Yaniv Rosen

Highly effective superconducting vortex pinning in conformal crystals

We have investigated the vortex dynamics in superconducting thin film devices with non-uniform patterns of artificial pinning centers (APCs). The magneto-transport properties of a conformal crystal and a randomly diluted APC pattern are compared with that of a triangular reference lattice. We have found that in both cases the magneto-resistance below the first matching field of the triangular reference lattice is significantly reduced. For the conformal crystal, the magneto-resistance is below the noise floor indicating highly effective vortex pinning over a wide magnetic field range. Further, we have discovered that for asymmetric patterns the R vs. H curves are mostly symmetric. This implies that the enhanced vortex pinning is due to the commensurability with a stripe in the non-uniform APC pattern and not due to a rearrangement and compression of the whole vortex lattice.

Interaction-induced anisotropy in the onion-to-vortex transition in dense ferromagnetic nano-ring arrays

We show that the onion-to-vortex switching field in dense arrays of nanostructured ferromagnetic rings is strongly dependent on the angle between the applied magnetic field and the arrays main axis. The variations in switching field of up to 8 mT are connected to the anisotropy produced by dipolar interactions between domain walls in the rings. The interactions stabilize the onion state in aligned arrays but assist domain wall rotation and onion-to-vortex switching in rotated arrays. These results are established using magneto optical Kerr effect measurements of major and minor hysteresis loops together with micromagnetic simulations.

Superconducting heterostructures: from antipinning to pinning potentials

We study vortex lattice dynamics in a heterostructure that combines two type-II superconductors: a niobium film and a dense triangular array of submicrometric vanadium (V) pillars. Magnetic ac susceptibility measurements reveal a sudden increase in ac penetration, related to an increase in vortex mobility above a magnetic field, , that decreases linearly with temperature. Additionally, temperature independent matching effects that occur when the number of vortices in the sample is an integer of the number of V pillars, strongly reduce vortex mobility, and were observed for the first and second matching fields, and . The angular dependence of , and shows that matching is determined by the normal applied field component, while is independent of the applied field orientation. This important result identifies with the critical field boundary for the normal to superconducting transition of V pillars. Below , superconducting V pillars repel vortices, and the array becomes an ?antipinning? landscape that is more effective in reducing vortex mobility than the ?pinning? landscape of the normal V sites above . Matching effects are observed both below and above , implying the presence of ordered vortex configurations for ?antipinning? or ?pinning? arrays.

Proximity effects and vortex dynamics in superconducting heterostructures

Vortex lattice dynamics has been studied in thin Nb superconducting films sputtered on top of a dense triangular array of V dots with and without an intermediate SiO2 insulating layer. While the insulating layer modifies only slightly the Nb film corrugation, it reduces superconducting commensurability effects (CE) substantially. This implies that superconducting commensurability is dominated by proximity effects. Moreover, the HC2 (T) phase diagram of the sample without an insulating layer shows a parabolic temperature dependence near TC and critical temperature oscillations with the periodicity of the matching field. Therefore, strong proximity effects locally suppress superconductivity leading to a superconducting mesh. When the proximity effect is decreased by an insulating layer, HC2(T) follows the expected linear T dependence.

Erratum: Vortex ratchet reversal: role of interstitial vortices [Phys. Rev. B 83, 174507 (2011)]

We have discovered a calibration error in the measurements performed at UCSD and presented in Fig. 1. The 1 micrometer bar shown in Fig. 1 corresponds to 1.16 μm. This implies that the minima shown in the magnetoresistance curves correspond well to the matching fields. This means that static interstitial vortices cannot be inferred from these minima. Consequently, the following parts of the text should be corrected: (a) The second sentence of the Abstract, which reads “Collective pinning with a vortex-lattice configuration different from the expected fundamental triangular ‘Abrikosov state’ is found,” should be deleted. (b) Page 174507-2, first column, lines 3–5 should have “sides close to 600 nm” changed to “700 ± 25 nm” and “periodicity of around 700 nm” changed to “810 ± 30 nm.” (c) Page 174507-2, second column, lines 13 (starting with “However, . . .”) to 31 (ending with “. . . . significant”) are incorrect and should be changed to the following: “The matching fields predicted from the geometry of Fig. 1, 36 ± 3 Oe, are in excellent agreement with measurements of minima in Fig. 2. This implies that the density of the vortex lattice matches the density of the pinning sites. Therefore, the system is exactly at the matching condition.” (d) Page 174507-03, first column, lines 7 (starting with “Therefore. . .”) to 15 (ending with “. . .dc ratchet”) should be replaced by “For the higher driving force, the signal of the dc voltage is reversed to positive values. In previous papers,4 the existence of this ratchet reversal was associated with the presence of interstitial vortices.” (e) Page 174507-4, second column, after line 14, add the following paragraph: “One possible explanation is that, although interstitial vortices do not exist statically, they are produced in the triangular array of triangles as follows. At the first appearance of the ratchet minimum (see Fig. 3), i.e., the negative Vdc, the vortices are driven out of their static pinning sites into an interstitial position. Once they are located there, the ac drive is not large enough to move them back into the asymmetric pinning sites. At this stage, they are forced to move along the interstitial lattice, which forms triangles pointing in the opposite direction of those in the pinning site lattice. Once the driving force becomes big enough to drive them back into the asymmetric pinning sites, the ratchet reverses sign. The ratchet reversal is still connected to the appearance of interstitial vortices. Note, however, that the detailed mechanism, present in the square array of triangles (Ref. 4), is different. In that case, the geometry forces the vortices to move from strong pinning site to strong pinning site. Thus, the ratchet reversal appears at a filling factor of 4 where interstitial vortices are produced statically.”