Ferromagnetic Ga Research Articles

Location of Mn sites in ferromagnetic Ga1−xMnxAs studied by means of X-ray diffuse scattering holography

A three-dimensional image of the local neighbourhood of Mn atoms in a Ga1−xMnxAs (x= 0.02) layer has been obtained by using X-ray diffuse scattering holography. The first and second nearest neighbours of the Mn atoms correspond to the local structure around Ga atoms in the zinc-blende GaAs structure. Accordingly, the Mn atoms are situated in substitutional positions.

Magnetic behavior of Mn 3GaN precipitates in ferromagnetic Ga 1− xMn xN layers

Ga 1− x Mn x N layers (where x ≈ 10.4–12.5%) containing Mn 3GaN precipitates were grown on (0 0 0 1) sapphire substrates using molecular beam epitaxy, and were shown to exhibit two ferromagnetic phases. These p-type GaMnN films were revealed to have a well defined hysteresis loop up to T ∼ 200 K, which originates from the appreciable number of Mn 3GaN precipitates imbedded in the GaN matrix. The precipitated GaMnN samples exhibited a ferromagnetism due to Mn 3Ga clusters mixed with Mn 3GaN precipitates in the high temperature region (above 200 K). The Mn 3GaN precipitates and Mn 3Ga clusters were identified by X-ray diffraction and transmission electron microscopy. Mn 3GaN precipitates in the samples with a high Mn concentration have a characteristic transition temperature at T ∼ 200 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.

Mn L 3 , 2 x-ray absorption and magnetic circular dichroism in ferromagnetic Ga1−xMnxP

We have measured the x-ray absorption and x-ray magnetic circular dichroism (XMCD) at the MnL3,2 edges in ferromagnetic Ga1−xMnxP films for 0.018⩽x⩽0.042. Large XMCD asymmetries at the L3 edge indicate significant spin polarization of the density of states at the Fermi energy. The temperature dependence of the XMCD and moment per Mn of 2.67±0.45μB calculated using sum rules are consistent with magnetometry values. The spectral shapes of the x-ray absorption and XMCD are nearly identical with those for Ga1−xMnxAs indicating that the hybridization of Mn d states and anion p states is similar in the two materials.

Correlation of electrical properties with magnetic properties for ferromagnetic (Ga1−xMnx)As epilayer

Abstract The correlation of electrical properties with magnetic properties for the (Ga 1− x Mn x )As epilayer was investigated. For electrical transport measurements, it was clearly observed that the electrical mobility is increased with decreasing temperatures for both the (Ga 0.974 Mn 0.026 )As/LT-GaAs epilayer and the LT-GaAs:Be epilayer. However, a different behavior was observed at the cryogenic temperature region. The electrical mobility of (Ga 0.974 Mn 0.026 )As/LT-GaAs epilayer increases with decreasing temperature, while the mobility of LT-GaAs:Be epilayer decreases with decreasing temperature. In Arrhenius plots of carrier mobility for the (Ga 0.974 Mn 0.026 )As/LT-GaAs epilayer, the critical point is observed at 69 K, and this value is almost the same as the T C . This result indicates that the carrier transport in ferromagnetic (Ga 1− x Mn x )As epilayers might be related to a spin-ordering effect because the spins will be arranged with the same direction below the T C , and this will lead to reducing the probability of spin-disorder scattering. Therefore, the observation of a gradual drop below the T C in the temperature-dependent resistivity curve is expected to be a result of the spin-ordering effect in the ferromagnetic (Ga 0.974 Mn 0.026 )As/LT-GaAs epilayer.

Optical studies of MOCVD‐grown GaN‐based ferromagnetic semiconductor epilayers and devices

