H. Jaffrès

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/Al2O3/GaAs interface with electrical detection. The application of a transverse magnetic field induces a large voltage drop DeltaV at the interface as high as 1.2 mV for a current density of 0.34 nA.microm(-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 Al2O3/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 traveling back into the ferromagnetic contact reproduces accurately the experimental results.

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

Although the creation of spin polarization in various nonmagnetic 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 that, for semiconductor/Al(2)O(3)/ferromagnet tunnel structures based on Si or GaAs, local magnetostatic fields arising from interface roughness dramatically alter and even dominate the accumulation and dynamics of spins in the semiconductor. Spin precession in inhomogeneous magnetic fields is shown to reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous and noncollinear with the injector magnetization. The inverted Hanle effect serves as the experimental signature. This interaction needs to be taken into account in the analysis of experimental data, particularly in extracting the spin lifetime tau(s) and its variation with different parameters (temperature, doping concentration). It produces a broadening of the standard Hanle curve and thereby an apparent reduction of tau(s). For heavily doped n-type Si at room temperature it is shown that tau(s) is larger than previously determined, and a new lower bound of 0.29 ns is obtained. The results are expected to be general and to occur for spins near a magnetic interface not only in semiconductors but also in metals and organic and carbon-based materials including graphene, and in various spintronic device structures.

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 layer through an AlAs barrier. The resulting spin polarization in GaAs is detected by measuring how the tunneling current, to a second GaMnAs ferromagnetic electrode, depends on the orientation of its magnetization. Our results can be accounted for by sequential tunneling with the nonrelaxed spin splitting of the chemical potential, that is, spin accumulation, in GaAs. We discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we observe.

Semiconductors Between Spin-Polarized Sources and Drains

Injecting spins into a semiconductor channel and transforming the spin information into a significant electrical output signal is a long-standing problem in spintronics. This is the prerequisite of several concepts of spin transistor. In this paper, we discuss the general problem of spin transport in a nonmagnetic channel between source and drain. Two problems must be mastered: 1) In diffusive regime, the injection/extraction of a spin-polarized current into/from a semiconductor beyond the ballistic zone at the interface with a magnetic metal requires the insertion of a spin-dependent and large enough interface resistance. 2) 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 (DeltaR/Rap1 or larger). 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

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.

Field sensing using the magnetoresistance of IrMn exchange-biased tunnel junctions

An original concept of high sensitivity magnetic field sensor using the spin-dependent tunneling effect has been investigated. The required crossed-biased configuration is obtained by combining both shape energy originating from vicinal step bunched Si substrates and unidirectional exchange anisotropy supplied by an Ir20Mn80 film in the “top-biased” geometry. We demonstrate a linear and reversible signal at room temperature and above. The smooth loss of sensitivity at higher temperature is shown to be correlated to the thermal dependence of the exchange bias property when IrMn is deposited above the insulating Al2O3 barrier.An original concept of high sensitivity magnetic field sensor using the spin-dependent tunneling effect has been investigated. The required crossed-biased configuration is obtained by combining both shape energy originating from vicinal step bunched Si substrates and unidirectional exchange anisotropy supplied by an Ir20Mn80 film in the “top-biased” geometry. We demonstrate a linear and reversible signal at room temperature and above. The smooth loss of sensitivity at higher temperature is shown to be correlated to the thermal dependence of the exchange bias property when IrMn is deposited above the insulating Al2O3 barrier.

Irreversible magnetization switching using surface acoustic waves

An analytical and numerical approach is developped to pinpoint the optimal experimental conditions to irreversibly switch magnetization using surface acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and two switching mechanisms are considered. In preces-sional switching, a small in-plane field initially tilts the magnetization and the passage of the SAW modifies the magnetic anisotropy parameters through inverse magneto-striction. The SAW triggers precession, and eventually reversal. Using the micromagnetic parameters of a fully characterized layer of the magnetic semiconductor (Ga,Mn)(As,P), we then show that there is a large window of accessible experimental conditions (SAW amplitude/wave-vector, field amplitude/orientation) allowing irreversible switching. As this is a resonant process, the influence of the detuning of the SAW frequency to the magnetic systems eigenfrequency is also explored. Finally, another-non-resonant-switching mechanism is briefly contemplated, and found to be applicable to (Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small perpendicular field is applied opposite the initial magnetization and the passage of the SAW lowers the domain nucleation barrier.

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 2u2009nm 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.

Spin pumping and inverse spin Hall effect in germanium

We have measured the inverse spin Hall effect (ISHE) in

Review of recent results on spin polarized tunneling and magnetic switching by spin injection

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