only basic research has been conducted. And then, most studies on DMSs have focused on II–VI semiconductors such as (Cd,Mn)Te and (Zn,Mn)Se. Although these DMSs are relatively easy to prepare, II–VI-based DMSs are difficult to dope to create p- and n-types, which made the material lowly attractive for applications [1,6]. phys.scichina.com
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Figure 1 (a) A magnetic semiconductor, (b) a diluted magnetic semiconductor, and (c) a non-magnetic semiconductor. Reprinted with permission from ref. [1]. Copyright (1998) by AAAS.
More recently, III–V semiconductors such as (In,Mn)As and (Ga,Mn)As attracted research interest [6]. The magnetic Mn ion, which occupies the site of the cation (In or Ga) in InAs and GaAs systems, provides a localized spin and simultaneously acts as an acceptor. In this way, carrier-induced FM can be realized. Experiments also show high reproducibility. However, a crucial obstacle in the III–V systems is the low Tc. To date, the maximum Tc is only 200 K [7]. This low Tc value limits the use of DMSs in practical applications. The above discussion indicates that increasing Tc to above 300 K is a key factor in the application of DMSs. The search for new high-Tc DMSs was stimulated by theoretical considerations. A pioneering set of calculations based on the Zener mean-field of FM by Dietl et al. [8] predicted that 5% Mn-doped p-type ZnO and p-type GaN with acceptor concentrations of 3.5×1020 cm3 should exhibit Tc values in excess of RT. This theory considered a coupling between carrier spins and local atomic moments of magnetic impurities and, in parallel to the mean-field theory, provided an estimate of the ordering temperature. Sato et al. [9] investigated the FM of ZnO-based DMSs by ab initio calculations. The Korringa-Kohn-Rostoker Green’s function method based on the local density approximation is used for all calculations. The stability of the ferromagnetic state can be examined by the total energy differences E=TE (antiferromagnetic)TE (ferromagnetic ordering). ZnO systems doped with V, Cr, Fe, Co, and Ni (but not Mn) showed ferromagnetic ordering without additional carrier doping. Then, Sato et al. [10] found that the ferromagnetic state was stabilized by hole doping in Mn-doped ZnO, which agreed with the prediction of Dietl et al. [8]. The stability of the ferromagnetic state was also increased by electron doping in Fe-, Co-, or Ni-doped ZnO systems. n-Type ZnO is more easily realized than p-type ZnO. Therefore, the calculation of Sato et al. [10] provided confidence in the preparation of ZnO- based DMSs with RT FM. Theoretical prediction opens a window for experimental attempts to prepare DMSs with RT FM. Ueda et al. [11] first grew some Co-doped ZnO films exhibiting ferromagnetic behaviors with Tc above RT. The reproducibility of the samples was poor (less than 10%). Therefore, more research was required. Sharma et al. [12] reported the first observation of FM above RT for diluted (<4 at%) Mn-doped ZnO. Mn was found to carry an average magnetic moment of 0.16B per Mn ion. Co-doped TiO2 DMSs exhibiting FM
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above RT were also first grown [13]. Also, Fe, Cu codoped In2O3 bulk and films were prepared by solid-state reaction and pulsed laser deposition (PLD), respectively [14,15]. Magnetic Fe ions were found to have a high thermodynamic solubility (up to 20%) in the In2O3 host compound. The samples with high Fe concentration were found to be ferromagnetic, and Tc was around 750 K. Then, in the Cr-doped In2O3 system [16], the electrical and magnetic behaviors (from ferromagnetic metal-like to ferromagnetic semiconducting to paramagnetic insulator) could be effectively tuned by varying the defect concentrations. Currently, more than a decade has been spent on investigating various DMSs, particularly diluted magnetic oxides (DMOs) such as ZnO [11,12], In2O3 [14–16], TiO2 [13], SnO2 [17–19], CuO [20,21] and HfO2 [22–25], etc. These oxides are wide band gap semiconductors. Although some key problems in these DMO systems cannot be determined as of yet, the considerable amount of work performed on DMOs established a basis for future applications in spintronics devices.
2 Preparation and characterization DMO films are commonly prepared by PLD [11,26–29]. In addition to PLD, other methods such as magnetron sputtering [30–33], laser molecular-beam epitaxy [34,35], ion implantation [36,37], and ion beam sputtering [38] are also used. The microstructures and magnetic properties are reported to be sensitive to depositional methods. However, the best method is difficult to determine. In addition to the depositional method, the structures and magnetism of DMO films strongly depend on the preparation conditions, such as substrate type, substrate temperature, deposition atmosphere, and post-annealing in different atmospheres at different temperatures [39]. R-cut and c-cut sapphires are usually chosen as the substrates for depositing DMO films because of their closely matching lattice constants between substrates and films. Experiments about the influence of substrate variation on the magnetic properties of Co-doped ZnO insulating films were designed. In these experiments, many types of substrates such as ferroelectric and piezoelectric crystals, some other single crystals, and amorphous glass were chosen. The reorganization of defects was found to be because of the magnetoelectric coupling between Co:ZnO films and the ferroelectric crystal substrates, which may significantly enhance the magnetic ordering [40]. In addition to these inorganic substrates, even a plastic substrate was used to investigate the magnetic anisotropy of the ZnO:Cu samples under strain by bending the flexible samples to various directions [41]. The substrate temperature (Ts) is also an important parameter to tune the structure and magnetism of DMO films. If Ts is low, the energies may favor the formation of ho-
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mogenous DMS films, where the TM ions substitute for the cations in oxides. However, poor crystallinity exists when Ts is too low. When Ts is high, TM ions having sufficient energy can diffuse, which may provide more opportunities for the formation of the secondary phase [27,30,42]. Different FM behaviors at different Ts values were observed, which may arise from the opposing temperature dependence of Zn interstitial (Zni) and oxygen vacancy (VO) related defect concentrations (free-carrier concentration), as well as crystal symmetry and lattice defects (affecting the free-carrier mobility) [43]. Therefore, an appropriate Ts is important. To investigate DMOs further, various gases such as oxygen (O2) and nitrogen (N2) are introduced into the chamber when growing films. Many reports indicated that introducing O2 usually reduced the conductivity of the films, even making it act as an insulator, and that the corresponding saturation magnetization (Ms) decreased [27,42,44–48]. Conversely, when the films were deposited in a typical oxygen-deficient environment, a certain number of VOs or/and Znis are produced. They act not only as the defects of the ZnO crystal structure but also as ionized donors and contributors of free electrons. For this reason, the conductivity and Ms can be enhanced. N2 is considered to be a promising acceptor for p-type ZnO. Some results also indicated that doping with N2 can induce holes as N substitutes O, which significantly enhanced the coercivity, remanence, and Ms of ZnMnO films [49,50]. This observation was in good agreement with the original prediction by Dietl et al. [8] and Sato et al. [10]. However, some results were discrepant. The introduction of N2 led to a decrease in Ms of n-type Cu-doped ZnO films from 0.36B/Cu to 0.15B/Cu, and Tc decreased from 350 K to 320 K [51,52]. This phenomenon was because of the N2 doping-induced decrease in electron density, which weakened the ferromagnetic interactions. This finding also indicated that electron density played a critical role in FM. Thus, the introduction of various gases (O2, N2, etc.) during growing films strongly influenced their transport properties, such as carrier density and mobility, and accordingly influenced their magnetic properties. The relationship between the transport properties and magnetism is further discussed later. In addition to changing the above-discussed deposition conditions, post-annealing is also usually performed in DMO systems. Schwartz et al. [53,54] discovered reversible 300 K ferromagnetic ordering in Co2+:ZnO that can be switched between “on” and “off” states with quantitative reproducibility by introducing and removing Zni, a native n-type defect of ZnO (Figure 2). This phenomenon indicated the important contribution of Zni defects to FM. The influence of annealing in reducing atmosphere in the presence or absence of Zn vapor on FM in Co-doped ZnO films was also studied. The results demonstrated the gradual reduction of Ms with annealing time by sequential annealing in a reducing atmosphere. However, Ms was enhanced when it was annealed in a Zn vapor atmosphere. This result again indi-
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Figure 2 Quantitatively reversible cycling of Ms (300 K; solid solid red circles), conductivity (, blue diamonds), IR absorbance (□), and ligand field absorption () for a 9% Co2+:ZnO film with alternating air (“off”) and Zn vapor (“on”) annealing. Reprinted with permission from ref. [54]. Copyright (2006) by the American Physical Society.
cated that Zni, not VO, defects are critical to FM in Codoped ZnO [55]. However, many reports [27,38,47,56–58] show that VOs play an important role in inducing FM in DMOs. For instance, the enhancement (suppression) of FM was strongly correlated with the increase (decrease) in VOs in Co-doped ZnO [38]. The ZnMnO systems exhibited RT FM when in the conducting “as-deposited” state. However, upon high temperature annealing in excess oxygen, they became insulating and exhibited non-ferromagnetic behavior at RT [56]. Ms was also enhanced in Zn0.97Cu0.03O films annealed in a vacuum/Zn vapor. When they were annealed in air, their FM drastically decreased [27]. After annealing in a vacuum and Zn vapor, a film of nominal composition Zn0.97Cu0.03O can be written in the forms of Zn0.97Cu0.03O1a and Zn0.97+b Cu0.03O because of the introduction of VOs and Znis, respectively. In the two forms, the ratio of the total of Zn and Cu to O was greater than 1:1 in the ideal ZnO. When they were annealed in air, Ms rapidly decreased. Annealing in air eliminated a certain number of VOs and Znis of the films, further illustrating that the donor defects of VO and Zni played key roles in inducing FM in DMOs. The removal and introduction of defects require a certain energy, which implies that the annealing temperature (Ta) may be a critical factor. A sufficient high Ta easily leads to the appearance of the secondary phase in an oxide semiconductor host, whereas the defects cannot be adequately formed when Ta is too low. Therefore, we must also choose a proper Ta that is similar to the effect of Ts [30]. The various aforementioned conditions, including the substrate temperature, deposited atmosphere and annealing atmosphere and temperature can strongly influence the microstructures of the films, including the crystallinity, growth orientation, ratio of cation to O in oxide, categories, and
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number of defects. Accordingly, their magnetic and transport properties can be tuned. Therefore, obtaining homogeneous DMOs by the optimization of these deposition conditions is desirable. Gaining precise knowledge of the atomic-scale microstructures of samples is also targeted. From this point of view, film characterizations are significant in the progress of studying DMOs. An increasing number of techniques are being developed and used to characterize the microstructures and composites of DMOs. Such techniques include X-ray diffraction, transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structures, etc. Magnetic circular dichroism (MCD) spectroscopy and field-cooled/zero-field-cooled measurements are usually used to determine the existence of a magnetic impurity phase in DMO systems [39]. According to the concept of DMO, it is a type of material in which TM ions are incorporated into the lattice of the oxide semiconductor host. The TM ions are then uniformly substituted for the cations in oxides on the atomic scale. Therefore, directly observing the precise substitution between ions by any characterization technique is challenging. Almost all available techniques have their own merits and limitations. Therefore, in characterizing DMOs, researchers should use as many techniques as possible, and then combine the results for analysis. This can help determine the exact microstructures of samples as well as the origin and mechanism of magnetism.
