Henri-Jean Drouhin

Spin-polarized photoemission from AlGaAs/GaAs heterojunction: A convenient highly polarized electron source

We analyze the operation of a spin‐polarized electron source, consisting of a 100 A GaAs cap on top of Al0.3Ga0.7As, excited at 300 or 120 K by a He‐Ne laser. The cap allows easy activation to negative electron affinity while the alloy permits gap matching to the light source, and thus large electron spin polarization (30% at 300 K, 36% at 120 K). We compare yield curves, energy distribution curves, and polarized energy distribution curves obtained on samples with 100 and 1000 A caps and on bulk GaAs. The X conduction minimum position in the alloy is also determined.

Anisotropic magnetothermopower : Contribution of interband relaxation

Spin injection in metallic normal/ferromagnetic junctions is investigated taking into account interband relaxation and the consequences in terms of thermoelectric power. On the basis of a generalized two-channel model, it is shown that there is an interface resistance and thermoelectric power contribution due to anisotropic scattering, besides spin accumulation and giant magnetoresistance. The corresponding expression of the thermoelectric power is derived and compared with the expression accounting for the thermoelectric power produced by the giant magnetoresistance. Measurements of anisotropic magnetothermoelectric power are presented in electrodeposited Ni nanowires contacted with Ni, Au, and Cu. It is shown that a thermoelectric power is generated at the interfaces of the nanowire and that the experimental results strongly support the model.

Spin-dependent scattering in transition metals

The spin-dependent electron inelastic mean free path (IMFP) in transition metals is studied in the energy range 5–50 eV above the Fermi level. It is shown that the spin-dependent IMFP is simply related to the number of holes in both d spin subbands, whatever the detail of the d-band structure. This analysis allows us to disentangle the different scattering channels. Many experimental determinations of the spin-dependent part of the electron scattering cross section, from several teams, are analyzed in the framework of this model. The strong energy dependence of the exchange matrix element is clearly evidenced.

Spin transfer in an open ferromagnetic layer: from negative damping to effective temperature

Spin transfer is a typical spintronics effect that allows a ferromagnetic layer to be switched by spin injection. All experimental results concerning spin transfer (quasi-static hysteresis loops or AC resonance measurements) are described on the basis of the Landau–Lifshitz–Gilbert equation of the magnetization, in which additional current dependent terms are added, like current dependent effective fields and current dependent damping factors, that can be positive or negative. The origin of these terms can be investigated further by performing stochastic experiments, like one-shot relaxation experiments under spin injection in the activation regime of the magnetization. In this regime, the Neel–Brown activation law is observed which leads to the introduction of a current dependent effective temperature. In order to define these counterintuitive parameters (effective temperature and negative damping), a detailed thermokinetic analysis of the different sub-systems involved is performed. This report presents a thermokinetic description of the different forms of energy exchanged between the electric and the ferromagnetic sub-systems at a normal/ferromagnetic junction. The derivation of the Fokker–Planck equation in the framework of the thermokinetic theory allows the transport parameters to be defined from the entropy variation and refined with the Onsager reciprocity relations and symmetry properties of the magnetic system. The contribution of the spin polarized current is introduced as an external source term in the conservation laws of the ferromagnetic layer. Due to the relaxation time separation, this contribution can be reduced to an effective damping. The flux of energy transferred between the ferromagnet and the spin polarized current can be positive or negative, depending on the spin accumulation configuration. The effective temperature is deduced in the activation (stationary) regime, provided that the relaxation time that couples the magnetization to the spin polarized current is shorter than the relaxation to the lattice.

Anisotropic magnetothermal transport and spin Seebeck effect

The angular dependence of the thermal transport in insulating or conducting ferromagnets is derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the non-magnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magneto-voltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.

Spin-Currents and Spin-Pumping Forces for Spintronics

A general definition of the Spintronics concept of spin-pumping is proposed as generalized forces conjugated to the spin degrees of freedom in the framework of the theory of mesoscopic non-equilibrium thermodynamics. It is shown that at least three different kinds of spin-pumping forces and associated spin-currents can be defined in the most simple spintronics system: the Ferromagnetic/Non-Ferromagnetic metal interface. Furthermore, the generalized force associated with the ferromagnetic collective variable is also introduced on an equal footing to describe the coexistence of the spin of the conduction electrons (paramagnetic spins attached to s-band electrons) and the ferromagnetic-order parameter. The dynamical coupling between these two kinds of magnetic degrees of freedom is presented and interpreted in terms of spin-transfer effects.

Origin of anomalous energy spreads in photoelectron beams

The use of photocathodes as intense monoenergetic electron sources seems very promising. Yet, many experimenters have observed large energy spreads, increasing with the emitted current, which remained unexplained. Here, we analyze a mechanism which accounts well for these effects.

Magnetization reversal driven by spin injection: A diffusive spin-transfer effect

An out-of-equilibrium description of spin-transfer effect is proposed, based on the spin-injection mechanism occurring at the junction with a ferromagnet. The effect of spin injection is to locally modify, in the ferromagnetic configuration space, the density of magnetic moments. The corresponding gradient leads to a currentdependent diffusion process of magnetization. In order to describe this effect, the dynamics of the magnetization of a ferromagnetic single domain is reconsidered in the framework of the thermokinetic theory of mesoscopic systems. Assuming an Onsager cross coefficient that couples the currents, it is shown that spindependent electric transport leads to a correction of the Landau–Lifshitz–Gilbert equation of the ferromagnetic order parameter with supplementary diffusion terms. The consequence of spin injection in terms of the activation process of the ferromagnet is deduced, and the expressions of the effective energy barrier and of the critical current are derived. Magnetic fluctuations are calculated: the correction to the fluctuations is similar to that predicted for the activation. These predictions are consistent with the measurements of spin transfer obtained both in the activation regime and for ferromagnetic resonance under spin injection.

Probability-current definition in presence of spin-orbit interaction

A novel definition of the probability-current operator, based on a momentum power expansion of effective Hamiltonians, is presented. This operator can be applied in the presence of the Rashba term but also when the D’yakonov–Perel contribution is included. We exploit this formalism to find new boundary conditions and we apply our findings to the interface between two semi-infinite media, with on one side a free-electron-like material and on the other side a [110]-oriented GaAs barrier.

Structure of spin-split evanescent states in the fundamental gap of zinc-blende-type semiconductors

We calculate evanescent waves in GaAs throughout the forbidden band gap, taking into account both the absence of inversion symmetry and the spin-orbit coupling. In this case, the energy bands are spin split. We find that the evanescent wave functions only exist in limited energy and wave-vector domains. We show that no evanescent state associated with a purely imaginary wave vector exists in some simple directions such as [110], which has implications concerning the tunneling mechanism itself. Finally, we show that a GaAs tunnel barrier can be used as a spin injector in solid-state devices.