Ab Initio Study of Curie Temperatures of Diluted Magnetic Semiconductors

The Curie temperature of diluted (Ga,Mn)As magnetic semiconductors in the presence of As antisites is studied from first principles. We map total energies associated with rotations of Mn-magnetic moments onto the effective classical Heisenberg Hamiltonian which is treated in the mean-field approximation to find the Curie temperature. The presence of donors strongly reduces the Curie temperature and gives rise to a ground state with a partial disorder of local moments. We show that the observed dependence of the Curie temperature on the Mn concentration indicates that the concentration of As antisites increases with the Mn content.     

基于从头算法的TM金属共掺杂的ZnO基稀磁半导体的磁性研究(英文)

本文采用从头计算的方法研究了基于过渡性金属共掺杂Ⅱ-Ⅵ族稀释半导体的磁性和电子结构.并系统的研究了氧化锌基的稀释半导体铁磁态的稳定性和对其材料设计.在所有的共掺杂体系中,发现(Mn,Co),(Co,Ni)和(Mn,Ni)共掺杂体系是铁磁态的,而(Fe,Ni)共掺杂体系是自旋玻璃态.另一方面,Fe-,Co-和Ni掺杂ZnO基系统的稳态是铁磁态.同时,本文研究了ZnO基稀释半导体的载流子传导铁磁性,计算分析了电子态密度,铁磁态的稳定性.结合双交换和超交换理论解释共掺杂稀释半导体的磁性机理.     

Transport and Optical Properties of Diluted Magnetic Semiconductors

We investigated the transport and optical properties of diluted magnetic semiconductors theoretically by using a simple model where carriers move in a single band. In this model the carrier feels a nonmagnetic potential at a magnetic impurity site, and its spin interacts with the localized spins of the magnetic impurities through exchange interactions. The electronic states of a carrier were calculated by using the coherent potential approximation (CPA). The magnetism was investigated by minimizing the free-energy and the electrical conductivity was calculated by using the Kubo formula. We examined the results in several typical cases which correspond to (Ga_1–xMn_x)As with x = 0.05.     

溶胶-凝胶法制备ZnO基稀释磁性半导体薄膜

用溶胶－凝胶法制备了具有良好光学性质和C轴取向的ZnO:Fe薄膜。ZnO:Fe薄膜具有尖锐的带边发光,禁带宽度约为3.3eV,半高宽13nm。磁性测量表明,ZnO：Fe薄膜在室温下具有铁磁性,饱和磁化强度约为10^-3emu量级,矫顽力为30奥斯特（Oe)。     

Diluted Magnetic Semiconductors in the Low Carrier Density Regime

This paper, based on a presentation at the Spintronics 2001 conference, provides a review of our studies on II–VI and III–V Mn-doped Diluted Magnetic Semiconductors. We use simple models appropriate for the low carrier density (insulating) regime, although we believe that some of the unusual features of the magnetization curves should qualitatively be present at larger dopings (metallic regime) as well. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.     

Magneto-Optical Study of Multiple Layers of Self-Assembled Quantum Dots Involving Diluted Magnetic Semiconductors

Magneto-optical experiments were carried out on structures comprised of multiple layers of self-assembled quantum dots (QDs) involving diluted magnetic semiconductors (DMSs). Photoluminescence (PL) from interband ground state transitions was clearly observed in these DMS-based QD systems. The PL energy from QD multilayers appears at a lower energy than that emitted by a single QD layer, suggesting that there exists electronic coupling between the QD layers. When an external magnetic field is applied, the PL peaks from QDs both in single-layer and in multilayer form exhibit large Zeeman shifts and a significant enhancement of intensity, a behavior that is typical for many low dimensional systems involving DMSs. In contrast to this behavior, however, we have observed a decrease of the PL intensity as a function of magnetic field in multilayer structures where alternating QW layers contain DMS and non-DMS QDs. We will show evidence that this effect arises from carrier transfer between pairs of QDs from adjacent layers (double QDs) due to the large Zeeman shifts of the conduction and valence bands characteristic of DMS QDs.     

