Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor

We report on spin injection experiments at a Co/Al$_2$O$_3$/GaAs interface with electrical detection. The application of a transverse magnetic field induces a large voltage drop $Delta V$ at the interface as high as 1.2mV for a current density of 0.34 nA.$mu m^{-2}$. This represents a dramatic increase of the spin accumulation signal, well above the theoretical predictions for spin injection through a ferromagnet/semiconductor interface. Such an enhancement is consistent with a sequential tunneling process via localized states located in the vicinity of the Al$_2$O$_3$/GaAs interface. For spin-polarized carriers these states act as an accumulation layer where the spin lifetime is large. A model taking into account the spin lifetime and the escape tunneling time for carriers travelling back into the ferromagnetic contact reproduces accurately the experimental results.

Spin-Polarized Tunneling as a probe of (Ga,Mn)As electronic properties

We present magnetic and tunnel transport properties of (Ga,Mn)As/(In,Ga)As/(Ga,Mn)As structure before and after adequate annealing procedure. The conjugate increase of magnetization and tunnel magnetoresistance obtained after annealing is shown to be associated to the increase of both exchange energy $Delta$$_{exch}$ and hole concentration by reduction of the Mn interstitial atom in the top magnetic electrode. Through a 6x6 band k.p model, we established general phase diagrams of tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) textit{vs.} (Ga,Mn)As Fermi energy (E$_F$) and spin-splitting parameter (B$_G$). This allows to give a rough estimation of the exchange energy $Delta$$_{exch}$=6B$_G$$simeq$120 meV and hole concentration p$simeq1.10^{20}$cm$^{-3}$ of (Ga,Mn)As and beyond gives the general trend of TMR and TAMR textit{vs.} the selected hole band involved in the tunneling transport.