3 Magnetic and transport properties To achieve spin injection in semiconductors, various wide band-gap oxide semiconductors such as ZnO, In2O3, TiO2, SnO2, CuO, HfO2, etc. have been chosen as hosts for doping with TMs to realize FM. These studies can be divided into three categories: 1) DMOs doped with 3d TMs, 2) DMOs co-doped with two elements, and 3) oxide materials that do not contain ions with partially filled d or f bands. This FM is also called “d 0” FM, which includes pure oxide and non-ferromagnetic element-doped oxides. Firstly, DMOs doped with 3d TMs. For ZnO, which is a typical oxide semiconductor, almost all 3d TMs have been used to realize RT FM [36,47,48,51,59–64]. Venkatesan et al. [47,64] deposited films from targets of (Zn0.95TM0.05)O (TM=Sc–Cu) by PLD. The RT perpendicular and parallel Ms in μB per dopant cation is plotted in Figure 3, where two distinct peaks appear: one at Ti and V, and another at Co. Ms was close to zero for Cr, Mn, and Cu. Remarkably, FM was observed for Sc in ZnO. These different Ms values may be because of the varied numbers of electrons in the 3d orbital of the varied 3d TM ions. Accordingly, the distribution of the electrons (spins) differs. Thus, the number of net spins in the high-spin state differs, which leads to altered Ms values. Even if the same TM element in ZnO is doped, the
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Ms values of the samples with different concentrations may differ. Figure 4 shows how the average Ms in ZnO:Co films prepared at 104 mbar varied with the Co concentration [47]. The fall-off in Ms with increased Co concentration can be explained by the Co ions with their spin-only moment, randomly distributed over the cation sites in the wurtzite lattice. Isolated ions contribute to the full moment. Pairs and most groups of four are antiferromagnetically coupled and made no net contribution. Numerous reports indicate that even when the same element is doped with the same concentration, the Ms values of samples may differ. Some films show FM, some show antiferromagnetism, and some even show spin-glass state or paramagnetism. The ferromagnetic samples also show different Ms values. The maximum Ms is even about several orders of magnitude higher than the lowest one. This finding may be related to the deposition methods and conditions. Thus, the reason for the discrepant magnetic properties is the microstructures of the films. For instance, ensuring a fully homogeneous substitution of TM in oxides using the present experimental techniques, and the avoidance of the occurrence of TM clusters as well as their related oxide phase are difficult. Even if we can obtain the real DMOs on an atomic scale, such high Ms per cation is inexplicable in terms of possible known ferromagnetic phases. These obstacles challenge our understanding of traditional magnetism in oxides [39]. Secondly, DMOs co-doped with two elements. Typically, oxides are co-doped with a 3d TM and a main element. In this case, the substitution of 3d TM ions for cations in the oxide supplies net spins because of their partially filled 3dn orbital. Donors or acceptors can also be produced because of the substitution of a main element for the cations or anions in the oxides, which makes the system n- or p-type conducting. By adjusting the doping concentration of the main element, the transport properties such as carrier densi-
Figure 3 Ms of (Zn0.95TM0.05)O films, for TM=Sc–Cu, measured at RT. Solid circles are for the field applied perpendicularly to the film plane, and open circles are for the field applied in the plane of the film. Reprinted with permission from ref. [47]. Copyright (2004) by the American Physical Society.
