Element resolved spin configuration in ferromagnetic manganese-doped gallium arsenide

We report induced Ga and As moments in ferromagnetic Ga(1-x)MnxAs detected using x-ray magnetic circular dichroism at the Mn, Ga, and As L(3,2) edges. Across a broad composition range, we find As and Ga dichroism signals which indicate an As 4s moment coupled antiparallel to the Mn 3d moment, and a smaller parallel Ga 4s moment. The Ga moment follows that of Mn in both doping and temperature dependence. These results are consistent with recent predictions of induced GaAs host moments and support the model of carrier-mediated ferromagnetic ordering involving As-derived valence band states.     

MFM and Raman studies in PEMBE-grown (Ga,Mn)N thin films showing room-temperature ferromagnetism

We present the room-temperature ferromagnetism in the (Ga,Mn)N films grown on n-type GaN templates by plasma-enhanced molecular beam epitaxy for semiconductor spintronic device applications. Despite of the possible interface effects between the (Ga,Mn)N layers and n-type GaN templates, the (Ga,Mn)N films were found to exhibit the ferromagnetic ordering above room temperature. The magnetic force microscopy identified the magnetic domains with the different magnetic orientations at room temperature, indicating the existence of the ferromagnetic long-range ordering. In Raman spectra, an additional peak at 578 cm⁻¹ was observed, which is attributed to the local vibration of substitutional Mn in the (Ga,Mn)N lattice. Therefore, it is believed that the ferromagnetic ordering in (Ga,Mn)N is due to the carrier-mediated Ruderman-Kittle-Kasuya-Yosida interaction.     

Diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As: the effect of a charge state

Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.     

Investigation of photoluminescence in GaAs quantum well coupled with a ferromagnetic semiconductor layer of (Ga,Mn)As

We have investigated circular-polarized photoluminescence (CPL) from a novel quantum structure in which a ferromagnetic semiconductor (Ga,Mn)As is placed adjacent to the GaAs quantum well. By eliminating the contribution of the magneto-circular dichroism effect of the (Ga,Mn)As top layer from the observed CPL, we found a small but nonnegligible contribution of quantum mechanical coupling between the GaAs quantum well states and the spin-polarized states in (Ga,Mn)As.     

Angle-dependent x-ray magnetic circular dichroism from (Ga,Mn)As: anisotropy and identification of hybridized states

Remarkably anisotropic Mn L2,3 x-ray magnetic circular dichroism spectra from the ferromagnetic semiconductor (Ga,Mn)As are reported. States with cubic and uniaxial symmetry are distinguished by careful analysis of the angle dependence of the spectra. The multiplet structures with cubic symmetry are qualitatively reproduced by calculations for an atomiclike d5 configuration in tetrahedral environment, and show zero anisotropy in the orbital and spin moments within the experimental uncertainty. However, hybridization with the host valence bands is reflected by the presence of a preedge feature with a uniaxial anisotropy and a marked dependence on the hole density.     

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.     

Nature of magnetic coupling between Mn ions in As-grown Ga1-xMnxAs studied by X-ray magnetic circular dichroism

The magnetic properties of as-grown Ga1-xMnxAs have been investigated by the systematic measurements of temperature and magnetic field dependent soft x-ray magnetic circular dichroism (XMCD). The intrinsic XMCD intensity at high temperatures obeys the Curie-Weiss law, but a residual spin magnetic moment appears already around 100 K, significantly above the Curie temperature (T_{C}), suggesting that short-range ferromagnetic correlations are developed above T_{C}. The present results also suggest that the antiferromagnetic interaction between the substitutional and interstitial Mn (Mn_{int}) ions exists and that the amount of the Mn_{int} affects T_{C}.     

Observation of giant magnetic linear dichroism in (Ga, Mn) As

Giant magnetic linear dichroism (MLD) is observed in the ferromagnetic semiconductor Ga(0.98)Mn(0.02)As. The contribution to this effect induced by the spontaneous magnetization can be clearly identified by azimuthal dependencies. The spectral dependence of the effect in the range from 1.4 to 2.4 eV shows that the MLD induced by the spontaneous magnetization is strongly enhanced for excitations from the electronic states that are responsible for the ferromagnetism in this material. This spectral sensitivity and the size of the effect makes MLD a powerful tool for the study of (III, Mn)V alloys and similar novel ferromagnetic semiconductors.     

Identification of different electron screening behavior between the bulk and surface of (Ga,Mn)As

We report x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of both temperature and Mn doping. Analysis of Mn 2p core level spectra reveals the presence of a distinct electronic screening channel in the bulk, hitherto undetected in more surface sensitive analysis. Comparison with model calculations identifies the character of the Mn 3d electronic states and clarifies the role, and the difference between surface and bulk, of hybridization in mediating the ferromagnetic coupling in (Ga,Mn)As.     

Hybrid ferromagnetic/semiconductor heterostructures for spintronics

Heterostructures that integrate conventional semiconductors with ferromagnetic semiconductors and ferromagnetic metals are important for developing a framework for semiconductor spintronics. We describe recent efforts to study ‘hybrid’ ferromagnetic/semiconductor heterostructures that combine conventional III-V and II-VI semiconductors with the ferromagnetic semiconductor (Ga,Mn)As and the ferromagnetic metal MnAs. We focus on the characteristics of two novel classes of heterostructures: (a) (Ga,Mn)As/AlAs/MnAs magnetic tunnel junctions (MTJs) that provide an all-electrical scheme for probing spin injection from metals into GaAs and (b) n-ZnSe/(Ga,Mn)As heterojunction diodes that surprisingly exhibit a magnetically-driven photoconductivity.     

