17 research outputs found
Saturation of Spin-Polarized Current in Nanometer Scale Aluminum Grains
We describe measurements of spin-polarized tunnelling via discrete energy
levels of single Aluminum grains. In high resistance samples (),
the spin-polarized tunnelling current rapidly saturates as a function of the
bias voltage. This indicates that spin-polarized current is carried only via
the ground state and the few lowest in energy excited states of the grain. At
the saturation voltage, the spin-relaxation rate of the highest
excited states is comparable to the electron tunnelling rate: and in two samples. The ratio of
to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet
scaling, an evidence that spin-relaxation in Al grains is governed by the
spin-orbit interaction.Comment: 5 pages, 4 figure
Crossover from Kondo assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers
Recently, it has been shown that magnetic tunnel junctions with thin MgO
tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR)
values at room temperature1, 2. However, the physics of spin dependent
tunneling through MgO barriers is only beginning to be unravelled. Using planar
magnetic tunnel junctions in which ultra-thin layers of magnetic metals are
deposited in the middle of a MgO tunnel barrier here we demonstrate that the
TMR is strongly modified when these layers are discontinuous and composed of
small pancake shaped nanodots. At low temperatures, in the Coulomb blockade
regime, for layers less than ~1 nm thick, the conductance of the junction is
increased at low bias consistent with Kondo assisted tunneling. In the same
regime we observe a suppression of the TMR. For slightly thicker layers, and
correspondingly larger nanodots, the TMR is enhanced at low bias, consistent
with co-tunneling.Comment: Nano Letters (in press
Spin effects in single electron tunneling
An important consequence of the discovery of giant magnetoresistance in
metallic magnetic multilayers is a broad interest in spin dependent effects in
electronic transport through magnetic nanostructures. An example of such
systems are tunnel junctions -- single-barrier planar junctions or more complex
ones. In this review we present and discuss recent theoretical results on
electron and spin transport through ferromagnetic mesoscopic junctions
including two or more barriers. Such systems are also called ferromagnetic
single-electron transistors. We start from the situation when the central part
of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle.
Transport characteristics reveal then single-electron charging effects,
including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations.
Single-electron ferromagnetic transistors based on semiconductor quantum dots
and large molecules (especially carbon nanotubes) are also considered. The main
emphasis is placed on the spin effects due to spin-dependent tunnelling through
the barriers, which gives rise to spin accumulation and tunnel
magnetoresistance. Spin effects also occur in the current-voltage
characteristics, (differential) conductance, shot noise, and others. Transport
characteristics in the two limiting situations of weak and strong coupling are
of particular interest. In the former case we distinguish between the
sequential tunnelling and cotunneling regimes. In the strong coupling regime we
concentrate on the Kondo phenomenon, which in the case of transport through
quantum dots or molecules leads to an enhanced conductance and to a pronounced
zero-bias Kondo peak in the differential conductance.Comment: topical review (36 figures, 65 pages), to be published in J. Phys.:
Condens. Matte
Modeling of the current lines in discontinuous metal/insulator multilayers
Discontinuous magnetic metal/insulator multilayers are formed of equally
spaced layers of magnetic particles embedded in an insulating matrix. Their
electronic transport properties result from spin-polarized electron tunneling
and Coulomb blockade effect. The current-in-plane (CIP) and current-perpendicular-to plane (CPP) resistances change by several orders of
magnitude when the thicknesses of the metallic or insulating layers are
varied. Calculations of the shape of the current lines in these multilayers
are presented. It is shown that pure CIP or CPP transport occur in these
systems only when the CIP or CPP resistances are very different in magnitude.
If the two resistances are of the same order of magnitude, then the measured
transport properties in both geometries are a combination of CIP and CPP
transport
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Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers.
Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices