Sp-d Exchange Interaction Research Articles

Anisotropic Zeeman splitting and Stark shift of In1−yMnyAs1−xNx oblate quantum dots

The electronic structure, Zeeman splitting, and Stark shift of In1−yMnyAs1−xNx oblate quantum dots are studied using the ten-band k⋅p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level.

Electronic structure of paramagnetic In1-xMnx As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In1-xMnxAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero kz (kz is the wave vector in the wire direction), and the spin-splitting bands cross at kz = 0, whose kz-positive part and negative part are symmetrical. A proper magnetic field makes the kz-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero kz. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.

Negatively charged excitons in semimagnetic CdSe/ZnSe/ZnMnSe quantum dots

Low-temperature (T = 1.6 K) photoluminescence (PL) of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) with different magnitudes of the sp-d exchange interaction between the magnetic impurity ions and charge carriers has been studied in a magnetic field up to 12 T applied in the Faraday and Voigt geometry. The magnitude of the interaction was controlled by changing the fraction (ηe, h) of the squared wave function of charge carriers in the semimagnetic barrier by means of variation of the nonmagnetic (ZnSe) layer thickness. It is established that the sp-d exchange interaction leads to a change in the sign of the effective hole g factor even for ηe, h ∼ 5%, while further increase in the interaction magnitude is accompanied by a rapid growth in the magnitude of spin splitting for both electrons and holes. The quantum yield of PL exhibits a significant decrease due to nonradiative Auger recombination with the excitation of Mn ions only for ηe, h ∼ 12%, while the rate of the holes spin relaxation starts growing only for still higher ηe, h values. In a strong magnetic field perpendicular to the sample plane, the alignment of Mn spins leads to suppression of the Auger recombination only in the excited spin state. For a small rate of the hole spin relaxation, this leads to a rather unusual result: the emission from an excited trion state predominates in strong magnetic fields.

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.

Diluted magnetic semiconductor properties of Zn1 − x Cox O: 1 at% Al thin films prepared by pulsed DC magnetron sputtering

Zn1 − x CoxO: 1 at% Al(x = 0−0.3) films were grown on corning 7059 glass by asymmetrical bipolar pulsed dc magnetron sputtering. The c-axis orientation along the (002) plane was enhanced with increasing Co concentration. The ZnCoO thin films are grown to the fibrous grains of tight dome shape. The transmittance spectra showed that sp-d exchange interactions and typical d-d transitions become activated with increasing Co concentration. The electrical resistivity of ZnCoO films increased ranging from ∼10− 3 to ∼10−2 Ω ⋅ cm with increasing Co concentration, especially it increased greatly at 30 at% Co. X-ray photoelectron spectroscopy and alternating gradient magnetometer analyses indicated that no Co metal cluster in the ZnCoO films is formed and room temperature ferromagnetism is exhibited. These electrical and magnetic properties of ZnCoO films suggest a potential application of dilute magnetic semiconductor devices.

Magneto-optical properties for bulk Cd0.63−yMn0.37HgyTe single crystals

We investigated the magneto-optical properties of diluted magnetic semiconductor Cd1−x−yMnxHgyTe (x∼0.37) single crystals grown using a vertical Bridgman method. This material crystallizes in the zinc-blende structure for values of y⩽0.07. The band gap energy increases with increasing y (⩽0.03), while decreases with increasing y (⩾0.03). An anomalous behavior was also observed in the magnetic susceptibility and electrical resistivity. Faraday rotation as a function of photon energy with various Hg compositions for CdMnHgTe in the temperature range of 10–300K has been measured. The Verdet dispersion curves were enhanced near the fundamental band gap energy. The Verdet constant becomes larger since the effect of the sp-d exchange interaction is enhanced as the temperature is lowered. At 10K, the Verdet constants of the Faraday rotation for the samples with y=0, 0.01, 0.03, 0.05, and 0.07 were observed to be −1.09, −2.57, −3.94, −0.97, and −0.87deg∕Gcm at 613nm, respectively. The Verdet constant showed a maximum for the sample with y=0.03, and the value is approximately four times larger than that of the Cd0.63Mn0.37Te crystal. The Verdet constant has a maximum around Hg compositions with y∼0.03, and this result agrees well with the behavior of the magnetic susceptibility. This result reflects that Hg ion mediates and enhances the spin-spin interaction between Mn2+ ions with increasing Hg composition up to y∼0.03.

