Ferromagnetic Ga Research Articles

Colossal negative magnetoresistance from hopping in insulating ferromagnetic semiconductors

Ferromagnetic semiconductor Ga1–x Mn x As1–y P y thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers. In this regime, we report a colossal negative magnetoresistance (CNMR) coexisting with a saturated magnetic moment, unlike in the traditional magnetic semiconductor Ga1– x Mn x As. By analyzing the temperature dependence of the resistivity at fixed magnetic field, we demonstrate that the CNMR can be consistently described by the field dependence of the localization length, which relates to a field dependent mobility edge. This dependence is likely due to the random environment of Mn atoms in Ga1–x Mn x As1–y P y which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.

Direct observation of the magnetic ordering process in the ferromagnetic semiconductor Ga1−xMnxAs via soft x-ray magnetic circular dichroism

In order to understand the mechanism of the ferromagnetism in the ferromagnetic semiconductor Ga1−xMnxAs [(Ga,Mn)As], we have investigated the magnetic behavior on a microscopic level through systematic temperature (T)- and magnetic field (H)-dependent soft x-ray magnetic circular dichroism (XMCD) experiments at the Mn L2,3 absorption edges. The T and H dependences of XMCD intensities have been analyzed using a model consisting of the ferromagnetic (FM), paramagnetic, and superparamagnetic (SPM) components. Intriguingly, we have found a common behavior for the ferromagnetic ordering process in (Ga,Mn)As samples with different Mn concentrations (4% and 10.8%) and different Curie temperature (TC) values (65, 120, and 164 K). In particular, the SPM component develops well above TC, indicating that local FM regions are formed well above TC. The present findings indicate that the onset of ferromagnetic ordering is triggered by local electronic states around the substitutional Mn ions rather than uniform electronic states considered by mean-field theories. Insight into the most representative ferromagnetic semiconductor, (Ga,Mn)As, provided by the present study will be an important step in understanding the mechanism of ferromagnetic ordering in various ferromagnetic semiconductor families.

Magneto-Optical Detection of the Intrinsic Ferromagnetism and Phase Separation in Diluted Magnetic Semiconductors

Abstract—Spectral and temperature dependences of the magneto-optical transversal Kerr effect (TKE) of GaMnAs layers prepared by different methods are reported in this work. The GaMnAs layers prepared by pulsed laser sputtering at 300°С demonstrate a ferromagnetic behavior below 80 K, which is due to the presence of local ferromagnetic (Ga,Mn)As areas in the paramagnetic matrix. The Ga(In)MnAs layers prepared by ion implantation and subsequent pulsed laser annealing were found to exhibit a high TKE response at low temperatures. The presence of the characteristic band in the TKE spectrum in the range of transitions near the L point of the band structure of Ga(In)As confirms the intrinsic ferromagnetism. The temperature dependences of TKE measured for different spectrum ranges demonstrate nonmonotonic behavior, which indicates the magnetic inhomogeneity of the layers. The peculiarities of the magneto-optical spectra of GaMnAs, which were not observed previously, were explained by taking the magnetic and phase inhomogeneity of the layers into account. The sensitivity of TKE to the phase inhomogeneity of the Ga(In)MnAs layers and the efficiency of TKE in studying the electron spectrum and magnetic structure of diluted magnetic superconductors are demonstrated.

Thermal stability of interstitial and substitutional Mn in ferromagnetic (Ga,Mn)As

In (Ga,Mn)As, a model dilute magnetic semiconductor, the electric and magnetic properties are strongly influenced by the lattice sites occupied by the Mn atoms. In particular, the highest Curie temperatures are achieved upon thermal annealing in a narrow temperature window around $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, by promoting the diffusion of interstitial Mn towards the surface. In this work, we determined the thermal stability of both interstitial and substitutional Mn in ferromagnetic (Ga,Mn)As thin films, using the emission channeling technique. At a higher Mn concentration, the temperatures at which substitutional and interstitial Mn become mobile not only decrease, but also become closer to each other. These findings advance our understanding of self-compensation in (Ga,Mn)As by showing that the strong dependence of the Curie temperature on annealing temperature around $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ is a consequence of balance between diffusion of interstitial Mn and segregation of substitutional Mn.

