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Our study of magnetization switching in crystalline (Ga,Mn)(As,P) ferromagnetic semiconductor (FMS) film by spin-orbit torque (SOT) has revealed an unexpected increase in critical switching current as the in-plane magnetic bias field is increased beyond a certain point. This intriguing behavior is ascribed to depolarization of spin-polarized current induced by the application of bias field perpendicular to the direction of current carrier spins. This is particularly interesting, because the bias field is itself a necessary requirement for achieving the deterministic SOT magnetization switching. To gain understanding of this unexpected behavior, we incorporated the process of spin depolarization into micromagnetic simulation study of SOT magnetization switching in the (Ga,Mn)(As,P) system. Through simulations that include effects of spin depolarization, we were able to replicate the observed increase in the required critical switching current as the in-plane bias field is increased. Furthermore, our study demonstrates that the dependence of critical switching current on bias field can be quantitatively described by adjusting magnetic anisotropy parameters of the film. This study not only enhances our understanding of SOT phenomena but also offers valuable insights for tailoring and optimizing FMS materials for spintronic applications. [ABSTRACT FROM AUTHOR]

Topologically protected surface current is highly promising for next-generation low-dissipation and disorder-tolerant quantum electronics and computing. Yet, electric transport from the co-existing bulk state dominates the responses of the Dirac surface state, especially at elevated temperatures relevant to technological applications. Here, we present an approach that convincingly showcases the generation, disentanglement, and precise control of enduring surface charge carriers on a topological insulator, Bi2Se3, with high bulk conductivity, all achieved at room temperature. By using pump–probe modulation spectroscopy under ultrabroadband driving tunable from 4 meV to 1.55 eV, we show the terahertz (THz) field-induced surface carriers by discovering their initial temporal responses dominant over high density trivial bulk carriers. Strikingly, the response of the induced surface carrier responses persists for more than ∼5 ps and is enhanced by reducing pump photon energy. The dynamics and lifetime of the distinct surface response manifest themselves as the enhanced THz pump-induced THz transmission, which directly correlates with the transient negative THz conductivity. Increasing the THz driving field reduces the induced surface carrier lifetime and identifies, particularly, an optimal pump field of Es ∼ 224 kV cm−1 for generating the dominant surface response relative to the bulk. This surface carrier dominant regime is suppressed by a joint effect of enhanced surface-bulk scattering and a more rapid saturation of surface excitation compared to the bulk that sets in above Es. The controllability of room temperature topologically surface carriers through pump photon energy offer compelling possibilities for extending this approach to other topological complex materials. [ABSTRACT FROM AUTHOR]

We report an investigation of current-induced spin–orbit fields (SOFs) in a crystalline (Ga,Mn)As ferromagnetic film with four-fold in-plane anisotropy. By designing a Hall device with current channels along crystallographic 〈100〉 directions, we observe the effects of SOFs in all four magnetization transitions over the four 〈110〉 hard axes of the film. The SOF effects are observed in the form of opposite shifts of angles at which the magnetization of the film switches sign for opposite current polarities in anisotropic magnetoresistance (AMR) measurements carried out as the applied magnetic field was rotated. Note that the Dresselhaus- and Rashba-type SOFs generated by currents flowing along the 〈100〉 directions—as in the device designed for the present experiments—are orthogonal to one another and can thus be separated by appropriate analysis of AMR. The analysis of experimental results based on magnetic free energy, in which the effects of SOFs are included, reveals that the Dresselhaus-type SOF is much stronger than the Rashba-type SOF. The values of the two SOFs obtained independently with currents flowing either along the [100] or [010] channels are consistent with each other, showing the reliability of the above-mentioned approach for investigating SOFs in ferromagnetic films. [ABSTRACT FROM AUTHOR]

We have investigated the effect of annealing on the magnetic anisotropy of MBE-grown GaMnAs1−yPy film in which phosphorus content varies from 0% to 24% along the growth direction. Such variation is achieved by growing a series of GaMnAs1−yPy layers in which y is successively increased. Hall effects measurements on an as-grown graded film reveal that the bottom 80% of the film has in-plane easy axes, 10% has both in-plane and perpendicular easy axes, and the remaining 10% has a vertical easy axis. Such gradual change of magnetic anisotropy in the film from in-plane to perpendicular with increasing P concentration is in accordance with the continuous variation of strain from compressive to tensile as the P concentration increases the bottom of the film to tensile toward its tip surface. However, thermal annealing significantly changes the magnetic anisotropy of the graded GaMnAs1−yPy film. In particular, the intermediate region having both in-plane and perpendicular easy axes nearly disappears in the film after annealing, so the film is divided into two types of layers having either only in-plane or only perpendicular anisotropy. These dramatic changes in magnetic anisotropy of the graded GaMnAs1−yPy film introduced by annealing suggest that one can strategically use this process to realize orthogonal magnetic bilayers consisting of in-plane and perpendicular easy axes. [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]

We report the observation of current induced spin–orbit torque (SOT) switching of magnetization in a (Ga,Mn)(As,P) film using perpendicular magnetic anisotropy. Complete SOT switching of magnetization was achieved with current densities as low as 7.4 × 105 A/cm2, which is one to two orders of magnitude smaller than that normally used for SOT switching in ferromagnet/heavy metal bilayer systems. The observed magnetization switching chirality during current scans is consistent with SOT arising from spin polarization caused by the Dresselhaus-type spin–orbit-induced (SOI) fields. The magnitudes of effective SOI fields corresponding to the SOT were obtained from shifts of switching angles in angular dependent Hall measurements observed for opposite current polarities. By measuring effective SOI fields for the [ 1 1 ̄ 0 ] and the [110] current directions, we were then able to separate the values of the Dresselhaus-type ( H e f f D ) and Rashba ( H e f f R ) SOI fields. At a current density of 6.0 × 105 A/cm2, these values are H e f f D = 6.73 O e and H e f f R = 1.31 O e , respectively. The observed ratio of about 5:1 between Dresselhaus-type and Rashba SOI fields is similar to that observed in a GaMnAs film with an in-plane magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Using spectroscopic ellipsometry, the dielectric functions of a series of topological insulators, including Bi2Te3, Bi2Se3, and their ternary alloys, were determined. The ellipsometry measurements were obtained using an IR-spectroscopic ellipsometer, spanning a spectral range between 2000 and 35 000 nm. A standard inversion technique was used to model the ellipsometry spectra, which produced the dielectric functions of each of the topological insulator films. These dielectric functions were analyzed further to obtain characteristics such as their bandgap, carrier concentration, and effective mass. Specifically, Kramers–Kronig consistent oscillators were used to represent the Drude contribution as well as the energy gap. The authors found that the bandgap of Bi2(Te1−xSex)3 ternary alloys are larger than their binary constituents (i.e., Bi2Te3 and Bi2Se3). By exploring the temperature dependence of the bandgap, the authors deduced the electron–phonon coupling parameters for the Bi2(Te1−xSex)3 system, using the Bose–Einstein occupation distributions. Compared with Bi2Te3, the authors found that the electron–phonon coupling parameters of the ternary alloys of Bi2(Te1−xSex)3 are smaller. [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]

Magnetization precession induced by linearly polarized optical excitation in ferromagnetic (Ga,Mn)As was studied by time-resolved magneto-optical Kerr effect measurements. The superposition of thermal and non-thermal effects arising from the laser pulses complicates the analysis of magnetization precession in terms of magnetic anisotropy fields. To obtain insight into these processes, we investigated compressively-strained thin (Ga,Mn)As films using ultrafast optical excitation above the band gap as a function of pulse intensity. Data analyses with the gyromagnetic calculation based on Landau-Lifshitz-Gilbert equation combined with two different magneto-optical effects shows the non-equivalent effects of in-plane and out-of-plane magnetic anisotropy fields on both the amplitude and the frequency of magnetization precession, thus providing a handle for separating the effects of non-thermal and thermal processes in this context. Our results show that the effect of photo-generated carriers on magnetic anisotropy constitutes a particularly effective mechanism for controlling both the frequency and amplitude of magnetization precession, thus suggesting the possibility of non-thermal manipulation of spin dynamics through pulsed laser excitations. [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]

We studied the thermal conductivity of Bi2Se3, Bi2Te3, and their alloy Bi2(sexTe1-x)3 at room temperature using time-domain thermoreflectance measurements. The Bi2(sexTe1-x)3 films with various concentrations of Se and Te prepared by molecular beam epitaxy on GaAs substrates were investigated to study the dependence of thermal conductivity on film composition. We observed that the Bi2(sexTe1-x)3 ternary alloys can have much lower thermal conductivity values compared to those of Bi2Se3 and Bi2Te3. These results may provide useful information for developing and engineering low thermal conductivity materials for thermoelectric applications. [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]

We discuss growth and magnetic properties of high-quality two dimensional (2D) Sn1-xMnxSe2 films. Thin films of this 2D ternary alloy with a wide range of Mn concentrations were successfully grown by molecular beam epitaxy. Mn concentrations up to x ≈ 0.60 were achieved without destroying the crystal structure of the parent SnSe2 2D system. Most important, the specimens show clear weak ferromagnetic behavior above room temperature, which should be of interest for 2D spintronic applications. [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]

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]

A series of Ga1-xMnxAs1-yPy thin films grown on GaAs (100) substrates by molecular beam epitaxy were systematically investigated to establish the effect of phosphorous on structural and magnetic properties of the alloy. Detailed characterization of both as-grown and annealed samples by x-ray diffraction and magnetometry were carried out. Reciprocal space map scans confirmed that the quaternary alloy is fully strained by the substrate throughout its thickness. Magnetization measurements revealed a clear trend of decreasing Curie temperature with increasing P concentration, and revealed that the magnetic easy axis gradually turns from in-plane to out-to-plane orientation as P concentration increases. [ABSTRACT FROM AUTHOR]

A systematic study of low temperature (LT) annealing III1-xMnxV of (Ga1-xMnxAs and Ga1-xMnxAs1-yPy) thin films in situ with different capping layers (Se, Te, or As) was carried out without exposure to the atmosphere. Experimental results show that a correct in situ annealing approach can lead to significant increases of the Curie temperature, carrier concentration, and magnetic moment, similar to the ex situ LT-annealing experiments achieved in earlier studies. Moreover, this approach allowed us to successfully deposit high-quality semiconductor layers on top of such in situ annealed films, demonstrating great potential for designing high quality III1-xMnxV-based multilayers for spintronic applications optimized by the benefits of the LT-annealing. [ABSTRACT FROM AUTHOR]

This paper reports the molecular beam epitaxial growth and characterization of high-reflectivity and broad-bandwidth distributed Bragg reflectors (DBRs) made of ZnTe/GaSb quarter-wavelength (λ/4) layers for optoelectronic applications in the midwave infrared spectral range (2-5 μm). A series of ZnTe/GaSb DBRs has been successfully grown on GaSb (001) substrates using molecular beam epitaxy (MBE). During the MBE growth, a temperature ramp was applied to the initial growth of GaSb layers on ZnTe to protect the ZnTe underneath from damage due to thermal evaporation. Post-growth characterization using high-resolution x-ray diffraction, atomic force microscopy, and transmission electron microscopy reveals smooth surface morphology, low defect density, and coherent interfaces. Reflectance spectroscopy results show that a DBR sample of seven λ/4 pairs has a peak reflectance as high as 99.0% centered at 2.56 μm with a bandwidth of 517 nm. [ABSTRACT FROM AUTHOR]

A single-step method for the preparation of metastable ε-Fe 3 N nanoparticles by combustion of reactive gels containing iron nitrate (Fe(NO 3 ) 3 ) and hexamethylenetetramine (C 6 H 12 N 4 ) in an inert atmosphere is reported. The results of Fourier transform infrared spectroscopy (FTIR) and thermal analysis coupled with dynamic mass spectrometry revealed that the exothermic decomposition of a coordination complex formed between Fe(NO 3 ) 3 and HMTA is responsible for the formation of ε-Fe 3 N nanoscale particles with sizes of 5-15 nm. The magnetic properties between 5 and 350 K are characterized using a superconducting quantum interference device (SQUID) magnetometer, revealing a ferromagnetic behavior with a low-temperature magnetic moment of 1.09 μ B /Fe, high room temperature saturation magnetization (∼80 emu/g), and low remanent magnetization (∼15 emu/g). The obtained value for the Curie temperature of ∼522 K is close to that (∼575 K) for bulk ε-Fe 3 N reported in the literature.

We investigate the process of selectively manipulating the magnetization alignment in magnetic layers in the Fe/GaAs/GaMnAs structure by current-induced spin-orbit (SO) magnetic field. The presence of such fields manifests itself through the hysteretic behavior of planar Hall resistance observed for two opposite currents as the magnetization in the structure switches directions. In the case of the Fe/GaAs/GaMnAs multilayer, hystereses are clearly observed when the magnetization switches direction in the GaMnAs layer, but are negligible when magnetization transitions occur in Fe. This difference in the effect of the SO-field in the two magnetic layers provides an opportunity to control the magnetization in one layer (in the presence case in GaMnAs) by a current, while the magnetization in the other layer (i.e., Fe) remains fixed. Owing to our ability to selectively control the magnetization in the GaMnAs layer, we are able to manipulate the relative spin configurations in our structure between collinear and non-collinear alignments simply by switching the current direction even in the absence of an external magnetic field.

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe 2 , chemical vapor deposition (CVD)-grown MoS 2 , and MBE SnSe 2 are compared. Both MBE-grown WSe 2 and CVD-grown MoS 2 display a selective air exposure response at the step edges, consistent with oxidation on WSe 2 and adsorption of hydrocarbon on MoS 2 , while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe 2 , an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe 2 is decomposed to SnO x and SeO x , as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe 2 being driven by the weak bonding of SnSe 2 .

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.  Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.  The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.  The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.  We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.   This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

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 Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 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.

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn 2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.