We fabricated a device that controls the motion of flux quanta in a niobium superconducting film grown on an array of nanoscale triangular pinning potentials. The controllable rectification of the vortex motion is due to the asymmetry of the fabricated magnetic pinning centers. The reversal in the direction of the vortex flow is explained by the interaction between the vortices trapped on the magnetic nanostructures and the interstitial vortices. The applied magnetic field and input current strength can tune both the polarity and magnitude of the rectified vortex flow. Our ratchet system is explained and modeled theoretically, taking the interactions between particles into consideration.

A generic electromagnetic model for the working principles of triboelectric nanogenerators derived using Maxwell's equations, to a universally applicable framework.

We present tunneling experiments on Fe͑001͒/MgO͑20 Å͒/FeCo͑001͒ single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on ͑001͒-oriented Fe/amorphous-Al 2 O 3 /FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surfaceFe͑001͒ in this case-but depends on the actual electronic structure of the entire electrode/barrier system.

A vortex lattice ratchet effect has been investigated in Nb films grown on arrays of nanometric Ni triangles, which induce periodic asymmetric pinning potentials. The vortex lattice motion yields a net dc voltage when an ac driving current is applied to the sample and the vortex lattice moves through the field of asymmetric potentials. This ratchet effect is studied taking into account the array geometry, the temperature, the number of vortices per unit cell of the array, and the applied ac currents. DOI: 10.1103/PhysRevB.71.024519 PACS number͑s͒: 74.78.Ϫw, 05.60.Ϫk, 74.40.ϩk Feynman used, in his Lectures on Physics, 1 a ratchet to show how anisotropy never could lead to net motion in an equilibrium system. Since then, asymmetric sawtooth potentials are called ratchet potentials and, in general, a device with broken inversion symmetry is called a ratchet device. The ratchet effect occurs when asymmetric potentials induce outward particle flow under external fluctuations in the lack of any driving direct outward forces. The ratchet effect changes an ac source in a dc one. Ratchet effect spans from Nature phenomena to laboratory fabricated devices. In a ratchet, the energy necessary for net motion is provided by raising and lowering the barriers and wells, either via an external time-dependent modulation, for example an ac current injected in a superconducting film with asymmetric pinning centers, 2 or by energy input from a nonequilibrium source, such as a chemical reaction, as for instance in biological motors. 3 During the past years, ratchet effect has called the attention of many researchers. A state of the art on the related topics Brownian motion and ratchet potential could be found in Ref. 4.The use of ratchetlike pinning potentials in superconductors has been the subject of theoretical approaches which deal with very different topics, for instance, to remove flux trapped in superconducting devices, 5 fluxon optic, 6 logic devices, 7 etc. From the experimental point of view, some progress has been reported related to superconducting circuits, [8][9][10] and very recently vortex motion ratchet effect has been reported in superconducting films with artificially fabricated arrays of asymmetric pinning centers. 2 In the present paper, we will address some of the properties of this superconducting ratchet effect. We will explore the dependence of the ratchet with the applied alternating current, the array shape, the temperature, and the number of vortices per array unit cell. We will show that periodic asymmetric potentials are crucial to produce the ratchet behavior, that the effect is enhanced decreasing the temperature, and finally, that the effect decreases when the applied magnetic field ͑number of vortices per unit cell of the array͒ increases. The paper is organized as follows: First, we will summarize some results on the behavior of vortex lattice on artificially induced pinning potentials. After this, we will present the fabrication method and main characteristics of the films. Finally, the experim...

Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensive research, particularly after recent demonstration that the vortex polarity can be controlled by in-plane magnetic field. This has stimulated the proposals of nonvolatile vortex magnetic random access memories. In this work, we demonstrate that triangular nanodots offer a real alternative where vortex chirality, in addition to polarity, can be controlled. In the static regime, we show that vortex chirality can be tailored by applying in-plane magnetic field, which is experimentally imaged by means of variable-field magnetic force microscopy. In addition, the polarity can be also controlled by applying a suitable out-of-plane magnetic field component. The experiment and simulations show that to control the vortex polarity, the out-of-plane field component, in this particular case, should be higher than the in-plane nucleation field. Micromagnetic simulations in the dynamical regime show that the magnetic vortex polarity can be changed with short-duration magnetic field pulses, while longer pulses change the vortex chirality.

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple phi(4) model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.

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Keywords: nanomaterials, carbon nanotubes, photothermal chemical vapor deposition, low temperature growth 2 For carbon nanotubes (CNTs) to be exploited in electronic applications the growth of high quality material on conductive substrates at low temperatures (< 450 o C) is required. CNT quality is known to be strongly degraded when growth is conducted on metallic surface at low temperatures using conventional chemical vapor deposition (CVD). Here, we demonstrate production of high quality vertically-aligned CNTs at low substrate temperatures (350 -440 o C) on conductive TiN thin film using photo-thermal CVD by confining the heat required for growth to just the catalyst using an array of optical lamps and by optimizing the thickness of the TiN under-layer. The thickness of the TiN plays a crucial role in determining various properties including diameter, material quality, number of shells and metallicity. The highest structural quality with a visible Raman D-to G-band intensity ratio as low as 0.13 is achieved for 100 nm TiN thickness grown at 420 o C; a record low value for low temperature CVD grown CNTs. Electrical measurements of high density CNT arrays show the resistivity to be 1.25×10 -2 Ωcm represents some of the lowest values reported. Finally, the broader aspects of using this approach as a scalable technology for carbon nanomaterial production are also discussed.3