Optimization of Semimagnetic Semiconductor-Based Nanostructures for Spintronic Applications

We have analyzed the spin-filtering effects of the electron current in asymmetric ZnSe/Zn1-xMnxSe multilayer structures, under the influence of both an external magnetic field and a bias voltage. In this type of semiconductor systems, conduction band electrons interact with 3d electrons of the magnetic Mn2+ ions. Because of this sp-d exchange interaction, an external magnetic field modulates the effective potential profile seen by spin-up and spin-down electrons, giving rise to a large Zeeman effect. It is found that the degree of spin polarization changes significantly when the electrical bias is switched from forward to reverse, thus the proposed structure displays obvious behavior of spin-filter diode. This originates from the effective “lifting” of the potential for spin-up electrons, which tunnel through actual potential barriers. Structural parameters optimization, with the goal of maximizing the spin-filtering coefficient, was performed by using simulated annealing algorithm. The described effect may be important for designing new tunable spin-based multifunctional semiconductor devices.

Nature of the abnormal band gap narrowing in highly crystalline Zn1−xCoxO nanorods

Highly crystalline Zn1−xCoxO nanorods were prepared using a hydrothermal method. With increasing Co2+ dopant concentration, the lattice volume enlarged considerably, which is associated with the enhanced repulsive interactions of defect dipole moments on the wall surfaces. This lattice modification produced a significant decrease in band gap energies with its magnitude that followed the relationship, ΔEg=ΔE0∙(e−xB−1), where x and B are Co2+ dopant concentration and a constant, respectively. The abnormal band gap energies were indicated to originate from the sp-d exchange interactions that are proportional to the square of lattice volume.

Valence‐band mixing induced by sp‐d exchange interaction in CdMnTe quantum wires

Anisotropic magneto-optical effects have been studied theoretically by using multi-band effective mass method. We focus on a model of an ideal CdMnTe quantum wire surrounded by infinite potential barriers. The mixing of heavy- and light-hole bands is caused by both the one-dimensional quantum confinement and the sp-d exchange interaction. As a result of the valence-band mixing characteristic to the one-dimensional diluted magnetic semiconductor, the Zeeman splitting in the magnetic field parallel to the wire direction becomes smaller than that in the perpendicular magnetic field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sol-Gel 법으로 제작한 Zn1-xCoxO 박박의 미세조직 및 자기적 특성

Zn 1-x Co x O (x=0.05-0.20) films were grown on Corning 7059 glass by sol-gel process. A homogeneous and stable Zn 1-x Co x O sol was prepared by dissolving zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), cobalt acetate tetrahydrate ((CH₃CO₂)₂·Co·4H₂O) and aluminium chloride hexahydrate (AlCl₃·6H₂O) as solute in solution of isopropanol ((CH₃)₂·CHOH) and monoethanolamine (MEA:H₂NCH₂CH₂OH). The films grown by spin coating method were postheated in air at 650℃ for 1 h and annealed in the condition of vacuum (5×10 -6 Torr) at 300℃ for 30 min and investigated the nature of c-axis preferred orientation and physical properties with different Co concentrations. Zn 1-x Co x O thin films with different Co concentrations were well oriented along the c-axis, but especially a highly c-axis oriented Zn 1-x Co x O thin film was grown at 10 at% Co concentration. The transmittance spectra showed that Zn 1-x Co x O thin films occur typical d-d transitions and sp-d exchange interaction became activated with increasing Co concentration. The electrical resistivity of the films at 10 at% Co had the lowest value due to the highest c-axis orientation. X-ray photoelectron spectroscopy and alternating gradient magnetometer analyses indicated that no Co metal cluster was formed, and the ferromagnetic properties appeared, respectively. The characteristics of the electrical resistivity and room temperature ferromagnetism of Zn 1-x Co x O thin films suggested the possibility for the application to dilute magnetic semiconductors.

Spin-orbit and exchange interaction in a HgMnTe magnetic two-dimensional electron gas

Spin-orbit interaction and spd exchange interaction are investigated by analyzing beating patterns of Shubnikov-de Hass oscillations in a HgMnTe magnetic two-dimensional electron gas (2DEG) at various temperatures.The results show that (1) spin-orbit interaction results in zero-magnetic-field spin splitting in 2DEG; (2) spin-orbit interaction is dominant at low magnetic field, and the total spin-splitting decreases with increasing magnetic field due to the influence of Landau splitting and Zeeman splitting; (3) at a high magnetic field, the spd exchange interaction tends to saturate and the spin splitting corresponds to Zeeman splitting. The experiments demonstrate that the spin splitting deduced from fast Fourier transformation (FFT) of SdH oscillations versus. 1/B is different from zero-magnetic-field spin splitting when Zeeman spin splitting is comparable to zero-magnetic-field spin splitting.

Effect of alloy potential fluctuations on the exciton magnetic polaron in the bulk diluted magnetic semiconductorsCd1−xMnxTe

A theory for exciton magnetic polarons (EXMP's) in diluted magnetic semiconductors localized by the sp-d exchange interaction with assist of alloy potential fluctuations (APF's) is presented within the molecular-field approximation. First, we show that the self-trapped EXMP in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ is rarely realized except at quite low temperatures below \ensuremath{\sim}1 K. Then, we incorporate the effect of APF's with the optimal fluctuation method; the calculated results turn out to be consistent with the photoluminescence data above 1 K observed after band-edge excitation. This strongly suggests the necessity of APF's to localization of EXMP's. The calculation with APF's further demonstrates that the luminescence due to the EXMP in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ is observed clearly from $x\ensuremath{\sim}0.05,$ which is also consistent with the experiment.

Stability of the self-trapped exciton magnetic polaron in diluted magnetic semiconductors:Cd1−xMnxTe

The self-trapped exciton magnetic polaron (ST-EXMP) in diluted magnetic semiconductors (DMS's) has been the subject of intensive investigations related to the observed $L2$ photoluminescence during the past two decades, although the stability seems still in controversy. In this paper we have studied the stability condition for ST-EXMP theoretically: we applied the molecular-field approximation to begin with, and then incorporated the effects of the thermodynamic fluctuations of magnetization. The calculation suggests that the exciton magnetic polaron (EXMP) in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ $(x=0.05--0.2)$ is not self-trapped except at quite low temperatures below \ensuremath{\sim}1 K, but the other primary localization mechanisms, for example, alloy potential fluctuations, are necessary for the localization at the temperatures above \ensuremath{\sim}1 K. This conclusion is different from a recent theoretical report [J. Appl. Phys. 81, 6297 (1997)] that the ST-EXMP in DMS's is stable up to relatively high temperatures $(T<~30\mathrm{K})$ over a wide range of Mn concentrations without any other primary localization. The reason for this difference is clarified. It is also discussed briefly that when the higher-order sp-d exchange interaction is taken into account, the ST-EXMP in DMS's may become more unstable than that studied in the present paper.

