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Electrical detection of spin accumulation in p-type GaAs quantum well

For spin injection into semiconductors, magnetic semiconductors are an alternative to ferromagnetic metals, with the advantage that the difficulties resulting from the conductivity mismatch just as well the chemical reactivity between metal and semiconductor can be avoided. On the other hand, magnetic tunnel junctions with magnetic semiconductor electrodes can be used to test spin injection since the existence of magnetoresistance (TMR) is a signature of the transmission os spin polarized carriers between the electrodes. We have fabricated single and double barrier magnetic tunnel junctions with GaMnAs electrodes by a low temperature molecular beam epitaxy procedure. The two magnetic electrodes (Ga_(1-x)Mn_xAs) are separated by thin AlAs (1.7 nm) tunnel barrier (single barrier MTJ) or by a AlAs (1.7 nm)/GaAs (5nm)/ AlAs (1.7 nm) trilayer (double barrier MTJ). Antiparallel arrangement of the ferromagnetic electrodes at low field is obtained by playing with the thickness and manganese concentration in the GaMnAs layers to obtain different coercive fields. GaAs layers have been inserted between electrodes and tunnel barrier in order to avoid the diffusion of the manganese into the tunnel barrier. We observe large TMR effects, 38% at low field associated with the switching between the parallel (P) and antiparallel (AP) configurations of the remanent magnetization. The existence of similar large MR ratios in the double junction is a new and interesting effect, never observed in metallic double junctions when the intermediate electrode is nonmagnetic. Our result can be accounted for the non-relaxed spin splitting of the chemical potential, that is spin accumulation, in GaAs well. We will discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we have observed.

semiconductor spintronics; magnetic semiconductors; spin accumulation; quantum well

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