Exchange bias in ferromagnetic bilayers with orthogonal anisotropies: the case of GaMnAsP/GaMnAs combination

We report the observation of exchange bias in a ferromagnetic Ga0.94Mn0.06As0.77P0.23/ Ga0.94Mn0.06As bilayer, in which the easy axis in one layer is oriented out-of-plane, and in the other in-plane. Magnetization reversal in this system is explored using planar Hall effect (PHE) measurements under various initial conditions and with various field-cooling orientations. Our results show that the two magnetic layers are ferromagnetically exchange-coupled, and that such coupling results in pronounced exchange-bias-like shifts of magnetic hysteresis loops during reversal of in-plane magnetization. The presence of exchange bias in this system can be understood on the basis of magnetic closure domains formed in the layer with the out-of-plane easy axis.

Engineering the breaking of time-reversal symmetry in gate-tunable hybrid ferromagnet/topological insulator heterostructures

Studying the influence of broken time-reversal symmetry on topological materials is an important fundamental problem of current interest in condensed matter physics and its understanding could also provide a route toward proof-of-concept spintronic devices that exploit spin-textured topological states. Here we develop a new model quantum material for studying the effect of breaking time-reversal symmetry: a hybrid heterostructure wherein a ferromagnetic semiconductor Ga1−xMnxAs, with an out-of-plane component of magnetization, is cleanly interfaced with a topological insulator (Bi,Sb)2(Te,Se)3 by molecular beam epitaxy. Lateral electrical transport in this bilayer is dominated by conduction through (Bi,Sb)2(Te,Se)3 whose conductivity is a few orders of magnitude higher than that of highly resistive Ga1−xMnxAs. Electrical transport measurements in a top-gated heterostructure device reveal a crossover from weak antilocalization to weak localization as the temperature is lowered or as the chemical potential approaches the Dirac point. This is accompanied by a systematic emergence of an anomalous Hall effect. These results are interpreted in terms of the opening of a gap at the Dirac point due to exchange coupling between the topological insulator surface state and the ferromagnetic ordering in Ga1−xMnxAs. The experiments described here show that well-developed III–V ferromagnetic semiconductors could serve as valuable components of artificially designed quantum materials aimed at exploring the interplay between magnetism and topological phenomena.

Investigation of a possible electronic phase separation in the magnetic semiconductors Ga1−xMnxAs and Ga1−xMnxP by means of fluctuation spectroscopy

We present systematic temperature-dependent resistance noise measurements on a series of ferromagnetic Ga$_{1-x}$Mn$_{x}$As epitaxial thin films covering a large parameter space in terms of the Mn content $x$ and other variations regarding sample fabrication. We infer that the electronic noise is dominated by switching processes related to impurities in the entire temperature range. While metallic compounds with $x>2$ % do not exhibit any significant change in the low-frequency resistance noise around the Curie temperature $T_\mathrm{C}$, we find indications for an electronic phase separation in films with $x<2$ % in the vicinity of $T_\mathrm{C}$, manifesting itself in a maximum in the noise power spectral density. These results are compared with noise measurements on an insulating Ga$_{1-x}$Mn$_{x}$P reference sample, for which the evidence for an electronic phase separation is even stronger and a possible percolation of bound magnetic polarons is discussed. Another aspect addressed in this work is the effect of ion-irradiation induced disorder on the electronic properties of Ga$_{1-x}$Mn$_{x}$As films and, in particular, whether any electronic inhomogeneities can be observed in this case. Finally, we put our findings into the context of the ongoing debate on the electronic structure and the development of spontaneous magnetization in these materials.

