Spin-polarized electron transport through nanometer-scale Al grains

Abstract

We investigate spin-polarized electron tunnelling through ensembles of nanometer scale Al grains embedded between two Co-reservoirs at 4.2K, and observe tunnelling-magnetoresistance (TMR) and effects from spin-precession in the perpendicular applied magnetic field (the Hanle effect). The spincoherence time (T ⋆ 2 ) measured using the Hanle effect is of order ns. The dephasing is attributed to electron spin-precession in local magnetic fields. Dephasing process does not destroy T M R, which is strongly asymmetric with bias voltage. The asymmetric TMR is explained by spin relaxation in Al grains and asymmetric electron dwell times.Long spin relaxation times for polarized carriers are necessary for development of spintronic devices.In quantum dots, spin relaxation times are strongly enhanced compared to bulk, and the spin of an electron confined in a quantum dot is a candidate quantum bit [1,2]. Unfortunately, the spin-coherence time T ⋆ 2 measured in a semiconducting quantum dot is only ∼ ns, [3] despite the fact that the spin-relaxation time (T 1 ) is extremely long, up to a ∼ ms [4,5]. In GaAs quantum dots, dephasing is caused by spin precession around an effective magnetic field created by nuclear spins.Spin-injection and detection from ferromagnetic electron reservoirs is a well-known technique to measure spin relaxation time [6]. In this paper we study electron spininjection in tunnelling junctions containing a large number of embedded Al-grains.Our tunnelling device is sketched in Fig. 1-A. The top and the bottom layers are 100Å thick Co films. The width/length is 1.5mm/15mm and 1mm/20mm for the bottom and the top layer, respectively. The sample cross-section, sketched in Fig.

Saturation of spin-polarized current in nanometer scale aluminum grains

“…[13] and references therein. Spin-coherent electron tunnelling via nanometer scale normal metallic grains has been confirmed in arrays [14,15] and in single grains. [16] However, the electron spin-relaxation time T 1 in a metallic grain has not been reported yet.…”

Magneto-Coulomb effect in spin-valve devices

“…1 Recently, quite a bit of progress has been made in studying spin devices in the presence of the Coulomb blockade. [2][3][4][5][6][7][8][9][10][11][12] The interpretation of the two-terminal data in these reports has focused mainly on spin transport and spin accumulation. Here, we discuss another influence on the two-terminal resistance in ferromagnetically contacted nanostructures, namely the magnetoCoulomb effect ͑MCE͒ discovered by Ono et al 13 In this contribution, we consider a confined conductor weakly connected to two ferromagnets, F 1 and F 2 ͓see Fig.…”

Exchange effects on electron transport through single-electron spin-valve transistors

“…Measurements were done on systems with nonmagnetic islands contacted to ferromagnetic leads [1][2][3][4][5][6] as well as for all-ferromagnetic systems. [7][8][9] Much of the theoretical work focuses on F ͉ N ͉ F spin valves, in which the island is a normal metal ͑N͒ and the contacts are ferromagnets ͑F͒ with variable magnetization directions.…”

“…There is evidence from several experiments that the spin-flip times in metallic nanoparticles can be much longer than in bulk systems, which implies that the effects of a spin accumulation on the island should be taken into account. 4,6,9 For later convenience we introduce the spin-flip conductance parameter G sf ϵ N e 2 / ͑2 sf ͒. We assume that the energy relaxation on the island is fast compared to the electron dwell time.…”