Artificial ordered pinning in superconductors

The study of flux pinning in type II superconductors has shown a rich phenomenology, which is interesting from both the fundamental point of view and for its technological applications. The dynamics of vortex-lattices is useful to study the general problem of interacting “particles” (such as colloids, charge density waves, etc) or elastic media moving on different types of pinning potentials. E-beam lithography and porous alumina techniques allow us to obtain ordered arrays of magnetic nanodots with sizes and distances comparable to the superconducting coherence length x and the magnetic penetration depth l. The geometry of the pinning potential landscapes induced with these arrays can be designed at will, and thus a variety of different problems can be studied: commensurability, lattice correlation lengths, order-disorder transitions, ratchets, etc [1].

Fig. 1: Periodic matching effects in a) rectangular pinning lattice, b) square pinning lattice.
For several years we have been studying the commensurability effects (observed in the magnetoresistance and in the critical current dependence with the applied field, see figure 1) induced by periodic arrays of magnetic nanodots in Nb thin films [2,3], and the different pinning mechanism involved (magnetic, proximity, structural) [4]. We have also investigated vortex-lattice directional motion guided by periodic potentials [5,6], and quasiperiodic potentials [7]. Work is currently being done on the ratchet effect where an asymmetric potential causes an alternating current to rectify and produce a DC voltage [8]. This voltage can be controlled by the amplitude of the current and can actually be forced to undergo a ratchet reversal where the sign of the DC voltage changes [9] (figure 2).

Fig. 2: a) Scanning electron microscope picture of an asymmetric pinning potential - a triangular array of Ni triangles. b) Ratchet effect in Nb film with array of Ni triangular pinning sites for different magnetic fields.

Another of our current ongoing projects involves studying the effects of disorder on the vortex lattice. We have been able to engineer controlled levels of disorder into the pinning potential and by varying it have seen novel effects on the pinning of the vortex lattice [10,11] (figure 3).

Fig. 3: Scanning electron microscope pictures of artificially disordered arrays. In (a) order can be seen in the sample (the yellow dots are a guide to the eye), whereas in (b) there is clustering and large empty areas.

[1] M. Vélez, J.I. Martín, J.E. Villegas, c, A. Hoffmann, E.M. González, J.L. Vicent, Ivan K. Schuller, Journal of Magnetism and Magnetic Materials 320(21), 2547 (2008).

[2] J. I. Martín, M. Vélez, J. Nogués, and Ivan K. Schuller, Phys. Rev. Lett. 79, 1929 (1997).

[3] O. M. Stoll, M. I. Montero, J. Guimpel, Johan J. Åkerman, and Ivan K. Schuller Phys. Rev. B 65, 104518 (2002).

[4] M. I. Montero, Johan J. Åkerman , A. Varilci and Ivan K. Schuller, Europhys. Lett. 63, 118 (2003).

[5] J. E. Villegas, E. M. González, M. I. Montero, I. K. Schuller, J. L. Vicent, Phys. Review B 68, 224504 (2003).

[6] J. E. Villegas, E. M. Gonzalez, M. I. Montero, Ivan K. Schuller, and J. L. Vicent , Phys. Review B 72, 064507 (2005).

[7] J. E. Villegas, M.I. Montero, C.-P. Li and Ivan K. Schuller, Phys. Rev. Lett. 97, 027002 (2006).

[8] 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 (2009).

[9] D. Perez de Lara, M. Erekhinsky, E. M. Gonzalez, Y. J. Rosen, Ivan K. Schuller, and J. L. Vicent, Phys. Rev. B 83, 174507 (2011).

[10] Y. J. Rosen, A. Sharoni, and Ivan K. Schuller, Phys. Rev. B 82, 014509 (2010).

[11] C. Chiliotte, G. Pasquini, V. Bekeris, J. E. Villegas, C.-P. Li, and Ivan K. Schuller, Superconductor Science & Technology 24(6), 065008 (2011).

(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig