ZnO基稀磁半导体磁性机理研究进展

稀磁半导体是指在非磁性化合物半导体中通过掺杂引入部分磁性离子所形成的一类新型功能材料.目前,稀磁半导体的磁性来源和机理一直是该领域的研究热点,掺杂的磁性离子通过怎样的交换方式实现铁磁性一直存有争议.本文对近几年来ZnO基稀磁半导体磁性机理研究进展作一综述,着重阐述了代表性的RRKY理论、平均场理论、双交换理论和磁极子理论,对实验和理论方面的热点和存在问题作一评价,对磁性理论的研究提出了新思路.     

Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO

The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.     

Influence of ageing on H-induced ferromagnetism in Zn1 − xMxO (M = Co, Fe, Mn)

Many dilute magnetic semiconductors, when annealed in hydrogen atmosphere, have been reported to show a giant ferromagnetism. The H-induced lattice defects, mainly the oxygen vacancies, have been suggested to mediate this ferromagnetic coupling. We have also observed huge magnetic induction in paramagnetic Co and Fe-doped ZnO systems, upon hydrogenation; however, the re-heating caused this magnetism to vanish quickly. In the present work, we have furthered these studies on their longevity aspect i.e. the ageing effect. Alarmingly, the magnetic properties of hydrogenated samples stored in dry atmosphere degrade gradually with ageing. On the other hand, the 2% Mn doped ZnO, which depicts only a weak ferromagnetism, and no enhancement in magnetization upon hydrogenation, exhibits no such ageing effect.     

A new class of magnetic materials: Sr2FeMoO6 and related compounds

Ordered double perovskite oxides of the general formula A₂BB′O₆ have been known for several decades to have interesting electronic and magnetic properties. However, a recent report of a spectacular negative magnetoresistance effect in a specific member of this family, namely Sr₂FeMoO₆, has brought this class of compounds under intense scrutiny. It is now believed that the origin of the magnetism in this class of compounds is based on a novel kinetically-driven mechanism. This new mechanism is also likely to be responsible for the unusually high temperature ferromagnetism in several other systems, such as dilute magnetic semiconductors, as well as in various half-metallic ferromagnetic systems, such as Heussler alloys.     

Ferromagnetism of ZnO and GaN: A Review

The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 10²⁰ cm⁻³, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.     

Magnetic effects at the interface between non-magnetic oxides

The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interface-induced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida interactions, the Kondo effect and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.     

Tailoring ferromagnetic chalcopyrites

If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that bandgaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and bandgaps are related in magnetic semiconductors. Here we explore theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V(2) chalcopyrites (where II, IV and V represent elements from groups II, IV and V, respectively); three of these compounds are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. On the basis of our results for structural, electronic and magnetic properties, we identify a small number of new stable chalcopyrites with excellent prospects for ferromagnetism.     

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-Doped Tungsten Diselenide Monolayers at Room Temperature

The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step toward modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe₂ are shown. As vanadium concentration increases, the saturation magnetization increases, which is optimal at ≈4 at% vanadium; the highest doping level ever achieved for V-doped WSe₂ monolayers. The TISF occurs at ≈175 K and becomes more pronounced upon increasing the temperature toward room temperature. The TISF can be manipulated by changing the vanadium concentration. The TISF is attributed to the magnetic-field- and temperature-dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory study. The findings pave the way for the development of novel spintronic and valleytronic nanodevices and stimulate further research.     

氧化物稀磁半导体材料的理论与实验研究进展

以氧化物宽禁带半导体为基体,通过掺杂磁性元素,可将非磁性半导体转变成铁磁性半导体,利用这些铁磁性半导体,能将新型的自旋电子器件集成到传统的微电子器件上,构成功能丰富的新型器件.由于稀磁半导体材料在自旋电子学中的重要作用,近年来受到广泛的关注.简要总结了有关氧化物稀磁半导体研究的发展状况;分析了制备条件对其磁性的可能影响;重点介绍了该系统中有关磁性起源的理论模型,包括双交换机制、磁极化子模型、RKKY模型等;比较了2种磁极化子理论模型,并对这些模型的适用范围进行了分析讨论.另外,还介绍了该体系微结构和磁结构的一些检测方法以及与磁性相关的输运性质、反常霍尔效应等.     

Dilute Doping,Defects, and Ferromagnetism in Metal Oxide Systems

Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non‐magnetic metal oxides such as TiO₂ and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto‐transport and magneto‐optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re‐emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.     

Ferromagnetic transition metal implanted ZnO: A diluted magnetic semiconductor?

