Ferromagnetism in dilute magnetic semiconductors and new materials for spintronics

Spintronics materials may be classified under concentrated magnetic semiconductors, semimetals and half-metals, semimagnetic semiconductors, and dilute magnetic semiconductors (DMS). The nature of ferromagnetism, that occurs in p-type DMS with an increase in the transition metal content, is governed by the proposed kinematic exchange involving the kinetic energy gain of the heavy hole carriers caused by their hybridization with 3d electrons of impurities. The synthesis of DMS (In,Mn)Sb is proposed on the basis of hint at its T_C from kinematic mechanism. The effect of the dimensionality-driven T_C increase is derived for spintronics materials such as delta-doped DMS (DDMS) and DMS heterostructures. The state-of-the-art in the field of synthesis and research of “new” DMS with announced “high T_C” is also outlined with particular attention to chalcopyrite-based systems.     

Rare-earth doped III-nitride semiconductors for semiconductor spintronics

InGaGdN layers and InGaGdN/GaN superlattice (SL) structures were grown by plasma-assisted molecular beam epitaxy. InGaGdN layers exhibited photoluminescence emission at room temperature and its peak wavelength was red-shifted with the increase of In composition. Clear hysteresis and saturation were observed in the magnetization versus magnetic field curves at room temperature for the InGaGdN layers. Si co-doping into InGaGdN layers increased the electron carrier concentration and enhanced the magnetization. In the InGaGdN/GaN SL samples, enhanced magnetization was also observed. Si doping into wide bandgap GaN layers in these SL structures further increased the magnetization, where InGaGdN layers were not doped with Si. All these results can be understood with the carrier-mediated ferromagnetism.     

Diluted magnetic oxides

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 II-VI oxide semiconductors with multiple band gaps

We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.     

How to make semiconductors ferromagnetic: a first course on spintronics

The rapidly developing field of ferromagnetism in diluted magnetic semiconductors, where a semiconductor host is magnetically doped by transition metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_1−xMn_xAs with x≈1-10%), is discussed with the emphasis on elucidating the physical mechanisms underlying the magnetic properties. Recent key developments are summarized with critical discussions of the roles of disorder, localization, band structure, defects, and the choice of materials in producing good magnetic quality and high Curie temperature. The correlation between magnetic and transport properties is argued to be a crucial ingredient in developing a full understanding of the properties of ferromagnetic semiconductors.     

稀磁半导体--自旋和电荷的桥梁

稀磁半导体可能同时利用载流子的自旋和电荷自由度构造将磁、电集于一体的半导体器件．尤其是铁磁半导体材料的出现带动了半导体自旋电子学的发展．室温铁磁半导体材料的制备，半导体材料中有效的自旋注入，以及自旋在半导体结构中输运和操作已成为目前半导体自旋电子学领域中的热门课题．稀磁半导体呈现出强烈的自旋相关的光学性质和输运性质，这些效应为人们制备半导体自旋电子学器件提供了物理基础．     

同步辐射X射线磁二色性在自旋电子学研究中的应用

文章对同步辐射X射线磁圆二色谱和X射线磁线二色谱的独特优点进行了综合分析,并通过一些典型应用实例说明同步辐射磁性测量技术具有元素分辨能力、高灵敏度、纳米量级空间分辨能力和皮秒量级时间分辨能力,并可用于测量铁磁性和反铁磁性,因而在自旋电子学材料和器件研究领域具有广泛的应用前景.     

Diluted magnetic semiconductor superlattices and heterostructures

Diluted magnetic semiconductors (DMS) are mixed semiconducting crystals whose lattice is made up in part of substitutional magnetic ions. Cd_1−xMn_xTe and Hg_1−xMn_xTe are examples of such materials. Their structural and band parameters can be “tuned” by composition over a wide range. They can thus be exploited in situations completely similar to those involving Ga_1−xAl_xAs. Using molecular beam epitaxy, we have grown Cd_1−xMn_xTe superlattices with alternating Mn content, having up to 150 layers, with layer thickness ranging from 50 to 100 Å. The superlattice structure is clearly revealed by transmission electron microscopy and by zone-folding of the phonon spectrum observed in Raman scattering. Photoluminescence observed on Cd_1−xMn_xTe superlattices is several orders of magnitude greater than that from a Cd_1−xMn_xTe film with uniform Mn content, or from bulk Cd_1−xMn_xTe specimens. The presence of localized magnetic moments in DMS results in a strong exchange interaction between these moments and band electrons. This in turn leads to gigantic Zeeman splittings of impurity states, exciton levels, Landau levels, and the bands themselves. Zeeman splittings as large as 20 meV (which in non-magnetic semiconductors would require unrealistic megagauss fields) are easily achieved in DMS in fields of several kilogauss. Since the magnitude of this exchange-induced splitting in DMS can be comparable to the binding energies and to the minigaps encountered in multiple quantum wells, DMS superlattices hold promise of a host of novel effects of both fundamental and applied interest.     

