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This paper describes the contributions made by Władysław (Władek) Walukiewicz and his colleagues to the field of ferromagnetic semiconductor (FMS) alloys, such as (Ga,Mn)As. We focus on two key accomplishments. First, this team has predicted the formation of Mn interstitials in these materials, which have a profound effect on ferromagnetism in semiconductors. Additionally, identifying the conditions at which interstitials form has provided grounds for optimizing their ferromagnetic properties. Second, by applying the approach of band anticrossing to ferromagnetic semiconductors, this team has mapped out the properties of an Mn-derived impurity band in these materials. This is of particular importance in the field, because holes, which reside in the Mn-derived impurity band, are the very mechanism responsible for ferromagnetic order in FMSs. We discuss the effect that these accomplishments have on our understanding of FMSs and how they have contributed to progress in this area. We then describe the pathways that these achievements have opened up toward further progress in both basic and applied fronts of ferromagnetism in semiconducting systems; and we present our perspective on where additional work along the lines initiated by Władek Walukiewicz should be extended to further benefit this field. [ABSTRACT FROM AUTHOR]

We report different growth modes and corresponding magnetic properties of thin EuSe films grown by molecular beam epitaxy on BaF2, Pb1−xEuxSe, GaAs, and Bi2Se3 substrates. We show that EuSe grows predominantly in the (001) orientation on GaAs(111) and Bi2Se3, but along the (111) crystallographic direction on BaF2 (111) and Pb1−xEuxSe (111). High resolution transmission electron microscopy measurements reveal a sharp and highly crystalline interface for both (001) and (111) EuSe films. In agreement with previous studies, ordered magnetic phases include antiferromagnetic, ferrimagnetic, and ferromagnetic phases. In contrast to previous studies, we found a strong hysteresis of the antiferromagnetic–ferrimagnetic transition. The ability to grow epitaxial films of EuSe on Bi2Se3 and of Bi2Se3 on EuSe enables further investigation of interfacial exchange interactions between various phases of an insulating metamagnetic material and a topological insulator. [ABSTRACT FROM AUTHOR]

Effect of spin-orbit-induced (SOI) field on magnetization reversal in ferromagnetic semiconductor GaMnAs films with 4-fold in-plane magnetic anisotropy has been investigated by planar Hall resistance (PHR) and anisotropic magnetoresistance (AMR) with current along the <110> and <100> crystallographic directions, respectively. Field scan hysteresis of PHR and AMR measured with current of 2.5 mA were observed to be asymmetric respect to zero field, showing different transition fields between positive and negative field regions. In addition, the hysteresis asymmetry is reversed as the sign of the current is reversed from positive to negative. Such dependence of asymmetry on current polarity implies that the current-induced spin-orbit field, which is known to depend on the current direction, significantly affects magnetization reversal. The asymmetry observed for currents along [100], [010], [110], and [ 1 1 ̄ 0 ] crystallographic directions is fully consistent with the effects arising from the Dresselhaus-type SOI fields in GaMnAs films. [ABSTRACT FROM AUTHOR]

We have studied spin-orbit torque (SOT) magnetization switching in a (Ga,Mn)(As,P) film with vertically-graded magnetic anisotropy. The magnetization switching chirality during current scans reveals that strain-induced Dresselhaus-type spin-orbit field does the major role for spin polarization of carriers causing SOT in the system. The volume fraction of SOT magnetization switching significantly depends on the magnitude and direction of the applied bias field. This feature leads to the realization of stable multilevel magnetic states in composition-graded (Ga,Mn)(As,P) film. The experiment demonstrates that multiple magnetic state can be robustly set by using appropriate bias fields. This characteristic can be used to realize SOT-driven multi-state memories and/or memristor devices, which are key ingredients for neuromorphic computing. [ABSTRACT FROM AUTHOR]

