Dilute Magnetic Semiconductor Quantum Research Articles

Spin Dynamics in Mn:ZnSe Quantum Dots: A Pulsed High-Frequency EPR Study

Dilute magnetic semiconductor quantum dots (DMSQDs) have emerged as potential materials for future spin-based technologies. A critical parameter for the development of this technology is a relative...

Volume and Concentration Scaling of Magnetism in Dilute Magnetic Semiconductor Quantum Dots

Investigation of the magnetism of dilute magnetic semiconductors (DMSs) by changing particle dimensionality and doping concentration and its complete understanding is a major step toward their application in multifunctional devices. The importance of effects, such as magnetization reversal with size and magnetic invariance with respect to doping concentration for an appropriate functioning of DMS systems, is empirically well-known. However, explicit demonstration of these effects, specifically in nanomaterials, has so far not been studied mainly due to the lack of synthetic handle. In this work, we have demonstrated the prerequisites of DMS materials by isolating origins of magnetism arising due to clustering of magnetic dopant ions as well as sp–d exchange interaction with the host. We have studied magnetism with varying concentrations of dopant ions and have shown that the magnetism arising due to exchange interaction with the host is invariant up to 10% doping concentration, demonstrating the concentra...

Doping-Concentration-Induced Ferromagnetism and Antiferromagnetism in In2S3:Dy3+ Quantum Dots

Diluted magnetic semiconductor (DMS) quantum dots have been researched extensively due to their potential applications in next-generation spin-based devices. Herein, the cubic In2S3:Dy3+ DMS quantum dots (3–5 nm) with different doping concentrations were synthesized via a gas–liquid phase chemical deposition method. The effect of Dy3+ content on the photoluminescence (PL) and ferromagnetism was investigated. The PL emission spectra exhibit a blue-shift compared with those reported previously due to the increased quantum size confinement and enhanced intensity attributed to the Dy3+ doping. The distinct and stronger room-temperature ferromagnetism is observed from vibrating sample magnetometer (VSM) measurement. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases and antiferromagnetic interation plays a dominant position after a certain doping concentration value can be further confirmed according to the zero field cooling/field cooling (ZFC/FC) curves. As revealed in the magnetic origin study from first-principles calculations, the ferromagnetism obtained arises not only from the Dy atoms but also from the In vacancies. In addition, we also proposed a spontaneous mechanism based on the bound magnetic polaron theory to explain the change of saturation magnetizations along with Dy3+ doping concentration. This work provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote development of spintronic applications.

Ballistic conductance in a diluted magnetic semiconductor quantum ring

In the present paper, we have calculated the ballistic conductance in a quantum ring in a diluted magnetic semiconductor (DMS) material in the presence of a magnetic field by using the tunneling Hamiltonian method. We take into account the effect of the Rashba spin-orbital interaction, the exchange interaction and the Zeeman term on the ballistic conductance. We have calculated the energy spectrum and wave functions of the electrons. Moreover we have calculated the ballistic conductance dependency on the flux and Mn concentration at zero temperature of a diluted magnetic semiconductor quantum ring. It has been shown that the dependency of the conductance as a function of flux exhibits resonances whose positions are determined by the quasi-bound discrete levels of the electron in the DMS quantum ring.

Diffusion doping in quantum dots: bond strength and diffusivity.

Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M2+ (Fe2+, Ni2+, Co2+, Mn2+). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.

Graphene activating room-temperature ferromagnetic exchange in cobalt-doped ZnO dilute magnetic semiconductor quantum dots.

Control over the magnetic interactions in dilute magnetic semiconductor quantum dots (DMSQDs) is a key issue to future development of nanometer-sized integrated "spintronic" devices. However, manipulating the magnetic coupling between impurity ions in DMSQDs remains a great challenge because of the intrinsic quantum confinement effects and self-purification of the quantum dots. Here, we propose a hybrid structure to achieve room-temperature ferromagnetic interactions in DMSQDs, via engineering the density and nature of the energy states at the Fermi level. This idea has been applied to Co-doped ZnO DMSQDs where the growth of a reduced graphene oxide shell around the Zn(0.98)Co(0.02)O core turns the magnetic interactions from paramagnetic to ferromagnetic at room temperature, due to the hybridization of 2p(z) orbitals of graphene and 3d obitals of Co(2+)-oxygen-vacancy complexes. This design may open up a kind of possibility for manipulating the magnetism of doped oxide nanostructures.

The electromagnetic properties of dilute magnetic GeSi:Mn quantum rings

We present a synthesis of GeSi:Mn dilute magnetic semiconductor (DMS) quantum rings and their electromagnetic properties. The GeSi:Mn rings were grown by means of a chemical vapor deposition process, and then doped with Mn using a magnetic sputtering technique. The as-grown and annealed DMS ring samples present wide and open magnetic hysteresis loops with large remnant magnetizations. The largest magnetic moment per Mn atom of 4.2 μB was obtained for the annealed sample at room temperature. The voltage and magnetic resistance differences in the presence or absence of an applied magnetic field were 4 V and 163 kΩ, respectively.

