arXiv subjects

J. -M. George

Publications and source records attributed to J. -M. George.

36 records · Page 2Linked to original sources

arXiv2016

Highly efficient and tuneable spin-to-charge conversion through Rashba coupling at oxide interfaces

The spin-orbit interaction couples the electrons' motion to their spin. Accordingly, passing a current in a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice-versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronics functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics hetero- and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism - the Rashba effect - in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency. Through spin-pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES.

cond-mat.mtrl-sci↗

arXiv2015

Spin transport in molybdenum disulfide multilayer channel

Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nano-electronic, opto-electronic and spintronic applications. However, demonstrating spin-transport through a semiconducting MoS2 channel is challenging. Here we demonstrate the electrical spin injection and detection in a multilayer MoS2 semiconducting channel. A magnetoresistance (MR) around 1% has been observed at low temperature through a 450nm long, 6 monolayer thick channel with a Co/MgO spin injector and detector. From a systematic study of the bias voltage, temperature and back-gate voltage dependence of MR, it is found that the hopping via localized states in the contact depletion region plays a key role for the observation of the two-terminal MR. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel for in-plan spin injection. The underestimated long spin diffusion length (~235nm) and large spin lifetime (~46ns) open a new avenue for spintronic applications using multilayer transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2015

Perpendicular magnetization reversal in Pt/[Co/Ni]$_3$/Al multilayers via the Spin Hall Effect of Pt

We experimentally investigate the current-induced magnetization reversal in Pt/[Co/Ni]$_3$/Al multilayers combining the anomalous Hall effect and magneto-optical Kerr effect techniques in crossbar geometry. The magnetization reversal occurs through nucleation and propagation of a domain of opposite polarity for a current density of the order of 0.3 TA/m$^2$. In these experiments we demonstrate a full control of each stage: i)the Ørsted field controls the domain nucleation and ii) domain-wall propagation occurs by spin torque from the Pt spin Hall effect. This scenario requires an in-plane magnetic field to tune the domain wall center orientation along the current for efficient domain wall propagation. Indeed, as nucleated, domain walls are chiral and Néel like due to the interfacial Dzyaloshinskii-Moriya interaction.

cond-mat.mtrl-sci↗

arXiv2015

Evaluation of the spin diffusion length of AuW alloy by spin absorption experiments in the limit of large spin-orbit interactions

The knowledge of the spin diffusion length $λ_{A}$ is a prerequisite for the estimation of the spin Hall angle. We investigate spin current absorption of materials with small $λ_{A}$ using AuW stripes inserted in lateral spin-valves. Width variations of the AuW stripe lead to drastic changes of the spin absorption, which cannot be explained by conventional analysis. We show that the spin-current polarization and the spin accumulation attenuation in the vicinity of the spin absorber must to be precisely taken into account for accurate estimation of $λ_{A}$. We propose an analytical model supported by numerical calculations that allows to extract proper $λ_{A}$ values of spin Hall effect materials.

cond-mat.mes-hall↗

arXiv2015

Spin-pumping into surface states of topological insulator α-Sn, spin to charge conversion at room temperature

We present experimental results on the conversion of a spin current into a charge current by spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) α-Sn[1-3]. By angle-resolved photoelectron spectroscopy (ARPES) we first confirm that the Dirac cone at the surface of α-Sn (0 0 1) layers subsists after covering with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into α-Sn layers induces a lateral charge current that can be ascribed to the Inverse Edelstein Effect[4-5]. Our observation of an Inverse Edelstein Effect length[5-6] much longer than for Rashba interfaces[5-10] demonstrates the potential of the TI for conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of TI for spin to charge conversion and the conditions to reach it.