Switching of magnetization by spin–orbit torque in the (Ga,Mn)(As,P) film was studied with currents along ⟨100⟩ crystal directions and an in-plane magnetic field bias. This geometry allowed us to identify the presence of two independent spin–orbit-induced magnetic fields: the Rashba field and the Dresselhaus field. Specifically, we observe that when the in-plane bias field is along the current (I ∥ Hbias), switching is dominated by the Rashba field, while the Dresselhaus field dominates magnetization reversal when the bias field is perpendicular to the current (I ⊥ Hbias). In our experiments, the magnitudes of the Rashba and Dresselhaus fields were determined to be 2.0 and 7.5 Oe, respectively, at a current density of 8.0 × 105 A/cm2. [ABSTRACT FROM AUTHOR]

We describe the structural evolution of dilute magnetic (Sn,Mn)Se films grown by molecular beam epitaxy on GaAs (111) substrates, as revealed by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. When the Mn concentration is increased, the lattice of the ternary (Sn,Mn)Se films evolves quasi-coherently from a SnSe2 two-dimensional (2D) crystal structure into a more complex quasi-2D lattice rearrangement, ultimately transforming into the magnetically concentrated antiferromagnetic MnSe 3D rock-salt structure as Mn approaches 50 at. % of this material. These structural transformations are expected to underlie the evolution of magnetic properties of this ternary system reported earlier in the literature. [ABSTRACT FROM AUTHOR]

Topological superconductors that support Majorana fermions have been predicted when one-dimensional semiconducting wires are coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor-superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radiofrequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ?V ?=?hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge- 2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge- e quasiparticles. This is a unique signature of the Majorana fermions, predicted almost 80 years ago. [ABSTRACT FROM AUTHOR]

Spin–orbit-induced (SOI) effective magnetic field in GaMnAs film with in-plane magnetic anisotropy has been investigated by planar Hall effect measurements. The presence of SOI field was identified by a shift between planar Hall resistance (PHR) hystereses observed with positive and negative currents. The difference of switching fields occurring between the two current polarities, which is determined by the strength of the SOI field, is shown to depend on the external field direction. In this paper we have developed a method for obtaining the magnitude of the SOI fields based on magnetic free energy that includes the effects of magnetic anisotropy and the SOI field. Using this approach, the SOI field for a given current density was accurately obtained by fitting to the observed dependence of the switching fields on the applied field directions. Values of the SOI field obtained with field scan PHR measurements give results that are consistent with those obtained by analyzing the angular dependence of PHR, indicating the reliability of the field scan PHR method for quantifying the SOI-field in GaMnAs films. The magnitude of the SOI field systematically increases with increasing current density, demonstrating the usefulness of SOI fields for manipulation of magnetization by current in GaMnAs films. [ABSTRACT FROM AUTHOR]

We have investigated the exchange bias (EB) effect occurring in (Ga, Mn)As/(Ga, Mn)(As, P) bilayers, in which the (Ga, Mn)As layer has an in-plane (IP) magnetic anisotropy, and the magnetic anisotropy of the (Ga, Mn)(As, P) layer is out-of-plane (OP). Planar Hall resistance (PHR) measured during magnetization reversal in this system showed a clear shift of the hysteresis loops, indicating the presence of EB in the (Ga, Mn)As layer. The EB significantly depends on the direction of the field used in the field-cooling process. The observed EB occurring in a bilayer in which the constituent layers have orthogonal magnetic anisotropies can be understood in terms of the formation of magnetic closure domains in the (Ga, Mn)(As, P) layer during field cooling. We show that such EB can be effectively controlled by the process of field cooling, suggesting the possibility of memory device applications based on bilayer systems with orthogonal magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Highlight • The planar Hall resistance measurements revealed two GaMnAs layers have opposite sign of AMR. • Alignment of magnetizations between GaMnAs have been identified form Hall measurements. • GaMnAs trilayer showed GMR-like effect that arises from the noncollinear configurations. Abstract The magnetotransport properties of a GaMnAs trilayer in which two GaMnAs layers are separated by a nonmagnetic GaAs:Be spacer has been investigated during the magnetization reversal process. Interestingly, the planar Hall resistance (PHR) reveals that the two GaMnAs layers in a trilayer structure have opposite signs of anisotropic magnetoresistance (AMR). The magnetoresistance is observed to be significantly different for the collinear and noncollinear magnetic alignment, which indicates that the presence of the giant magnetoresistance (GMR)-like effect in the system. This GMR-like effect was systematically investigated during the process of magnetization reversal using field and angle scans of MR measurements. [ABSTRACT FROM AUTHOR]

