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.     

Pump–Probe Spectroscopy of Exciton Spin Injection Process in Diluted Magnetic Quantum Wells

Ultrafast spin dynamics of excitons is studied in a double quantum well composed of Cd_0.92Mn_0.08Te and CdTe wells with a Cd_0.80Mg_0.20Te tunnel barrier, in magnetic fields, by pump-and-probe absorption spectroscopy. The excitonic injection process is clarified with the injection time of 30 ps from the Cd_0.92Mn_0.08Te spin aligner to the CdTe spin detector. The time dependencies of circularly polarized differential absorbances show directly the spin injection into the CdTe well. The spin relaxation of the injected excitons is observed as a function of energy in the exciton band of the CdTe well. In addition, ultrafast relaxation processes of spin-polarized carriers in the Cd_0.78Mn_0.05Mg_0.17Te barrier are studied, when it is stacked directly with the CdTe well.     

Transient Spectroscopy of Optical Spin Injection in ZnMnSe/ZnCdSe Quantum Structures

We show, by time-resolved magneto-photoluminescence (PL) spectroscopy in combination with selective laser excitation, that optical polarization of the ZnCdSe spin detector induced by spin injection from the ZnMnSe spin injector persists over a much longer time scale than the lifetime of the ZnMnSe excitons. This finding provides compelling experimental evidence that the dominant mechanism for the observed spin injection in the ZnMnSe/ZnCdSe structures should not be due to injection of the excitonic spins of the diluted magnetic semiconductor (DMS). It is rather due to e.g. a delayed spin injection arising from tunneling of individual carriers or/and trapped spins in ZnMnSe.     

The effect of ferromagnetic Mn-delta-doped layer on the emission properties of GaAsSb/GaAs and InGaAs/GaAsSb/GaAs heterostructures

We have studied the emission characteristics and circularly polarized electroluminescence of light-emitting diodes based on heterostructures with a single (GaAs/GaAsSb/GaAs) or two-layer (GaAs/InGaAs/GaAsSb/GaAs) quantum well (QW) and a Mn-delta-doped layer in the GaAs barrier. The ferromagnetic effect of the delta-layer of Mn on the spin polarization of carriers in QWs based on type-II heterostructures has been observed and studied for the first time. The observed phenomena are described using a model of the exchange interaction of Mn ions in the barrier and holes in the QW.     

Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.

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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.     

Spin‐Orbit Torque Switching of a Nearly Compensated Ferrimagnet by Topological Surface States

Utilizing spin‐orbit torque (SOT) to switch a magnetic moment provides a promising route for low‐power‐dissipation spintronic devices. Here, the SOT switching of a nearly compensated ferrimagnet Gd_x(FeCo)_1?x by the topological insulator [Bi₂Se₃ and (BiSb)₂Te₃] is investigated at room temperature. The switching current density of (BiSb)₂Te₃ (1.20 × 10⁵ A cm^?2) is more than one order of magnitude smaller than that in conventional heavy‐metal‐based structures, which indicates the ultrahigh efficiency of charge‐spin conversion (>1) in topological surface states. By tuning the net magnetic moment of Gd_x(FeCo)_1?x via changing the composition, the SOT efficiency has a significant enhancement (6.5 times) near the magnetic compensation point, and at the same time the switching speed can be as fast as several picoseconds. Combining the topological surface states and the nearly compensated ferrimagnets provides a promising route for practical energy‐efficient and high‐speed spintronic devices.     

Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover (SCO) complex [Fe(HB(tz)₃)₂] (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.     

Spin current,spin accumulation and spin Hall effect

Abstract

Nonlocal spin transport in nanostructured devices with ferromagnetic injector (F1) and detector (F2) electrodes connected to a normal conductor (N) is studied. We reveal how the spin transport depends on interface resistance, electrode resistance, spin polarization and spin diffusion length, and obtain the conditions for efficient spin injection, spin accumulation and spin current in the device. It is demonstrated that the spin Hall effect is caused by spin–orbit scattering in nonmagnetic conductors and gives rise to the conversion between spin and charge currents in a nonlocal device. A method of evaluating spin–orbit coupling in nonmagnetic metals is proposed.     