Mn distribution and spin polarization in Ga(1-x)Mn x As

Using the density functional full-potential linearized augmented plane wave approach, the x-ray absorption and magnetic circular dichroism (XMCD) spectra of Ga(1-x)Mn x As are calculated. Significantly, XMCD of Mn is highly sensitive to the change in environment, and thus can be utilized to characterize impurity distribution. The nature of Mn-induced spin polarization on Ga and As sites, vital for the carrier mediated magnetic ordering, is discussed in light of computational and experimental results.     

Ferromagnetism in Ga(1-x)Mn(x)P: evidence for inter-Mn exchange mediated by localized holes within a detached impurity band

We report an energy gap for hole photoexcitation in ferromagnetic Ga(1-x)Mn(x)P that is tunable by Mn concentration (x < or = 0.06) and by compensation with Te donors. For x approximately 0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (TC) of 60 K and by thermally activated hopping below TC. Magnetization measurements reveal a moment of 3.9 +/- 0.4 muB per substitutional Mn while the large anomalous Hall signal demonstrates that the ferromagnetism is carrier mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.     

室温下(Ga,Mn)As中载流子的自旋弛豫特性

运用飞秒时间分辨抽运-探测克尔光谱技术,研究了室温下退火及未退火(Ga,Mn)As的载流子自旋弛豫的激发能量密度依赖性,发现电子自旋弛豫时间随激发能量密度增加而增大,而在同一激发能量密度下,退火样品比未退火样品具有更短的载流子复合时间、电子自旋弛豫时间和更大的克尔转角,显示DP机理是室温下(Ga,Mn)As的电子自旋弛豫的主导机理.退火(Ga,Mn)As的超快克尔增强效应显示其在超高速全光自旋开关方面的潜在应用价值,也为(Ga,Mn)As铁磁性起源的p-d交换机理提供了证据. 关键词： (Ga Mn)As稀磁半导体 时间分辨克尔光谱 电子自旋弛豫 DP机理     

Role of defect sites and Ga polarization in the magnetism of Mn-doped GaN

We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for Ga1-xMnxN.     

Dilute moment n-type ferromagnetic semiconductor Li(Zn,Mn)As

We propose to replace Ga in (Ga,Mn)As with Li and Zn as a route to high Curie temperature, carrier mediated ferromagnetism in a dilute moment n-type semiconductor. Superior material characteristics, rendering Li(Zn,Mn)As a realistic candidate for such a system, include high solubility of the isovalent substitutional Mn impurity and carrier concentration controlled independently of Mn doping by adjusting Li-(Zn,Mn) stoichiometry. Our predictions are anchored by ab initio calculations and comparisons with the familiar and directly related (Ga,Mn)As, by the physical picture we provide for the exchange interaction between Mn local moments and electrons in the conduction band, and by analysis of prospects for the controlled growth of Li(Zn,Mn)As materials.     

Mixed magnetic phases in (Ga,Mn)As epilayers

Two different ferromagnetic-paramagnetic transitions are detected in (Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition at a higher temperature results from (Ga,Mn)As cluster phases with [110] uniaxial anisotropy and that at a lower temperature is associated with a ferromagnetic (Ga,Mn)As matrix with 100 cubic anisotropy. A change in the magnetic easy axis from [100] to [110] with increasing temperature can be explained by the reduced contribution of 100 cubic anisotropy to the magnetic properties above the transition temperature of the (Ga,Mn)As matrix.     

Prediction of enhanced ferromagnetism in (Ga,Mn)As by intrinsic defect manipulation

In the diluted magnetic semiconductor (Ga,Mn)As the excess of As incorporated as As antisites (As_Ga) is responsible for the hole compensation. The As_Ga defect can be transformed into a As interstitial–Ga vacancy pair (As_i–V_Ga) upon illumination. In this paper we study the effects of such a transition on the ferromagnetism of (Ga,Mn)As using density functional theory within the local spin density approximation. We find that the ferromagnetic order in (Ga,Mn)As is strongly enhanced if As_Ga are transformed into As_i–V_Ga pairs, since the hole compensation is reduced. This suggests a valuable way to tune the carrier concentration and hence the T_c in (Ga,Mn)As, without changing the Mn concentration nor the microscopic configuration of the Mn ions.     

Proximity induced enhancement of the Curie temperature in hybrid spin injection devices

We investigate the increase of the Curie temperature T(C) in a lateral spin injection geometry where the ferromagnetic (Ga,Mn)As injector and detector contacts are capped by a thin iron film. Because of interlayer coupling between Fe and (Ga,Mn)As T(C) gets enhanced by nearly 100% for the thinnest (Ga,Mn)As films. The use of the proximity effect might pave the way for practical implementation of spintronic devices.     

Picosecond dynamics of the photoinduced spin polarization in epitaxial (Ga,Mn)as films

Static and time-resolved magneto-optical spectra of the ferromagnetic semiconductor (Ga,Mn)As show that a pulsed photoexcitation with a fluence of 10 microJ/cm(2) is equivalent to the application of an external magnetic field of about 1 mT, which relaxes with a decay time of 30 ps. This relaxation is attributed to the spin relaxation of electrons in the conduction band and is found to be not affected by interactions with Mn ions.