Spin Electronics: From Concentrated to Diluted Magnetic Semiconductors and Beyond

This presentation will review physical properties, relevant to spintronics, of concentrated magnetic semiconductors. Examples from the open literature and current research utilizing thin film EuS will be used as illustrations. New research on potential spin injection materials and spin detection will also be described. This latter work, carried out at MARTECH, the Center for Materials Research and Technology, demonstrates that carriers from the half-metal CrO₂ may be injected across an insulator without loss of (100%) spin polarization. It also features the development of a Hall gradiometer capable of detecting as few as 10⁵ spins.     

Magnetic Properties of Chalcopyrite-Based Diluted Magnetic Semiconductors

We calculated the chemical trends of transition metal-doped chalcopyrite DMS (diluted magnetic semiconductors) by the use of KKR–CPA–LDA method. The ferromagnetism was stable in V- and Cr-doped chalcopyrite DMS. In the case of Fe and Co doping, however, the spinglass-like state was realized. On the other hand, in the cases of Mn doped I-III-VI₂ and II-IV-V₂ type DMS, the ground state was ferromagnetic and spinglass-like, respectively.     

自燃烧法合成的ZnO基稀磁半导体纳米颗粒研究进展

稀磁半导体制备方法与磁性起源的研究是当前凝聚态物理的一项热门课题.首先介绍了自燃烧合成法的原理和优点,然后以Co和Mn掺杂ZnO为重点,总结了国内外采用自燃烧法合成的ZnO基稀磁半导体纳米颗粒晶体结构、磁性能相关的研究进展,讨论了所得纳米颗粒磁性能的内在物理机制.通过对自燃烧法合成的更宽掺杂范围ZnO基稀磁半导体纳米颗粒的研究,使我们能够更加系统地了解过渡金属掺杂ZnO材料的结构与磁性能,并探讨所得实验现象的内在物理机制.     

Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors

Spin injection processes in the double quantum dots of ZnSe-based diluted magnetic semiconductors are discussed. Double quantum dots are fabricated from ZnSe-based double quantum wells by electron beam lithography and wet etching. In these samples, the photo-excited carriers in the magnetic dots are injected into the non-magnetic dots. The circular polarization degrees of photoluminescence from the non-magnetic dots are measured by micro-photoluminescence measurement system under the magnetic field up to 5 T. The maximum spin polarization degrees of injected carriers determined from our experiment are 10% for double quantum wells and 15% for double quantum dots. The spin injection efficiency was estimated both from the observed circular polarization degree and the diffusion length of carriers. We concluded that the spin injection efficiency is increased in the double quantum dots.     

Mn- and Cr-Doped InN: A Promising Diluted Magnetic Semiconductor Material

Cr- and Mn-doped InN films were successfully grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. Low temperature GaN buffer layers grown by metal-organic vapor-phase epitaxy were used to accommodate the large lattice mismatch between InN and sapphire. A high n-type carrier concentration of 1.5×10²⁰ cm^–3 was measured in InN films with 3% Cr-doping. Films of this type exhibit a well-defined in-plane magnetic hysteresis loop and remanence for temperatures varying from 5 to 300K. The Mn-doped films, however, turned out to exhibit less clear magnetic properties. Thus, ferromagnetism in Cr-doped InN can be concluded from our measurements.     

锰掺杂氧化铜稀磁半导体薄膜的微结构和磁性

采用射频磁控溅射制备Mn掺杂CuO薄膜样品。X射线衍射结果说明薄膜样品为单相结构且沿（111）取向生长。通过样品的XRD精修得到样品的结构和晶格参数,掺杂后薄膜晶体结构有微小畸变。薄膜的高分辨透射电镜研究证明了对结构和晶粒大小等的精修结果,且同时说明Mn以替代Cu的形式掺入了CuO晶格中。通过对样品M1T曲线的分析,得到样品的居里温度为96．5K,近邻Mn离子之间的耦合为铁磁性,并由居里外斯拟合得到Mn离子的有效磁矩为3．1μa。这说明磁性不是来自于团聚的Mn原子或铜锰的其它氧化物,而很可能来自于替位的锰离子所形成的Mn-O-Cu-O-Mn之间的铁磁性耦合。     

稀磁半导体材料的研究进展及应用前景

综述了稀磁半导体的研究现状,从其分类、各自特点、物理性质和应用现状等各方面详细阐述了稀磁半导体的基本特点,并展望了今后稀磁半导体的研究重点与实用方向.     