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Figure 4 Ms of Zn1xCoxO measured at RT for films prepared at 104 mbar for different x values. The solid line is based on a weighted random distribution of cobalt ions over the cation sites of the wurtzite lattice, with strong antiferromagnetic coupling of the nearest-neighbor Co cations and a constant contribution attributed to defects. The inset shows how the constant contribution is deduced by extrapolating the magnetic moment to x=0. Reprinted with permission from ref. [47]. Copyright (2004) by the American Physical Society.
ty and the magnetism of the system can be tuned. The ions of the main elements such as N and P can substitute for O sites. Li and Na can also substitute for Zn sites in the ZnO system. By doping with these four elements, p-type ZnO can be hoped to be prepared [49,50,65–67]. Conversely, IIIA elements such as Al and Ga are often used as doping elements to prepare n-type ZnO. We obtained a large RT Ms with 4.36B/Mn and 1.69B/Co in ZnMnO and ZnCoO systems by systematically changing the Al concentration and O2 pressure [46]. Doping Sn in TM-doped In2O3 can also make the carrier density reach 1022 cm3, which explained the tunable transport and magnetic properties by co-doping Sn in Mn- or Fe-doped In2O3 films. We should mention a “non-compensated n-p co-doping” concept [68]. This concept was first proposed using firstprinciple calculations in the Si and Ge system [68]. This system was energetically favorable and kinetically accessible to an interstitial Mn to occupy a substitutional site neighboring an n-type dopant, thereby increasing the concentration of substitutional Mn ions. The concept was also applicable in enhancing the visible-light photoactivity of TiO2 by narrowing its band gap [69]. The introduction of “non-compensated n-p co-doping” in DMOs has two advantages. Firstly, the Coulombic attraction between n- and p-type dopants with opposite charge states substantially enhanced the solubilities of the dopant pairs in concerted substitutional doping. Secondly, the non-compensated nature of n-p pairs consisting of different number of acceptors and donors also ensured a certain carrier density that further influenced the FM of oxide systems. For instance, Mn was usually used as a p-type dopant because the carrier density of Mn-doped ZnO was found to be lower than that of pure ZnO semicon-
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ductor [70]. Using Al as the n-type dopant was its individual distinct feature. When Mn and Al were co-doped in ZnO, the Mn2+ ions not only supplied the net spins but also behaved as acceptors. The Al3+ ions behaved as donors. When the number of donors and acceptors cannot compensate for one another, “non-compensated n-p co-doping” occurs. We obtained Ms of 4.36B and carrier density of 1021 cm3 in Mn-Al co-doped ZnO film by adjusting the doping concentration [46]. In Sn-Mn co-doped In2O3 system, FM can be tuned by controlling the carrier density by varying the Sn concentrations [71]. The aim of applying the concept to DMOs was to enhance the solubility of TM elements, change the transport properties (such as the carrier type and carrier density), and accordingly tune the magnetization. To date, a large number of studies suggest a strong link between magnetic and transport properties. Figure 5 is a typical illustration of this relationship [72]. FM was observed in both insulating and metallic films, but not when the carrier density was intermediate. The insulating films exhibited variable range hopping at low temperatures and were FM at RT because of the interaction of the localized spins with static localized states. However, the magnetism was quenched when carriers in the localized states became mobile. In the metallic (degenerate semiconductor) range, robust FM reappeared together with very strong magneto-optic signals and RT anomalous Hall data. A Hall effect, particularly an anomalous Hall effect, is usually measured to provide dependable information on the magnetic ordering in DMOs [73–75]. Therefore, the conduction bands were polarized and ZnO behaved as a genuine magnetic semiconductor when it was doped into the metallic regime. Figure 5 indicates two distinct mechanisms that can lead to FM in doped ZnO: magnetic polarons and carrier-mediated exchange. These two mechanisms result in diluted magnetic insulator and DMS behaviors, respectively. Yamada et al. [76] first realized electric field-induced FM at RT in the magnetic oxide semiconductor (Ti,Co)O2.
Figure 5 RT Ms of ZnO films with 5% Co and varied Al doping as a function of the mobile carrier density. Reprinted with permission from ref. [72]. Copyright (2008) by the American Physical Society.
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They used electric double-layer gating with high-density electron accumulation (>1014 per cm2). By applying a few volts of gate voltage, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state (Figure 6), thereby revealing the considerable role of carriers in high-temperature FM and demonstrating a method that can realize RT FM. The electric field effect in ferromagnetic semiconductors enables the switching of the magnetization state, which is a key technology for spintronic applications. Recently, Sinova et al. [77] suggested that one of the five emerging sub-fields in spintronics was current-induced torque (CIT), also known as spin-transfer torque. The manipulation of magnetization by CIT originated in angular momentum conservation, which twisted the layer receiving the angular momentum carried by the spin current. CIT is a key element for next-generation magnetic random access memories, logic-in-memory architectures, and high-density memory devices. Thirdly, “d 0” FM. Magnetic order in an oxide requires cations to have partially filled shells of d or f electrons. However, theoretical and experimental studies have indicated the existence of FM in undoped and non-TM-doped oxide semiconductors. An influential discovery was that of HfO2 film, an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic, as sho- wn in Figure 7 [22]. The Tc of the film was above 500 K and Ms was about 0.15B per HfO2 formula unit. The magnetization was also remarkably anisotropic. Reports on the FM of pure oxides and oxides doped with N, C, and other non-metal elements are numerous. FM is enhanced in pure ZnO upon thermal annealing with Tc above RT. This enhancement in FM can be attributed to the formation of VO clusters [78]. The ZnO films were prepared under N2 atmosphere with low Ts showed clear hysteresis loops up to 290 K [79]. The FM of C-doped ZnO films can also be supported by theoretical considerations as well as experimental results [28,80,81]. It was suggested that the
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Figure 7 RT magnetization curves at for HfO2 film with the field parallel (‖) or perpendicular (⊥) to the plane of the substrate. Insets: top left, proposed coupling scheme for orbital, spin and impurity-band magnetism in HfO2 (blue circles represent electrons, arrows show direction of their spin moment, the orbital and spin moments l and s of an electron in a 5d state couple antiparallel to form a j=3/2 state); bottom right, magnetization curves at different temperatures (triangles, 5 K; circles, 400 K). Reprinted with permission from ref. [22]. Copyright (2004) by Nature Publishing Group.
