Ferromagnetic Ga Research Articles

Stability and diffusion of interstitial and substitutional Mn in GaAs of different doping types

We report on ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission channeling experiments on the lattice location and thermal stability of Mn impurities ($l$0.05$%$) in both semi-insulating and heavily $n$-type doped GaAs. In addition to the majority of the Mn impurities substituting for Ga, up to 30$%$ occupy tetrahedral interstitial sites with As nearest neighbors. Whereas the interstitial fraction is stable up to 400${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C, with an activation energy for diffusion of 1.7\char21{}2.3 eV, substitutional Mn diffuses only at $\ensuremath{\sim}$700${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C with an activation energy of $\ensuremath{\sim}$3 eV. By comparing these results to those of recent emission channeling experiments on heavily $p$-type doped GaAs [L. M. C. Pereira et al., Appl. Phys. Lett. 98, 201905 (2011)], we conclude that the observed high thermal stability of the interstitial fraction cannot be ascribed to trapping by charged defects, but is an intrinsic characteristic of isolated interstitial Mn in the low doping regime ($l$0.05$%$ Mn). Compared to ferromagnetic Ga${}_{1\ensuremath{-}x}$Mn${}_{x}$As (few percent Mn), for which a significantly lower activation energy has been reported, these findings motivate a comprehensive assessment of how the thermal stability and the diffusion of interstitial Mn are affected by the Mn concentration.

Room Temperature Ferromagnetism in Ga1−xHoxN (x=0.0 and 0.05) Diluted Magnetic Semiconductor Thin Films

Holmium doped GaN diluted magnetic semiconductor thin films have been prepared by thermal evaporation technique and subsequent ammonia annealing. X-ray diffraction measurements reveal all peaks belong to the purely hexagonal wurtzite structure. Surface morphology and composition analysis were carried out by scanning electron microscopy and energy dispersive spectroscopy respectively. The room temperature ferromagnetic properties of Ga1−xHoxN (x=0.0, 0.05) films were analyzed using vibrating sample magnetometer at room temperature. Magnetic measurements showed that the undoped films (i.e. GaN) exhibited diamagnetic behavior, while the Ho-doped (Ga0.95Ho0.05N) film exhibited a ferromagnetic behavior.

Thickness dependence of magneto-optical effects in (Ga,Mn)As epitaxial layers

Rotation angle (θMO) of a linearly polarized light reflected from in-plane, ferromagnetic (Ga,Mn)As layers was measured precisely using a magneto-optical microscope. The θMO value varies non-linearly as a function of (Ga,Mn)As layer thickness d, showing a maximum at d = 50–60 nm. The thickness dependent θMO was analyzed quantitatively with a model based on an interference effect incorporating birefringence and dichroism, and it has been concluded that the contribution of magnetization-vector dependent refractive index, a magnetic birefringence, is responsible for the observed magneto-optical effect. The magnitude of magnetic birefringence appears to be comparable to those of uniaxial birefringence crystals.

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.

Direct identification of interstitial Mn in heavily p-type doped GaAs and evidence of its high thermal stability

We report on the lattice location of Mn in heavily p-type doped GaAs by means of β− emission channeling from the decay of M56n. The majority of the Mn atoms substitute for Ga and up to 31% occupy the tetrahedral interstitial site with As nearest neighbors. Contrary to the general belief, we find that interstitial Mn is immobile up to 400 °C, with an activation energy for diffusion of 1.7–2.3 eV. Such high thermal stability of interstitial Mn has significant implications on the strategies and prospects for achieving room temperature ferromagnetism in Ga1−xMnxAs.

Mechanism of ultrahigh Mn concentration in epitaxially grown wurtzite Ga1−xMnxN

The mechanism of Mn doping in epitaxially grown wurtzite Ga1−xMnxN has been investigated by using first-principles calculations. Although the Mn solubility limit is calculated to be as low as 0.032% under thermodynamical equilibrium, it can be increased by about two orders due to the role of growth surface, reaching several atomic percent, and can be further increased by hydrogen passivation. This work indicates that Ga1−xMnxN can be grown epitaxially with Mn concentration as high as that were used in most theoretical predictions, making it reasonable to expect practical room-temperature ferromagnetic Ga1−xMnxN suitable for spin injector.

Nanoscale Potential Fluctuations in (GaMn)As/GaAs Heterostructures: From Individual Ions to Charge Clusters and Electrostatic Quantum Dots

During growth of the dilute p-type ferromagnetic semiconductor Ga1-xMnxAs, interstitial manganese, Mni(2+), is formed when x exceeds 2%. The double donor Mni(2+) compensates the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of Mni(2+) in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual Mni(2+) ions, and show that the ions are doubly charged.

Influence of uniaxial anisotropy on the domain pinning fields of ferromagnetic Ga1−xMnxAs films

The domain pinning fields of ferromagnetic Ga1−xMnxAs films were investigated using the planar Hall effect (PHE). Two in-plane components of GaMnAs films’ anisotropy fields (H4∥ and HU∥), which determine the direction of magnetic easy axes in the (001) plane, were obtained from the PHE’s angular dependence fitted with magnetic free energy within the scheme of the Stoner–Wohlfarth model. The domain pinning fields were obtained both with and without consideration of the deviation angle, δ, of magnetic easy axes from the ⟨100⟩ crystallographic direction of each sample. The values of domain pinning fields are clearly different between the two methods of analysis and the discrepancy increases with δ. This indicates that the correct direction of the magnetic easy axis (i.e., the influence of uniaxial anisotropy) must be considered to obtain precise values of pinning fields in GaMnAs films.

Raman scattering study of vibrational modes and hole concentration in GaxMn1−xSb

The Raman scattering study of vibrational modes and hole concentration in a ferromagnetic semiconductor Ga1−xMnxSb grown by Mn ion implantation, deposition and post-annealing has been presented. The experiments are performed both in implanted and unimplanted regions before and after etching the samples. The Raman spectra measured from the unimplanted region show only GaSb-like phonon modes. On the other hand, the spectra measured from the implanted region show additional phonon modes approximately at 115, 152, 269, 437 and 659 cm−1. The experimental results demonstrate that the extra modes are associated with surface defects, crystal disorder and blackish layer that is formed due to Mn ion implantation, deposition and annealing processes. Furthermore, we have determined the hole concentration as a function of laser probing position by modeling the Raman spectra using coupled mode theory. The contributions of GaSb-like phonon modes and coupled LO-phonon plasmon mode are taken into consideration in the model. The hole-concentration-dependent CLOPM is resolved in the spectra measured from the implanted and nearby implanted regions. The hole concentrations determined by Raman scattering are found to be in good agreement with those measured by the electrochemical capacitance–voltage technique.

Heat flow model for pulsed laser melting and rapid solidification of ion implanted GaAs

In order to further understand the pulsed-laser melting (PLM) of Mn and N implanted GaAs, which we have used to synthesize thin films of the ferromagnetic semiconductor Ga1−xMnxAs and the highly mismatched alloy GaNxAs1−x, we have simulated PLM of amorphous (a-) and crystalline (c-) GaAs. We present a numerical solution to the one-dimensional heat equation, accounting for phase-dependent reflectivity, optical skin depth, and latent heat, and a temperature-dependent thermal conductivity and specific heat. By comparing the simulations with experimental time-resolved reflectivity and melt depth versus laser fluence, we identify a set of thermophysical and optical properties for the crystalline, amorphous, and liquid phases of GaAs that give reasonable agreement between experiment and simulation. This work resulted in the estimation of thermal conductivity, melting temperature and latent heat of fusion of a-GaAs of 0.008 W/cm K at 300 K, 1350 K, and 2650 J/cm3, respectively. These materials properties also allow the prediction of the solidification velocity of crystalline and ion-amorphized GaAs.

All-optical 90-degree switching of magnetization in a ferromagnetic Ga0.98Mn0.02As microbar

All-optical, 90° switching of magnetization has been carried out successfully in a laterally magnetized (Ga,Mn)As microbar with a scanning laser magneto-optical microscope. No external magnetic field was applied during switching. Light was used primarily as local excitation source to elevate the lattice temperature (>TC/2) at which cubic and uniaxial anisotropies become near equal. Lithography-induced magnetic anisotropy plays a key role in that it breaks the degeneracy of fourfold cubic anisotropy of (Ga,Mn)As to a considerable extent.

Asymmetry in the reorientation process of magnetization for crossing the [11¯] and the [110] directions in Ga1−xMnxAs epilayers

The magnetization reversal processes of ferromagnetic Ga1−xMnxAs films with different Mn concentrations have been investigated using the planar Hall effect. The field scan of the planar Hall resistance (PHR) showed an asymmetric behavior for the reorientation of magnetization crossing the [11¯0] and the [110] directions. The magnetic anisotropy fields and the domain pinning field distributions of the films are obtained from the angle dependence of the PHR measurements. The magnetic anisotropy and pinning field distribution in the samples provided explanation for the observed asymmetric behavior in the magnetization reorientation process of Ga1−xMnxAs film.