Erratum

Erratum to “Tcn and Tcn@C70 Endohedral Metalofullerenes: ab initio Spin-density-functional Calculations” from Li-jing Zeng, Ke Deng [Volume 28, Issue 4, Page 481, (2015)]. We regret to announce that the author's English name was found incorrect after publication due to the author's carelessness. “Li-jing Zeng” should be corrected as “Li-jin Zeng”. Erratum to “Influence of Rare Earth Ho3+ Doping on Structural, Microstructure and Magnetic Properties of ZnO Bulk and Thin Film Systems from “Ghulam Murtaza Rai, Muhammad Azhar Iqbal, Yongbing Xu, Iain Gordon Will, Wen Zhang” [Volume 24, Issue 3, Page 353, (2011)] and “Room Temperature Ferromagnetism in Ga1−xHoxN (x=0.0 and 0.05) Diluted Magnetic Semiconductor Thin Films” from “Ghulam Murtaza Rai, Muhammad Azhar Iqbal, Yong-bing Xu, Iain Gordon Will, Qasim Mahmood” [Volume 25, Issue 3, Page 313, (2012)]. We regret to announce that the author's name “Ghulam Murtaza Rai” was found to be inconsistent with his social identity after publication due to the author's carelessness. “Ghulam Murtaza Rai” should be “Ghulam Murtaza”.

Emergence of half-metallic ferromagnetism in Ga1−x Cr x As

We have studied the structural, electronic and half-metallic ferromagnetic properties of Ga1−xCrxAs compounds at dopant concentrations x = 0.25, 0.125 and 0.0625. First principle calculations based on density functional theories as implemented in SIESTA code using LDA + U (local density approximation + U) as exchange correlation potential have been used to study the properties of these compounds. Here, U is the Hubbard’s parameter. The calculated results predict that Cr-doped GaAs diluted magnetic semiconductors exhibit half-metallic properties at different concentrations, in which Cr atoms form deep levels in forbidden energy gap. The results also predict that with increase of fraction of Cr atoms, half-metallic energy band gap of Ga1−xCrxAs decreases. Total magnetic moment of these compounds is due to Cr states, and also p–d hybridization between Ga-p and Cr-d induces small magnetic moment on nonmagnetic atoms (Ga and As) for all concentrations.

Zeeman Splitting of Ferromagnetic GaMnAs on InP Observed by Magnetic Circular Dichroism in Reflection Mode

Systematic investigations of Ga1−xMnxAs grown on InP with different Mn concentrations have been conducted using magnetic circular dichroism (MCD) in reflection mode. The MCD spectrum of Ga0.97Mn0.03As/InP was decomposed into two dispersion curves originating from E1 and E1 + Δ1 optical transitions using the energy derivative of a Gaussian function. The Zeeman splitting energy E1 at the L critical point (0.6 meV) of ferromagnetic Ga0.97Mn0.03As/InP was estimated using a rigid band shift model. Based on the relationship between E1 and E0 (Γ critical point) observed in Cd1−xMnxTe dilute magnetic semiconductor (DMS), the Zeeman splitting energy E1 (9.6 meV) of ferromagnetic Ga1−xMnxAs/InP was calculated. In addition, it was established that the peaks in the MCD spectra at L critical points shift toward the lower energy side as the Mn concentration is increased, and the observed shift saturates for Mn content of x = 0.001. Furthermore, the measured absorption spectra for Ga1−xMnxAs/InP did not show noticeable peak shifts with increasing Mn content. This suggests that the s, p–d exchange interaction induced in Ga1−xMnxAs/InP has localized nature due to the presence of a Mn rigid sphere of influence.

Investigations on the origin of ferromagnetism in Ga1−xCrxN and Si-doped Ga1−xCrxN films: Experiments and theory

In this paper, we combined experiments and first-principles calculations to investigate the physical properties especially the magnetic properties of Ga1−xCrxN and Si-doped Ga1−xCrxN thin films grown by molecular beam epitaxy. The results of experiments indicate that Si doping improved the crystallinity by improving the substitution of Ga in Si-doped Ga1−xCrxN. And the size and/or the number of vacancy defects are reduced in the Si-doped Ga1−xCrxN thin films. All the samples show room-temperature ferromagnetism, and the measured saturation magnetizations are 15 emu/cm3 (2.26 μB per Cr atom) and 10 emu/cm3 (1.31 μB per Cr atom) respectively for Ga1−xCrxN and Si-doped Ga1−xCrxN thin films. First-principles calculations based density-functional theory show that there is a strong magnetic coupling between the Ga vacancies and the impurities of Cr and Si ions. And Si doping reduced the saturation magnetization in Si-doped Ga1−xCrxN thin films which is in agreement with the experiment results. This intrinsic defect derived magnetic interaction plays an important role on improving the ferromagnetism in Ga1−xCrxN and Si-doped Ga1−xCrxN, and the ferromagnetism of the system is the result of long-range mediation between VGa, Cr and Si ions.

Magneto-Optical Evidence for Intrinsic Ferromagnetism in (Ga,Mn)As Layers Grown by Pulsed Laser Deposition

GaMnAs layers fabricated on GaAs(001) substrates by laser ablation technique were studied using the magneto-optical transversal Kerr effect (TKE) (E = 0.5 - 4.0 eV) and spectral ellipsometry (E = 1.4 - 4.5 eV). Obtained spectral, temperature and magnetic field TKE dependences shown that the used technique allowed us to obtain ferromagnetic (Ga,Mn)As layers, whose composition was close to single-phase one (without MnAs inclusions). Spectral dependences of the off-diagonal components (e’ = e’1 - ie’2) of the permittivity tensor and also of the magnetic circular dichroism (MCD) were calculated for ferromagnetic (Ga,Mn)As sample. The calculated dependences were compared with the published magneto-optical spectra of (Ga,Mn)As layers grown by molecular beam epitaxy.

Localized sp–d exchange interaction in ferromagnetic Ga1−xMnxAs observed by magnetic circular dichroism spectroscopy of L critical points

We have conducted systematic investigations of magnetic circular dichroism (MCD) spectra and optical absorption spectra of the III–V diluted magnetic semiconductor (DMS) Ga1−xMnxAs. With an increase in Mn concentration x, the MCD structure originating from the L-critical points of the zinc-blend type band structure shifted towards lower energy while the corresponding absorption spectrum did not show the shift although they are expected to show the same dependence. This indicates that the s, p–d exchange interaction of Ga1−xMnxAs has a very localized nature, because MCD is only active in a region where band structure is affected by Mn spins, and optical absorption results from the overall sample response. The MCD spectrum of a ferromagnetic Ga0.97Mn0.03As sample was decomposed into contributions from Γ-critical points (E0 and E0 + Δ0), L-critical points (E1 and E1 + Δ1), and impurity band-related optical transitions. Using the intensities of MCD and optical absorption, the Zeeman splitting energy at E1 was estimated to be larger than +8 meV at 6 K. By assuming the same ratio between the Zeeman splitting energies at E0 and E1, which is experimentally reported in a II–VI paramagnetic DMS Cd1−xMnxTe, the Zeeman splitting at E0 of Ga0.97Mn0.03As was estimated to be larger than +120 meV at 6 K.

Magnetoresistance in Mn ion-implanted GaAs:Zn nanowires

We have investigated the magnetoresistance (MR) in a series of Zn doped (p-type) GaAs nanowires implanted with different Mn concentrations. The nanowires with the lowest Mn concentration (∼0.0001%) exhibit a low resistance of a few kΩ at 300 K and a 4% positive MR at 1.6 K, which can be well described by invoking a spin-split subband model. In contrast, nanowires with the highest Mn concentration (4%) display a large resistance of several MΩ at 300 K and a large negative MR of 85% at 1.6 K. The large negative MR is interpreted in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins. Sweeping the magnetic field back and forth for the 4% sample reveals a hysteresis that indicates the presence of a weak ferromagnetic phase. We propose co-doping with Zn to be a promising way to reach the goal of realizing ferromagnetic Ga1−xMnxAs nanowires for future nanospintronics.

Measurement of magnetization of Ga1−xMnxAs by ferromagnetic resonance

In this paper, we extend ferromagnetic resonance (FMR) studies of thin layers of the ferromagnetic semiconductor Ga1−xMnxAs to the analysis of the integrated intensity of the resonance in order to obtain information on the total spin in the sample directly involved in ferromagnetically-ordered magnetization. A theoretical model is proposed for the dependences of the FMR integrated intensity and linewidth on the orientation of the applied magnetic field as the field direction is varied from in-plane to normal-to-the-plane of the Ga1−xMnxAs layer. The strain-induced magnetic anisotropy of Ga1−xMnxAs presents a significant challenge to conventional FMR linewidth and integrated intensity models. The new model predicts that the integrated FMR intensity is proportional to the saturation magnetization MS of the sample, with the constant of proportionality varying as a function of the polar and azimuthal angles of the applied magnetic field. The angular and temperature behaviors of the integrated intensity and linewidth of the FMR predicted by the proposed model are in good qualitative agreement with measurements.

