Ferromagnetic Semiconductor GaMnAs Research Articles

Effect of spin-orbit field on magnetization reversal in GaMnAs single layers with 4-fold in-plane magnetic anisotropy

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 [11̄0] crystallographic directions is fully consistent with the effects arising from the Dresselhaus-type SOI fields in GaMnAs films.

Creation of GaMnAs ferromagnetic semiconductor by combined laser method

GaMnAs layers made by pulsed laser deposition in vacuum on semi-insulating GaAs substrates have been studied. During the creation of the structures, the substrate temperature varied in the range from 200 to 350oC, and the thickness of the layers was ~ 50 nm. Samples of the manufactured structures were annealed by pulses of an excimer KrF-laser. The analysis of the Raman scattering spectra of annealed samples using their approximation by Lorentz functions showed, in addition to peaks in the LO- and TO-GaAs modes, the presence of a coupled phonon-plasmon mode. As a result of annealing, a significant increase in the hole conductivity of the layers is observed (the resistance decreases from Rs~107-109 Ω to Rs~ 900-3000 Ω). The type of magnetic field dependence of magnetization at room temperature changes significantly: there is a transition from a nonlinear characteristic with a hysteresis loop for the initial sample (appearing due to the presence of MnAs compound inclusions with a Curie temperature above room temperature) to a linear behavior for the annealed sample. The study of galvanomagnetic properties at temperatures from 10 to 150 K shows the existence of ferromagnetism in GaMnAs layers with a Curie temperature reaching 90 K. The observed effects allow us to conclude that the applied pulsed laser action leads to modification (dissolution) of MnAs inclusions, electrical activation of Mn and, as a consequence, to the formation of a single-phase ferromagnetic semiconductor GaMnAs. Keywords:: pulsed laser deposition, pulsed laser annealing, ferromagnetic semiconductor.

Ultrafast carrier and spin dynamics in GaMnAs across the Curie temperature

The carrier and spin dynamics of the diluted ferromagnetic semiconductor GaMnAs are crucial to understanding its ferromagnetic nature, especially near the Curie temperature (TC). Herein, transient differential reflectivity measurements and the temporal evolution of Kerr rotation are shown using ultrafast optical spectroscopy at various temperatures. The energy and angular momentum transfer routes after femtosecond laser pumping are revealed. The hole dynamics consist of two processes: a fast process that arises from carrier recombination and a slow process caused by carrier–lattice interactions. The spin dynamics are determined from the spin–orbital coupling and spin–lattice interactions in the ferromagnetic and paramagnetic phases, respectively. Critical phenomena are observed from the competition between the hole-mediated ferromagnetic orders and thermal perturbations near TC. These experimental results provide deep insights into the ultrafast decay behavior in GaMnAs, which is regarded as one of the most promising spintronic materials.

Revealing angular momentum transfer channels and timescales in the ultrafast demagnetization process of ferromagnetic semiconductors

Ultrafast control of magnetic order by light provides a promising realization for spintronic devices beyond Moore's Law and has stimulated intense research interest in recent years. Yet, despite 2 decades of debates, the key question of how the spin angular momentum flows on the femtosecond timescale remains open. The lack of direct first-principle methods and pictures for such process exacerbates the issue. Here, we unravel the laser-induced demagnetization mechanism of ferromagnetic semiconductor GaMnAs, using an efficient time-dependent density functional theory approach that enables the direct real-time snapshot of the demagnetization process. Our results show a clear spin-transfer trajectory from the localized Mn-d electrons to itinerant carriers within 20 fs, illustrating the dominant role of [Formula: see text] interaction. We find that the total spin of localized electrons and itinerant carriers is not conserved in the presence of spin-orbit coupling (SOC). Immediately after laser excitation, a growing percentage of spin-angular momentum is quickly transferred to the electron orbital via SOC in about 1 ps, then slowly to the lattice via electron-phonon coupling in a few picoseconds, responsible for the 2-stage process observed experimentally. The spin-relaxation time via SOC is about 300 fs for itinerant carriers and about 700 fs for Mn-d electrons. These results provide a quantum-mechanical microscopic picture for the long-standing questions regarding the channels and timescales of spin transfer, as well as the roles of different interactions underlying the GaMnAs demagnetization process.

Observation of two-critical-angle phenomenon in spin wave resonances in diluted ferromagnetic semiconductor GaMnAs thin films

We have studied surface anisotropy energy and surface pinning parameters of ferromagnetic semiconductor GaMnAs film by analyzing spin wave resonance (SWR) measurements carried out on this material. The SWR spectra were observed in various polar and azimuthal angular orientations of external static magnetic field with respect to crystal axes of an annealed GaMnAs film with a Mn concentration of 8.0 at.%). The observed SWR spectra were interpreted in terms of a recently developed spherical surface pinning (SSP) theory. As predicted by the SSP theory, our data clearly show the existence of two critical angles in specific resonance configurations. The experimental confirmation of this key prediction of the SSP model brings new insights into the behavior of magnetic surface anisotropy of GaMnAs films. In particular, the agreement of our experimental results with SWR theory shows that, by including higher-order terms with cubic symmetry in the description of the surface free energy, our analysis will improve our understandings of the magnetic surface anisotropy of GaMnAs films.

