We have fabricated nanometer sized magnetic tunnel junctions using a new nanoindentation technique in order to study the transport properties of a single metallic nanoparticle. Coulomb blockade effects show clear evidence for single electron tunneling through a single 2.5 nm Au cluster. The observed magnetoresistance is the signature of spin conservation during the transport process through a non magnetic cluster.PACS numbers: 85.75.-d, 75.47.-m, 73.63.-b Spintronics debuted with the discovery of giant magnetoresistance[1] effect in magnetic multilayers in which a single dimension was reduced to the nanometer range. This field was then extended to structures with two reduced dimensions like nanowires[2] and nanopillars [3,4] or nanotubes [5]. Today, a challenge for spintronics is the study of spin transport properties in structures based on 0D elements in which the three dimensions have been reduced. In particular, we have in mind systems in which the reduction of the size leads to both Coulomb blockade and spin accumulation effects [6,7,8]. Transport studies on systems including mesoscopic islands [9,10,11] or granular films [12,13,14] have paved the way to understanding the effect of confinement on charge and spin transport properties in metallic nano-objects. However, so far, very few techniques allow to contact a single isolated nanometer sized object [15,16,17] to study the effect of confinement on spin transport.In this letter we present the experimental achievement of a new technique allowing us to inject and detect spins in a single isolated nanometer sized cluster. We then obtain information on both spin and single electron transport in the nanoparticle. In this technique, a ferromagnetic nanocontact with a cross section of ∼ 5 − 10 nm in diameter is created on a bilayer associating a cobalt layer and an ultrathin alumina layer in which a 2D assembly of gold nanoparticles is embedded (see Fig 1 for a sketch). As we will show, this structure allows the tunneling of electrons into and out of a single Au nanoparticle.The whole structure is elaborated in a sputtering system (base pressure 5 × 10 −8 mbar) with Ar gas at a dynamic pressure of 2.5 × 10 −3 mbar. The deposition of a bilayer of Co(15nm)/Al(0.6nm) is followed by the oxidization of the Al layer in pure O 2 (50 mbar for 10 min) * Now at Laboratorio de Física de Sistemas Pequeños y Nanotecnología, Consejo Superior de Investigaciones Científicas, Serrano 144, 28006 Madrid, Spain † Also at Université d'Evry, Bat. des Sciences, rue du pere Jarlan, 91025 Evry, France to form the first tunnel barrier. Then, an ultrathin layer of Au (0.2 nm nominal thickness) is deposited on top of the bilayer. The 3D growth (see [18]) of the sputtered gold on top of alumina produces a self-formed nanoparticles layer. A plane view transmission electron microscopy (TEM) picture of the Au nanoparticles 2D self-assembly is shown in Figure 2. The size distribution of the Au nanoparticles is characterized by a 2 nm diameter mean value and a 0.5 nm standard deviation with a d...

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