Recent advances in the theory of magnetism of NiAs-type transition-metal chalcogenides and pnictides

Magnetism of NiAs-type transition-metal compounds is studied from the viewpoint of the itinerant electron. Electronic nonmagnetic bands of MAs and MSb (M = Cr, Mn, Fe, Co, Ni) and ferromagnetic bands of MnAs and MnSb are calculated by the self-consistent APW method. The obtained ferromagnetic moments are in good agreement with observations. The anomalous temperature dependence of the paramagnetic susceptibility observed in MnAs, MnAs_1−xP_x, FeAs and CoAs is well explained on the basis of spin fluctuation theory, by taking account of characteristic behavior of the density of states obtained for each compound.     

Electron paramagnetic resonance study of the nuclear spin dynamics in an AlAs quantum well

The nuclear spin dynamics in an asymmetrically doped 16-nm AlAs quantum well grown along the [001] direction has been studied experimentally using the time decay of the Overhauser shift of paramagnetic resonance of conduction electrons. The nonzero spin polarization of nuclei causing the initial observed Overhauser shift is due the relaxation of the nonequilibrium spin polarization of electrons into the nuclear subsystem near electron paramagnetic resonance owing to the hyperfine interaction. The measured relaxation time of nuclear spins near the unity filling factor is (530 ± 30) min at the temperature T = 0.5 K. This value exceeds the characteristic spin relaxation times of nuclei in GaAs/AlGaAs heterostructures by more than an order of magnitude. This fact indicates the decrease in the strength of the hyperfine interaction in the AlAs quantum well in comparison with GaAs/AlGaAs heterostructures.     

Spin-dependent localization effects in GaAs:Mn/MnAs granular paramagnetic–ferromagnetic hybrids at low temperatures

We compare the magneto-transport in paramagnetic–ferromagnetic GaAs:Mn/MnAs granular hybrids and paramagnetic GaAs:Mn reference samples. The differences in the hole transport between the two systems at low temperatures arise due to carrier localization effects at the cluster–matrix interface in the hybrids. The localization is caused by a Schottky barrier formation at the interface as well as spin-dependent shifts of the hole bands caused by the stray field of the ferromagnetic clusters. The application of an external magnetic field leads to a delocalization of the carriers and thus a negative magneto-resistance effect. These effects can be simulated using a network model approach.     

Magnetic force microscopy of GaAs:Mn ferromagnetic semiconductors

The dependence of magnetic properties of GaAs:Mn and MnAs epitaxial films grown on GaAs (001) by laser ablation of Mn and undoped GaAs in a hydrogen atmosphere under the growth conditions has been studied by magnetic force microscopy (MFM). Magnetic probe calibration for quantitative MFM measurements was performed by scanning across the slit of the magnetic-head of a tape recorder through which controlled direct current was passed. The dipole approximation was used to describe the magnetic properties of the MFM probe. Nonuniformity of the magnetization of GaAs:Mn films related to the formation of MnAs nanoinclusions, which are ferromagnetic at 300 K, has been observed. The typical scales of the spatial nonuniformity of the magnetization of GaAs:Mn films were varied from 270 to 550 nm depending on the film-growth conditions. The MnAs phase was identified by MFM measurements at an elevated temperature (up to 80°N).     

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100 mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.     

Growth and Properties of Ferromagnet-SemiconDuctor Hetero-Structures for Spin Injection

Molecular-beam epitaxial growth and interface formation is investigated and optimized for the material systems Fe-on-GaAs(001) and MnAs-on-GaAs(001), which represent model systems for the integration of magnetic materials with semiconductors and the investigation of spin injection. In view of interface reactions as a key problem for the first system and in order to optimize the surface morphology, the Fe films are grown at reduced temperature. For MnAs-on-GaAs, an abrupt interface with an anisotropic lattice-mismatch accommodation mechanism is observed, which explains the unexpected high quality of the films. It is shown that the phase transition between paramagnetic g MnAs and ferromagnetic f MnAs during cooling after growth plays an important role and leads, at appropriate conditions, to strain-mediated self-organized structures. Electrical injection of spin polarized electrons through the ferromagnet-semiconductor interfaces is observed by analyzing the electroluminescence signal of GaAs/(In, Ga)As light emitting diodes capped with Fe or MnAs.     

Spin Dynamics in Ferromagnet/10-nm-Thick N-Type GaAs Quantum Well Junctions

Spin dynamics in several different types of ferromagnetic metal(FM)/10-nm-thick n-type GaAs quantum well(QW) junctions is studied by means of time-resolved Kerr rotation measurements. Compared with the MnGa/insitu doped 10-nm-thick n-type GaAs QW junction, the spin lifetime of the MnGa/modulation-doped 10-nm-thick n-type GaAs QW junction is shorter by a factor of 5,consistent with the D'yakonov-Pcrel' spin relaxation mechanism. Meanwhile, compared with the spin lifetime of the MnAs/in-situ doped 10-nm-thick n-type GaAs QW junction, the MnGa/in-situ doped 10-nm-thick n-type GaAs QW junction is of a spin lifetime longer by a factor of 4.2. The later observation is well explained by the Rashba effect in the presence of structure inversion asymmetry, which acts directly on photo-excited eleetron spins. We demonstrate that MnGa-like FM/in-situ doped 10-nm-thick n-type GaAs QW junctions, which possess relatively low interfacial potential barriers, are able to provide long spin lifetimes.     

Hybrid ferromagnetic/semiconductor heterostructures for spintronics

Heterostructures that integrate conventional semiconductors with ferromagnetic semiconductors and ferromagnetic metals are important for developing a framework for semiconductor spintronics. We describe recent efforts to study ‘hybrid’ ferromagnetic/semiconductor heterostructures that combine conventional III-V and II-VI semiconductors with the ferromagnetic semiconductor (Ga,Mn)As and the ferromagnetic metal MnAs. We focus on the characteristics of two novel classes of heterostructures: (a) (Ga,Mn)As/AlAs/MnAs magnetic tunnel junctions (MTJs) that provide an all-electrical scheme for probing spin injection from metals into GaAs and (b) n-ZnSe/(Ga,Mn)As heterojunction diodes that surprisingly exhibit a magnetically-driven photoconductivity.     

Coherent spin dynamics of carriers in ferromagnetic semiconductor heterostructures with an Mn δ layer

The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW.     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

Study of the “metal–insulator” transition induced by the impurity fluctuation potential using the Shubnikov–de Haas effect

Heterostructures with a GaAs/InGaAs/GaAs quantum well and aMn magnetic impurity layer separated from it, which have different conductivity types, are studied. At a Mn content not exceeding the amount corresponding to 0.5 monolayer of MnAs, a percolation cluster formed in the quantum well plane is not simply connected, but consists of metal drops separated by low-conductivity interspaces. Despite the absence of the simply connected conducting channel, Shubnikov–de Haas oscillations are observed in all studied systems, which are controlled by carrier properties in conducting drops, independent of Mn content. The estimate of drop sizes corresponds to theoretical values.     

Electronic structure at rubrene metal interfaces

Many studies have been done on low energy (1–200 keV) and high dose (10¹⁶–10¹⁷) implantation of Mn in GaAs. This study is an attempt to incorporate Mn ions in GaAs through implantation of 1 MeV Mn⁺¹ ions in semi-insulating GaAs substrates at doses of 3×10¹⁵/cm² and subsequent annealing. This was done to find out if any alloy of Mn–Ga–As, or binary compounds of Mn–Ga or Mn–As form due to annealing of Mn⁺¹ ions implanted in GaAs substrates. High Resolution XRD (HRXRD) performed before annealing shows a possibility of Ga–Mn–As alloy formation, and after annealing at 800°C, except for GaAs main peaks no other phase peaks were detected. Scanning electron microscopy (SEM) shows nanostructures of various dimensions which are thought to be formed due to the defects generated due to implantation. Fourier Transform Infrared (FTIR) study shows the shift in bandgap due to Mn doping. Raman spectroscopy shows the red shift in LO and TO peak positions of GaAs after annealing, which indicates the presence of disorder and damage due to implantation. Resistivity measurement shows a thermally activated semiconductor character of charge conduction with an activation energy of 51 meV and this activation may have occurred through the transitions from impurity band to valence band. Large positive (∼25%) magnetoresistance and a mixture of ferromagnetic and paramagnetic behavior obtained in the magnetization measurement indicate the presence of ferromagnetic MnAs nanoclusters embedded in paramagnetic GaAs:Mn matrix.     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

Spin control of drifting electrons using local nuclear polarization in ferromagnet-semiconductor heterostructures

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations of 5π rad during transport.     

