arXiv subjects

T. Jungwirth

Publications and source records attributed to T. Jungwirth.

At least 91 records · Page 5Linked to original sources

arXiv2013

Solving and refining novel thin film phases using Cu X-ray radiation: the epitaxy-induced CuMnAs tetragonal phase

We present a combined experimental and computational method which enables the precise determination of the atomic positions in a thin film using CuKα radiation, only. The capabilities of this technique surpass simple structure refinement and allow solving unknown phases stabilized by substrate-induced stress. We derive the appropriate corrections to transform the measured integrated intensities into structure factors. Data collection was performed entirely on routinely available laboratory diffractometers (CuKα radiation); the subsequent analysis was carried out by single-crystal direct methods (δ recycling procedure) followed by the least-squares refinement of the structural parameters of the unit cell content. We selected an epitaxial thin film of CuMnAs grown on top of a GaAs substrate, which formed a crystal structure with tetragonal symmetry, differing from the bulk material which is orthorhombic. Here we demonstrate the new tetragonal form of epitaxial CuMnAs grown on GaAs substrate and present consistent high-resolution scanning transmission electron microscopy and stoichiometry analyses.

cond-mat.mtrl-sci↗

arXiv2012

Influence of Magnetic Anisotropy on Laser-induced Precession of Magnetization in Ferromagnetic Semiconductor (Ga,Mn)As

The laser-induced precession of magnetization in (Ga,Mn)As samples with different magnetic anisotropy was studied by the time-resolved magneto-optical method. We observed that the dependence of the precession amplitude on the external magnetic field depends strongly on the magnetic anisotropy of (Ga,Mn)As and we explain this phenomenon in terms of competing cubic and uniaxial anisotropies. We also show that the corresponding anisotropy fields can be deduced from the magnetic field dependence of the precession frequency.

cond-mat.mtrl-sci↗

arXiv2012

Correspondence on "Controlling the Curie temperature in (Ga,Mn)As through location of the Fermi level within the impurity band" by M. Dobrowolska et al

Comment on the recent Nature Materials article by M. Dobrowolska et al., arXiv:1203.1852. We present experimental data showing that the Curie temperature and conductivity of high quality (Ga,Mn)As samples are maximized at low compensation, and thus the magnetic order in (Ga,Mn)As is not consistent with the isolated impurity band scenario.

cond-mat.mtrl-sci↗

arXiv2012

Establishing micromagnetic parameters of ferromagnetic semiconductor (Ga,Mn)As

(Ga,Mn)As is at the forefront of research exploring the synergy of magnetism with the physics and technology of semiconductors, and has led to discoveries of new spin-dependent phenomena and functionalities applicable to a wide range of material systems. Its recognition and utility as an ideal model material for spintronics research has been undermined by the large scatter in reported semiconducting doping trends and micromagnetic parameters. In this paper we establish these basic material characteristics by individually optimizing the highly non-equilibrium synthesis for each Mn-doping level and by simultaneously determining all micromagnetic parameters from one set of magneto-optical pump-and-probe measurements. Our (Ga,Mn)As thin-film epilayers, spannig the wide range of accessible dopings, have sharp thermodynamic Curie point singularities typical of uniform magnetic systems. The materials show systematic trends of increasing magnetization, carrier density, and Curie temperature (reaching 188 K) with increasing doping, and monotonous doping dependence of the Gilbert damping constant of ~0.1-0.01 and the spin stiffness of ~2-3 meVnm^2. These results render (Ga,Mn)As well controlled degenerate semiconductor with basic magnetic characteristics comparable to common band ferromagnets.

cond-mat.mtrl-sci↗

arXiv2012

Experimental observation of the optical spin-orbit torque

Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) on the magnetization. The phe- nomenon belongs to the area of spintronics research focusing on manipulating magnetic moments by electric fields and is the basis of the emerging technologies for scalable magnetoresistive random access memories. In our previous work we have reported experimental observation of the optical counterpart of STT in which a circularly polarized pump laser pulse acts as the external polarizer, allowing to study and utilize the phenomenon on several orders of magnitude shorter timescales than in the electric current induced STT. Recently it has been theoretically proposed and experimentally demonstrated that in the absence of an external polarizer, carriers in a magnet under applied electric field can develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) acting on the magnetization. In this paper we report the observation of the optical counterpart of SOT. At picosecond time-scales, we detect excitations of magnetization of a ferromagnetic semiconductor (Ga,Mn)As which are independent of the polarization of the pump laser pulses and are induced by non-equilibrium spin-orbit coupled photo-holes.

cond-mat.mtrl-sci↗

arXiv2012

Spin gating electrical current

We use an aluminium single electron transistor with a magnetic gate to directly quantify the chemical potential anisotropy of GaMnAs materials. Uniaxial and cubic contributions to the chemical potential anisotropy are determined from field rotation experiments. In performing magnetic field sweeps we observe additional isotropic magnetic field dependence of the chemical potential which shows a non-monotonic behavior. The observed effects are explained by calculations based on the $\mathbf{k}\cdot\mathbf{p}$ kinetic exchange model of ferromagnetism in GaMnAs. Our device inverts the conventional approach for constructing spin transistors: instead of spin-transport controlled by ordinary gates we spin-gate ordinary charge transport.