Efficient full spin\u2013orbit torque switching in a single layer of a perpendicularly magnetized single-crystalline ferromagnet

Spin–orbit torque (SOT), which is induced by an in-plane electric current via large spin-orbit coupling, enables an innovative method of manipulating the magnetization of ferromagnets by means of current injection. In conventional SOT bilayer systems, the magnetization switching efficiency strongly depends on the interface quality and the strength of the intrinsic spin Hall Effect. Here, we demonstrate highly efficient full SOT switching achieved by applying a current in a single layer of perpendicularly magnetized ferromagnetic semiconductor GaMnAs with an extremely small current density of ∼3.4 × 105 A cm−2, which is two orders of magnitude smaller than that needed in typical metal bilayer systems. This low required current density is attributed to the intrinsic bulk inversion asymmetry of GaMnAs as well as its high-quality single crystallinity and large spin polarization. Our findings will contribute to advancements in the electrical control of magnetism and its practical application in semiconductor devices.

Interlayer Exchange Coupling Between Fe and GaMnAs Ferromagnetic Semiconductor

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.

Spin–Orbit-Induced Effective Magnetic Field in GaMnAs Ferromagnetic Semiconductor

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.

Large spin-valve effect in a lateral spin-valve device based on ferromagnetic semiconductor GaMnAs

We investigate the spin-dependent transport properties of a lateral spin-valve device based on the ferromagnetic semiconductor GaMnAs. This device is composed of a GaMnAs channel layer grown on GaAs with a narrow trench across the channel. Its current–voltage characteristics show tunneling behavior. Large magnetoresistance (MR) ratios of more than ∼10% are obtained. These values are much larger than those (∼0.1%) reported for lateral-type spin metal–oxide–semiconductor field-effect transistors. The magnetic field direction dependence of the MR curve differs from that of the anisotropic magnetoresistance of GaMnAs, which confirms that the MR signal originates from the spin-valve effect between the GaMnAs electrodes.

Determination of current-induced spin-orbit effective magnetic field in GaMnAs ferromagnetic semiconductor

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.

Temperature-induced transition of magnetic anisotropy between in-plane and out-of-plane directions in GaMnAs film

Abstract We used the Hall effect and magnetization measurements to investigate the temperature dependence of the magnetic anisotropy of a ferromagnetic semiconductor GaMnAs film grown on a (001) GaAs substrate. The Hall effect was systematically measured by applying an external magnetic field within and normal to the film plane. The switching behavior of the magnetization during the reversal process revealed the coexistence of in-plane and out-of-plane magnetic anisotropies in the film. However, these two types of magnetic anisotropies strongly depended on the temperature. Specifically, the out-of-plane anisotropy was dominant in the low-temperature region (i.e., 3–10 K), whereas the in-plane anisotropy became dominant in the temperature region higher than 15 K. This temperature dependent change in the magnetic anisotropy was further confirmed using direct magnetization measurements.

Electronic structure near the Fermi level in the ferromagnetic semiconductor GaMnAs studied by ultrafast time-resolved light-induced reflectivity measurements

Clarification of the electronic structure near the Fermi level is important in understanding the origin of ferromagnetism in the prototypical ferromagnetic semiconductor GaMnAs. Here, we perform ultrafast transient reflectivity spectra measurement, which is a powerful tool for selective detection of absorption edges in GaMnAs. The results show that the Fermi level of GaMnAs exists in the band gap. By using the Kramers-Kronig relation, we find the Mn-induced electronic states around the Fermi level, confirming that the ferromagnetism is stabilized by spin-polarized impurity-band holes.

Spin-dependent transport properties of a GaMnAs-based vertical spin metal-oxide-semiconductor field-effect transistor structure

We fabricate a vertical spin metal-oxide-semiconductor field-effect transistor (spin-MOSFET) structure, which is composed of an epitaxial single-crystal heterostructure with a ferromagnetic-semiconductor GaMnAs source/drain, and investigate its spin-dependent transport properties. We modulate the drain-source current IDS by ∼±0.5% with a gate-source voltage of ±10.8 V and also modulate IDS by up to 60% with changing the magnetization configuration of the GaMnAs source/drain at 3.5 K. The magnetoresistance ratio is more than two orders of magnitude higher than that obtained in the previous studies on spin MOSFETs. Our result shows that a vertical structure is one of the hopeful candidates for spin MOSFET when the device size is reduced to a sub-micron or nanometer scale.