Magnetic anisotropy of Ga1−xMnxAs films with additional nonmagnetic donor doping

We have investigated the magnetic anisotropy properties of a series of ferromagnetic Ga1−xMnxAs:Si films by transport measurements. The angular dependences of the planar Hall resistance (PHR) were analyzed in terms of the magnetic free energy to obtain the anisotropy fields. The cubic component of the magnetic anisotropy is much stronger than the uniaxial component at low temperature, although its dominance rapidly decreases with increasing temperature. The direction of the uniaxial anisotropy at 16 K lies along either the [1¯11] or the [110] direction depending on the Mn composition in the series. The uniaxial anisotropy direction, however, changed along the [110] direction regardless of the Mn concentration in the samples as the temperature increases.

Temperature induced in-plane/out-of-plane magnetization transition in ferromagnetic Ga0.93Mn0.07As0.94P0.06/(100)GaAs thin films

Phosphorous alloying in the y=0.06 range allows to engineer ferromagnetic Ga1−xMnxAs1−yPy/(100)GaAs thin films with standard x=0.07 Mn doping in which the easy axes of magnetization can be changed from in-plane to out-of-plane by a small (10 K) temperature rise. Ferromagnetic resonance investigations show the reorientation to be induced by the different temperature dependence of the cubic and uniaxial anisotropy constants.

Mapping a magnetization switching field in Ga0.98Mn0.02As microbars with a scanning laser magneto-optical microscope

Local information on 90° magnetization switching in ferromagnetic Ga0.98Mn0.02As microbars with in-plane magnetization was obtained successfully by a homemade scanning laser magneto-optical microscopy. The switching occurs from the edge regions that include edges whose directions are perpendicular to the initial magnetization direction. The local switching field depends strongly on the direction of initial magnetization. The observed results suggest that demagnetization is one of the important factors for magnetization switching in micron-scale (Ga,Mn)As structures.

Analysis of Raman scattering of Ga 1−xMn xAs dilute magnetic semiconductor

Ferromagnetic Ga 1− x Mn x As layers (where x=1.4–3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga 1− x Mn x As increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10 19 and 8.3×10 20 cm −3, respectively.

Photoluminescence investigation of ferromagnetic Ga 1− xMn xN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga 1− x Mn x N layers (where x ≈ 0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga 1− x Mn x N layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga 1− x Mn x N layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10 −4, 2.7 × 10 −4, 3.4 × 10 −4 eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga 1− x Mn x N layers increased, the temperature dependence of the band-gap energy was clearly reduced.

Magneto-Transport Characterization of Four-Arm Nanostructure Based on Ferromagnetic (Ga,Mn)As

We report on results of magneto-transport measurements performed on four-arm nanostructure fabricated from p-type ferromagnetic Ga0.92Mn0.08As layer. The results reveal hysteresis-like behaviors of low field magneto resistance. We interpret the magnetoresistance in terms of domain walls, which are expected to be trapped inside the nanostructure at some particular positions and which contribute to the total resistance.

Charge trapping effects in photovoltage measurements of (Ga,Mn)As

Abstract We report charge trapping effects in a ferromagnetic (Ga,Mn)As thin film observed during measurements of the photovoltage under inhomogeneous illumination conditions. Using a laser diode as the excitation source, the temperature-dependent response has been observed, with open-circuit voltages as high as −1.4 V. The unexpectedly large and sharp temperature-dependent responses are explained in terms of the excitation of photo-generated charges and their trapping in localised states, or separation of the charges across the film/substrate interface. The sharp features found in the signals suggest their possible use for studying localised trapping states in semiconductors using inhomogeneous illumination.

Thermoelectric studies of electronic properties of ferromagnetic GaMnAs layers

Thermoelectric power, electrical conductivity, and high field Hall effect were studied over a broad temperature range in ferromagnetic Ga(1-x)Mn(x)As epitaxial layers (0.015<x<0.06). Thermoelectric power analysis gives the information about carrier transport mechanisms in layers with both metallic and non-metallic type of conductivity and allows determination of the Fermi energy and carrier concentration. At high temperatures (T>70 K) the thermoelectric power in GaMnAs linearly increases with increasing temperature. That indicates the presence of a degenerate hole gas with the Fermi energy EF=220+-25 meV, nearly independent of Mn content (for 0.02<x<0.05). At lower temperatures GaMnAs layers with metallic-type conductivity show an additional contribution to the thermoelectric power with the maximum close to the Curie temperature. The layers exhibiting insulating electrical properties show 1/T-type increase of thermoelectric power at low temperatures.