Highlights • Investigation of magnetic properties of GaMnAsP films with two different thicknesses. • The Curie temperature was higher in thicker GaMnAsP layer. • Annealing increases the Curie temperature of GaMnAsP layers. • The out-of-plane magnetic anisotropy becames stronger after annealing. • 3-D magnetic anisotropy energy diagrams show easy axes of GaMnAsP films. Abstract We have investigated the magnetic properties of ferromagnetic semiconductor GaMnAsP films with two different thicknesses. Temperature scans of the resistance revealed that the Curie temperature was higher in the 50-nm GaMnAsP layer than in the 16-nm layer, and was increased in both layers after annealing. The Hall effect was measured during the magnetization reversal process to identify the anisotropy of the films. The hysteresis observed from the as-grown samples in anomalous Hall resistance measurements implied the presence of out-of-plane magnetic anisotropy in both samples. The out-of-plane magnetic anisotropy in both samples became much stronger after thermal annealing. Quantitative values of the magnetic anisotropy of the GaMnAsP films were obtained from angular dependent Hall effect measurements. By constructing magnetic anisotropy energy diagrams for the films, we clearly show the change of magnetic anisotropy depending on the film thickness and thermal annealing in GaMnAsP films. [ABSTRACT FROM AUTHOR]

Spin–orbit-induced (SOI) field of crystalline GaMnAs ferromagnetic film has been investigated by using planar Hall effect measurements. The presence of SOI field manifested itself as an asymmetry in the magnetization reversal process between positive and negative current directions. A significant Joule heating caused by current has been observed and its effect was carefully considered to obtain the magnetic anisotropy of the GaMnAs film. The switching of magnetization by SOI field in the GaMnAs film was demonstrated in the absence of an external field. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) between ferromagnetic (FM) metal Fe and FM semiconductor GaMnAs has been investigated by using magnetotransport measurements. The realization of diverse magnetization alignments, including collinear and non-collinear configurations, between Fe and GaMnAs layers are observed during the magnetization reversal process measured by the planar Hall effect. Minor loop scan reveals the presence of FM IEC between Fe and GaMnAs, which systematically decreases as the temperature increases. [ABSTRACT FROM AUTHOR]

We discuss tunneling magnetoresistance (TMR) phenomena observed in hybrid magnetic tunnel junctions (MTJs) fabricated from a Fe/GaAlAs/GaMnAs trilayer. The TMR clearly shows abrupt changes of resistance that depends on the relative alignments of magnetization in the two magnetic layers comprising the MTJ. The TMR ratio of the structure strongly depends on the bias voltage, reaching values up to ~45% when the bias voltage is low (~0.2mV). In addition, the device exhibits intermediate TMR values, which correspond to non-collinear alignments of magnetization in the GaMnAs and the Fe layers. Such alignments are possible due to the presence of two magnetic easy axes both in both magnetic layers originating from their strong cubic magnetic anisotropy. The TMR states realized in such Fe/GaAlAs/GaMnAs MTJs are excellent candidates for use in multi-valued memory storage devices. [ABSTRACT FROM AUTHOR]

A series of Ga1-xMnxAs1-yPy films grown by MBE on GaAs (100) substrates was systematically studied by ferromagnetic resonance (FMR). Magnetic anisotropy parameters were obtained by analyzing the angular dependence of the FMR data. The results clearly show that the easy axis of the films shifts from the in-plane [100] direction to the out-of-plane [001], indicating the emergence of a strong tensile-strain-induced perpendicular anisotropy when the P content exceeds y ≈ 0.07. Multiple resonances were observed in Ga1-xMnxAs1-yPy films with thicknesses over 48 nm, demonstrating the existence of exchange-dominated non-propagating spin-wave modes governed by surface anisotropy. [ABSTRACT FROM AUTHOR]

