Magnetic pinning of flux lattice in superconducting-nanomagnet hybrids

Abstract

Strong superconducting pinning effects are observed from magnetic landscapes produced by arrays of circular rings with varying magnetic remanent states. The collective and the background pinning of superconducting Nb films is strongly enhanced by the stray magnetic field produced by an array of circular Ni rings magnetized to form "onion" (bidomain) states. On the other hand, when the same rings are magnetized into vortex (flux-closed) states, or are randomly magnetized, the superconducting pinning is much smaller. The greatest pinning is produced when the superconducting vortex lattice motion is along a direction in which there is a strong magnetic field variation. Vortices occur in a wide range of quantum fluids. 1,2 Due to their mutual interactions, superconducting vortices order into regular vortex lattices in defect-free materials. The structure and dynamics of the vortex lattice is significantly affected by interactions with intrinsic and extrinsic artificially prepared pinning centers. The motion of the normal cores of superconducting vortices produces dissipation. This plays a key role in superconductor properties such as fluxflow resistance, critical currents, magnetization, and magnetic susceptibility. These properties are important ingredients in superconductor based applications.The effect of magnetic pinning centers has been investigated using a variety of techniques, 3 and the influence of magnetic domains and domain walls on various superconducting properties has been reported. [4][5][6][7] In spite of this, no clear-cut experimental evidence has emerged to prove that magnetic interaction between the superconducting vortices and magnetic pinning centers plays a determining role. We show here, by manipulating the magnetic state of a pinning array of magnetic nanorings, the magnetotransport of superconducting films can be controlled and changed substantially. Previously, 8 we studied ratchet effects in similar samples to investigate the role played by magnetic potentials with broken reflection symmetry on the vortex lattice dynamics.In this work, we have studied the magnetism and magnetotransport of hybrid superconducting Nb thin films grown on top of square-symmetry arrays of circular magnetic Ni rings [see Figure 1(a) for dimensions]. The Ni rings were prepared by electron-beam lithography on (100) Si substrates as described elsewhere. 9 The 20 nm thick Ni film was deposited by ion-beam sputtering while the 100 nm Nb was magnetron-sputtered. Nb electrical contacts were patterned to form a 40 lm  40 lm bridge.The magnetic hysteresis of the rings was measured using alternating gradient force magnetometry. A similar array of Ni rings without a Nb layer was studied by atomic force microscopy [AFM, Fig. 1(a)], magnetic force microscopy [MFM, Fig. 1(b)], and magneto-optical Kerr effect (MOKE, Fig. 2) using the first-order reversal curve (FORC) method. 10 The 30 lm diameter MOKE laser beam probed approximately 10 3 rings simultaneously. Micromagnetic simulations were performed using the 2D OOMMF code 11...

Reconfigurable Pinwheel Artificial-Spin-Ice and Superconductor Hybrid Device

“…Vortices can be trapped by nanoscale defects introduced by ion irradiation, , grain boundaries, , nanostrained regions, and patterned nanostructures via advanced nanofabrication. − However, in the aforementioned approach, the spatial vortex pinning potential landscapes produced by defects are fixed, once the samples are synthesized. In contrast, superconducting vortices can also interact with magnetic nanostructures. − The controlled magnetization of magnetic nanostructures with an applied magnetic field allows in situ tuning of the vortex pinning potential, enabling reconfigurable superconducting functionalities. − Recently, a particular type of magnetic nanostructure, called artificial spin ice (ASI), was used as nanoscale reconfigurable magnetic landscapes to modulate the behavior of vortices in a hybrid ferromagnetic/superconducting structure. , The highly tunable and wide spectrum of magnetic configurations of ASI structures allow the tuning of vortex motion and superconducting transport properties in a more flexible way. For example, a reprogrammable vortex diode has been recently realized by patterning a triaxial ASI on top of a superconducting film, and a reversible vortex ratchet effect was demonstrated by using an artificial kagome spin ice .…”

“…In both cases, the I c ( B ) curves show clear peaks/kinks at the magnetic fields of B 0 and 0.5 B 0 . These are typical features of vortex-matching effects induced by dynamical matching of an ordered vortex lattice to the regularly distributed pinning potentials. − To unveil the microscopic distribution and dynamics of the vortices, we conduct molecular dynamics (MD) simulations (details on the method are described in ref ). The simulations (Figure S1) show ordered vortices at the magnetic fields of B 0 and 0.5 B 0 .…”

Control of dissipation in superconducting films by magnetic stray fields

“…Aladyshkin et al (1), Perez et al (3), and Van Bael et al (4,5) have emphasized the role stray field plays in determining the behavior of the superconductor. Furthermore, periodic arrays of nanoelements embedded in superconductors have been shown to significantly alter their intrinsic properties, for instance, changing the dynamic phases of the vortex matter (6,7) or vortex channeling and commensurability effects (8 -13).…”

Reversible ratchet effects for vortices in conformal pinning arrays

“…Ratchet effects on asymmetric substrates have been extensively studied in colloidal systems [3][4][5] , granular matter 6,7 , and polymers 8,9 . Ratchet effects also appear in ac-driven vortices in type-II superconductors in the presence of an asymmetric substrate [10][11][12][13][14][15] , such as a quasi-one-dimensional periodic array produced by asymmetrically modulating the sample thickness 10,[16][17][18][19] , etching funnel-shaped channels for vortex flow 11,[20][21][22][23][24] , introducing asymmetry to the sample edges 25 , or adding periodic pinning arrays in which the individual pinning sites have some form of intrinsic asymmetry 13,14,[26][27][28][29][30][31][32][33][34][35] . At lower vortex densities when collective interactions between vortices are weak, the ratchet effect produces a dc flow of vortices in the easy flow direction of the asymmetric substrate; however, when collective effects are present it is possible to have reversals of the ratchet effect where for one set of parameters the vortices move in the easy direction while for another set of parameters they move in the hard direction [13][14][15][16]…”