substitution of C atoms at O sites in ZnO introduced holes in O2p states, which coupled with the parent C2p localized spins by a p-p interaction. This p-p interaction led to the appearance of additional mixed band levels derived from the semiconductor valence band. The coupling ‘‘pushes’’ the minority p-p mixed state upward and the opposite spin state downward, decreasing the total energy of the system. The wave function of the C2p states was spatially extended to neighboring O2p states and coupled with the O2p states. By this p-p interaction, holes mediated the spin alignment of parent C atoms, leading to an indirect ferromagnetic coupling of C atoms [80]. Thus, more than a decade has been spent studying magnetism (Ms and Tc) and transport properties (carrier density, mobility, and anomalous Hall effect), as well as the strong relationship between them in DMOs.
4 Origin and mechanism of FM
Figure 6 Illustration of an electrically induced change from a paramagnetic state without gate voltage to a ferromagnetic state with finite gate voltage by accumulating electron carriers that mediate ferromagnetic coupling between localized spins. Reprinted with permission from ref. [76]. Copyright (2011) by AAAS.
Studies have reported FM in DMSs, including DMOs. Ordered magnetic moments per 3d cation at low concentrations are unprecedented, exceeding those of almost all known oxides, alloys, and pure magnetic metals. Such high moments per cation are inexplicable in terms of possible known ferromagnetic phases. These results challenge our understanding of oxide magnetism [64]. To date, numerous models have been proposed to explain the origin of FM. In this section, we introduce three typical models: carrier-induced FM, bound magnetic polarons (BMPs), and chargetransfer FM (CTF). Carrier induced FM was proposed by Dietl et al. [8]
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based on the Zener model. Zener [82] first proposed an FM model driven by the exchange interaction between carriers and localized spins. However, this model was later abandoned as neither the itinerant character of the magnetic electrons nor the quantum (Friedel) oscillations of the electron spin polarization around the localized spins were taken into account. These two factors are now established to be critical to the theory of magnetic metals. However, in the case of semiconductors, the effect of Friedel oscillations averages zero because the mean distance between the carriers is greater than that between the spins. Thus, the Zener model becomes equivalent [83] to the approach developed by Ruderman, Kittel, Kasuya, and Yosida, in which the presence of oscillations is explicitly taken into account. On this basis, Dietl et al. [8] theoretically predicted that p-type ZnO doping with 5 at% Mn would exhibit RT FM. Tc was then determined from the competition between long-range carrier-mediated FM coupling and short-range Mn-Mn anti-FM exchange interactions. Following this prediction, Sato et al. [9,10] calculated the FM of ZnO doping with various TMs, in which additional holes were introduced by replacing O with N, and electrons were added by replacing some Zn atoms with Ga. Ferromagnetic ordering was found to be induced by hole doping in Mn-doped ZnO. The ferromagnetic state was stabilized by electron doping in the case of Fe-, Co-, or Ni-doped ZnO. The model of carrier-induced FM has been verified by many subsequent experiments. However, this model cannot explain the FM of all DMOs. Coey et al. [64] proposed that ferromagnetic exchange was mediated by shallow donor electrons that formed BMPs, which overlaped to create a spin-split impurity band. An electron associated with a particular defect can be confined in a hydrogenic orbital of a specific radius. The depth of the electron traps was the order of a few tenths of an electron volt. As the donor concentration increased, 1s orbitals overlapped to form an impurity band. The electrons remained localized at first because of the influence of correlations and potential fluctuations in a narrow band. However, a critical donor concentration, at which the impurity band states became delocalized and metallic conduction set in, existed. The donors tended to form BMPs, coupling the 3d moments of the ions within their orbits. The basic idea is illustrated in Figure 8. The cations presented an extra random potential that extended the localized region with increased 3d concentration. Provided that the radius of the hydrogenic orbital was sufficiently large, the overlap between a hydrogenic electron and the cations within its orbit led to ferromagnetic exchange coupling between them. Generally, the coupling between the cation and the donor electron is ferromagnetic when the 3d shell is less than half-full. Either way, the coupling between two similar impurities within the same donor orbital is ferromagnetic. As defect density increased, hydrogenic orbitals associated with the randomly positioned defects overlapped. Assuming that they were randomly packed spherical objects, percola-
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tion occurred when they filled about 16% of the space. Provided that the number of magnetic cations within a donor orbital was sufficiently large, FM occurred when > p and x < xp, where x and are the concentrations of magnetic cations and donors, respectively; and xp and p are the cation and donor polaron percolation thresholds, respectively. Coey et al. [84] proposed the CTF model (Figure 9). This model was developed for defect-related FM, which involves a spin-split defect band populated by charge transfers from a proximate charge reservoir. In the case of DMOs, the 3d dopants can serve as the reservoir provided they are able to coexist in different valence states such as Ti3+/Ti4+, Mn3+/ Mn4+, Fe2+/Fe3+, Co2+/Co3+ or Cu+/Cu2+, etc. The CTF model embodies three crucial components: firstly, a defect-based band with a high density of states in the vicinity of the Fermi level; secondly, a charge reservoir to or from which electrons can be easily transferred; thirdly,
Figure 8 Representation of magnetic polarons. Cation sites are represented by small circles. Oxygen is not shown; unnoccupied oxygen sites are represented by squares. Reprinted with permission from ref. [64]. Copyright (2005) by Nature Publishing Group.
