Jean-Marie George

Strain control of the magnetic anisotropy in (Ga,Mn) (As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10%) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects strongly affect the domain wall propagation.

Spin Signal in Metallic Lateral Spin Valves Made by a Multiple Angle Evaporation Technique

We report on the fabrication of metallic lateral spin valves using different evaporation directions for respective ferromagnetic and nonmagnetic materials. In this way, we can fabricate and connect different nanowires through the same resist mask in a single evaporation sequence, avoiding interface contamination. With this technique and by reducing the wire widths down to 50 nm, we obtained spin signals as large as 24 mΩ at 77 K in devices with transparent interfaces. We studied the influence of the nonmagnetic metal. Spin signals are found to be much larger for Al and Cu than for Au-based devices.

Electrical spin injection and detection 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, the demonstration of an electron spin transport through a semiconducting MoS2 channel remains challenging. Here we show the evidence of the electrical spin injection and detection in the conduction band of a multilayer MoS2 semiconducting channel using a two-terminal spin-valve configuration geometry. A magnetoresistance around 1% has been observed through a 450 nm long, 6 monolayer thick MoS2 channel with a Co/MgO tunnelling spin injector and detector. It is found that keeping a good balance between the interface resistance and channel resistance is mandatory for the observation of the two-terminal magnetoresistance. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel with an in-plane spin polarization. The long spin diffusion length (approximately ∼235 nm) could open a new avenue for spintronic applications using multilayer transition metal dichalcogenides.

Evidence for spin-to-charge conversion by Rashba coupling in metallic states at the Fe/Ge(111) interface

The spin–orbit coupling relating the electron spin and momentum allows for spin generation, detection and manipulation. It thus fulfils the three basic functions of the spin field-effect transistor. However, the spin Hall effect in bulk germanium is too weak to produce spin currents, whereas large Rashba effect at Ge(111) surfaces covered with heavy metals could generate spin-polarized currents. The Rashba spin splitting can actually be as large as hundreds of meV. Here we show a giant spin-to-charge conversion in metallic states at the Fe/Ge(111) interface due to the Rashba coupling. We generate very large charge currents by direct spin pumping into the interface states from 20 K to room temperature. The presence of these metallic states at the Fe/Ge(111) interface is demonstrated by first-principles electronic structure calculations. By this, we demonstrate how to take advantage of the spin–orbit coupling for the development of the spin field-effect transistor.

Spin–Charge Conversion Phenomena in Germanium

The spin–orbit coupling relating the electron spin and momentum allows for spin generation, detection and manipulation. It thus fulfils the three basic functions of the spin field-effect-transistor made of semiconductors. In this paper, we review our recent results on spin–charge conversion in bulk germanium and at the Ge(111) surface. We used the spin pumping technique to generate pure spin currents to be injected into bulk germanium and at the Fe/Ge(111) interface. The mechanism for spin–charge conversion in bulk germanium is the spin Hall effect and we could experimentally determine the spin Hall angle θSHE, i.e., the spin–charge conversion efficiency, in heavily doped n-type and p-type germanium. We found very small values at room temperature: θSHE ≈ (1–2) × 10−3 in n-Ge and θSHE ≈ (6–7) × 10−4 in p-Ge. Moreover, we pointed out the essential role of spin dependent scattering on ionized impurities in the spin Hall effect mechanism. We concluded that the spin Hall effect in bulk germanium is too weak to...

Theoretical and experimental investigation of optically spin-injected VECSEL

We report theoretical and experimental analysis of spin-injected VECSELs. First, we fabricate and characterize an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well VECSEL. The structure is designed to allow the integration of a Metal-Tunnel-Junction ferromagnetic spin-injector for future electrical injection. We report here the control at room temperature of the VECSEL polarization using optical spin injection in the active medium. The switching between two highly circular polarization states had been demonstrated using an M-shaped extended cavity in multi-modes lasing. This first result witnesses an efficient spin-injection in the active medium of the laser. Then, we report birefringence measurements of the VECSEL in oscillating conditions. The proposed technique relies on the measurement in the microwave domain of the beatnote between the oscillating mode and the amplified spontaneous emission of the cross-polarized non-lasing field lying in the following longitudinal mode. This technique is shown to offer extremely high sensitivity and accuracy enabling to track the amount of residual birefringence according to the laser operation conditions. Finally, we discuss the compensation of the residual linear phase anisotropy by controlling the birefringence of an intracavity electro-optical crystal. A 44-fold birefringence reduction is obtained. Besides, we study the modification of the laser polarization eigen states with birefringence compensation: a rotation of the linear polarization state is observed when the total phase anisotropy is reduced. An elliptical polarization eigen state is obtained at the minimum of the birefringence into the laser cavity, more favorable for spin injection.