Ruderman-Kittel-Kasuya-Yosida interaction between diluted magnetic semiconductor quantum dots embedded in semiconductor

Using the Keldysh Green's function method, we calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interaction between the localized spins of two diluted magnetic semiconductor (DMS) quantum dots embedded in a two-dimensional semiconductor. It was shown that the RKKY interaction is strongly dependent on the hybridization between the quantum-confined states in DMS quantum dots and the band in the semiconductor, the discrete energy levels in DMS quantum dots and the carrier density of the semiconductor. Since the carrier energy levels in DMS quantum dots may be adjusted by the applied voltage gates, the RKKY interaction of the present system is gate-controllable. It provides an alternate way to get the controllable RKKY magnetic interaction in semiconductor nanostructures for potential application in quantum information processing.

Interface inversion asymmetry in Cd1–xMnxTe quantum wells

AbstractSpin‐flip Raman spectroscopy (SFRS) reveals an intrinsic in‐plane anisotropy of the conduction band spin splitting in dilute magnetic semiconductor quantum wells based on cadmium telluride, containing a twodimensional electron gas, and intended for studies of extrinsic anisotropies induced through nanomagnets.We consider various possible sources of this intrinsic anisotropy (interface, bulk or structural inversion asymmetries) and we ascribe the present results to an interface‐related inversion asymmetry of the quantum wells. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on the spin polarization of a current through a hybrid resonant tunneling diode

The effects of spin-splitting energy and temperature on the electric transport properties through a hybrid resonant tunneling diode (RTD) with ferroelectric barriers and a diluted magnetic semiconductor (DMS) quantum well are studied by using the non-equilibrium Green's function method. The results show that the period of spin polarization oscillation as a function of a bias voltage increases by increasing the spin-splitting energy of the DMS quantum well. The spin polarization of the current near the Fermi level is highly influenced by a change in temperature. The RTD with a ferromagnetic quantum well is more efficient than that with a ferromagnetic emitter to manipulate the spin direction of the current and to enhance the spin polarization of the current without changing the direction of an external magnetic field.

Multi-spin-subband structure of dilute magnetic semiconductor quantum wells: Feedback mechanism

Using a fully self-consistent envelope function approach, we focus on wide conduction band NMS (non-magnetic semiconductor)/DMS (dilute magnetic semiconductor)/NMS quantum wells, under weak external parallel magnetic field, where many spin-subbands are usually present. We concentrate on small values of the magnetic field because we want to investigate the influence of the feedback mechanism due to the difference of the concentrations of spin-up and spin-down carriers which could induce spontaneous spin-polarization i.e. in the absence of a magnetic field. We study the spin-subband structure, the spin-subband populations and the spin-polarization as functions of the sheet carrier concentration, N s , for different values of the magnitude of the exchange interaction, | J | , between the itinerant carriers and the magnetic impurities. Our calculations for 0.01 T show that at 20 K the values of | J | necessary to make this feedback mechanism sufficiently strong are too high compared to the | J | values of common Mn-doped systems in the conduction band. However, the feedback mechanism will be sufficiently strong at low enough temperatures below 20 K for realistic values of | J | . Moreover, we explain how increasing the sheet carrier concentration the heterostructure is transformed from an almost square quantum well to a system of two coupled heterojunctions with an intermediate soft barrier.

Local Definition of Spin Polarization in a Semiconductor by Micro-scale Current Loops

We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn2+ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements.

Coherent Raman Spectroscopy of Cd1-xMnxTe Quantum Wells

The first coherent Raman electron spin resonance (ESR) results obtained in dilute magnetic semiconductor quantum wells are presented and the sensitivity and selectivity of the coherent Raman-detected ESR (CRESR) technique for probing the local environment of magnetic ions in a semiconductor host is demonstrated. The lineshapes and linewidths that are observed are considered in the context of previous work on bulk Cd1-x,MnxTe alloys and on CdTe/MnTe heterostructures.

Local spin manipulation in ferromagnet-semiconductor hybrids

The authors demonstrate the usage of magnetic fringe fields from nanoscale ferromagnets to locally control the spin degree of freedom in a semiconductor. Fringe fields stemming from Fe∕Tb multilayer ferromagnets induce a local, remanent out-of-plane magnetization in a ZnCdMnSe dilute magnetic semiconductor quantum well, which in turn aligns the spin of photogenerated carriers via sp-d exchange interaction. The authors achieve a local exciton spin polarization of up to ±12% at 4K without the need of an external magnetic field. The spin polarization can be controlled in sign and magnitude via the magnetization of the ferromagnet and is observable up to T=80K.