Figure 9 Schematic of the idea underlying CTF. Electron transfer to or from the charge reservoir to the defect band leads to fulfillment of the Stoner criterion, and spontaneous ferromagnetic splitting occurs. Reprinted with permission from ref. [84]. Copyright (2008) by IOP.
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an effective exchange integral associated with the defect states. The defect band is closely analogous to the impurity band in semiconductors. Unlike a classical Stoner ferromagnet, a charge-transfer ferromagnet is not uniformly magnetized. Only defect-containing regions become ferromagnetic. The relevant density of states is not necessary at the Fermi level in the un-split defect band. A decrease in the Fermi level near the maximum density of states would be considered unlikely. A high density of states above or just below the Fermi level suffices in conjunction with the charge reservoir [85]. Some possible defect distributions are illustrated in Figures 10(a)–(d). The defects may be uniformly distributed throughout the material (Figure 10(a)), or they may aggregate by spinodal decomposition (Figure 10(b)). They can be associated with grain boundaries (Figure 10(d)), with the surface or interface of a thin film (Figure 10(c)), or with the surface of a nanoparticle. In this manner, only a small fraction of the total volume of the sample needs to be involved. Electrons are transferred from the charge reservoir to the defect band, or vice versa, which can increase the density of states at the Fermi level in the defect band to the point where the Stoner criterion is satisfied. The shaded regions in Figure 10 then become ferromagnetically ordered. To summarize, the CTF model is also a Stoner model with a defect-based impurity band; however, another charge reservoir exists in the system. This reservoir allows for the easy transfer of electrons to or from the impurity band to create a filling that leads to spontaneous spin-splitting. In DMOs, this reservoir is associated with the dopant ions. In the carrier-mediated FM model, the local moments on the TM dopants are ferromagnetically coupled with each other through polarized mobile carriers. In the BMP model, the local moments on the TM dopants are ferromagnetically coupled with each other through polarized carriers trapped by defects, such as electrons associated with a VO. These models predict moments contributed by both TM ions and mediating polarized carriers (mobile or localized). In contrast, the CTF model predicts that moments are entirely localized on itinerant carriers confined in small regions, possibly grain boundaries. Although each model has its merits
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Figure 10 Possible distributions of defects: (a) random distribution of point defects at the percolation threshold, (b) spinodal decomposition, (c) interface defects, and (d) grain boundary defects. Reprinted with permission from ref. [85]. Copyright (2010) by IOP.
in certain aspects, a complete understanding of the ferromagnetic behavior in DMOs still requires further microscopic insight into the defect structures, transport properties, and moment distributions in these systems. We studied the combined roles of VO and electron doping in mediating local and non-local magnetic ordering of Fedoped In2O3 and Co-doped ZnO using the first-principle density functional theory [86]. We chose ZnO as an example and enlarged the supercell to 4 × 3 × 3 and 6 × 3 × 2 to consider two BMPs (Figure 11(a)). The distance between the two BMPs was different. Figure 11(b) shows ∆E for Co-doped ZnO as a function of the BMP–BMP distance with and without Al doping. With Al doping, the ∆E values were negative and decreased in magnitude with decreased distance, which implied the ferromagnetic exchange coupling between the two BMPs. The results indicated that carrier doping played a dual role of further enhancing the ferromagnetic stability of local polarons and mediating the non-local magnetic coupling between two magnetic polarons.
5 Other related work In addition to the above discussion on DMOs, another related work, such as computational work on DMOs and studies on the heterostructures of oxides and DMOs is also in progress. Computational work based on ab initio calculations can
Figure 11 (a) The supercell of ZnO used to study the long-range ferromagnetic coupling of two BMPs. The gray and red spheres are Zn and O atoms, respectively, and the green and yellow spheres are Co and Al atoms, respectively. The dotted circles indicate oxygen vacancies. (b) ∆E of Co-doped ZnO as a function of the BMP–BMP separation. The filled and unfilled squares represent doping and not doping additional electrons, respectively. Reprinted with permission from ref. [86]. Copyright (2011) by the American Physical Society.
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provide some predictions of magnetic states as well as explanations for the electronic structure and nature of FM ordering in TM-doped oxide semiconductors. The total energy differences (E) per unit supercell, electron structures, and Tc are usually calculated. The prediction of Sato et al. [9,10] was based on the calculation of ∆E. The ferromagnetic state was stable when E > 0 and vice versa. Subsequently, considerable efforts concerning ∆E were reported. E between the FM and anti-FM states were studied in a Co-doped ZnO system, which was less than 3 meV per Co atom and almost independent of the Co-Co distance [87]. This finding indicated that couplings between two Co ions were negligible for distances larger than 4.6 Å, and that the range of magnetic interactions was rather short. The effect of defects on ∆E was also extensively studied theoretically because defects usually existed in DMOs. For instance, Zn vacancy and O substitution by N yielded ferromagnetic interactions [88]; the role of VO in stabilizing the FM of Co-doped ZnO and Fe-doped In2O3 can be verified by the calculation of E [89,90]. These findings were also confirmed by many theoretical and experimental studies. Electron structures such as the density of state and spin density are advantageous in analyzing the origin of FM. Figures 12(a)–(d) present the density of state spectra of ZnO:Co:VO (ZCO-VO) and ZnAlO:Co:VO (ZCAO-VO) with FM ordering. Doping with Al caused a dramatic downward shift of the valence band and the movement of the Fermi level into the conduction band, resulting in the appearance of more mobile electrons. Figures 12(c) and (d) show that strong coupling existed between the 2p orbital of the oxygen atoms and the 3s orbital of the Al atom in ZCAO-VO, making the FM state more stable than in ZCO-VO. Tc can also usually be calculated. Dietl et al. [8] made a prediction about Tc. Pemmaraju et al. [91] theoretically investigated the FM of ZnO:Co and calculated Tc using a combination of state-of-the-art density functional theory electronic structure method and Monte Carlo simulations for an effective Hamiltonian. The heterostructures of oxides or DMOs mainly include multilayers and DMO-based magnetic tunnel junctions (MTJs). Studies on these heterostructures focus on the magnetoresistance (MR) effect. In fact, MR has been observed in many DMO films. Defects, carriers, and mobility are widely studied to explain the FM of DMOs. Magnetic and transport properties have been found to be strongly related, and has thus attracted research interest. MR is the change in resistance with an applied magnetic field, and probes the interaction between itinerant carriers and defect ions. ZnOand In2O3-based DMOs usually exhibit positive or negative changes in the MR sign at low temperatures. Jin et al. [92] found that ZnO films doped with different 3d TM elements exhibited different MR properties. The MR of ZnO:Cr and ZnO:Mn films was negative and positive at high and low magnetic fields, respectively. ZnFeO, ZnNiO, and ZnCuO
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Figure 12 Total density of states (a) and projected density of states ((b)–(d)) of Co 3d, O 2p, and Al 3s for ZCO-VO (dashed lines) and ZCAO-VO. Reprinted with permission from ref. [86]. Copyright (2011) by the American Physical Society.
films only exhibited negative MR. The MR of ZnCoO films was negative at 20 K. However, a decrease in temperature resulted in a combined negative and positive MR at high and low magnetic fields, respectively. A negative MR of 95% at 2 K and a positive MR of 48% at 10 K were also found in Zn0.94Mn0.03Co0.03O films [93]. The MR of DMOs was related with the temperature, carrier density, and doping concentration of 3d TMs [94–96]. The mechanism is considered complex. Various models were proposed to explain the origin of MR. Positive MR may be caused by the large s-d spin-splitting of the conduction band in terms of the weak-localization theory [46,97–100]. Negative MR can be attributed to a weak localization effect [92,94,99], a precursor effect on magnetic polaron formation [93,97], the scattering of spin-polarized charge carriers at isolated magnetic impurities [101], or s-d exchange coupling [102]. Generally, MR in DMOs with low TM concentrations can only be found at low temperatures. With increased temperature, MR values sharply decrease, which hinder the application of DMOs in spintronics. Recently, MR was also found in “dense” ferromagnetic semiconductors with high TM concentrations. Yan et al. [103–105] successfully deposited Co/ZnO and Fe/ZnO films by alternately sputtering thin metal layers and ZnO layers. A large negative MR