Structural Control of Rashba Spin–Orbit Coupling in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As Quantum Wells

We investigated the correlation between the Rashba spin–orbit coefficient α and potential shape of the quantum wells (QW), where α values are experimentally deduced from the weak antilocalization analysis. We studied the gate I–V properties of the QW samples and have obtained results consistent with the potential shapes predicted for these QWs.     

Giant Zeeman Splitting of Excitons in a III–V Nonmagnetic Quantum Well Electronically Coupled With a II–VI Semimagnetic Quantum Well

We report on design, fabrication by molecular beam epitaxy, and photoluminescence (PL) studies of GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum wells (QWs), where resonant electronic coupling occurs through a heterovalent interface. The resonant conditions achieved in the properly designed sample facilitate penetration of the electron wave function from the nonmagnetic GaAs QW into the diluted magnetic semiconductor ZnCdMnSe QW. It results in the sign reversal and drastic increase of a GaAs QW excitonic g factor. The exciton spin splitting observed in the magneto-PL spectra is in general agreement with the calculation performed within the envelope function approximation, taking into account both the inter-well electron coupling and Brillouin-like paramagnetic behavior of the Mn²⁺ ions.     

Spin Wave Excitation,Detection, and Utilization in the Organic-Based Magnet,V(TCNE)x (TCNE = Tetracyanoethylene)

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin-wave propagation needed for information processing devices. The organic-based magnet [V(TCNE)_x; TCNE = tetracyanoethylene; x ≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non-coherent spin waves on various modes in V(TCNE)_x is demonstrated and show that the angular momentum carried by microwave-excited coherent spin waves in a V(TCNE)_x film can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)_x film. In addition, it is shown that non-coherent spin waves in a V(TCNE)_x film, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)_x. Combining coherent and non-coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic-based magnets.     

Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

Tuning optical properties of high In content InGaAs/GaAs capped InAs quantum dots by post growth rapid thermal annealing

The effect of rapid thermal annealing on InAs quantum dots (QDs) capped with In_0.4Ga_0.6As/GaAs layer has been investigated by photoluminescence (PL). An unusual red shift of the PL emission peak has been observed for an annealing temperature (T_a) of 650 °C together with a pronounced improvement of the PL from the quantum well like heterocapping layer (QW). This behavior is attributed to the strain induced phase separation of the hetero-capping alloy. However, for T_a = 750 °C, a blue shift of the QDs PL peak has been observed with respect to that of the as-grown sample. For this annealing temperature the PL intensity of the QW exceeds that of the QDs indicating a relatively prominent In/Ga interdiffusion. When annealed at 850 °C, only the PL arising from the QW can be detected in addition to a broadened low energy side band indicating the dissolution of the QDs at that temperature.     

Filtering Spin with Tunnel-Coupled Hole Quantum Wires

Creating spin-polarized currents in nonmagnetic semiconductors is one of the key prerequisites for realizing spintronics devices. We have shown previously that the k-linear Rashba spin splitting present in two-dimensional (2D) electron systems can be utilized in a momentum-selective tunneling geometry to design a spin filter without using magnetic fields or ferromagnetic contacts. Motivated by the fact that spin–orbit effects are typically much stronger in 2D hole systems, we consider quantum wires formed by additional confinement of the lowest (heavy-hole) 2D valence subband. Its k ³-type Rashba term gives rise to a k-linear spin splitting for holes in the quantum wire. Implementation of the spin-filter design is then analogous to the electron case but, in the hole system, requires less momentum selectivity and should therefore be easier to realize.     

