Noncollinear single-electron spin-valve transistors

Abstract

We study interaction effects on transport through a small metallic cluster connected to two ferromagnetic leads ͑a single-electron spin-valve transistor͒ in the "orthodox model" for the Coulomb blockade. The nonlocal exchange between the spin accumulation on the island and the ferromagnetic leads is shown to affect the transport properties such as the electric current and spin-transfer torque as a function of the magnetic configuration, gate voltage, and applied magnetic field. DOI: 10.1103/PhysRevB.72.020407 PACS number͑s͒: 85.75.Ϫd, 72.25.Mk, 73.23.Hk Magnetoelectronics is a contender to fulfill the technological need for faster and smaller memory and sensing devices. The drive into the nanometer regime brings about an increasing importance of electron-electron interaction effects. Small metallic clusters ͑islands͒ that are electrically contacted to metallic leads by tunnel junctions and capacitively coupled to a gate electrode can behave as "singleelectron transistors ͑SETs͒." In the Coulomb-blockade regime, the charging energy needed to change the electron number on the island by one exceeds the thermal energy, and transport can be controlled on the level of the elementary charge. 1 In a spin-valve SET ͑SV-SET͒, the contacts to the cluster consist of ferromagnetic metals ͑F͒. We focus here on F͉N͉F structures with normal-metal ͑N͒ islands ͓see Fig. 1͑a͔͒, 2 since these "spin valves" display giant magnetoresistance and spin-current-induced magnetization reversal. 3 Other combinations such as F͉F͉F ͑Ref. 4͒, N͉N͉F ͑Ref. 5͒, or F͉F͉N ͑Refs. 6 and 7͒ are of interest as well.Several theoretical studies have been devoted to the binary magnetoresistance ͑MR͒ of SV-SETs, i.e., the difference in the electric resistance between parallel and antiparallel configurations of the magnetization directions. 8,9 Interaction effects in magnetic devices have been studied as well for spin valves with a Luttinger liquid island 10 and for single-level quantum dots 11-13 with noncollinear magnetic configurations.A necessary condition for a significant MR in F͉N͉F structures is a spin accumulation on the normal-metal island, viz. a sufficiently long spin-flip relaxation time sf . Seneor et al. 14 have measured the MR of SV-SETs with gold islands with a sf ϳ 800 ps. A sf in the microsecond regime has been reported for Co nanoclusters. 6 The long spin-flip times in small clusters is not yet fully understood; it might simply be due to the probability of finding zero impurities in a given small cluster.In this Rapid Communication we discuss aspects of electron transport in metallic SV-SETS with noncollinear magnetization directions. It turns out that an effective exchange effect between the spin accumulation and the magnetizations has to be taken into account.We take the junction resistances sufficiently larger than the quantum resistance R Q = h / e 2 so that the Coulomb blockade can be treated by lowest-order perturbation theory. We furthermore disregard the size quantization of states in the clusters, thus adopting the well-esta...

Probing Spin Accumulation in Ni/Au/Ni Single-Electron Transistors with Efficient Spin Injection and Detection Electrodes

“…Given the uncertainty in the interpretation of our TMR signal, we emphasize that this estimate is only an upper bound for RE τ . Nevertheless this conclusion is still significant and complementary to other recent magnetotransport measurements on F-SETs with smaller Au nanoparticles, which report a RE τ of the order of 1 ns 29,31 . It should also be noted that our deduced spin-relaxation time is much larger than the spin-orbit scattering time SO τ estimated in Au nanoparticles of comparable size by investigating the individual g-factors of the non-interacting electron states 32 .…”

Spin effects in single-electron tunnelling

“…Most of the works concerned theoretical description of spin-polarized transport in the weak coupling regime, as well as in the strong coupling regime, where the Kondo physics emerges [137,138,139,140,141,142,143,144,145]. Sequential transport through a single-level quantum dot coupled to ferromagnetic leads was studied for both collinear [146,147] and non-collinear [148,149,150,151,152,153] configurations of the electrodes' magnetic moments. Spin-polarized transport in the cotunneling regime has also been addressed for collinear systems [154,155,156,157], as well as for systems magnetized non-collinearly [158,159,160,161].…”

Magnetoresistance of a quantum dot with spin-active interfaces

“…The problem of the effects of interactions on the transport properties of a central region connected to ferromagnetic contacts has already been considered in various regimes, like e.g. the Coulomb blockade regime 6,21,22,23 , the Kondo regime 24,25 , the Luttinger liquid regime 5,26 and the marginal Fermi liquid regime 27 . This article develops an approach suitable for the limit of Ref.…”

“…This Spin-Dependence of Interfacial Phase Shifts (SDIPS) can modify significantly the behavior of mesoscopic circuits. First, when a mesoscopic conductor is connected to several F leads with non collinear polarizations, the SDIPS produces an interfacial precession of spins which can modify current transport in the device 4,5,6,7,8 . Secondly, in collinear configurations, precession effects are not relevant, but the SDIPS can modify mesoscopic coherence effects.…”