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of 11% was observed at RT. MR was increased with decreasing temperature and reached 36% at 4.8 K in Co-ZnO films, in which Co-rich and ZnO-rich regions existed at the nanometer scale. No layered structures were observed because of the diffusion between metal and ZnO, although the films were grown by alternately depositing very thin metal layers and ZnO layers. The large, negative MR may be related to spin-dependent variable range hopping. Magnetic semiconductor films exhibiting RT MR were also deposited by a co-sputtering and ion beam sputtering system. However, metallic particles were found in these films [106,107]. Recently, we studied Co/ZnO and Co/ZnAlO films with Co concentrations from 20% to 60% by alternately sputtering Co layers and ZnO or ZnAlO layers [108–110]. High RT MR ratios of 11.9% and 12.3% were obtained in the Co/ZnO and Co/ZnAlO films, respectively. Metallic Co particles were detected in the films by TEM and XPS. The temperature dependence of the resistivity () showed semiconductor behavior. The observed linear relation between lnρ and temperature (T1/2) below 180 K also indicated an inter-particle tunneling conduction mechanism in the sample. The ZnO matrix provided a tunneling barrier between the metallic Co regions. In the MCD data for Co/ZnO films, the ZnO electrons themselves were partially polarized, which could be responsible for the high MR ratio. This finding provides a clue to the spin injection at the metal/ semiconductor interface. Apart from multilayers, DMO-based MTJs were studied. Song et al. [111–113] deposited fully epitaxial (Zn,Co)O/ ZnO/(Zn,Co)O MTJs with a single barrier and (Zn,Co)O/ ZnO/(Zn,Co)O/ZnO/(Zn,Co)O MTJs with a double barrier by a simple and coherent magnetron sputtering technique. This formed the all-oxide based heterostructure configuration involving ZnO as the magnetic electrode and the barrier. A positive tunneling MR (TMR) of 20.8% in (Zn,Co)O/ ZnO/(Zn,Co)O MTJs was obtained at 4 K, which can resist RT with the small TMR ratio of 0.35% because of the improved crystallinity of barriers and electrode/barrier interfaces. When changing from the ZnO barrier to the MgO barrier, the TMR of (Zn,Co)O/MgO/(Zn,Co)O MTJs was as high as 46.8% at 4 K; however, the TMR did not persist up to RT [114]. (Zn,Co)O/ZnO/(Zn,Co)O MTJs deposited by PLD only exhibited positive MR at low temperatures [115]. From the above discussion, MR effect can be observed in both DMOs and MTJs with DMOs electrodes. Accordingly, whether MR originates from DMOs or DMO-based MTJs needs to be determined. The ZnCoO/Al2O3/Co MTJs were designed and studied, and the results provided evidence of the spin injection of electrons from ZnCoO to Al2O3 [116]. An MR value of 11% at low temperature was also observed in TiCoO2/Al-O/FeCo MTJs [117,118]. An ultrathin Co:TiO2 layer (0–1 nm thick) was observed to be a paramagnetic insulator and acted as an additional tunnel barrier when it was inserted at the SrTiO3/Co interface in LaSrMnO3/SrTiO3/Co tunnel junctions [119]. Studies on DMO-
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based MTJs are limited, and most MR effects can only be observed at low temperatures. The mechanisms of DMObased MTJs require further study.
6 Conclusions The prediction of Tc well above RT for wide-band gap oxides doped with a small amount of TM cations has triggered intense interest in these materials as potential DMSs. Consequently, research efforts have yielded interesting results but also raised new questions. To realize the substitution of TM ions for cations in oxides, various preparation methods were adopted. Almost all prepared conditions were optimized, all TM elements were chosen, and possible characterization techniques (including those for microstructures, magnetism, transport, and optical properties) were used. FM was found to be strongly correlated with the TM concentration, carrier density, defect number, oxide semiconductor crystallinity, and environments of the short ordering of TM atoms. However, the uniform incorporation of TM atoms into the lattice of oxides is difficult because of the oxide characteristics and equipment limitations. Issues on reproducibility, stability, nonintrinsic effects, and possible roles of defects should be addressed. Over the past decade, a considerable amount of experimental data, computational data, and corresponding mechanisms have been accumulated. Although DMOs are far from being applied in practical devices, these results can pave the way for breakthroughs in basic research and the development of spintronic devices. However, many fundamental problems, such as achieving efficient spin-injection and manipulation, should be urgently solved. Addressing these issues can drive the development of semiconductor spintronics and accelerate their commercial application. The magnetization of DMS materials can be reversed using electric currents that transport spin angular momentum, which shows great potential for enhancing the functionality of semiconductor devices. This finding presents new opportunities for spin-based device applications that may considerably affect future information-processing technologies. This work was supported by China National Funds for Distinguished Young Scientists (Grant No. 51025101), the National Natural Science Foundation of China (Grant Nos. 11274214, 11104173 and 61204097), the Research Fund for the Doctoral Program of Higher Education (Grant No. 20101404120002) and the Youth Science Foundation of Shanxi Province (Grant Nos. 2011021021-1, 2011021021-2 and 2012021020-2). 1 Ohno H. Making nonmagnetic semiconductors ferromagnetic. Science, 1998, 281: 951–956 2 Prinz G A. Magnetoelectronics. Science, 1998, 282: 1660–1663 3 Wolf S A, Awschalom D D, Buhrman R A, et al. Spintronics: A spinbased electronics vision for the future. Science, 2001, 294: 1488–
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