Ionic Liquid Gating Control of Spin Reorientation Transition and Switching of Perpendicular Magnetic Anisotropy

Electric field (E‐field) modulation of perpendicular magnetic anisotropy (PMA) switching, in an energy‐efficient manner, is of great potential to realize magnetoelectric (ME) memories and other ME devices. Voltage control of the spin‐reorientation transition (SRT) that allows the magnetic moment rotating between the out‐of‐plane and the in‐plane direction is thereby crucial. In this work, a remarkable magnetic anisotropy field change up to 1572 Oe is achieved under a small operation voltage of 4 V through ionic liquid (IL) gating control of SRT in Au/[DEME]⁺[TFSI]^?/Pt/(Co/Pt)₂/Ta capacitor heterostructures at room temperature, corresponding to a large ME coefficient of 378 Oe V^?1. As revealed by both ferromagnetic resonance measurements and magnetic domain evolution observation, the magnetization can be switched stably and reversibly between the out‐of‐plane and in‐plane directions via IL gating. The key mechanism, revealed by the first‐principles calculation, is that the IL gating process influences the interfacial spin–orbital coupling as well as net Rashba magnetic field between the Co and Pt layers, resulting in the modulation of the SRT and in‐plane/out‐of‐plane magnetization switching. This work demonstrates a unique IL‐gated PMA with large ME tunability and paves a way toward IL gating spintronic/electronic devices such as voltage tunable PMA memories.     

Effect of barrier layer composition and thickness on structural and optical properties of TlInGaAsN/TlGaAs(N) triple quantum wells

Influence of the barrier layer composition and thickness on the structural and optical properties of TlInGaAsN Triple quantum wells (TQWs) was studied. Three types of TlInGaAsN TQW structures with different barriers were grown by gas-source molecular-beam epitaxy. Strong photoluminescence emission was obtained at room temperature from the TlInGaAsN TQW samples having 10 nm-thick TlGaAsN barriers, compared with the TlInGaAsN TQWs of 26 nm-thick TlGaAs barriers and those of 30 nm-thick TlGaAsN barriers. Structural investigations revealed that the TQWs with 10 nm-thick TlGaAsN barriers have good structural qualities. On the other hand, the other two samples showed the composition modulations at the interface between the lower side of the 3rd QW layer and the barrier layer. It was found that the addition of nitrogen into barrier layers and the decrease of barrier layer thickness significantly improve the crystalline quality and in turn the luminescence properties of the TlInGaAsN TQWs.     

An active lasing region with a quantum well and a quantum dot array

A combined active lasing region of the new type, containing an In_0.2Ga_0.8As quantum well (QW) and a single-layer array of InAs quantum dots (QDs) located outside the QW, was studied. In this system, the QW accumulates the injected charge carriers and the QD array serves as a radiator. The energy levels of electrons and holes in a QD were calculated. It is shown that the QDs can be filled by the resonance tunneling of holes from the QW to an unoccupied QD. The electron energy level in an unoccupied QD is markedly higher than that in the QW, but occupation of the QD by a hole leads to a resonance of the electron levels. Theoretical conclusions agree with the results of observations on a prototype laser with a combined active region.     

Anisotropic Spin Splitting and Spin Relaxation in AlGaAs/GaAs Quantum Structures

Spin relaxation due to the D'yakonov–Perel' mechanism is intimately related with the spin splitting of the electronic states. We calculate the spin relaxation rates from anisotropic spin splittings of electron subbands in n-(001)-AlGaAs/GaAs quantum structures obtained in a self-consistent multiband approach. The giant anisotropy of spin relaxation rates found for different spin components in the (001) plane can be ascribed to a mutual compensation of terms because of the asymmetry of the bulk crystal and the quantum well structure.     

Highly spin-polarized materials and devices for spintronics∗

Abstract

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co₂Cr_{1 ? x}Fe_xAl (CCFA(x)) and Co₂FeSi_{1 ? x}Al_x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co₂FeSi_0.5Al_0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2₁ than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe₂ film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe₂O₄ (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe₂ (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe₂ /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of - 124% at 10 K when the configuration of the magnetizations of CFO and CoFe₂ changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The - 124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.