Science 302, 1188 (Nov. 14, 2003)

See also perspectives in Science 302, 1159, by H. Hilgenkamp, V. V. Moshchalkov, P. Kes;
and Highlights: Science 302, 1111
 

Reversible Rectifier that Controls the Motion of Magnetic Flux Quanta in Superconductors
 
 

J.E. Villegas (1), Sergey Savel’ev (2), Franco Nori (2,3), E.M. Gonzalez (1), J.V. Anguita (4), R. García (4), J.L. Vicent(1)

1 Departamento de Física de Materiales, Facultad Ciencias Físicas, Universidad Complutense, 28040, Madrid, Spain.
 2 Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama, 351–0198, Japan.
 3 Center for Theoretical Physics, Department of Physics, University of Michigan, Ann Arbor, MI, 48109–1120, USA.
 4 Instituto de Microelectrónica de Madrid, Centro Nacional Mícroelectrónica, Consejo Superior de Investigaciones Científicas, Isaac Newton 8, Tres Cantos, 28760
 Madrid, Spain.



Abstract

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 considering the interactions between particles.



      
Schematics of vortex motion in nano-magnetic triangles.


Introduction

Motor proteins play a key role in the transport of materials at the cellular level. These biological motors are anisotropic devices that, driven by non-equilibrium fluctuations, bias the motion of particles, and are inspiring a new generation of solid-state devices that open new avenues for controlling the motion of, for example, electrons, colloidal particles, and magnetic flux quanta. We consider superconducting devices with anisotropic pinning, where the dc transport of magnetic flux quanta may be driven by an ac or unbiased current. The control of the motion of vortices using asymmetric pinning can be useful for applications in superconductivity, including field-dependent reversible vortex diodes and the removal of unwanted trapped flux in devices. Several different ways of using asymmetric pinning in superconductors to control vortex motion have been recently proposed. However experiments in this area have been difficult to control.

Our device allows a versatile control of the motion of vortices in superconducting films. Simple modifications and extensions of it would allow the pile up (magnetic “lensing”), shaping, or “sculpting” of micro-magnetic profiles inside superconductors. Thus, vortex lenses made of oppositely-oriented triangles would provide a strong local increase of the vortex density at its “focus” regions. Thus, extensions of these type of systems should open new avenues for motion control of flux quanta and provide a first step towards a new generation of devices based on the flow of magnetic flux quanta.

Importance

As integrated circuits become smaller and smaller, it becomes increasingly difficult to create the many "guiding channels" that act like wires to move electrons around the circuit components.

This difficulty in wiring nano-circuits must be overcome if researchers are to continue developing the microscopic machines and sensors that represent the wave of the future in nanotechnology. A similar problem exists for researchers developing better magnetic imaging tools for medical diagnostics. Here the goal is to control the motion of magnetic field lines within the superconducting material, so that their motion does not produce noise that degrades the performance of the diagnostic device. A new approach and several novel devices described in a recent article in Nature Materials offer hope that the noise challenge has been overcome.

Magnetic fields penetrate superconducting materials via lattices of quantized magnetic flux, called vortices because electrons whirl around them without dissipating energy. Electrical currents, externally applied to superconducting devices, induce the motion of these magnetic flux quanta. This vortex motion produces noise that degrades the device performance in practical applications, such as the sensitive measuring of the magnetic fields produced by the brain. Therefore, the precise control of the motion of these vortices is of central importance for applications involving superconducting materials.

By controlling the motion of quanta inside superconducting materials, the new devices allow the design of micro-machines such as "pumps," "diodes" and "lenses" of magnetic flux quanta to create specific magnetic profiles within a given sample or device. This would give designers the ability to remove unwanted flux trapped inside superconducting devices and enable researchers to increase the magnetic field in designated target regions inside materials, which would "magnetically focus" nearby magnetic particles.

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