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

3m R ≃ 0.9m S . This allows to reverse the Fe layer magnetizationfrom +m R to −m R by pulsed-current operationof an in-situ magnetic coils and to measure the spindependenttransmitted current at zero external magneticfield.The spin-polarized electron beam is produced by a GaAsphotocathode activated to negative electron affinity bycesium and oxygen deposition. Under excitation with acircularly-polarized near-band gap laser light (of energyhν = 1.58eV), this source yields an electron beam oflongitudinal spin-polarization P 0 , which can be switchedbetween +25% and −25% by reversing the light polarization.This electron beam passes through a cylindricalelectrostatic deflector to convert the longitudinal spin polarizationinto a transverse one aligned along the Fe-layermagnetization axis. The beam is then focused onto thesample by electrostatic electron optics. The electron injectionenergy E 0 , referred to the metal Fermi level, iscontrolled by the sample potential V 0 . A typical incidentcurrent of 200nA, with a 200meV energy distributionwidth, is injected into the junction.By analogy with the three terminal transistor-like devices,the currents flowing in the metallic layer (“base”)and in the semiconductor (“collector”) are labeled I B andI C respectively. The injected current is labeled I 0 . Thesethree currents are independently measured using homemadeisolated current amplifiers, which may operate at1kV with an electronic noise of 30fA/ √ Hz. The independentmeasurement of is performed by disconnectingone of the two (base or collector) terminals, and thenusing the sample as a collecting anode. Fig.2 shows theexperimental variation of I 0 , I B and I C with the injectionenergy E 0 . The injected current I 0 is constant assoon as the sample potential is significantly larger thanthe potential of the last electrode of the electron optics(40V). However, at low sample potential, the injectionefficiency is at most reduced by 40%. The current conservationrelation I 0 = I B + I C is experimentally verifiedover the whole probed energy range. At low injectionenergy, the base current I B is almost equal to I 0 sincethe transmitted current I C is very small. But at high injectionenergy, I C strongly increases and becomes largerthan the injected current I 0 above E 0 = 712eV . At thesame time, the base current I B decreases, drops down tozero at E 0 = 712eV and then becomes negative. SinceI C increases and overcomes the value of the injected current,it is clear that the transport through the metal ismainly governed by electron-electron scattering, providingelectron multiplication by excitation of a secondaryelectron cascade.The spin-dependent component of the transmitted currentΔI C = I + C − I− Cis the difference between the valuesI + C and I− Cof the collector current for an incident electronspin-polarization of respectively +P 0 and −P 0 . Asan example, the measurement of ΔI C , when the injectionenergy is set at 712eV (the energy where I C = I 0and I B = 0), is shown in the inset of Fig.2. For this measurement,the incident electron polarization was flippedFIG. 2: Variation of the injection current I 0, of the base currentI B measured at the metallic layer terminal, and of thetransmitted current I C measured in the semiconductor collectorversus the injection energy E 0. The inset shows thecollected current change due to the spin-dependent electrontransmission. For this measurement, the energy was set atE 0 = 712eV and the incident electron polarization modulatedat 400Hz. The magnetization was several times flipped overby pulsed current operation of the magnetic coil. The configurations[+m R, +P 0] and [−m R, −P 0] gives the same value ofI C = ĪC − ΔIC/2, showing a low instrumental asymmetry.between +P 0 and −P 0 by modulating the incident lightpolarization between σ + and σ − at a frequency of 400Hz.For each value of the injection energy, ΔI C is systematicallymeasured for the two opposite magnetizations +m Rand −m R of the Fe layer. This allows to get rid of instrumentalasymmetry. As shown in the inset of Fig.2,instrumental asymmetry is very low and reversing the incidentelectron polarization or the sample magnetizationproduces the same transmitted current variation. Indeed,the spin-filtering effect only depends on the orientationof the incident electron polarization relative to that ofthe magnetization.For the analysis of the experimental data, we will considerthe dimensionless quantities:i) the transmission T = I C /I 0 ,ii) the spin dependent transmission ΔT =ΔI C /I 0 ,iii) the transmission spin asymmetryA C =(I + C − I− C )/(I+ C + I− C ) ≈ ΔT/2TNote that these three normalized quantities are not affectedby the decrease in the injection efficiency at low