Spin dephasing of a two-dimensional electron gas in a GaAs quantum well near odd filling factors

The coherent spin dynamics of a two-dimensional electron gas in a GaAs/AlGaAs quantum well is experimentally studied near the filling factors ν = 3 and 1. The nonmonotonic character of the dependence of the spin dephasing time of a Goldstone spin exciton on the filling factor is found experimentally. The observed effect can be due to the formation of a new spin relaxation channel, when the main state of the two-dimensional electron system is a spin-textured liquid.     

Magnetic properties of (Ga,Mn)N ternaries and structural,electronic, and magnetic properties of cation-mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries

ABINIT simulation package with built in local density, generalized gradient, and spin local density approximations was used to investigate the structural, electronic, and magnetic properties of cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal and fixed compositions of Ga, In, and Mn atoms. In particular, total energy minimization approach was used to compute the equilibrium structural parameters of zinc-blende (GaAs, InAs, and MnAs), wurtzite (GaN, InN, and MnN) binary parent compounds, as well as, the corresponding equilibrium parameters of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternary systems. The band structures of zinc-blende GaAs, InAs, and MnAs binary parent compounds were computed and analyzed. Spin polarized band structures of the cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal compositions of Ga, In, and Mn cations were computed and analyzed using spin local density approximation based calculations. Moreover, the magnetic properties of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal concentration of Ga, In, and Mn cations were investigated. Our results suggest that the two quaternary systems are nonmagnetic. An interpretation of our results is presented. In addition, the magnetic properties of (Ga,Mn)N nanocrystal ternaries constructed from doping GaN with one or two Mn atoms were investigated using Vienna Ab-initio Simulation Package (VASP) and compared with those of (Ga,Mn)(As,N) quaternaries.     

Spin susceptibility of an ultra-low-density two-dimensional electron system

We determine the spin susceptibility in a two-dimensional electron system in GaAs/AlGaAs over a wide range of low densities from 2x10(9) cm(-2) to 4x10(10) cm(-2). Our data can be fitted to an equation that describes the density dependence as well as the polarization dependence of the spin susceptibility. It can account for the anomalous g factors reported recently in GaAs electron and hole systems. The paramagnetic spin susceptibility increases with decreasing density as expected from theoretical calculations.     

Full electrical control of the electron spin relaxation in GaAs quantum wells

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by time-resolved photoluminescence spectroscopy. By applying an external electric field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under a transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.     

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an In_xGa_1?x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T _m ～ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.     

The source of room temperature ferromagnetism in granular GaMnAs layers with zinc blende clusters

Granular GaAs:(Mn,Ga)As films were prepared by annealing the Ga_0.985Mn_0.015As/GaAs layers at 500 °C or 600 °C. It is commonly accepted that this processing should result in the formation of cubic or hexagonal MnAs clusters, respectively. We demonstrate that such a priori assumption is not justified. If in the as grown sample there are not many defects with the interstitial Mn atoms, only small cubic clusters can be formed even after annealing at 600 °C. Moreover, in a sample containing solely cubic GaMnAs clusters, the Mn ions are ferromagnetically coupled at room temperature. This fact was explained by the existence of GaMnAs solid solution in the clusters, with content of Mn close to 20% (higher than ever found in the layers) as was confirmed by experiment and theory. Extended X‐ray absorption spectroscopy studies excluded the possibility of formation of the hypothetic zinc blende MnAs clusters. Not more than one Mn atom was detected in the second shell around central Mn atom. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)