We carried out a systematic study of magnetic ordering and magnetic interlayer coupling in Ga1-xMnxAs1-yPy/Ga1-xMnxAs bilayers using superconducting quantum interference device magnetometry and ferromagnetic resonance. Such bilayers are interesting, because the easy axis of the constituent materials are orthogonal. Our results show that the bilayers are strongly exchange-coupled at the interface, that manifests itself in the form of horizontal exchange-bias-like shifts of the hysteresis loops of the Ga1-xMnxAs layer, as observed in field-cooled magnetic measurements. [ABSTRACT FROM AUTHOR]

MoTe 2 is the least explored material in the Molybdenum-chalcogen family. Molecular beam epitaxy (MBE) provides a unique opportunity to tackle the small electronegativity difference between Mo and Te while growing layer by layer away from thermodynamic equilibrium. We find that for a few-layer MoTe 2 grown at a moderate rate of ∼6 min per monolayer, a narrow window in temperature (above Te cell temperature) and Te:Mo ratio exists, where we can obtain pure phase 2 H -MoTe 2 . This is confirmed using reflection high-energy electron diffraction (RHEED), Raman spectroscopy and X-ray photoemission spectroscopy (XPS). For growth on CaF 2 , Grazing incidence X-ray diffraction (GI-XRD) reveals a grain size of ∼90 Å and presence of twinned grains. In this work, we hypothesis the presence of excess Te incorporation in MBE grown few layer 2 H -MoTe 2 . For film on CaF 2 , it is based on >2 Te:Mo stoichiometry using XPS as well as ‘a’ and ‘c’ lattice spacing greater than bulk 2 H -MoTe 2 . On GaAs, its based on observations of Te crystallite formation on film surface, 2 × 2 superstructure observed in RHEED and low energy electron diffraction, larger than bulk c-lattice spacing as well as the lack of electrical conductivity modulation by field effect. Finally, thermal stability and air sensitivity of MBE 2H-MoTe 2 is investigated by temperature dependent XRD and XPS, respectively. [ABSTRACT FROM AUTHOR]

We report a method for accurate determination of the strength of the current-induced spin-orbit (SO) field in ferromagnetic GaMnAs films. The SO-field manifests itself in the form of a hysteresis between planar Hall resistances (PHR) measured with positive and negative currents as an applied magnetic field is rotated in the sample plane at constant field strength. The width of the hysteresis, which is related to the strength of the SO-field, is observed to change significantly for different values of the rotating external field strength. Since the SO field occurring at a given current is an intrinsic property of the crystal, such a field dependence of the hysteresis indicates that the width of the hysteresis measured with a single field strength is insufficient for determining the SO field. However, using a model based on magnetic free energy that includes the effects of magnetic anisotropy and the SO-field as developed in the present paper, we show that the SO field for a given current density can be accurately established by fitting to the experimentally observed dependence of transition angles of PHR measured with different applied field strengths. Using the known dependence of magnetic anisotropy of GaMnAs on temperature, we also show that this method applies reliably as the temperature varies. [ABSTRACT FROM AUTHOR]

We have investigated magnetic properties of Ni film specimens deposited either on GaAs or on Bi2Se3 surfaces. The magnetization saturation field along the out-of-plane direction observed in the Hall data was weaker for the Ni/Bi2Se3 sample than for the Ni/GaAs sample. On the other hand, planar Hall resistance measurements showed the larger in-plane coercive field for Ni/Bi2Se3 than for Ni/GaAs. These changes of magnetic hardness observed along the out-of-plane and in-plane directions in the Ni/Bi2Se3 specimen are consistent with effects arising from spin–orbit interaction at the surface of the Bi2Se3 topological insulator. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) of GaMnAs-based multilayers has been investigated by varying the structural parameters, such as spacer thickness, carrier doping in the nonmagnetic layers, and repetition number of GaMnAs/GaAs bilayers. The type of IEC in the structure (i.e., either ferromagnetic or antiferromagnetic IEC) was identified from the magnetotransport measurements, which show either anisotropic magnetoresistance or giant magnetoresistance-like behavior. The investigation revealed that the parameters of the ferromagnetic GaMnAs layer (i.e., Mn composition and thickness) are less important than those of the non-magnetic GaAs spacer layer in the structure. In particular, it was found that the presence of carriers in the spacer layer plays a crucial role in determining the type and strength of the IEC in GaMnAs-based multilayer systems. [ABSTRACT FROM AUTHOR]