Storage moduli, loss moduli and damping factor of GaAs and Ga1−xMnxAs thin films using DMA 2980

The spin injector part of spintronic FET and diodes suffers from fatigue due to rising heat on the depletion layer. In this study the stiffness of Ga1−xMnxAs spin injector in terms of storage modulus with respect to a varying temperature, 45°C≤T≤70°C was determined. It was observed that the storage modulus for MDLs (Manganese Doping Levels) of 0%, 1% and 10% decreased with increase in temperature while that with MDLs of 20% and 50% increase with increase in temperature. MDLs of 20% and 50% appear not to allow for damping but MDLs ≤20% allow damping at temperature range of 45°C≤T≤70°C. The magnitude of storage moduli of GaAs is smaller than that for ferromagnetic Ga1−xMnxAs systems. The loss moduli for GaAs were found to reduce with increase in temperature. Its magnitude of reducing gradient is smaller than Ga1−xMnxAs systems. The two temperature extremes show a general reduction in loss moduli for different MDLs at the study temperature range. From damping factor analysis, damping factors for ferromagnetic Ga1−xMnxAs was found to increase with decrease in MDLs contrary to GaAs which recorded the largest damping factor at 45°C≤T≤70°C. Hence, MDL of 20% shows little damping followed by 50% while MDL of 0% has the most damping in an increasing trend with temperature.

Study of the structural, electric and magnetic properties of Mn-doped Bi2Te3 single crystals

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ∼ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.

First-principles investigations of structural, elastic, electronic and magnetic properties of Ga1−x Mn x P and In1−x Mn x P

We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga1−x Mn x P and In1−x Mn x P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga0.75Mn0.25P and In0.75Mn0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 μB per unit cell for both Ga0.75Mn0.25P and In0.75Mn0.25P, which agree with the Slater–Pauling rule quite well, and we observe that p–d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N 0α and N 0β exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga0.75Mn0.25P and In0.75Mn0.25P as a function of lattice constant is also investigated.

Electron spin dynamics of ferromagnetic Ga1−x Mnx As across the insulator-to-metal transition

Electron spin dephasing dynamics of ferromagnetic Ga1−xMnxAs with Mn concentration spanning from 0.5% to 15% across the insulator-to-metal transition is systematically studied using time-resolved magneto-optical Kerr effect measurements. In the insulating Ga1−xMnxAs, the impurity scattering and s-d exchange scattering induced by the magnetic impurity Mn ions are responsible for the electron spin dephasing process. While in the metallic and near metallic regime, the electron-electron Coulomb scattering becomes dominant over the impurity scattering on the spin dephasing process with D'yakonov-Perel' mechanism. Our findings are important for better engineering of Mn impurity doping in order to achieve potential Ga1−xMnxAs-based spintronics application.

Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices

We have fabricated electric double-layer field-effect devices to electrostatically dope our active materials, either $x$=0.015 Ga$_{1-x}$Mn$_x$As or $x$=3.2$\times10^{-4}$ Ga$_{1-x}$Be$_x$As. The devices are tailored for interrogation of electric field induced changes to the frequency dependent conductivity in the accumulation or depletions layers of the active material via infrared (IR) spectroscopy. The spectra of the (Ga,Be)As-based device reveal electric field induced changes to the IR conductivity consistent with an enhancement or reduction of the Drude response in the accumulation and depletion polarities, respectively. The spectroscopic features of this device are all indicative of metallic conduction within the GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show enhancement of the far-IR itinerant carrier response and broad mid-IR resonance upon hole accumulation, with a decrease of these features in the depletion polarity. These later spectral features demonstrate that conduction in ferromagnetic (FM) Ga$_{1-x}$Mn$_x$As is distinct from genuine metallic behavior due to extended states in the host VB. Furthermore, these data support the notion that a Mn-induced impurity band plays a vital role in the electron dynamics of FM Ga$_{1-x}$Mn$_x$As. We add, a sum-rule analysis of the spectra of our devices suggests that the Mn or Be doping does not lead to a substantial renormalization of the GaAs host VB.