Bases and experimental validation of a novel VSPIN model: towards functional spin-controlled VCSELs

Optimization of spin-lasers relies on the proper design of the active medium but also on a thorough understanding of the vectorial dynamics of the electromagnetic field in the laser cavity itself. A vectorial approach based the Jones formalism associated to the resonant condition of the field in the laser cavity is derived in order to draw the main guidelines for developing functional spin-controlled VCSELs. This general modelling framework, which accounts for spin injection effects as a gain circular dichroism in the active medium, shows that any residual phase anisotropy in the laser has a detrimental role on polarization switching. The same framework, is used to propose two solutions enabling to overcome this drawback: either by compensating the phase anisotropy or by preparing the laser cavity so that its eigenstates are circularly polarized. Moreover, unlike in spin-LED, we show that the leverage effect existing in the laser due to eigenmodes coupling makes it possible to switch the laser from a polarized oscillation to the orthogonal one despite the weak spin injection efficiency due to spin decoherence. All these predictions are confirmed using external cavity VCSELs which offer an ideal playing field for experimental investigations. Based on these developments, future trends towards the achievement of efficient and compact spin-lasers will be given.

Demonstration of spin injection in a CW VECSEL at RT with a dynamic and accurate control of its polarization state

Thanks to the recent spintronics advancement, new applications should emerge, like fast communications with spin coding. For instance, data transmission by coding the polarization state of a spin-injected laser source [1] is enabling new modulation formats and wide frequency bandwidths. In comparison to semiconductor edge-emitting lasers where only one polarization is guided, vertical cavity surface emitting lasers (VCSELs) are more adapted to obtain a spin-laser. However, we recently showed, using an external cavity VCSEL (VECSEL), that even in such quasi-isotropic lasers, spin injection is inhibited by residual linear anisotropies due to the poor spin transfer efficiency [2]. These residual anisotropies set the laser polarization eigenstates to linear polarizations. Here, we propose to investigate theoretically and experimentally the impact of these anisotropies on the polarization eigenstates of a spin-VECSEL, so as to prepare the laser for spin injection. We finally highlight spin injection effects in a device where the linear birefringence and dichroism are reduced to the minimum. An original laser architecture is proposed to control dynamically the linear anisotropy of phase γlin and the linear dichroism of gain Δ Glin so as to control the laser polarization state (Figure 1, a). The laser that we developed includes a ½-VCSEL structure emitting at 1.5 mm at room temperature (RT), provided by the Ecole Polytechnique Fédérale de Lausanne [3]. It is optically pumped with a continuous wave (CW) single mode laser diode at 980 nm. A 500 mm PLZT electro-optic ceramic is added into the cavity to control its linear birefringence [4]. A 100 mm YAG etalon is also inserted in order to ensure both single frequency oscillation and minimization of the linear dichroism of the cavity. Experimentally, when we tune the linear birefringence from high to low values, the VECSEL polarization ellipticity is increased from 4 to 23° (Fig. 1, b, red circles). By fitting these results with a vectorial model, which takes into account three parameters (γlin, Δ Glin and the circular dichroism of gain Δ Glin), we can extract the values of Δ Glin = 5.10−4 and Δ Glin = 2.10−4 (Fig. 1, b, blue line). To highlight the effects of spin injection, we measure the laser polarization eigenstates modifications between a pumping with a linear polarization (unpolarized electrons) and a circular polarization (50% spin-polarized electrons) (Fig. 1, c). Compared to a linear pump polarization, an increase of the ellipticity of 22° is observed when spin down (right-handed circular pump polarization σ+) is injected into the gain medium and a decrease of 20° when spin up is injected (left-handed circular pump polarization σ–). The inset of the Figure 1, c) shows the switch of the laser polarization eigenstate when the VECSEL is optically injected with spin up and spin down. A threshold reduction of about 8 % is also obtained, as expected, witch spin injection. To conclude, spin injection in an optically pumped VECSEL operating at RT is demonstrated using a dynamic control of the laser residual linear anisotropies. We are now working in improving the spin injection efficiency so as to obtain a VECSEL polarization switch from right to left handed circular polarizations, that is, a switch of the VECSEL ellipticity from −45° to 45°.

Very efficient electrical spin injection (/detection) into quantum dots at zero magnetic field

In this paper, we demonstrate a very efficient electrical spin injection into an ensemble of InAs/InGaAs quantum dots at zero magnetic field. The circular polarization of the electroluminescence coming from the dots, which are embedded into a GaAs-based Spin Light Emitting diode reaches a value as large as 20% at low temperature. In this device, no external magnetic field is required in order to inject or read spin polarization thanks to the use of an ultrathin CoFeB electrode (1.1 nm), as well as p-doped quantum dots (with one hole per dot in average) as an optical probe. The electroluminescence circular polarization of the dots follows the hysteresis loop of the magnetic layer and decreases as a function of bias for large voltages. In a reverse way, we have also investigated the possibility to use such a device as a photodetector presenting a photon helicity-dependent photocurrent. We observe a weak asymmetry of photocurrent under right and left polarized light that follows the hysteresis cycle of the magnetic layer, and the effect decreases for increasing temperatures and can be controlled by the bias.

Demonstration of efficient spin injection in a CW VECSEL at RT and dynamic control of its polarization state

We report the first efficient spin injection in a CW optically pumped VECSEL, obtained with phase anisotropy dynamic control. A threshold reduction of 1.5 % and an ellipticity of 75% are obtained at RT. Theoretical and experimental investigation of the polarization eigenstates is presented.