We discuss magnetic properties of lithographically patterned nanoscale stripes of dilute ferromagnetic semiconductor (Ga,Mn)As aligned with [110] crystallographic direction, i.e., perpendicular to the easy axis of uniaxial magnetocrystalline anisotropy of this material. We find that at temperatures where uniaxial anisotropy becomes dominant, shape-induced effects strongly compete with this intrinsic property, tending to align the total magnetization along the stripe direction. The present study revealed, however, that low-temperature annealing of the stripes markedly reduces such shape-induced effects. This effect of annealing is not presently understood, and will require further detailed study. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) between GaMnAs layers in GaMnAs/InGaAs/GaMnAs tri-layers was studied by magnetization measurements. Minor hysteresis loops are observed to shift in a direction indicating the presence of ferromagnetic (FM) IEC in the structures. The strength of the FM IEC clearly exhibits an exponential decrease with respect to nonmagnetic InGaAs spacer thickness. The fitting of the spacer thickness dependence of the FM IEC to an exponential decay function provides a decay length of 3.3±0.3 nm, which is relatively large compared to metallic multilayers, indicating a long ranged IEC in systems based on GaMnAs. [ABSTRACT FROM AUTHOR]

We report a systematic investigation of ac magnetic susceptibility in (Ga,Mn)As films as a function of temperature and magnetic field, carried out in parallel with dc magnetization measurements. The temperature dependence of ac susceptibility \chi ac shows anisotropic behavior: 1) a single peak in \chi ac is observed close to TC for the [ 1\bar 10 ] orientation of the driving ac field; 2) a single peak is also seen close to 22 K for the field along [110]; and 3) peaks at both these temperatures are observed for the field applied along [100]. A detailed analysis of the ac and dc data unambiguously indicates that the peak near TC is related to the paramagnetic-to-ferromagnetic phase transition, with the ferromagnetic domains nucleating with their easy axes aligned with the [ 1\bar 10 ] direction, providing a clear picture of uniaxial domain behavior near TC . The peak near 22 K, on the other hand, is related to the onset of biaxial domain structure in (Ga,Mn)As induced by the competition between uniaxial and cubic anisotropy. More specifically, the ac susceptibility peak near TC involves 180° magnetization flips along the [ 1\bar 10 ] easy axis of the domains, while the peak near 22 K originates from magnetization wobbling between two biaxial easy axes separated by a small angle. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) between the GaMnAs layers in GaMnAs/GaAs:Be multilayer systems has been investigated using magnetotransport experiments. The observation of a stable antiparallel magnetization alignment state from the systems indicates the presence of antiferromagnetic (AFM) IEC between the GaMnAs layers. The transitions between parallel and antiparallel alignments of GaMnAs magnetic layers in the system were carefully investigated by measuring resistance change with increasing temperature under various bias magnetic fields. From the dependence of the transition temperature on bias fields, we have estimated the magnitude of AFM IEC and its temperature behavior of our GaMnAs/GaAs:Be multilayers. [ABSTRACT FROM AUTHOR]

Ultrafast laser-triggered coherent magnetization dynamics in ferromagnetic (Ga,Mn)As films have been investigated by time-resolved magneto-optical spectroscopy. Dynamic phase reversal in the magnetic precession process is observed when the ambient temperature or the external magnetic field is varied. This phenomenon is found to be sensitive to the spontaneous magnetization orientation, and is attributed to the giant magnetic linear dichroism (MLD) effect in (Ga,Mn)As. Our findings suggest that this effect will enable the sensitive measurement of the dynamic phase of in-plane magnetization precession on picosecond time scale in the collective spin excitation in (Ga,Mn)As, thus enabling efficient and ultrafast magneto-optical detection for magnetization dynamics in ferromagnetic semiconductor-based spintronic devices. [ABSTRACT FROM AUTHOR]