Fe/Tb multilayer ferromagnets for local semiconductor spin control

We investigated a hybrid structure consisting of microstructured Fe/Tb multilayer ferromagnets (FMs) and a ZnCdMnSe dilute magnetic semiconductor quantum well (DMS QW) by means of superconducting quantum interference device (SQUID) magnetometry and micro-photoluminescence (µPL) spectroscopy. The Fe/Tb multilayers show a strong perpendicular magnetic anisotropy (PMA) with a temperature dependent coercive field Hc reaching values of 2.5 T /μ0 at 4 K. The out-of-plane magnetic fringe field stemming from the Fe/Tb microstructures induces a locally varying magnetization in the underlying DMS QW, which polarizes the spins of optically excited charge carriers in the DMS. We demonstrate that the coupling between these magnetic systems enables us to control the local carrier spin polarization in the DMS via the magnetization of microscale Fe/Tb ferromagnetic wires. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Manipulation of exciton spin states in a dilute magnetic semiconductor by nanostructured ferromagnets

AbstractA hybrid of nanostructured Fe/Tb multilayer ferromagnets and ZnCdMnSe dilute magnetic semiconductor (DMS) quantum well was studied by magneto‐micro‐photoluminescence (µPL) spectroscopy. When the sample is magnetized in an external magnetic field an additional fringe field from the nanomagnets effectively modulates the local exciton spin states in the quantum well (QW) due to the giant Zeeman effect. Observation of 1.5% of remanent PL polarization degree from the QW gives a direct proof of remanent exciton spin polarization in the DMS. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MAGNETO-PHOTOLUMINESCENCE STUDY AT A FRACTIONAL QUANTUM HALL REGIME OF CHARGED EXCITONS IN A DILUTE MAGNETIC SEMICONDUCTOR

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1-x Mn x Te / Cd 1-y Mg y Te single quantum wells. In low magnetic fields (the electrons filling factor ν>1), we observed several photoluminescence lines associated with spin-singlet and spin-triplet charged excitons. In high magnetic fields (ν<1), we observed only one of the spin-triplet charged excitons that are called as a bright type due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells, where the photoluminescence intensity increased at a fractional value of [Formula: see text]. We consider that the screening effect plays a crucial role for this behavior in high magnetic fields.

Diluted magnetic semiconductor quantum dots: An extreme sensitivity of the hole Zeeman splitting on the aspect ratio of the confining potential

The valence band states confined in infinitely deep quantum dots made of diluted magnetic semiconductors (DMS) are considered theoretically. A complex anisotropic structure of the valence bands in DMSs with cubic symmetry described by the full Luttinger Hamiltonian is taken into account. It is found that the Zeeman splitting is very sensitive to the shape of the confining potential and, in particular, to its orientation relative to the direction of an external magnetic field. This sensitivity has its origin in a mixing of different spin components of a hole wave function which takes place for finite hole wave vectors $\mathbf{k}$. Several consequences of the effect are discussed, including a possibility to control the interdot tunneling by an external magnetic field. It is shown also that the polarizations of optical transitions in a single DMS quantum dot depend on details of geometry of its confining potential as well as on the strength of the magnetic field.

Spin sensitive dynamics of a charged exciton in magnetic fields

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1− x Mn x Te/Cd 1− y Mg y Te single quantum wells. We observed several transitions arising from spin-singlet and spin-triplet charged excitons. The transient photoluminescence data showed characteristic decay time depending on the spin configuration of the charged excitons. The spin-triplet charged excitons of a bright type are known to realize due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells. We have also found that the decay-time of charged excitons depends much on photon energies used for excitation.

Properties of conduction-band dilute-magnetic-semiconductor quantum wells in an in-plane magnetic field: A density of states profile that is not steplike

Leibniz Institute for Neurobiology, Special Lab for Non-Invasive Brain Imaging, Brenneckestrasse 6, D-39118 Magdeburg, Germanyand Physics Department, Solid State Section, University of Athens, Panepistimiopolis, Zografos, GR-15784 Athens, Greece~Received 6 September 2003; published 12 March 2004!We examine how an in-plane magnetic fieldB modifies the density of states ~DOS! in narrow-to-wide,conduction-band dilute-magnetic semiconductor quantum wells. We demonstrate that the DOS diverges sig-nificantly from the ideal steplike two-dimensional electron gas form and this causes severe changes to thephysical properties, e.g., to the spin-subband populations, the internal and free energy, the Shannon entropy,and the in-plane magnetizationM. We predict a considerable fluctuation of M in cases of vigorous competitionbetween spatial and magnetic confinement.DOI: 10.1103/PhysRevB.69.113302 PACS number~s!: 73.20.2r, 85.75.2d, 75.75.1a, 65.40.Gr