cond-mat.mes-hall↗

arXiv2014

Experimental evidences of a large extrinsic spin Hall effect in AuW alloy

We report an experimental study of a gold-tungsten alloy (7% at. W concentration in Au host) displaying remarkable properties for spintronics applications using both magneto-transport in lateral spin valve devices and spin-pumping with inverse spin Hall effect experiments. A very large spin Hall angle of about 10% is consistently found using both techniques with the reliable spin diffusion length of 2 nm estimated by the spin sink experiments in the lateral spin valves. With its chemical stability, high resistivity and small induced damping, this AuW alloy may find applications in the nearest future.

cond-mat.mes-hall↗

arXiv2014

Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces

Through combined ferromagnetic resonance, spin-pumping and inverse spin Hall effect experiments in Co|Pt bilayers and Co|Cu|Pt trilayers, we demonstrate consistent values of spin diffusion length $\ell_{\rm sf}^{\rm Pt}=3.4\pm0.4$ nm and of spin Hall angle $θ_{\rm SHE}^{\rm Pt}=0.051\pm0.004$ for Pt. Our data and model emphasize on the partial depolarization of the spin current at each interface due to spin-memory loss. Our model reconciles the previously published spin Hall angle values and explains the different scaling lengths for the ferromagnetic damping and the spin Hall effect induced voltage.

cond-mat.mes-hall↗

arXiv2013

Spin Pumping and Inverse Spin Hall Effect in Germanium

We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically lead to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.

cond-mat.mtrl-sci↗

arXiv2013

Crossover from spin accumulation into interface states to spin injection in the germanium conduction band

Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of $n$-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced down to a value compatible with spin diffusion model. More interestingly, we demonstrate in this regime a significant modulation of the spin signal by spin pumping generated by ferromagnetic resonance and also by applying a back-gate voltage which are clear manifestations of spin current and accumulation in the germanium conduction band.

cond-mat.mtrl-sci↗

arXiv2012

Electrical and thermal spin accumulation in germanium

In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any tunnel charge current. We show that temperature gradients yield larger spin accumulations than pure electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation in germanium remains surprisingly almost unchanged under the application of a gate voltage to the channel.

cond-mat.mtrl-sci↗

arXiv2012

Anisotropic magneto-resistance in a GaMnAs-based single impurity tunnel diode: a tight binding approach

Using an advanced tight-binding approach, we estimate the anisotropy of the tunnel transmission associated with the rotation of the 5/2 spin of a single Mn atom forming an acceptor state in GaAs and located near an AlGaAs tunnel barrier. Significant anisotropies in both in-plane and out-of-plane geometries are found, resulting from the combination of the large spin-orbit coupling associated with the p-d exchange interaction, cubic anisotropy of heavy-hole dispersion and the low C2v symmetry of the chemical bonds.

cond-mat.mes-hall↗

arXiv2011

Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor

We report on spin injection experiments at a Co/Al$_2$O$_3$/GaAs interface with electrical detection. The application of a transverse magnetic field induces a large voltage drop $ΔV$ at the interface as high as 1.2mV for a current density of 0.34 nA.$μm^{-2}$. This represents a dramatic increase of the spin accumulation signal, well above the theoretical predictions for spin injection through a ferromagnet/semiconductor interface. Such an enhancement is consistent with a sequential tunneling process via localized states located in the vicinity of the Al$_2$O$_3$/GaAs interface. For spin-polarized carriers these states act as an accumulation layer where the spin lifetime is large. A model taking into account the spin lifetime and the escape tunneling time for carriers travelling back into the ferromagnetic contact reproduces accurately the experimental results.

cond-mat.mtrl-sci↗

arXiv2011

Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface

Although the creation of spin polarization in various non-magnetic media via electrical spin injection from a ferromagnetic tunnel contact has been demonstrated, much of the basic behavior is heavily debated. It is reported here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs that local magnetostatic fields arising from interface roughness dramatically alter and even dominate the accumulation and dynamics of spins in the semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous and non-collinear with the injector magnetization. The inverted Hanle effect serves as experimental signature. This interaction needs to be taken into account in the analysis of experimental data, particularly in extracting the spin lifetime and its variation with different parameters (temperature, doping concentration). It produces a broadening of the standard Hanle curve and thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si at room temperature it is shown that the spin lifetime is larger than previously determined, and a new lower bound of 0.29 ns is obtained. The results are expected to be general and occur for spins near a magnetic interface not only in semiconductors but also in metals, organic and carbon-based materials including graphene, and in various spintronic device structures.

cond-mat.mes-hall↗

arXiv2007

Spin-Polarized Tunneling as a probe of (Ga,Mn)As electronic properties

We present magnetic and tunnel transport properties of (Ga,Mn)As/(In,Ga)As/(Ga,Mn)As structure before and after adequate annealing procedure. The conjugate increase of magnetization and tunnel magnetoresistance obtained after annealing is shown to be associated to the increase of both exchange energy $Δ$$_{exch}$ and hole concentration by reduction of the Mn interstitial atom in the top magnetic electrode. Through a 6x6 band k.p model, we established general phase diagrams of tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) \textit{vs.} (Ga,Mn)As Fermi energy (E$_F$) and spin-splitting parameter (B$_G$). This allows to give a rough estimation of the exchange energy $Δ$$_{exch}$=6B$_G$$\simeq$120 meV and hole concentration p$\simeq1.10^{20}$cm$^{-3}$ of (Ga,Mn)As and beyond gives the general trend of TMR and TAMR \textit{vs.} the selected hole band involved in the tunneling transport.

cond-mat.mtrl-sci↗

arXiv2006

Semiconductors between spin-polarized source and drain

Injecting spins into a semiconductor channel and transforming the spin information into a significant electrical output signal is a long standing problem in spintronics. Actually, this is the prerequisite of several concepts of spin transistor. In this tutorial article, we discuss the general problem of spin transport in a nonmagnetic channel between source and drain. Two problems must be mastered: i) In the diffusive regime, the injection of a spin polarized current from a magnetic metal beyond the ballistic transport zone requires the insertion of a spin dependent and large enough interface resistance. ii) In both the diffusive and ballistic regimes, and whatever the metallic or semiconducting character of the source/drain, a small enough interface resistance is the condition to keep the dwell time shorter than the spin lifetime and thus to conserve the spin accumulation-induced output signal at an optimum level. Practically, the main difficulties come from the second condition. In our presentation of experimental results, we show why the transformation of spin information into a large electrical signal has been more easily achieved with carbon nanotubes than with semiconductors and we discuss how the situation could be improved in the later case.

cond-mat.mtrl-sci↗

arXiv2005

Resonant tunneling magnetoresistance in epitaxial metal-semiconductor heterostructures

We report on resonant tunneling magnetoresistance via localized states through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of localized states in the ZnSe barrier and spatial symmetry. The averaging of conduction over all localized states in a junction under resonant condition is strongly detrimental to the magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2003

Magnetization reversal by injection and transfer of spin: experiments and theory

Reversing the magnetization of a ferromagnet by spin transfer from a current, rather than by applying a magnetic field, is the central idea of an extensive current research. After a review of our experiments of current-induced magnetization reversal in Co/Cu/Co trilayered pillars, we present the model we have worked out for the calculation of the current-induced torque and the interpretation of the experiments.

cond-mat.mtrl-sci↗

arXiv2002

Electrical detection of spin accumulation in a p-type GaAs quantum well

We report on experiments in which a spin-polarized current is injected from a $GaMnAs$ ferromagnetic electrode into a $GaAs$ quantum well through an AlAs barrier. The resulting spin polarization in the GaAs well is detected by measuring how the current, tunneling to a second $GaMnAs$ ferromagnetic electrode, depends on the orientation of its magnetization. Our results can be accounted for the non-relaxed spin splitting of the chemical potential, that is spin accumulation, in the $GaAs$ well. We discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we observe.

cond-mat.mtrl-sci↗