Speaker:Prof.LU Yuan,Centre national de la recherche scientifique
Time: 10:30a.m.Friday,December 16th
Binary information encoded within the spin of carriers can be transferred into corresponding right or left-handed circularly polarized photons emitted from an active semiconductor medium via carrier-photon angular momentum conversion. In order to attain maximized spin-injection at out-of-plane magnetic remanence, a number of material systems have been explored as possible solid-state spin injectors. However the electroluminescence circular polarization (PC) of emitted light was still limited at 3-4% at remanence.
In this talk, I would like to report our recent systematic develop of CoFeB/MgO in-plane and out-of-plane spin injector on GaAs quantum well based spin light emitting diodes (spin-LED). For the in-plane injector, we have used different growth techniques (sputtering and MBE) to prepare the MgO tunnel barrier . It is interesting to found that the maximal spin injection efficiency is comparable for both methods, however the bias dependence of Pc is quite different. In addition, the effect of annealing is also investigated. Both types of samples show the same trend: an increase of Pc with the increase of annealing temperature, followed by a saturation of Pc beyond 350°C annealing. Our study reveals that the control of CoFeB/MgO interface is essential important for an optimal spin injection into semiconductor.
For out-of-plane injector, we have reduced the thickness of CoFeB down to 1.2nm on MgO tunnel barrier (2.5nm) to obtain a strong perpendicular magnetization anisotropy (PMA). The maximum value of Pc measured at zero field is as large as 20% at 25K and still 8% at 300K, which is almost five times higher than any other types of PMA injectors . Our results show that this type of ultrathin perpendicular spin-injectors are of great interest i) to realize the electrical switching of the magnetization of the injector layer owing to the advanced spin-transfer torque properties of CoFeB layer and ii) to be directly embedded in optical cavities for spin lasers due to their very low optical absorption loss.
 P. Barate, et al., “Electrical spin injection into InGaAs/GaAs quantum wells: A comparison between MgO tunnel barriers grown by sputtering and molecular beam epitaxy methods”, Applied Physics Letters, vol.105, 012404 (2014).
 S. H. Liang, et al., “Large and robust electrical spin injection into GaAs at zero magnetic field using an ultrathin”, Physical Review B, vol.90, 085310 (2014).