Recent theoretical works have predicted that some semiconductors (e.g. ZnO) doped with magnetic ions are diluted magnetic semiconductors (DMS). In DMS, magnetic ions substitute cation sites of the host semiconductor and are coupled by free carriers, resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid–solubility limit of magnetic ions in semiconductor hosts. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5–10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism. Due to their different crystallographic orientation with respect to the host crystal, these nanocrystals in some cases are very difficult to be detected by a simple Bragg–Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films ranges between 10 and 40 at.% using different growth conditions pulsed laser deposition. Such diluted, Co-doped ZnO films show paramagnetic behavior. However, only the magnetoresistance of Co-doped ZnO films reveals possible s–d exchange interaction as compared to Co-implanted ZnO single crystals.     

Sonochemical Synthesis and Magnetism in Co-doped ZnO Nanoparticles

The understanding and control of ferromagnetism in diluted magnetic semiconducting oxides (DMO) is a special challenge in solid-state physics and materials science due to its impact in magneto-optical devices and spintronics. Several studies and mechanisms have been proposed to explain intrinsic ferromagnetism in DMO compounds since the theoretical prediction of room-temperature ferromagnetism. However, genuine and intrinsic ferromagnetism in 3d-transition metal-doped n-type ZnO semiconductors is still a controversial issue. Furthermore, for DMO nanoparticles, some special physical and chemical effects may also play a role. In this contribution, structural and magnetic properties of sonochemically prepared cobalt-doped ZnO nanoparticles were investigated. A set of ZnO samples was prepared varying cobalt molar concentration and time of ultrasonic exposure. The obtained results showed that single phase samples can be obtained by the sonochemical method. However, cobalt nanoclusters can be detected depending on synthesis conditions. Magnetic measurements indicated a possible ferromagnetic response, associated to defects and cobalt substitutions at the zinc site by cobalt. However, ferromagnetism is depleted at higher magnetic fields. Also, an antiferromagnetic response is detected due to cobalt oxide cluster at high cobalt molar concentrations.     

From nanoelectronics to nano-spintronics

Today's electronics uses electron charge as a state variable for logic and computing operation, which is often represented as voltage or current. In this representation of state variable, carriers in electronic devices behave independently even to a few and single electron cases. As the scaling continues to reduce the physical feature size and to increase the functional throughput, two most outstanding limitations and major challenges, among others, are power dissipation and variability as identified by ITRS. This paper presents the expose, in that collective phenomena, e.g., spintronics using appropriate order parameters of magnetic moment as a state variable may be considered favorably for a new room-temperature information processing paradigm. A comparison between electronics and spintronics in terms of variability, quantum and thermal fluctuations will be presented. It shows that the benefits of the scalability to smaller sizes in the case of spintronics (nanomagnetics) include a much reduced variability problem as compared with today's electronics. In addition, another advantage of using nanomagnets is the possibility of constructing nonvolatile logics, which allow for immense power savings during system standby. However, most of devices with magnetic moment usually use current to drive the devices and consequently, power dissipation is a major issue. We will discuss approaches of using electric-field control of ferromagnetism in dilute magnetic semiconductor (DMS) and metallic ferromagnetic materials. With the DMSs, carrier-mediated transition from paramagnetic to ferromagnetic phases make possible to have devices work very much like field effect transistor, plus the non-volatility afforded by ferromagnetism. Then we will describe new possibilities of the use of electric field for metallic materials and devices: Spin wave devices with multiferroics materials. We will also further describe a potential new method of electric field control of metallic ferromagnetism via field effect of the Thomas Fermi surface layer.     

Origin and control of high-temperature ferromagnetism in semiconductors

The extensive experimental and computational search for multifunctional materials has resulted in the development of semiconductor and oxide systems, such as (Ga,Mn)N, (Zn,Cr)Te and HfO(2), which exhibit surprisingly stable ferromagnetic signatures despite having a small or nominally zero concentration of magnetic elements. Here, we show that the ferromagnetism of (Zn,Cr)Te, and the associated magnetooptical and magnetotransport functionalities, are dominated by the formation of Cr-rich (Zn,Cr)Te metallic nanocrystals embedded in the Cr-poor (Zn,Cr)Te matrix. Importantly, the formation of these nanocrystals can be controlled by manipulating the charge state of the Cr ions during the epitaxy. The findings provide insight into the origin of ferromagnetism in a broad range of semiconductors and oxides, and indicate possible functionalities of these composite systems. Furthermore, they demonstrate a bottom-up method for self-organized nanostructure fabrication that is applicable to any system in which the charge state of a constituent depends on the Fermi-level position in the host semiconductor.     

