Optical studies of electron spin transmission

Abstract

We have discussed in detail evidence for room temperature spin filtering of spin polarised electrons at the FM/SC interface, in the context of optical studies of spin injection and detection. Hybrid FM/GaAs Schottky barrier structures with different FM layer materials, thicknesses and GaAs doping densities were investigated, as well as an AF Cr sample for reference. The magnetic field dependence of the helicity dependent photocurrent follows that of the polar MOKE signal in all cases, showing that I is determined by the magnetic properties of the FM layer. Any magnetic background from the SC substrate can therefore be ruled out. This is confirmed by the fact that no magnetic field dependent signal was observed for the Cr sample. A comparison of the bias dependence of I with that of the unpolarised photocurrent enabled us to separate MCD from the measured helicity dependent photocurrent and to isolate the true spin filtering signal ΔISF. We find that at reverse bias, when most of the photoexcited electrons travel into the bulk of the SC, I arises purely due to MCD. However, a peak in ΔISF is observed at forward bias, providing clear evidence for the spin filtering of spin polarised electrons propagating across the SC/FM interface. The finding of peaks instead of a monotonic increase in ΔISF with bias suggests that electron tunnelling is the spin dependent transport mechanism. A comparison of ΔISF for samples with different doping densities shows that peak position and peak width are strongly dependent on the depletion layer width and consequently the Schottky barrier width, aswould be expected for a tunnelling process. Significant spin filtering is observed for both NiFe and Fe, whereas magneto-optical effects dominate in Co/GaAs structures. In order to study the spin filtering mechanism at the FM/SC interface in more detail and to confirm our electron tunnelling model,MOSstructures were investigated. The dip in the bias dependent photocurrent observed at forward bias is indicative of the tunnelling of photoexcited electrons from the GaAs into the Fe layer through the AlOx barrier. Correspondingly, the helicity dependent photocurrent normalised by the photocurrent I/2Iph clearly shows a peak at the same forward bias value, i.e., the spin filtering effect is enhanced in the structures with the AlOx barrier due to spin polarised electron tunnelling. Further proof of the importance of electron tunnelling for spin filtering was added by temperature dependent measurements of band gap engineered FM/AlGaAs barrier/SC structures: spin dependent effects were only observed in the bias and temperature range where electron tunnelling occurs. This finding provides clear evidence that significant spin filtering effects can only be expected for tunnelling electrons. Furthermore we investigated spin dependent electron transport in hybrid spin valve/SC structures. A ∼ 2400% increase in helicity dependent photocurrent was observed on switching the spin valve from the parallel to the antiparallel configuration. The strong dependence of the spin filtering effect on the photon energy and the relative alignment of the FM layers shows that electrons ballistically propagating through the metal layer structure are involved in the transport process. The use of a spin valve instead of a single FM layer furthermore enabled us to separate the photocurrent across the FM/SC interface from the net measured signal, allowing for the observed spin filtering effect to be quantified. For the antiparallel configuration the polarisation of the photocurrent passing the spin valve was found to be close to the expected spin polarisation of the electrons photoexcited in the GaAs. This shows that spin polarised electrons are spin filtered in the spin valve structure with a high degree of efficiency. In summary these results unambiguously indicate that spin polarised electrons are efficiently transmitted from the SC to the FM. The investigated device structures were not designed to achieve large spin filtering efficiencies, but rather to study the underlying physical principles of spin dependent electron transport. However, the key role of electron tunnelling observed in our experiments suggests that the spin filtering effect could be significantly enhanced by eliminating the shunting current into the SC. Our current understanding of spin transport at the FM/SC interface therefore provides encouraging prospects for achieving efficient spin injection and detection in spin electronic devices using FM injector/detector electrodes at room temperature. The introduction of an insulating barrier at the interface clearly helps to achieve efficient spin injection and detection but a better understanding of the characteristics of the interface and barrier is needed. For spin detection, the spin valve structure may provide a promising basis for designing spin electronic devices, since we observed strongly spin dependent transmission effects associated with ballistic electron transport across the composite spin valve structure. Finally, the authors envisage that band gap engineering and spin engineering together offer a very promising new approach to the control of electron spins in semiconductors which is still very much at a pioneering stage.