Optimizing magneto-optical effects in the ferromagnetic semiconductor GaMnAs

A trilayer of the ferromagnetic semiconductor GaMnAs, a SiO2 buffer layer and a piezoelectric ZnO layer, is investigated in view of its use in device implementation to study surface acoustic wave-assisted magnetization switching. The magneto-optical properties: Kerr rotation and ellipticity and magnetic contrast in Kerr microscopy images are investigated as a function of temperature. While the ZnO layer prevents any good quality imaging of magnetic domains, we show that with the SiO2 layer only the polar Kerr rotation and the magnetic contrast are increased by a factor of 2. This result is in good quantitative agreement with calculations using an optical interference model and could be further improved. The detrimental effects of the dielectric layers capping on the Curie temperature and coercive field of the GaMnAs layer can be kept to a reasonable level.

Magnetoelectric effect in GaMnAs /P(VDF-TrFE) composite multiferroic nanostructures

The electric-field effect on the Curie temperature of the ferromagnetic semiconductor GaMnAs and ferroelectric polyvinylidene fluoride with trifluoroethylene P(VDF-TrFE) multiferroic nanostructures have been investigated. The Curie temperatures of GaMnAs layers were determined from the temperature dependencies of the resistivity at zero magnetic field and the anomalous Hall effect measurements. Under the electric-field potential with different polarities applied to the ferroelectric gate the shift of the Curie temperature in GaMnAs has been observed. The shift of TC is due to the variation of the hole concentration in the ferromagnetic layer induced by the gate electric field.

Magnetic depth profile in GaMnAs layers with vertically graded Mn concentrations

Controlled vertical grading of magnetization of the ferromagnetic semiconductor GaMnAs represents a significant step toward optimizing its magnetic properties for device applications. Quantitative control of such grading is difficult due to various competing effects, such as Mn diffusion, self-annealing, and diffusion of charge carriers. Furthermore, there are also several surface effects that can influence the magnetization profile, which should be considered in designing and fabricating graded GaMnAs specimens. However, we show that vertical magnetization gradients in GaMnAs layers can be readily achieved by appropriate growth strategies. In this paper we describe the preparation, magnetization measurements, and polarized neutron reflectometry studies of vertically graded GaMnAs layers, which provide direct evidence that vertical grading of Mn concentration has been successfully achieved in our GaMnAs samples. Our measurements also indicate that these graded samples exhibit magnetic “hardening” near the surface.

Quantitative investigation of magnetic domains with in-plane and out-of-plane easy axes in GaMnAs films by Hall effect

Magnetic anisotropy of ferromagnetic semiconductor GaMnAs film with a low Mn concentration grown on a (001) GaAs substrate was investigated by Hall effect measurements. The presence of domains with in-plane and out-of-plane easy axes was identified in the film by analyzing hysteresis loops observed via the Hall resistance measured in various geometries. Quantitative analysis of the planar Hall resistance showed that the fraction of the sample with magnetic domains having a dominant out-of-plane easy axis was about 6 times larger than the fraction corresponding to domains with easy axis in the sample plane.

Coexistence of magnetic domains with in-plane and out-of-plane anisotropy in a single GaMnAs film

Magnetic anisotropy of the ferromagnetic semiconductor GaMnAs film grown on a (001) GaAs substrate was investigated by using Hall effect and magnetization measurements. When field strength was swept at a fixed direction in the film plane, abrupt transitions in the Hall resistance appeared while reducing the field strength even before the field direction is reversed. We show that this phenomenon is related to the presence of magnetic domains with a vertical easy axis in the film, as identified via Hall measurements performed with a field applied normal to the plane. The coexistence of magnetic domains with both in-pane and out-of-plane anisotropies in a single film was further confirmed by direct measurement of the corresponding magnetization components of the film.

In-Plane Magnetic Anisotropy and Temperature Dependence of Switching Field in (Ga, Mn) as Ferromagnetic Semiconductors

We explore the magnetic anisotropy of GaMnAs ferromagnetic semiconductor by Planar Hall Effect (PHE) measurements. Using low magnitude of applied magnetic field (i.e., when the magnitude H is smaller than both cubic Hc and uniaxial Hu anisotropy field), we have observed various shapes of applied magnetic field direction dependence of Planar Hall Resistance (PHR). In particular, in two regions of temperature. At T < Tc/2, the "square-shape" signal and at T > Tc/2 the "zigzag-shape" signal of PHR. They reflect different magnetic anisotropy and provide information about magnetization reversal process in GaMnAs ferromagnetic semiconductor. The theoretical model calculation of PHR based on the free energy density reproduces well the experimental data. We report also the temperature dependence of anisotropy constants and magnetization orientations. The transition of easy axis from biaxial to uniaxiale axes has been observed and confirmed by SQUID measurements.

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.