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...

We study electron transport through single-electron spin-valve transistors in the presence of nonlocal exchange between the ferromagnetic leads and the central normal-metal island. The Coulomb interaction is described with the "orthodox model" for Coulomb blockade and we allow for noncollinear lead magnetization directions. Two distinct exchange mechanisms that have been discussed in the literature are shown to be of comparable strength and are taken into account on equal footing. We present results for the linear conductance as a function of gate voltage and magnetic configuration, and discuss the response of the system to applied magnetic fields.

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We propose to accelerate reversal of the ferromagnetic order parameter in spin valves by electronic noise. By solving the stochastic equations of motion we show that the current-induced magnetization switching time is drastically reduced by a modest level of externally generated current (voltage) noise. This also leads to a significantly lower power consumption for the switching process. DOI: 10.1103/PhysRevLett.96.127203 PACS numbers: 75.60.Jk, 72.70.+m, 75.75.+a The dynamics of the ferromagnetic order parameter persists to pose a challenging problem of fundamental and applied nature [1]. With increasing bit density of mass data storage devices and emergence of the magnetic random access memory (MRAM) concept, the speed and energy dissipation of the magnetization switching process have become important issues. In the present MRAM generation, magnetic bits are written by spatially extended Ørsted magnetic fields, which sets limits to bit size and power consumption. An attractive alternative method is the current-induced magnetization switching predicted by theoreticians [2,3] and confirmed experimentally in nanopillar devices [4 -6]. In these spin valves, which consist basically of an electrically connected ferromagnetic-normal-ferromagnetic metals sandwich, the electric current is polarized in a ''fixed'' layer of high magnetic coercivity and exerts a ''spin-transfer torque'' on the second magnetically soft ferromagnet as sketched in Fig. 1. Recently, time-resolved measurements of the current-induced magnetization dynamics have been reported [7]. Advanced theoretical models for the spin and charge transport in magnetic devices [8][9][10] lead to a reasonable description of the magnetization dynamics within the macro-spin model, in which the magnetization is assumed to move rigidly under external magnetic field and spin-transfer torques [11,12]. We should note that in larger devices evidence has been found for spin waves and more complicated excitations that require full micromagnetic simulations [13].The main obstacle that prevents wide application of current-induced switching is the high critical current needed to reverse the magnetization. There are proposals on how to reduce the critical current by sample design [12] and optimizing the switching process by a precessional switching strategy [14]. Since the spin-transfer torque vanishes for the collinear stable point of a spin valve, the switching time depends strongly on processes that induce a canting between the magnetizations such that the spintransfer torque starts to kick in. This happens, for example, by increasing temperature, and is the basic idea of the precharging strategy by Devolder et al. [15]. More advanced strategies used in conventional magnetization switching require pulse-shaped microwaves [16] and rely on precise knowledge of the magnetization dynamics with proper feedback.Electrical and magnetization noise usually degrades device and system performance and often efforts have to be undertaken to reduce it as much as possible. In nonline...

A method is presented for the reconstruction of both the background and signal in invariant-mass analyses for two-photon decays. The procedure does not make use of event mixing techniques and as such is based exclusively on an event-by-event analysis. Consequently, topological correlations of the event (e.g. jet structures) are automatically taken into account. By means of the decay process π 0 → γγ it will be demonstrated how the procedure allows for determination of the π 0 yield from the observed decay photons.