Nonmagnetic GaAs Research Articles

Spin-Current Oscillations in Diluted Magnetic Semiconductor Multibarrier GaMnAs/GaAs: Role of Temperature and Bias Voltage

This paper has studied the electronic properties of multi-diluted magnetic semiconductor (DMS) layers Ga(1 − x)MnxAs interposed between nonmagnetic GaAs layers. The asymmetry of confining potential on the transmission coefficient by tuning the temperature and the size of the (DMS) layers was discussed. The diluted magnetic layers Ga(1 − x)MnxAs behave as barriers for spin-up holes and quantum wells for spin-down holes. Furthermore, we have addressed the impact of an applied bias voltage and the temperature on the variation of the spin-polarization and spin current densities. Our findings reveal that the transmission coefficients present an oscillating behavior due to the resonant states and strongly depend on the temperature of the system and the number of magnetic layers. Furthermore, the obtained results demonstrated that the number of these states is multiplied by augmenting the magnetic layers. Moreover, we demonstrate that the asymmetric structure presents a completely different transmission of holes than the symmetric structure. Furthermore, the negative differential resistance (NDR) is demonstrated in the current density variations. Especially, this (NDR) was more intense for spin-up holes than spin-down holes. The findings in the present paper can be useful in manufacturing spin-filters by adjusting the values of the temperature and the external voltages.

Noncollinear magnetoresistance of trilayers consisting of two ferromagnetic GaMnAs layers and a nonmagnetic GaAs:Be spacer

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.

Large current modulation and tunneling magnetoresistance change by a side-gate electric field in a GaMnAs-based vertical spin metal-oxide-semiconductor field-effect transistor

A vertical spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) is a promising low-power device for the post scaling era. Here, using a ferromagnetic-semiconductor GaMnAs-based vertical spin MOSFET with a GaAs channel layer, we demonstrate a large drain-source current IDS modulation by a gate-source voltage VGS with a modulation ratio up to 130%, which is the largest value that has ever been reported for vertical spin field-effect transistors thus far. We find that the electric field effect on indirect tunneling via defect states in the GaAs channel layer is responsible for the large IDS modulation. This device shows a tunneling magnetoresistance (TMR) ratio up to ~7%, which is larger than that of the planar-type spin MOSFETs, indicating that IDS can be controlled by the magnetization configuration. Furthermore, we find that the TMR ratio can be modulated by VGS. This result mainly originates from the electric field modulation of the magnetic anisotropy of the GaMnAs ferromagnetic electrodes as well as the potential modulation of the nonmagnetic semiconductor GaAs channel layer. Our findings provide important progress towards high-performance vertical spin MOSFETs.

Interlayer exchange coupling in MBE-grown GaMnAs-based multilayer systems

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.

Dynamics of electron-spin injection in a heterovalent GaAs/AlGaAs/ZnMnSe structure with a coupled double quantum well of GaAs/AlGaAs

The dynamics of electron-spin injection in a heterovalent GaAs/AlGaAs/ZnMnSe structure with a coupled double quantum well (QW) of GaAs/AlGaAs has been studied by means of time-resolved circularly polarized photoluminescence (PL). The circularly polarized PL from the non-magnetic GaAs QW indicates the electron-spin injection from the ZnMnSe/ZnSSe spin generator, when photo-excitation within the ZnSSe barrier is performed by linearly polarized light. The circular polarization develops with an increase in the time delay after the pulsed excitation, which is indicative of the spin-injection dynamics. We find that the rise-time of the circular polarization, ranging from 0.05 to 0.5ns, as well as the maximum value of the polarization degree depends strongly on the power density of excitation light in the ZnSSe.

Exchange Coupling in Magnetic Semiconductor Multilayers and Superlattices

The study of ferromagnetic semiconductors continues to be of great interest because of their potential for spintronic devices. While there has been much progress in our understanding of ferromagnetic semiconductor materials particularly of the canonical III V system Ga1−xMnxAs many issues still remain unresolved. One of these is the nature of interlayer exchange coupling in GaMnAs-based multilayers, an issue that is important from the point of view of possible spintronic applications. In this connection, it is important to establish under what conditions the interlayer exchange coupling between successive GaMnAs layers is antiferromagnetic or ferromagnetic, since manipulation of such interlayer exchange coupling can then be directly applied to achieve giant magnetoresistance and other devices based on this material. In this review we will describe magneto-transport, magnetization, and neutron re ectometry experiments applied to two types of GaMnAs-based multilayer structures superlattices and tri-layers consisting of GaMnAs layers separated by non-magnetic GaAs spacers. These measurements serve to identify conditions under which AFM coupling will occur in such GaMnAs/GaAs multilayer systems, thus providing us the information which can be used for manipulating magnetization (and thus also giant magnetoresistance) in structures based on the ferromagnetic semiconductor GaMnAs.

Investigation of superlattices based on ferromagnetic semiconductor GaMnAs by planar Hall effect

Two ferromagnetic semiconductor GaMnAs-based superlattices (SLs) were investigated by measuring the planar Hall effect (PHE) with the external magnetic field applied in the plane of the sample. The two GaMnAs/GaAs SLs differed only by the Be doping of the nonmagnetic GaAs spacer layers. Both SLs showed a typical two-step transition behavior in PHE field scans at 4.0 K, essentially the same as that normally observed on single GaMnAs ferromagnetic layers with two in-plane magnetic easy axes. As the temperature increased to 30 K, the behaviors of the PHE changed differently in the two SL samples. The PHE in the undoped SL can be described simply by the temperature dependence of the magnetic anisotropy within the film plane of a GaMnAs film. However, the Be-doped SL revealed a completely different behavior, showing a transition of magnetization with a negative coercive field. The observation of this feature in a ferromagnetic multilayer indicates the presence of spontaneous anti-parallel interlayer exchange coupling between the GaMnAs magnetic layers.

Magnetotransport properties of ferromagnetic semiconductor GaMnAs-based superlattices

Two series of GaMnAs/GaAs superlattices (SSs) comprised of ferromagnetic semiconductor GaMnAs layers and non-magnetic GaAs spacers were investigated by the electronic transport measurements with an external magnetic field applied in the plane of the sample. The two SL series consisted of specimens that were structurally the same, but the GaAs spacers layers of one series were doped by Be, while in the second series the spacers were undoped. Although in field scans taken at 4 K all SLs showed a typical anisotropic magnetoresistance (MR) behavior dominated by magnetic anisotropy, similar to that normally observed in single GaMnAs ferromagnetic layers grown on GaAs (001) substrates, some of the SLs showed signatures of interaction between the GaMnAs layers in the form of broadening of the MR hysteresis. The effect of inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers became clear in the MR data taken at 30 K, where the strength of the magnetic anisotropy was reduced to the level of the inter-layer interaction. Specifically, MR measurements on two of the Be-doped SLs (BD2 and BD3) exhibited conspicuously large values of resistance at zero field, along with transitions of magnetization with negative coercive fields. The observation of these features in GaMnAs/GaAs SLs indicates the presence of spontaneous antiferromagnetic (AFM) inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers. The study further revealed that the IEC in the GaMnAs multilayers strongly depended on the properties of the non-magnetic GaAs spacers, such as their thickness and the density of carriers in the layers. Importantly, these IEC effects occurred on a longer range than that expected from current theoretical studies.