过渡金属掺杂ZnO稀磁半导体的发光特性

ZnO是一种宽带隙Ⅱ-Ⅵ族半导体,具有良好的光电耦合特性和稳定性,在光、电、磁功能集成等新型器件方面可获得重要应用.近来的研究表明,过渡金属掺杂的ZnO基半导体有望成为实现高居里温度稀磁半导体的候选材料,是目前研究的热点.总结了近几年人们在Fe、Co、Ni、Cu、Mn等过渡金属掺杂的ZnO基稀磁半导体的发光特性研究结果,讨论了过渡金属掺杂后ZnO中观察到的可见发光机制,分析认为过渡金属掺杂ZnO的可见光发射主要与这些发光过渡金属引入后所产生的缺陷有关,而紫外发光峰的变化则与过渡金属掺入后ZnO晶体质量与禁带宽度的改变相关.     

Giant Magneto-Optical Properties in Hybrid Nanostructures of Diluted Magnetic Semiconductors With Ferromagnetic Co Wires

Giant Zeeman effects of excitons were studied in hybrid nanostructures of diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co, where the DMS-QW wires with the width of 200 nm were sandwiched in the Co wires with the width of 400 nm. When the direction of the Co magnetization is normal to the wire plane, a magnetic field B ^⊥ _Co‐flux perpendicular to the well plane can be applied from the Co wires to all of the DMS-QW wire. In addition, the use of a Co/Pt multilayered film with the perpendicular magnetization at zero external field was examined instead of the pure Co film. The excitonic photoluminescence spectrum from the DMS-QW shows significant broadening in the hybrid structures, indicating the giant Zeeman shift of excitons induced by B ^⊥ _Co‐flux.     

Curie Temperatures of III–V Diluted Magnetic Semiconductors Calculated from First-Principles in Mean Field Approximation

Electronic structure and ferromagnetism in III–V compound-based diluted magnetic semiconductors (DMS) are investigated based on first-principles calculations by using the Korringa-Kohn-Rostoker method combined with the coherent-potential-approximation. The stability of the ferromagnetic phase in GaN-, GaAs-, GaP-, GaSb-based DMS is investigated systematically. The calculations show that 3d-impurities from the first-half of the transition metal series favor the ferromagnetic state, while impurities from the latter-half of the series exhibit spin-glass behavior. This chemical trend in the magnetism is explained by the double exchange mechanism taking the local symmetry at the impurity gap states into account. Curie temperatures of GaAs- and GaN-based DMS are estimated by using the Heisenberg model in a mean field approximation with the parameters calculated from first-principles. It is suggested that room-temperature ferromagnetism can be realized in these systems.     

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.     

Preparation and Characterization of IV–VI Diluted Magnetic Semiconductor Ge1?xCrxTe

IV–VI diluted magnetic semiconductor Ge_1–x Cr _x Te films with different Cr compositions up to x = 0.33 were successfully prepared by a sputtering method. Curie–Weiss behavior, giving that the paramagnetic Curie temperature is 29 K and the effective number of the Bohr magneton is 4.67, is clearly observed for a Ge_1–x Cr _x Te (x = 0.07) film. All the films show ferromagnetic order at low temperatures. The easy direction of the magnetization orients perpendicular to the plane. As the Cr composition increases, the ferromagnetic Curie temperature determined from the temperature dependence of the residual magnetization tends to increase up to 25 K for x = 0.33.     

Spin Carrier Exchange Interactions in (Ga,Mn)N and (Zn,Co)O Wide Band Gap Diluted Magnetic Semiconductor Epilayers

(Ga,Mn)N and (Zn,Co)O wide band gap diluted magnetic semiconductor epilayers have been investigated by magneto-optical spectroscopy. In both cases, absorption bands observed below the energy gap allow us to study the nature of the valence and spin state of the incorporated magnetic element. Exchange interactions between magnetic ions and carriers can be observed by analyzing the magnetic circular dichroism in transmission or the exciton Zeeman splitting in reflection for (Zn,Co)O. A first estimation of the exchange integrals can be given for both materials.