AbstractGa1–xMnxN epilayers and p‐i‐n device structures have been grown by metalorganic chemical vapor deposition. Optical studies were found to investigate the role of Mn concentration and the role of different co‐dopants to elucidate the origin of the room temperature ferromagnetism in Ga1–xMnxN epilayers. Increasing Mn concentration was found to significantly affect long‐range lattice ordering. This observation was supported by the existence of a disorder‐induced Raman mode at 300 cm–1 and local vibrational modes (LVMs) at 669 cm–1 that has been attributed to the formation of self‐compensating nitrogen. The E1(TO) phonon frequency was found to linearly increase with Mn composition, which is expressed by (558 + 2.7x) cm–1. The intensity and the linewidth of an absorption band near 1.5 eV was found to increase with Mn concentration, and is assigned to the e to t2 transition in the split 5T2 band of Mn3+. No absorption was detected in Si co‐doped Ga1–xMnxN. In this case, a significantly increased EPR signal of Mn2+ ions confirmed the trapping of electrons provided by the shallow Si donor and reduced the available states for ferromagnetic exchange. A change in the measured magnetization is observed under ultraviolet illumination, though the nature of this phenomenon is still not understood. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Giant planar Hall effect in ferromagnetic (Ga,Mn)As layers

We have performed a systematic investigation of the so-called planar Hall effect and the magnetization in thin layer of ferromagnetic Ga 0.94Mn 0.06As semiconductor, which has been subjected to post-growth annealing. The annealing process was performed under As capping in order to achieve high hole concentration exceeding 10 21 cm −3 and high Curie temperature of 125 K. At liquid helium temperature the planar Hall effect in the layer exhibits a giant magnitude, several orders of magnitude greater than analogous effect found in metallic ferromagnets. When sweeping the magnetic field the planar Hall resistance varies non-monotonously alternating its sign and displaying the appearance of single or double hysteresis loops, depending on the magnetic field orientation and the sweeping range of the magnetic field. It is demonstrated, by comparing with the magnetization data, that the planar Hall effect reproduces the complex behaviour of the magnetic anisotropy of the layer grown under biaxial compressive strain, which additionally displays uniaxial in-plane magnetization anisotropy.

Magneto-conductance through submicron constriction in ferromagnetic (Ga,Mn)As film

We fabricated a simple magnetoresistive microdevice formed by a narrow constriction of submicron width in the epitaxial film of a ferromagnetic (Ga,Mn)As semiconductor, and investigated magnetic properties of the film and the low-temperature charge-carrier transport through the constriction. We have revealed sharp jumps of a lowered conductance in a non-constricted sample and jumps of an enhanced conductance in the constricted one, which appeared when the sweeping magnetic field crossed the regions of the coercive field of the film. We argue that the both features result from a contribution of a magnetic domain wall to the conductance. While the spin-orbit interaction can be responsible for the negative contribution of a domain wall to the conductance, presumably the suppression of the weak localization effects by a domain wall located in the constriction results in the positive contribution to the conductance.

Temperature dependence of the anomalous Hall effect in ferromagnetic (Ga,Mn)As epilayers

We fabricated high quality (Ga,Mn)As epilayers by low-temperature molecular beam epitaxy and studied the magnetic and magneto-transport properties in detail. In particular, we investigated the relation between the longitudinal and the transverse Hall resistivity over a wide range of temperatures. It turned out that the anomalous Hall coefficient scaled linearly with the longitudinal resistivity, which seems to imply that the skew scattering is the primary source of the anomalous Hall effect in (Ga,Mn)As if we consider classical models only. However, we cannot rule out other mechanisms because of the narrow range of resistivity investigated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of diluted magnetic semiconductors: A minimal model

In this article we review our recent theoretical studies on the carrier induced ferromagnetism of DMS. We propose a minimal model, which is composed of a single tight-binding band of carriers and magnetic impurities, which randomly substitute the host sites. Both of nonmagnetic attractive potentials due to impurities and the exchange interactions between carrier and impurity spins are taken into account. We apply the coherent potential approximation in studying this system. The obtained phase diagrams illustrate the characteristic features of the carrier induced ferromagnetism in DMS. The role of nonmagnetic interaction for the enhancement of the Curie temperature is clarified. The results suggest that the ferromagnetism in Ga1−xMnxAs is caused by a double-exchange-like mechanism mediated by valence band holes. We also report the results of numerical simulations of the model.

Analysis of magnetic field dependent mobility in ferromagnetic Ga 1− xMn xAs layers

The magneto-transport properties of ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.

