Rocking ratchet induced by pure magnetic potentials with broken reflection symmetry

D. Perez de Lara, F. J. Castaño, B. G. Ng, H. S. Korner, R. K. Dumas, E. M. Gonzalez, Kai Liu, C. A. Ross, Ivan K. Schuller, and J. L. Vicent

Phys. Rev. B 80, 224510 – Published 15 December 2009

Abstract

A ratchet effect (the rectification of an ac injected current) which is purely magnetic in origin has been observed in a superconducting-magnetic nanostructure hybrid. The hybrid consists of a superconducting Nb film in contact with an array of nanoscale magnetic triangles, circular rings, or elliptical rings. The arrays were placed into well-defined remanent magnetic states by application of different magnetic field cycles. The stray fields from these remanent states provide a magnetic landscape which influences the motion of superconducting vortices. We examined both randomly varying landscapes from demagnetized samples and ordered landscapes from samples at remanence after saturation in which the magnetic rings form parallel onion states containing two domain walls. The ratchet effect is absent if the rings are in the demagnetized state or if the vortices propagate parallel to the magnetic reflection symmetry axis (perpendicular to the magnetic domain walls) in the ordered onion state. On the other hand, when the vortices move perpendicular to the magnetic reflection symmetry axis in the ordered onion state (parallel to the domain walls) a clear ratchet effect is observed. This behavior differs qualitatively from that observed in samples containing arrays of triangular Ni nanostructures, which show a ratchet of structural origin.

Received 3 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.224510

Authors & Affiliations

D. Perez de Lara¹, F. J. Castaño², B. G. Ng², H. S. Korner², R. K. Dumas³, E. M. Gonzalez¹, Kai Liu³, C. A. Ross², Ivan K. Schuller⁴, and J. L. Vicent¹

¹Departamento de Fisica de Materiales, Universidad Complutense, 28040 Madrid, Spain
²Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
³Physics Department, University of California, Davis, California 95616, USA
⁴Physics Department, University of California–San Diego, La Jolla, California 92093, USA

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Vol. 80, Iss. 22 — 1 December 2009

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Images

Figure 1
(Color online) Optical image of the cross-shaped bridge $(40 μ m \times 40 μ m)$ and electron micrographs of the arrays of elliptical and circular rings at higher magnifications.Reuse & Permissions
Figure 2
(Color online) Families of FORCs for an array of elliptical Ni rings (sample A in Table ) where the applied field is (a) parallel and (b) perpendicular to the long axis (major axis) of the ellipses. (c) FORCs measured on an array of circular Ni rings (sample B in Table ). The insets show a sketch of the onion magnetic states and the asymmetric magnetic reflection axis (double arrows).Reuse & Permissions
Figure 3
(Color online) [(a), (b), and (c)] OOMMF simulation of the remanent state of (a) a circular ring after 5000 Oe saturation, (b) an elliptical ring after 5000 Oe saturation along the long axis, and (c) an elliptical ring after 5000 Oe saturation along the short axis; [(d), (e), and (f)] topographic and [(g), (h), and (i)] magnetic remanent images of (d) and (g) the circular ring array at 65 nm lift height; [(e) and (h)] the elliptical ring array at 50 nm lift height, long axis field; [(f) and (i)] the elliptical array at 65 nm lift height, short axis field. Images are $6 μ m$ square. The white dotted lines indicate the locations of some of the rings in the MFM images.Reuse & Permissions
Figure 4
(Color online) Magnetoresistance of hybrid Nb film (100 nm thickness) on top of arrays of Ni triangles (full triangles), elliptical rings (solid dots, sample C, see Table ), and circular rings (open dots, sample D) measured with the magnetic nanostructure arrays initially in the demagnetized state. The temperature $T / T_{c}$ was 0.99. Current densities: $J = 8.0 \times 10^{2} A {cm}^{- 2}$ (triangle sample); $J = 2.5 \times 10^{3} A {cm}^{- 2}$ (elliptical rings); $J = 6.0 \times 10^{3} A {cm}^{- 2}$ (circular rings). The data for the triangles has been scaled by a factor of 2 for clarity.Reuse & Permissions
Figure 5
(Color online) Measurements of vortex ratchet effects in Nb/Ni hybrids with triangular and elliptical ring Ni nanomagnet arrays. The Ni triangles were in the demagnetized state. The Ni rings were measured in both the demagnetized state (open circles) and in parallel onion states after saturation along the long axis (solid circles). The vortex-lattice motion corresponds to translation from the triangle base to the apex in the array of triangles and along the major axis in the case of the array of elliptical rings. The frequency of the injected ac is 10 kHz and $T / T_{c} = 0.98$ . The inset shows ratchet reversal in the array of Ni triangles obtained with increasing ac drive current.Reuse & Permissions
Figure 6
(Color online) Experimental data showing the ratchet effect for samples with elliptical and circular Ni rings for different vortex motion directions and temperatures. In all cases the rings were magnetized into parallel onion states. The vortex motion is along either the major axis (LA) or minor axis (SA) in the case of the elliptical rings. All the measurements were made with the vortex motion along the direction of the DW, parallel to the initial in-plane saturating field, except for the data shown with solid circles in which there is no ratchet effect. The measurement temperatures were $0.98 T_{c}$ (solid triangles and solid diamonds) and $0.97 T_{c}$ (open triangles and open diamonds). $F_{L}$ is the Lorentz force on the vortices and $⟨ v ⟩$ their average velocity.Reuse & Permissions

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