cond-mat.mes-hall↗

arXiv2012

Spin Hall transistor with electrical spin injection

The realization of a viable semiconductor transistor and information processing devices based on the electron spin has fueled intense basic research of three key elements: injection, detection, and manipulation of spins in the semiconductor channel. The inverse spin Hall effect (iSHE) detection of spins injected optically in a 2D GaAs and manipulated by a gate-voltage dependent internal spin-orbit field has recently led to the experimental demonstration of a spin transistor logic device. The aim of the work presented here is to demonstrate in one device the iSHE detection combined with an electrical spin injection and manipulation. We use a 3D GaAs channel for which efficient electrical spin injection from Fe Schottky contacts has been demonstrated in previous works. In order to experimentally separate the strong ordinary Hall effect signal from the iSHE in the semiconductor channel we developed epitaxial ultrathin-Fe/GaAs contacts allowing for Hanle spin-precession measurements in applied in-plane magnetic fields. Electrical injection and detection is combined in our transistor structure with electrically manipulated spin distribution and spin current which, unlike the previously utilized electrical manipulations of the spin-orbit field or ballistic spin transit time, is well suited for the diffusive 3D GaAs spin channel. The magnitudes and external field dependencies of the measured signals are quantitatively analyzed using simultaneous spin detection by the non-local spin valve effect and modeled by solving the drift-diffusion and Hall-cross response equations for the parameters of the studied microstructure.

cond-mat.mes-hall↗

arXiv2012

Experimental observation of the optical spin transfer torque

The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.

cond-mat.mtrl-sci↗

arXiv2012

Direct measurement of the three dimensional magnetization vector trajectory in GaMnAs by a magneto-optical pump-and-probe method

We report on a quantitative experimental determination of the three-dimensional magnetization vector trajectory in GaMnAs by means of the static and time-resolved pump-and-probe magneto-optical measurements. The experiments are performed in a normal incidence geometry and the time evolution of the magnetization vector is obtained without any numerical modeling of magnetization dynamics. Our experimental method utilizes different polarization dependences of the polar Kerr effect and magnetic linear dichroism to disentangle the pump-induced out-of-plane and in-plane motions of magnetization, respectively. We demonstrate that the method is sensitive enough to allow for the determination of small angle excitations of the magnetization in GaMnAs. The method is readily applicable to other magnetic materials with sufficiently strong circular and linear magneto-optical effects.

cond-mat.mtrl-sci↗

arXiv2011

Electrical measurement of antiferromagnetic moments in exchange-coupled IrMn/NiFe stacks

We employ the recently discovered antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals the presence of an electrically measurable memory effect in an antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2011

CuMn-V compounds: a transition from semimetal low-temperature to semiconductor high-temperature antiferromagnets

We report on a theoretical and experimental study of CuMn-V antiferromagnets. Previous works showed low-temperature antiferomagnetism and semimetal electronic structure of the semi-Heusler CuMnSb. In this paper we present theoretical predictions of high-temperature antiferromagnetism in the stable orthorhombic phases of CuMnAs and CuMnP. The electronic structure of CuMnAs is at the transition from a semimetal to a semiconductor and we predict that CuMnP is a semiconductor. We show that the transition to a semiconductor-like band structure upon introducing the lighter group-V elements is present in both the metastable semi-Heusler and the stable orthorhombic crystal structures. On the other hand, the orthorhombic phase is crucial for the high Néel temperature. Results of X-ray diffraction, magnetization, transport, and neutron diffraction measurements we performed on chemically synthesized CuMnAs are consistent with the theory predictions.

cond-mat.mtrl-sci↗

arXiv2011

Non-thermal laser induced precession of magnetization in ferromagnetic semiconductor (Ga,Mn)As

Non-thermal laser induced spin excitations, recently discovered in conventional oxide and metal ferromagnets, open unprecedented opportunities for research and applications of ultrafast optical manipulation of magnetic systems. Ferromagnetic semiconductors, and (Ga,Mn)As in particular, should represent ideal systems for exploring this new field. Remarkably, the presence of non-thermal effects has remained one of the outstanding unresolved problems in the research of ferromagnetic semiconductors to date. Here we demonstrate that coherent magnetization dynamics can be excited in (Ga,Mn)As non-thermally by a transfer of angular momentum from circularly polarized femtosecond laser pulses and by a combination of non-thermal and thermal effects due to a transfer of energy from laser pulses. The thermal effects can be completely suppressed in piezo-electrically controlled samples. Our work is based on pump-and-probe measurements in a large set of (Ga,Mn)As epilayers and on systematic analysis of circular and linear magneto-optical coefficients. We provide microscopic theoretical interpretation of the experimental results.

cond-mat.mtrl-sci↗

arXiv2010

Detection of stacking faults breaking the [110]/[1-10] symmetry in ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P)