Effect of nitrogen incorporation in ferromagnetic (Ga,Mn)As epilayers

Physical properties of ferromagnetic p-type Ga1−xMnxAs1−yNy epitaxial films (x=0.045 and y=0–0.032) grown by molecular-beam epitaxy are described. It is found that the incorporation of nitrogen gradually suppresses the metallic behavior of p-(Ga,Mn)As, especially at low temperatures. The Curie temperature TC decreases accordingly with increasing y value. The reduction in TC is discussed in terms of the suppression of hole-mediated long-range spin-exchange interaction due to the enhancement of carrier localization. Perpendicular magnetic anisotropy appears at the region around y=0.02, reflecting the reduced lattice constant due to substitutionally incorporated nitrogen. No significant change was found in the magnetic circular dichroism spectra in the photon energy region around the band gap of (Ga,Mn)As. Our experimental results show that the theoretical prediction based on the double exchange interaction [V. A. Dinh et al., Jpn. J. Appl. Phys., Part 2 42, L888 (2003)] is hardly materialized in this system.

Perpendicular magnetization reversal, magnetic anisotropy, multistep spin switching, and domain nucleation and expansion in Ga1−xMnxAs films

We present a comprehensive study of the reversal process of perpendicular magnetization in thin layers of the ferromagnetic semiconductor Ga1−xMnxAs. For this investigation we have purposely chosen Ga1−xMnxAs with a low Mn concentration (x≈0.02), since in such specimens contributions of cubic and uniaxial anisotropy parameters are comparable, allowing us to identify the role of both types of anisotropy in the magnetic reversal process. As a first step we have systematically mapped out the angular dependence of ferromagnetic resonance in thin Ga1−xMnxAs layers, which is a highly effective tool for obtaining the magnetic anisotropy parameters of the material. The process of perpendicular magnetization reversal was then studied by magnetotransport (i.e., Hall effect and planar Hall-effect measurements). These measurements enable us to observe coherent spin rotation and noncoherent spin switching between the (100) and (010) planes. A model is proposed to explain the observed multistep spin switching. The agreement of the model with experiment indicates that it can be reliably used for determining magnetic anisotropy parameters from magnetotransport data. An interesting characteristic of perpendicular magnetization reversal in Ga1−xMnxAs with low x is the appearance of double hysteresis loops in the magnetization data. This double-loop behavior can be understood by generalizing the proposed model to include the processes of domain nucleation and expansion.

First-principles investigation of electronic structure and magnetic properties in ferromagnetic Ga1−xMnxN and Al1−xMnxN

In this paper, we present theoretical investigations of the electronic and magnetic properties of the ordered ferromagnetic Ga0.75Mn0.25N and Al0.75Mn0.25N in zinc-blende structure using all electron full-potential (linear) augmented plane waves plus local orbitals calculations within the density functional theory and local spin-density approximation. The analyses of the band structures, density of states, total energy, exchange interactions and magnetic moments reveal that both the alloys may exhibit a half-metallic ferromagnetism character and the valence band is ferromagnetically coupled to the Mn atoms. The total magnetization of the cell is evaluated as 4.0 μB.

Magnetotransport properties of ferromagnetic Ga1−xMnxAs layers on a (100) GaAs substrate

The magnetotransport properties of ferromagnetic Ga1−xMnxAs epilayers with Mn mole fractions in the range of x≈2.2%–4.4% were investigated using Hall effect measurements. The temperature-dependent Hall carrier concentration for a metallic sample with x≈2.2% was analyzed assuming an activation energy from two acceptor levels. It was found that the two acceptor levels with activation energies of 129.4 and 31.6 meV at B=0Oe decreased to 87.6 and 30.7 meV, respectively, at B=5kOe. The decrease in acceptor activation energy from 129.6 to 87.6 meV was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for the increase in carrier concentration. From magnetic-field-dependent Hall resistance data, the Curie temperature was estimated to be TC=60 and 70 K for Ga1−xMnxAs samples with x≈2.2 and x≈4.4%, respectively. The magnetoresistance measurements confirmed that the anomalous Hall effect existed in these samples that showed metallic and insulating behavior, respectively.

Identification of extrinsic Mn contributions in Ga1−xMnxAs by field-dependent magnetic circular X-ray dichroism

We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field-dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1−xMnxAs and observe different Mn constituents: ferromagnetic Mn with an nd>5 lineshape and paramagnetic Mn with distinct nd=5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency.