Vacancy defect-induced d0 ferromagnetism in undoped ZnO nanostructures: Controversial origin and challenges

ZnO-based dilute magnetic semiconductors have attracted great interest for their promising application potential in spintronics. Observation of ferromagnetic-like behavior in oxides in general directs the recent focus to defect-rich undoped ZnO thin films and nanostructures. Such magnetic properties are generally mediated by the defects exclusive of magnetic ion doping, thus called defect-induced d0 ferromagnetism (FM). However the intrinsic origin of d0 FM in such materials is controversially reported. In this review we aim to locate the root of the controversy by revisiting the way how the defects were characterized and correlated with the d0 FM in each situation. We found that the main cause of controversy is rooted in a long term debate on the nature of native defects concerning the unintentional n-type conductivity in as-grown ZnO. It is particularly manifested in the assignment of the green luminescence center in photoluminescence spectra and electron paramagnetic resonance signals near g = 1.96 and g = 2.0. Only through X-ray-based microscopy and spectroscopy analysis, can the intrinsic origin of d0 FM in undoped ZnO be unambiguously attributed to the O 2p orbitals arising from zinc vacancies, rather than the Zn 3d orbitals and oxygen vacancies. In spite of the complex defect state in the nanostructures, certain parameters that influence the d0 FM in undoped ZnO systems can be extracted from various reports. Finally, we summarize the challenges and general conclusions on the d0 FM in undoped ZnO nanostructures, followed by outlooks on potential device application in spintronics. It is clear that an important step to promote d0 FM in ZnO for spintronics is to stabilize enough V_Zn in ZnO nanostructures, either through acceptor doping or epitaxial growth of strained films, without diminishing the crystalline quality of the structure. Future research focusing on this direction will hopefully produce new breakthrough in device applications.     

Electronic structure origins of polarity-dependent high-TC ferromagnetism in oxide-diluted magnetic semiconductors

Future spintronics technologies based on diluted magnetic semiconductors (DMSs) will rely heavily on a sound understanding of the microscopic origins of ferromagnetism in such materials. Discoveries of room-temperature ferromagnetism in wide-bandgap DMSs hold great promise, but this ferromagnetism remains poorly understood. Here we demonstrate a close link between the electronic structures and polarity-dependent high-TC ferromagnetism of TM(2+):ZnO DMSs, where TM(2+) denotes 3d transition metal ions. Trends in ferromagnetism across the 3d series of TM(2+):ZnO DMSs predicted from the energies of donor- and acceptor-type excited states reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.     

Cluster ferromagnetism in Mn-doped InSb

We have studied cluster ferromagnetism in InSb〈Mn〉 and have refined kinematic exchange theory with application to such diluted magnetic semiconductors.     

Mn and other magnetic impurities in GaN and other III–V semiconductors – perspective for spintronic applications

III–V semiconductors doped with magnetic ions have been attracting interest of many laboratories all over the world during more than thirty years. At the beginning the reason was the will to understand influence of omnipresent unintentional, as well as intentionally introduced, impurities of transition metals or rare earths on electrical and optical properties of semiconductors commonly applied in electronic and optoelectronic devices. In the last years the subject of III–V semiconductors highly doped with magnetic ions, the so-called diluted magnetic semiconductors, has revived rapidly again in the context of the newborn branch of electronics, called spintronics. Diluted magnetic semiconductors based on III–V compounds are regarded as prospect candidates for applications in spintronic devices. The results of studies performed on III–V semiconductors, doped or diluted with different magnetic ions, are presented. Special attention is put to GaN because of a strong hope, based on theoretical calculations, for high temperature ferromagnetism. Reasons for difficulties with obtaining high temperature ferromagnetic semiconductors are shown. A possible mechanism of magnetic ordering in III–V semiconductors doped with Mn is presented.     

溶胶-凝胶法制备Cu/ZnO的室温铁磁性

为研究稀磁性半导体的室温铁磁性来源,采用溶胶-凝胶法制备了Cu掺杂ZnO半导体粉末。X射线衍射光谱显示Cu在ZnO中的固溶度小于0.08 (摩尔比);透射电子显微镜分析显示颗粒尺寸较为均匀,呈单结晶态;振动样品磁强计测试表明,Cu/ZnO具有室温铁磁性。由于Cu本身不具有任何磁性,样品的铁磁性来源为氧化锌晶格中的缺陷与Cu2+离子之间的交换作用。     

Warping a single Mn acceptor wavefunction by straining the GaAs host

Transition-metal dopants such as Mn determine the ferromagnetism in dilute magnetic semiconductors such as Ga(1-x)Mn(x)As. Recently, the acceptor states of Mn dopants in GaAs were found to be highly anisotropic owing to the symmetry of the host crystal. Here, we show how the shape of such a state can be modified by local strain. The Mn acceptors near InAs quantum dots are mapped at room temperature by scanning tunnelling microscopy. Dramatic distortions and a reduction in the symmetry of the wavefunction of the hole bound to the Mn acceptor are observed originating from strain induced by quantum dots. Calculations of the acceptor-state wavefunction in the presence of strain, within a tight-binding model and within an effective-mass model, agree with the experimentally observed shape. The magnetic easy axes of strained lightly doped Ga(1-x)Mn(x)As can be explained on the basis of the observed local density of states for the single Mn spin.