We report high resolution x-ray diffraction measurements of (Ga,Mn)As and (Ga,Mn)(As,P) epilayers. We observe a structural anisotropy in the form of stacking faults which are present in the (111) and (11-1) planes and absent in the (-111) and (1-11) planes. The stacking faults produce no macroscopic strain. They occupy 0.01 - 0.1 per cent of the epilayer volume. Full-potential density functional calculations evidence an attraction of Mn_Ga impurities to the stacking faults. We argue that the enhanced Mn density along the common [1-10] direction of the stacking fault planes produces sufficiently strong [110]/[1-10] symmetry breaking mechanism to account for the in-plane uniaxial magnetocrystalline anisotropy of these ferromagnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2010

Spin-orbit driven ferromagnetic resonance: A nanoscale magnetic characterisation technique

We demonstrate a scalable new ferromagnetic resonance (FMR) technique based on the spin-orbit interaction. An alternating current drives FMR in uniform ferromagnetic structures patterned from the dilute magnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P). This allows the direct measurement of magnetic anisotropy coefficients and damping parameters for individual nano-bars. By analysing the ferromagnetic resonance lineshape, we perform vector magnetometry on the current-induced driving field, observing contributions with symmetries of both the Dresselhaus and Rashba spin-orbit interactions.

cond-mat.mes-hall↗

arXiv2010

Modeling of diffusion of injected electron spins in spin-orbit coupled microchannels

We report on a theoretical study of spin dynamics of an ensemble of spin-polarized electrons injected in a diffusive microchannel with linear Rashba and Dresselhaus spin-orbit coupling. We explore the dependence of the spin-precession and spin-diffusion lengths on the strengths of spin-orbit interaction and external magnetic fields, microchannel width, and orientation. Our results are based on numerical Monte Carlo simulations and on approximate analytical formulas, both treating the spin dynamics quantum-mechanically. We conclude that spin-diffusion lengths comparable or larger than the precession-length occur i) in the vicinity of the persistent spin helix regime for arbitrary channel width, and ii) in channels of similar or smaller width than the precession length, independent of the ratio of Rashba and Dresselhaus fields. For similar strengths of the Rashba and Dresselhaus fields, the steady-state spin-density oscillates or remains constant along the channel for channels parallel to the in-plane diagonal crystal directions. An oscillatory spin-polarization pattern tilted by 45$^{\circ}$ with respect to the channel axis is predicted for channels along the main cubic crystal directions. For typical experimental system parameters, magnetic fields of the order of Tesla are required to affect the spin-diffusion and spin-precession lengths.

cond-mat.mes-hall↗

arXiv2010

Tunneling anisotropic magnetoresistance of NiFe/IrMn/MgO/Pt stack: An antiferromagnet based spin-valve

Spin-valve is a microelectronic device in which high and low resistance states are realized by utilizing both charge and spin of carriers. Spin-valve structures used in modern hard drive read-heads and magnetic random access memories comprise two ferromagnetic (FM) electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunneling magnetoresistance effect. In this paper we demonstrate >100$% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet (AFM) on one side and a non-magnetic metal on the other side of the tunnel barrier. FM moments in NiFe are reversed by external fields <50mT and the exchange-spring effect of NiFe on IrMn induces rotation of AFM moments in IrMn which is detected by the measured tunneling anisotropic magnetoresistance (TAMR). Our work demonstrates a spintronic element whose transport characteristics are governed by an AFM. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit coupling induced magneto-transport anisotropy using a single magnetic electrode. The AFM-TAMR provides means to study magnetic characteristics of AFM films by an electronic transport measurement.

cond-mat.mtrl-sci↗

arXiv2010

Plasmon mass and Drude weight in strongly spin-orbit-coupled 2D electron gases

Spin-orbit-coupled two-dimensional electron gases (2DEGs) are a textbook example of helical Fermi liquids, i.e. quantum liquids in which spin (or pseudospin) and momentum degrees-of-freedom at the Fermi surface have a well-defined correlation. Here we study the long-wavelength plasmon dispersion and the Drude weight of archetypical spin-orbit-coupled 2DEGs. We first show that these measurable quantities are sensitive to electron-electron interactions due to broken Galileian invariance and then discuss in detail why the popular random phase approximation is not capable of describing the collective dynamics of these systems even at very long wavelengths. This work is focussed on presenting approximate microscopic calculations of these quantities based on the minimal theoretical scheme that captures the basic physics correctly, i.e. the time-dependent Hartree-Fock approximation. We find that interactions enhance the "plasmon mass" and suppress the Drude weight. Our findings can be tested by inelastic light scattering, electron energy loss, and far-infrared optical-absorption measurements.

cond-mat.mes-hall↗

arXiv2010

Molecular Beam Epitaxy of LiMnAs

We report on the molecular beam epitaxy (MBE) growth of high crystalline quality LiMnAs. The introduction of a group-I alkali metal element Li with flux comparable to fluxes of Mn and As has not caused any apparent damage to the MBE system after as many as fifteen growth cycles performed on the system to date.

cond-mat.mtrl-sci↗