Magnetization Reversal by Electrical Spin Injection in Ferromagnetic (Ga,Mn)As-Based Magnetic Tunnel Junctions

We report current-induced magnetization reversal in a ferromagnetic semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared by molecular beam epitaxy on a p-GaAs(001) substrate. A change in magneto-resistance that is asymmetric with respect to the current direction is found with the excitation current of 106 A/cm2. Contributions of both unpolarized and spin-polarized components are examined, and we conclude that the partial magnetization reversal occurs in the (Ga,Mn)As layer having smaller magnetization with the spin-polarized tunneling current of 105 A/cm2.

First principles study of the ferromagnetism inGa1−xMnxAs semiconductors

We have performed ab initio calculations within the density-functional theory forGa1−xMnxAs diluted semiconductors. Total energy results unambiguously show that a quasi-localized hole, with predominant p-like character, surrounds the fully polarized Mn d5-electrons. The calculations indicate that the holes form a relativelydispersionless impurity band, thus rendering effective-mass descriptionsof hole states open to challenge. We obtain estimates both for thes = 1/2 holeand S = 5/2 Mn exchange coupling, and for the distance dependence of the effective Mn–Mn exchangeinteraction. The results demonstrate that the effective Mn–Mn coupling is alwaysferromagnetic, and thus non-RKKY, and is intermediated by the antiferromagneticcoupling of each Mn spin to the holes.

Properties of arsenic antisite defects in Ga1−xMnxAs

We report the results of optical absorption measurements on Ga1−xMnxAs layers grown by low-temperature molecular beam epitaxy. In the paramagnetic layers grown at very low temperatures (below 250 °C) the experiments reveal an absorption band at 1.2 eV arising from the presence of neutral arsenic antisites, AsGa. From the magnitude of the absorption we determine the concentration of AsGa to be between 4×1019 and 8×1019 cm−3 in these paramagnetic samples. These values are typical for GaAs specimens grown below 250 °C. Extrapolating the AsGa concentration from low-temperature-grown GaAs to Ga1−xMnxAs, we determine the concentration of this defect in ferromagnetic Ga1−xMnxAs layers grown at temperatures above 250 °C as 1×1019 down to 1×1018 cm−3. We conclude that the compensating role of arsenic antisites in Ga1−xMnxAs becomes gradually less important with increasing growth temperature.

Epitaxial overgrowth of (II,Mn)VI quantum well structures on ferromagnetic (Ga,Mn)As and MnAs films: topographical, magnetic and optical studies

We report on an overgrowth of shallowly buried quantum wells (QW) of diluted magnetic semiconductor (DMS) (with non-magnetic CdMgTe or ZnTe barriers) on ferromagnetic MnAs and GaMnAs films. Atomic force microscopy (AFM) of the structures grown on MnAs revealed protruded micrometer-size islands, surrounded by areas with small-scale roughness. Magnetic force microscopy (MFM) imaging at room temperature showed clear magnetic domains within these islands. The structures overgrown on GaMnAs exhibited smooth surfaces, but no magnetic signal was detectable. The luminescence (PL) emission attributable to CdMnTe quantum wells was observed from all overgrown structures. Spatially resolved μ-PL showed that the peak energy of emission depended on the position of the excitation and PL collection spot for the samples overgrown on MnAs and was slightly blue-shifted when recorded from the magnetized islands area.

High-temperature Hall effect inGa1−xMnxAs

The temperature dependence of the Hall coefficient of a series of ferromagnetic Ga(1-x)Mn(x)As samples is measured in the temperature range 80K < T < 500K. We model the Hall coefficient assuming a magnetic susceptibility given by the Curie-Weiss law, a spontaneous Hall coefficient proportional to rho_xx^2(T), and including a constant diamagnetic contribution in the susceptibility. For all low resistivity samples this model provides excellent fits to the measured data up to T=380K and allows extraction of the hole concentration (p). The calculated p are compared to alternative methods of determining hole densities in these materials: pulsed high magnetic field (up to 55 Tesla) technique at low temperatures (less than the Curie temperature), and electrochemical capacitance- voltage profiling. We find that the Anomalous Hall Effect (AHE) contribution to rho_xy is substantial even well above the Curie temperature. Measurements of the Hall effect in this temperature regime can be used as a testing ground for theoretical descriptions of transport in these materials. We find that our data are consistent with recently published theories of the AHE, but they are inconsistent with theoretical models previously used to describe the AHE in conventional magnetic materials.