同步辐射X射线磁二色性在自旋电子学研究中的应用

文章对同步辐射X射线磁圆二色谱和X射线磁线二色谱的独特优点进行了综合分析,并通过一些典型应用实例说明同步辐射磁性测量技术具有元素分辨能力、高灵敏度、纳米量级空间分辨能力和皮秒量级时间分辨能力,并可用于测量铁磁性和反铁磁性,因而在自旋电子学材料和器件研究领域具有广泛的应用前景.     

Diluted magnetic semiconductor at finite temperature

We studied the diluted magnetic semiconductor by the self-consistent Green's function approach, which treats the spin-wave kinematics appropriately at finite temperatures. Our approach leads to a simple formula for the critical temperature in a wide range of parameter space. In addition, the magnetization curve versus temperature in some regimes is concave, which is dramatically different from the usual convex shape. Finally, we discuss the possibility of generalizing the current theory to include the realistic band structure, electronic correlations and disorders in a systematic way.     

Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Novel germanium-based magnetic semiconductors

Epitaxial synthesis and properties of novel Co and Mn-doped Ge magnetic semiconductors were studied. Epitaxial growth of high quality films with high doping concentrations has been stabilized by the use of two dopants. The magnetic and transport properties of the system exhibit high T(C) and large magnetoresistance effects that can be controlled systematically by the doping concentration. The maximum T(C) achieved in the semiconducting materials is approximately 270 K at a composition of Co0.12Mn0.03Ge0.85.     

Giant magnetic moments in dilute magnetic semiconductors

The concentration dependence of the specific magnetic moment value at room temperature in dilute semiconductor titanium oxides doped with either Co or Fe has been investigated. This value was found to increase sharply at small concentrations of magnetic impurity. The magnetic moment of 22.9 μ_B per impurity atom has been revealed for TiO₂ doped with 0.15 at% Co, not yet reported in any semiconductor oxide systems. We conclude the observed giant magnetic moments are caused by the crystal lattice polarization at small impurity concentrations. The comparison with published data point to different types of the magnetization concentration dependence for various semiconductor matrixes that is probably related to the dielectric permittivity of the environment.     

Homogeneous and inhomogeneous magnetic oxide semiconductors

Magnetic oxide semiconductors are significant spintronics materials.In this article, we review recent advances for homogeneous and inhomogeneous magnetic oxide semiconductors.In the homogeneous magnetic oxide semiconductors,we focus on the various doping techniques including choosing different transition metals, codoping, non-magnetic doping,and even un-doping to realize homogeneous substitution and the clear magnetic origin.And the enhancement of the ferromagnetism is achieved by nanodot arrays engineering, which is accompanied by the tunable optical properties.In the inhomogeneous magnetic oxide semiconductors, we review some heterostructures and their magnetic and transport properties, especially magnetoresistance, which are dramatically modulated by electric field in the constructed devices.And the related mechanisms are discussed in details.Finally, we provide an overview and possible potential applications of magnetic oxide semiconductors.     

Electron propagation in magnetic semiconductors

A simple decoupling scheme is suggested and analyzed for the problem of the electron propagation in magnetic semiconductors treated in the s-d model. In most important cases, the method is shown to reduce to the Kubo form of the coherent potential approximation. It is then used to assess the role of higher order effects in the red shift of the optical absorption edge.     

Plasmon-magnon interaction in magnetic semiconductors

In appreciably doped semiconductors (e.g. EuO, CdCr₂S₄, etc.) plasmon and magnon energies are comparable. Therefore, there will be resonant interaction between these modes of excitations. On the basis of a new microscopic theory formulated for plasmon-magnon interaction, the effect of this interaction on the energies and lifetimes of plasmons and magnons has been calculated using the double-time Green’s function. The energy shifts are very small and the lifetimes of plasmons,τ ^p , and magnons,τ ^m , are of the order of 10⁻² and 10⁻³ sec respectively.     

Photomaganetization in diluted magnetic semiconductors

Starting from a many–body Hamiltonian for a system of photogenerated electrons and holes, spin-split by magnetic ions in diluted magnetic semiconductors, we derive, presumably for the first time, an expression for the photomagnetization as a function of the photon power, frequency, excitonic interaction and the magnetic ion concentration. Damping of nonequilibrium carriers and spin excitons is considered phenomenologically. Our results agree qualitatively with some of the systematics of the photomagnetization observed in Hg _1?x Mn _x Te.     

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.