We present a study of photoexcited magnetization dynamics in ferromagnetic (Ga,Mn)As films observed by time-resolved magneto-optical measurements. The magnetization precession triggered by linearly polarized optical pulses in the absence of an external field shows a strong dependence on photon frequency when the photoexcitation energy approaches the band edge of (Ga,Mn)As. This can be understood in terms of magnetic anisotropy modulation by both laser heating of the sample and by hole-induced nonthermal paths. Our findings provide a means for identifying the transition of laser-triggered magnetization dynamics from thermal to nonthermal mechanisms, a result that is of importance for ultrafast optical spin manipulation in ferromagnetic materials via nonthermal paths. [ABSTRACT FROM AUTHOR]

We report direct measurements of hole spin lifetimes in ferromagnetic GaMnAs carried out by time- and polarization-resolved spectroscopy. Below the Curie temperature, ultrafast photoexcitation of GaMnAs with linearly polarized light is shown to create a nonequilibrium hole spin population via dynamical polarization of the holes through p-d exchange scattering with ferromagnetically ordered Mn spins. The system is then observed to relax in a distinct three-step recovery process: (i) a femtosecond hole spin relaxation, on the scale of 160-200 fs; (ii) a picosecond hole energy relaxation, on the scale of 1-2 ps; and (iii) a coherent, damped Mn spin precession with a period of 250 ps. The transient amplitude of the hole spin relaxation component diminishes with increasing temperature, directly following the ferromagnetic order of GaMnAs, while the hole energy amplitude shows negligible temperature change. Our results serve to establish the hole spin lifetimes in the ferromagnetic semiconductor GaMnAs, at the same time demonstrating a spectroscopic method for studying nonequilibrium hole spins in the presence of magnetic order and spin-exchange interaction. [ABSTRACT FROM AUTHOR]

We report the realization of ten stable tunneling magnetoresistance (TMR) states in a single device. To achieve ten resistance states, we have used a magnetic tunnel junction (MTJ) structure that consists of two magnetic layers, which are Fe and GaMnAs ferromagnetic layers. Owing to the two in-plane magnetic easy axes that result from strong cubic anisotropies in both Fe and GaMnAs layers, noncollinear magnetic configurations between two magnetic layers were realized, in addition to the parallel and antiparallel configurations. Such noncollinear magnetic configurations provide stable intermediate TMR values between two extreme values corresponding parallel and antiparallel configurations. The number of stable TMR values was further increased by forming multidomain structures in the MTJ structure. We demonstrate that we can obtain up to ten stable TMR values, and they can be controlled by applying the appropriate magnetic field sequences. [ABSTRACT FROM AUTHOR]

Highlights: [•] Morphology dependence of dissociative adsorption of water on GaAs surfaces: simple planar crystal vs. nanowires. [•] In situ X-ray photoelectron spectroscopy of the water/semiconductor interface under near ambient condition. [•] Oxygenation and hydroxylation of GaAs surfaces. [ABSTRACT FROM AUTHOR]

The manipulation of semiconductor surfaces by tuning their electronic properties and surface chemistry is an essential ingredient for key applications in areas such as electronics, sensors, and photovoltaic devices. Here, in-situ surface reactions on gallium arsenide (GaAs) are monitored for two morphologies: a simple planar crystalline surface with (100) orientation and an ensemble of GaAs nanowires, both exposed to oxygen environment. A variety of oxide surface species, with a significant enhancement in oxidation states in the case of nanowires, are detected via near ambient pressure X-ray photoelectron spectroscopy. This enhancement in oxidation of GaAs nanowires is due to their higher surface area and the existence of more active sites for O2 dissociation. [ABSTRACT FROM AUTHOR]

Two-dimensional electron gas (2DEG) coexists with topological states on the surface of topological insulators (TIs), while the origin of the 2DEG remains elusive. In this work, electron density in TI thin films (Bi2Se3, Bi2Te3, and their alloys) were manipulated by controlling the density of electronically active native defects with particle irradiation. The measured electron concentration increases with irradiation dose but saturates at different levels for Bi2Se3 and Bi2Te3. The results are in quantitative agreement with the amphoteric defect model, which predicts that electronically active native defects shift the Fermi energy (EF) toward a Fermi stabilization level (EFS) located universally at ~4.9 eV below the vacuum level. Combined with thickness-dependent data, it is demonstrated that regardless of the bulk doping, the surface EF is always pinned at EFS, producing a band bending and 2DEG on TI film surfaces. Our work elucidates native defect physics of TIs with a model universally applicable to other semiconductors and has critical implications for potential device applications of TIs. [ABSTRACT FROM AUTHOR]