Magneto-optical properties of diluted magnetic semiconductor quantumdots

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magneticsemiconductor quantum dot (QD), with the Coulomb interaction and the sp–dexchange interaction included. The MP energy decreases rapidly with increasingmagnetic field at low magnetic field and saturates at high magnetic field for smallQDs, and the dependences of the MP energy on magnetic field are quite differentfor different QD radii due to the different carrier-induced magnetic fieldsBMP.The competition between the sp–d exchange interaction and the band gapshrinkage results in there being a maximum exhibited by the MP energy withincreasing temperature. Our numerical results are in good agreement withexperiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, ForchelA, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

Intracenter transitions of iron-group ions in II–VI semiconductor matrices

A review is made of studies of intracenter optical transitions in 3d shells of iron-group divalent (magnetic) ions. Attention is focused on the emission spectra of Mn2+ ions in CdTe, ZnS, and ZnSe crystals. An analysis is made of the structure of intracenter absorption and luminescence and of the effect that the elemental matrix composition, magnetic-ion concentration, temperature, hydrostatic pressure, and structural phase transitions exert on the intracenter transitions. The mutual influence of two electronic excitation relaxation mechanisms, interband and intracenter, is considered. The specific features of the intracenter emission of magnetic ions embedded in two-dimensional systems and nanocrystals associated with a variation in sp-d exchange interaction and other factors are discussed. Data on the decay kinetics over the intracenter luminescence band profile are presented as a function of temperature, magnetic ion concentration, and excitation conditions. The saturation of the luminescence and the variation of its kinetic properties under strong optical excitation, which are caused by excitation migration and the cooperative effect, as well as the manifestation of a nonlinearity in intracenter absorption, are studied.

Hole spin polarization in AlAs/Ga1-x-yMn xAl yAs structures

A self-consistent calculation of the electronic properties of GaAlAs:Mn magnetic semiconductor quantum well structures is performed including the Hartree term and the sp-d exchange interaction with the Mn magnetic moments. The spin polarization density is obtained for several structure configurations.

DILUTED MAGNETIC SEMICONDUCTORS FOR MAGNETO-OPTICAL APPLICATIONS (invited)

In diluted magnetic semiconductors (DMSs), extended sp-carriers strongly couple with localized d-electrons of transition metal ions. This sp-d exchange interaction causes interesting magneto-optical phenomena of DMSs. Large Faraday effect of Cd1-xMnxTe can be used for an integration of magneto-optical waveguide with semiconductor optical devices. The sp-d exchange interaction can be used for the manipulation of the motion of Mn2+ in Cd1-xMnxTe by ultrafast laser pulses. We also discuss magneto-optical properties of new DMSs.

Optical Spectroscopy on Single Semimagnetic Quantum Dots ? Probing the Interaction between an Exciton and Its Magnetic Environment

Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.

Magnetooptical properties of CdTe quantum well structures with quaternary CdMnMgTe barriers

The optical properties of CdTe single quantum wells (QWs) with barriers made of quaternary CdMnMgTe grown by molecular beam epitaxy have been investigated by use of photoluminescence and reflectivity spectroscopy. The spectra are characterized by small linewidths and small Stokes shifts indicating a high quality of the structures. Apart from optical features related to confined excitons, an additional line attributed to donor-bound excitons was also observed in majority of the structures. The use of a quaternary material for the barriers made it possible to keep the depth of the confining potential constant while varying the amount of the magnetic component. The latter, via the sp-d exchange interaction, is the main contributor to the spin splitting of the carriers. The Zeeman splitting of the excitons confined in the QWs was analysed under an assumption that the interfaces are not abrupt. The obtained values of the Mn intermixing lengths are about 5 Å which is typical for CdTe-based QWs with barriers made of ternary compounds.

Analysis of ellipsometric and photoemission spectra of diluted magnetic semiconductors by hybridization interaction mechanism

In a study of the E 1 and E 1+Δ 1 band gap for films of Zn 1− x (Mn, Fe, Co) x Se (0≤ x<0.14) using spectroscopic ellipsometry (SE), we demonstrated that a sp-d hybridization interaction gave a better description of the concentration dependence of the E 1 and E 1+Δ 1 band gaps than did an sp-d exchange interaction. We applied the sp-d hybridization interaction mechanism developed from this SE study to analyze photoemission spectra. The result shows that our model predicts the reversed ordering of the t 2 and e states when they are located inside the valence bands, contrary to the conventional understanding.