Etudes par microscopie en champ proche des phénomènes de ...

APS/123-QEDInjection energy dependence of spin-polarized hot-electron transport through aferromagnetic metal/oxide/semiconductor junctionN. Rougemaille, D. Lamine, ∗ G. Lampel, Y. Lassailly, and J. PerettiPhysique de la Matière Condensée, Ecole Polytechnique, CNRS, 91128 Palaiseau, France(Dated: September 30, 2007)Spin-polarized hot electron transport through a ferromagnetic metal/oxide/semiconductor junctionis studied as a function of the electron injection energy. The incident spin-polarized electronsare produced by a GaAs photocathode and are injected from vacuum into the thin metal layer. Thisallows a variation in the injection energy over a wide range (from a few eV up to 1keV). The currenttransmitted through the junction is measured in the semiconductor collector. A spin-dependentcomponent of the transmitted current is detected when reversing either the spin polarization of theincident electrons or the magnetization of the metal layer. For injection energy in the hundreds ofeV range, both the mean transmitted current and the spin-dependent transmitted current exhibita spectacular increase, over several orders of magnitude. A transport regime is reached where electrontransmission is larger than unity, providing a current gain, while the spin-selectivity of themagnetic layer is still very high (close to 100%). This variation is analyzed in the framework of atransport model, which accounts for the relaxation of the electron energy and velocity by secondaryelectron excitation. This model fits qualitatively and quantitatively with the experimental data andevidences the importance of the metal/oxide/semiconductor barrier shape on the spin-asymmetryof the transmitted current.PACS numbers: 72.25.-b, 73.40.Qv, 79.20.Hx, 85.75.-dI. INTRODUCTIONIn ferromagnetic metals, because of the spin asymmetryof the electron density of empty states, the inelasticmean-free-path is larger for majority-spin electrons thanfor minority-spin electrons in an energy range which extendsup to about 50eV above the Fermi Level. Thisfavors the transport of majority-spin hot electrons and isattheoriginofelectronspinfilteringeffectsinthinmagneticfilms. 1–8 Different experimental approaches havebeen developed to explore spin-dependent hot-electrontransport in thin magnetic films. They are based eitheron electron spectroscopy techniques 5–14 or on threeterminalsolid state devices, 15–21 and give access to thetransport properties in a wide energy range, typically between1eV up to several hundreds of eV. The principle ofall these experiments is basically the same : hot electronsare injected into a thin magnetic film and the intensity(or polarization) of the current transmitted through thefilm is measured as a function of the incident electron polarizationand/or of the magnetization state of the film.The largest spin-asymmetry values in hot-electron transmissionthrough thin ferromagnetic films are obtained atlow injection energy, i.e. a few eV above the metal Fermilevel. 9,10,12,14–21 In this energy range, the electron transmissionthrough the thin magnetic layer is almost ballisticand is described by an exponential attenuation withthe mean-free-path as characteristic attenuation length.From the spin asymmetry of the mean-free-path, one candefine a spin-discriminating length which is found to beas small as a few nanometers (typically 3 to 5nm) forelectrons of a few eV energy. 14,18 A magnetic layer ofthickness of the order of the spin discriminating lengthis therefore highly spin-selective. Spin-filtering efficiencyclose to unity has indeed been demonstrated in magneticlayers of only a few nanometers thickness. However, operatinga spin filter at low injection energy is limited bythe low total transmission efficiency. The mean transmittedcurrent is indeed generally orders of magnitudesmaller than the injected current mainly because of theweak exit probability of low energy electrons from themetal. 15–21At higher injection energy the situation is very different.The transmitted current is indeed dominated byelectrons which emerge from a secondary electron cascadeat energy much smaller than the injection energy.Therefore, the transport is governed by the electronicproperties over a wide energy range. When the injectionenergy ranges from a few eV to several tens of eV,the electron inelastic mean-free-path stiffly decreases sothat, electrons are very efficiently relaxed. The transportcan then be described by a simple model whichempirically combines the very fast excitation of a secondaryelectron cascade and the subsequent low-energyballistic transport through the magnetic layer. 9,10,12,14,18This scheme has two consequences : the overall transmittedcurrent increases because of the secondary electronmultiplication and the magnetic layer spin selectivityis still high because the transmission mainly occursat low energy. One can profit from these two propertiesin spin-valve structures. Indeed, in a spin valve structurecontaining only two magnetic layers (of about 1nmthickness each), large magneto-current asymmetry hasbeen obtained in the whole injection energy range froma few eV up to 100eV while the secondary electron multiplicationyielded a linear increase of the transmittedcurrent with injection energy. 14 Similar experiments performedin a single magnetic layer structure have shownthat the transmission asymmetry measured when inject-