Investigation of weak interlayer exchange coupling in GaMnAs/GaAs superlattices with insulating nonmagnetic spacers

A robust long-range antiferromagnetic coupling between ferromagnetic Ga0.97Mn0.03As layers has previously been realized via insertion of nonmagnetic Be-doped GaAs spacers between the magnetic layers. In this paper we report the observation of weak antiferromagnetic coupling between Ga0.97Mn0.03As layers through undoped GaAs spacers with thicknesses as large as 25 monolayers. The field and the temperature dependences of the sample magnetization suggest that the interlayer coupling in these systems substantially deviates from typical ferromagnetic behavior. Polarized neutron reflectivity measurements reveal antiferromagnetic alignment between Ga0.97Mn0.03As layers when a weak field is applied perpendicular to the magnetic easy axis during cooling below TC. The strength of the observed coupling between the magnetic layers is estimated to be weaker than 0.05 mT.

Antiferromagnetic exchange coupling between GaMnAs layers separated by a nonmagnetic GaAs:Be spacer

Interlayer exchange coupling (IEC) between two Ga0.95Mn0.05As layers separated by Be-doped GaAs spacers was investigated using magnetometry and neutron scattering measurements, which indicated the presence of robust antiferromagnetic IEC under certain conditions. We argue that the observed behavior arises from a competition between the IEC field and magnetocrystalline anisotropy fields intrinsic to GaMnAs layers. We estimate the magnitude of the IEC field and show how it decays with increasing temperature.

Beta-detected NMR study of the local magnetic field in epitaxial GaAs:Mn

A low energy beam of spin polarized 8 Li + has been employed to study the magnetic field distribution in an epitaxial thin film of 5.4% Mn doped GaAs(180nm) on a (100) GaAs substrate via beta-detected NMR. The spectrum is a strong function of the implantation energy in the range 28 keV to 3 keV. In the magnetic layer, there is no indication of a missing fraction, and even more remarkable, there is a broad negatively-shifted resonance. The spin lattice relaxation rate is, however, much faster in the Mn doped layer than in the substrate. A sharp peak characteristic of nonmagnetic GaAs is observed down to the lowest implantation energy, for which none of the Li should reach the substrate. This unexpected depth dependence is discussed.

Carrier spin polarization in ferromagnet–semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet–semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.

Magnetotransport in ferromagnetic schottky diodes made of Mn-doped GaAs

The Mn-doped GaAs thin films were grown on p-type GaAs substrates by using Molecular Beam Epitaxy technique. The Schottky contacts on top of the (Ga,Mn)As layer were made of Pt metal. In contrast to the non-magnetic GaAs Schottky diodes, a large negative magnetoresistance (MR) in the dc current was observed in the magnetic diodes at low temperatures. The contributions to the observed MR effects from the spin dependent tunnelling through the thin Schottky barrier and the negative MR of the semiconducting layer are discussed in detail.

Magnetization reversal of Ga1−xMnxAs layers separated by a nonmagnetic spacer

We have used polarized neutron reflectometry to individually examine the magnetization reversals of ferromagnetic Ga1−xMnxAs layers separated by a nonmagnetic GaAs spacer layer of varying thickness. For each of the samples studied, the top Ga1−xMnxAs layer is adjacent to a Be-doped Al0.25Ga0.75As capping layer on one side and the GaAs spacer on the other, while the bottom Ga1−xMnxAs layer is surrounded by GaAs on either side. For samples with spacer thicknesses of 12 and 6nm, antiparallel alignment of the two Ga1−xMnxAs layer magnetizations was observed at multiple fields, implying that hole doping from the capping layer strongly affects the coercivity of the top Ga1−xMnxAs layer but has a weaker effect on the coercivity of the bottom Ga1−xMnxAs layer. However, for a spacer thickness of 3nm, both top and bottom Ga1−xMnxAs layers appear to be equally influenced by the capping layer, as virtually identical coercivities were observed. This behavior is evidence of coupling between the Ga1−xMnxAs layers across the 3nm GaAs spacer.

Ferromagnetic Resonance Study of Exchange Coupled (Ga,Mn)As/GaAs/(Ga,Mn)As Heterostructures

Ferromagnetic resonance study of the exchange coupled (Ga,Mn)As/ GaAs/(Ga,Mn)As heterostructures is reported. The measurements were performed on the series of samples with varying thicknesses d(GaAs) of nonmagnetic GaAs spacer, established d(GaAs)-dependent extent of weak and strong interlayer exchange coupling, judging on the observation of one or two ferromagnetic resonance modes.

Circular polarization from a nonmagnetic p-i-n resonant tunneling diode

The authors investigate the circular polarization of the electro- and photoluminescence emissions from the quantum well and contact layers of a nonmagnetic GaAs–AlAs p-i-n resonant tunneling diode under an external magnetic field. The contact emission evidences the formation of a spin polarized two-dimensional electron gas at the n-accumulation layer. The quantum well electroluminescence presents a strong σ− degree of polarization, even for null Zeeman splitting energies, and a slight bias dependence. The observed circular polarization is mainly attributed to the spin polarization of the electrons injected into the quantum well from the two-dimensional electron gas.

Spin-Dependent Phenomena in (Ga,Mn)As Heterostructures

A model for spin-dependent tunneling in semiconductor heterostruc- tures, which combines a multi-orbital empirical tight-binding approach with a Landauer{Buttiker formalism, is presented. Using this approach we ex- plain several phenomena observed in modulated structures of (Ga,Mn)As, i.e., large values of the electron current spin polarization in magnetic Esaki{ Zener diode and the high tunneling magnetoresistance ratio. Next, the rel- evance of this theory to assess the tunneling anisotropic magnetoresistance efiect is studied. The results of applying the tight-binding model to describe the recently observed interlayer exchange coupling in (Ga,Mn)As-based su- perlattices are also shown. Gallium arsenide doped with manganese is a ∞ag member of the group of semiconductors which at low temperatures exhibit ferromagnetism. These so-called diluted ferromagnetic semiconductors are mainly III{V compounds with part of the cations substituted by magnetic ions. Since the discovery of such mate- rials at the 90-ties of previous century, the ferromagnetic p-type (Ga,Mn)As is by far the most studied and best understood | for (Ga,Mn)As also the highest tem- perature of the transition to the ferromagnetic phase (173 K) has been achieved (1). (Ga,Mn)As owes his unique position to the ability to form high quality junctions with nonmagnetic GaAs or AlAs, due to similar crystal structure of these com- pounds. Intensive studies of such multilayers with modulated magnetization have proven that most of the phenomena essential for realizing functional spintronic devices can be observed also in these all-semiconductor heterostructures. These include flrst of all an e-cient electrical injection of spin-polarized carriers | spin injection from p-(Ga,Mn)As into nonmagnetic semiconductor has been achieved

Spin-dependent transmission of holes in III–V diluted magnetic semiconductor based heterostructure

Based on the mean field approximation, we investigated analytically the spin-transport of holes through heterostructure formed by magnetic layers Ga(1−x)MnxAs separated by a non-magnetic spacer GaAs. Injected holes of up (down) spin have different transmission coefficient which oscillates for spin-down and increases fast for spin-up. The significant quantum size and (RKKY) interaction are considered simultaneously. The results indicate also that as the strength of the magnetic and non-magnetic layers increases, the spin-transmission changes. The results can be used to create efficient spin-filters.

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.

Resonant spin-dependent electron coupling in a III-V/II-VI heterovalent double quantum well

We report on design, fabrication, and magneto-optical studies of a III-V/II-VI hybrid structure containing a $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Zn}\mathrm{Se}∕\mathrm{Zn}\mathrm{Cd}\mathrm{Mn}\mathrm{Se}$ double quantum well (QW). The structure design allows one to tune the QW levels into the resonance, thus facilitating penetration of the electron wave function from the diluted magnetic semiconductor ZnCdMnSe QW into the nonmagnetic GaAs QW, and vice versa. Magneto-photoluminescence studies demonstrate level anticrossing and strong intermixing resulting in a drastic renormalization of the electron effective $g$ factor, in perfect agreement with the energy level calculations.