Hanle effect in transport through quantum dots coupled to ferromagnetic leads

– We suggest a series of transport experiments on spin precession in quantum dots coupled to one or two ferromagnetic leads. Dot spin states are created by spin injection and analyzed via the linear conductance through the dot, while an applied magnetic field gives rise to the Hanle effect. Such a Hanle experiment can be used to determine the spin lifetime in the quantum dot, to measure the spin injection efficiency into the dot, as well as proving the existence of intrinsic spin precession which is driven by the Coulomb interaction. Recent progress in nanofabrication technology opens the possibility for spintronic devices based on coherent manipulation of single spins in quantum dots [1]. Thereby the spatial confinement of the dot electrons suppresses spin decoherence [2]. While longitudinal spin relaxation times T1 up to microseconds were measured [3, 4], the size of the spin decoherence time T2 is still an open question. The spin coherence time T2 can be accessed in different experiments, for example by ESR techniques [5], or by the Hanle effect, i.e., the decrease of spin accumulation in the quantum dot due to precession about a static magnetic field. The optical realization of such a Hanle experiment involves the measurement of the fluorescent emission of polarized light from semiconductor quantum dots [6]. But this method requires an ensemble of spins, so the total signal varies with the spin dephasing time T ⋆ 2 rather than the decoherence time T2 > T ⋆ 2 . To void this ensemble averaging, we suggest to measure the Hanle effect in transport through an individual single level quantum dot. For preparation of the initial spin state, electronic spin injection from ferromagnetic leads into the dot can be used. This has been demonstrated in metallic bulk systems [7], quantum dots [8], Zener diodes [9], and metallic grains [10]. For the detection of the accumulated spin, we propose magnetoresistance measurements of the device as already shown for metallic systems [7, 10]. So an all electrical