Tunneling magnetoresistance (TMR) observed in Fe/GaAlAs/GaMnAs magnetic tunnel junctions depends strongly on relative configuration of magnetizations in the GaMnAs and Fe layers. Using a series of field orientations, we show that non-collinear alignments of magnetization in this structure occur when the magnetic field is applied away from the easy axes of the magnetic layers; and we find that the values of TMR observed for non-collinear spin configurations are very different from those of collinear configurations. The observed behavior suggests the possibility of using this effect for multivalued magnetic memory devices. [ABSTRACT FROM AUTHOR]

Abstract: We report experiments where magnetization in GaMnAs ferromagnetic semiconductor is manipulated via strain or electric current. In both cases, charge carrier holes become partially polarized due to the anisotropic modification of holes spectra caused by spin–orbit interactions, and this polarization exerts spin torque sufficient to rotate ferromagnetic domains. [Copyright &y& Elsevier]

CdSe/CdTe type-II superlattices grown on GaSb substrates by molecular beam epitaxy are studied using time-resolved and steady-state photoluminescence (PL) spectroscopy at 10 K. The relatively long carrier lifetime of 188 ns observed in time-resolved PL measurements shows good material quality. The steady-state PL peak position exhibits a blue shift with increasing excess carrier concentration. Self-consistent solutions of the Schrödinger and Poisson equations show that this effect can be explained by band bending as a result of the spatial separation of electrons and holes, which is critical confirmation of a strong type-II band edge alignment between CdSe and CdTe. [ABSTRACT FROM AUTHOR]

Aluminum doped ZnTe epitaxial layers have been grown on GaSb substrates by molecular beam epitaxy under different Te/Zn flux ratios, while the flux ratio is varied by keeping Te flux constant and changing Zn flux. The Te/Zn flux ratio ∼0.9 leads to the narrowest full width at half maximum of x-ray diffraction rocking curve. Indium deposition is applied to improve the contact conductivity, characterized by current-voltage measurements. The effect of annealing in forming gas (5% H2+ 95% Ar) has been studied and the secondary ion mass spectrometry results indicate Zn vacancies are suppressed in the order of 2 magnitudes by decreasing Te/Zn flux ratio from 2 to 0.5. This study assists the understanding and realization of effective n-type doping of ZnTe by the approach of suppressing Zn vacancies (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

ZnTe PN homojunctions are fabricated using thermal diffusion of Al into a ZnTe film grown on lattice-matched GaSb substrates by molecular beam epitaxy. Rectifying J-V characteristics and the photovoltaic effect are observed which suggests that n-type ZnTe has been successfully achieved. The reverse bias breakdown voltage is found to be dependent on annealing condition and acceptor concentration, and the trend agrees with PN junction theory. Secondary ion mass spectrometer depth profiles of the Al film show that Zn and Te out-diffuse into the Al film during annealing. This out-diffusion is undesirable because it may result in Zn vacancies which compensate Al donor atoms. Photoluminescence measurements of annealed ZnTe samples without the Al diffusion layer show strong band-edge luminescence at room temperature. Midgap luminescence also appears for annealed samples but the intensity is lower than the band-edge peak. This technique may lead to a better understanding of the n-type doping problem for ZnTe, and could lead to in-situdoping techniques and treatments. Successful n-type doping of ZnTe with low resistivity will enable high-efficiency optoelectronic devices operating at pure-green wavelengths (550 nm) (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)13 clusters with Mn2+ ions governs the Mn2+ incorporation during the nucleation stage. This highly efficient Mn2+ doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of ∼600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s–d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications. [ABSTRACT FROM AUTHOR]

The current state of information technology accentuates the dichotomy between processing and storage of information, with logical operations carried out by charge-based devices and non-volatile memory based on magnetic materials. The main obstacle for a wider use of magnetic materials for information processing is the lack of efficient control of magnetization. Reorientation of magnetic domains is conventionally carried out by non-local external magnetic fields or by externally polarized currents. The efficiency of the latter approach is enhanced in materials where ferromagnetism is carrier-mediated, because in such materials the control of carrier polarization provides an alternative means for manipulating the orientation of magnetic domains. In some crystalline conductors, the charge current couples to the spins by means of intrinsic spin–orbit interactions, thus generating non-equilibrium electron spin polarization tunable by local electric fields. Here, we show that magnetization can be reversibly manipulated by the spin–orbit-induced polarization of carrier spins generated by the injection of unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor. [ABSTRACT FROM AUTHOR]

The newly-developing spintronics technology requires materials that allow control of both the charge and the spin degrees of freedom of the charge carriers. Ferromagnetic semiconductors (SC) are considered suitable due to simultaneous presence of magnetic order and of semiconducting properties. GaMnAs is one of the most intensively studied ferromagnetic SC. In this paper we will review recent research and accomplishments regarding two technologically important properties – magnetic anisotropy and interlayer coupling — of GaMnAs-based multilayer structures, with an eye on their potential role in practical devices. [Copyright &y& Elsevier]

CdSe/CdTe superlattices are grown on GaSb substrates using molecular beam epitaxy. X-ray diffraction measurements and cross-sectional transmission electron microscopy images indicate high crystalline quality. Photoluminescence (PL) measurements show the effective bandgap varies with the superlattice layer thicknesses and confirm the CdSe/CdTe heterostructure has a type-II band edge alignment. The valence band offset between unstrained CdTe and CdSe is determined as 0.63 ± 0.06 eV by fitting the measured PL peak positions using the envelope function approximation and the Kronig-Penney model. These results suggest that CdSe/CdTe superlattices are promising candidates for multi-junction solar cells and other optoelectronic devices based on GaSb substrates. [ABSTRACT FROM AUTHOR]

In order to investigate the origin of the magnetic circular dichroism (MCD) in Ga1-xMnx, As, we prepared a series of paramagnetic samples with a very low Mn content (x ~ 0.0005 and 0.0003), grown at different substrate temperatures. The samples grown at low temperatures (< 300 °C) show a sign change in the MCD, while those grown at higher temperatures (> 300°C) reveal broad positive MCD spectra similar to those observed in ferromagnetic Ga1-xMnxAs. In addition, samples grown at high temperature (~600 °C) but with Si co-doping show a behavior typical to samples grown at low substrate temperatures, namely sign changing MCD spectra. Our results indicate that the negative component of the MCD does not originate from the interband transitions but from transitions involving ionized Mn acceptors. [ABSTRACT FROM AUTHOR]

In this paper, we report a systematic study on the temperature dependences of the exchange bias (EB) field HEB and coercivity HC in (Ga,Mn)As-MnO bilayers via magnetotransport measurements. Structures of (Ga,Mn)As-Mn were grown by molecular beam epitaxy, and then oxidized to obtain a top layer of MnO. Different methods were used to oxidize the Mn, and their effects on the behavior of HEB and HC were studied. It is a striking result that, once the structure is field-cooled from room temperature to 4.2 K, HEB retains its value even after warming the structure to 130 K, indicating that the exchange coupling sustains the ordering in MnO above the Néel temperature TN (110 K) of bare MnO. [ABSTRACT FROM AUTHOR]

In this paper, we present a detailed investigation of exchange-dominated nonpropagating spin-wave modes in a series of 100-nm Ga1-xMnxAs films with Mn concentrations x ranging from 0.02 to 0.08. The angular and Mn concentration dependences of spin-wave resonance (SWR) modes have been studied for both as-grown and annealed samples. Our results indicate that the magnetic anisotropy terms of Ga1-xMnxAs depend on the Mn concentration x, but are also strongly affected by sample growth conditions; moreover, the magnetic anisotropy of Ga1-xMnxAs films is found to be clearly linked to the Curie temperature. The SWR spectra consist of a series of well-resolved standing spin-wave modes. The observed mode patterns are consistent with the Portis volume-inhomogeneity model, in which a spatially nonuniform anisotropy field acts on the Mn spins. The analysis of these exchange-dominated spin-wave modes, including their angular dependences, allows us to establish the exchange stiffness constants for Ga1-xMnxAs films. [ABSTRACT FROM AUTHOR]