D. Perez de Lara

Control of the chirality and polarity of magnetic vortices in triangular nanodots

Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensive research, particularly after recent demonstration that the vortex polarity can be controlled by in-plane magnetic field. This has stimulated the proposals of nonvolatile vortex magnetic random access memories. In this work, we demonstrate that triangular nanodots offer a real alternative where vortex chirality, in addition to polarity, can be controlled. In the static regime, we show that vortex chirality can be tailored by applying in-plane magnetic field, which is experimentally imaged by means of variable-field magnetic force microscopy. In addition, the polarity can be also controlled by applying a suitable out-of-plane magnetic field component. The experiment and simulations show that to control the vortex polarity, the out- of-plane field component, in this particular case, should be higher than the in-plane nucleation field. Micromagnetic simulations in the dynamical regime show that the magnetic vortex polarity can be changed with short-duration magnetic field pulses, while longer pulses change the vortex chirality.

Transverse ratchet effect and superconducting vortices: simulation and experiment

A transverse ratchet effect has been measured in magnetic/ superconducting hybrid films fabricated by electron beam lithography and magnetron sputtering techniques. The samples are Nb films grown on top of an array of Ni nanotriangles. Injecting an ac current parallel to the triangle reflection symmetry axis yields an output dc voltage perpendicular to the current, due to a net motion of flux vortices in the superconductor. The effect is reproduced by numerical simulations of vortices as Langevin particles with realistic parameters. Simulations provide an intuitive picture of the ratchet mechanism, revealing the fundamental role played by the random intrinsic pinning of the superconductor.

Josephson device for simultaneous time and energy detection

The development of a detection device for simultaneous measurement of energy and impact time, to be used in time-of-flight mass spectrometry, is reported. In this device, two superconducting tunnel junctions are coupled through a passive network. The first junction operates in the quasiparticle regime in order to measure the energy of a molecule impact and to act as a proportional detector. The second one operates in the Josephson regime in order to act as a fast discriminator for the impact time of a molecule impinging on the detector junction. In this way, a very accurate time determination can be achieved limited only by the intrinsic detector response, thus improving the spectrometer mass resolution. To demonstrate the feasibility of this detection scheme in mass spectrometry, calibration measurements have been carried out using a 55Fe x-ray source to simulate the molecule impact. The experimental results successfully demonstrated simultaneous detection of energy and arrival time in coincidence with p...

Magnetic pinning of flux lattice in superconducting-nanomagnet hybrids

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.

Rocking ratchets in nanostructured superconducting?magnetic hybrids

Two rectification mechanisms in vortex lattice dynamics in Nb films have been studied. These two effects are based on ratchet effects, that is, an ac driving force induces a net dc vortex flow. In our case, an input ac current applied to the Nb films, grown on top of arrays of Ni nanotriangles, yields an output dc voltage. These two rectification effects occur when the vortex lattice moves in periodic asymmetric potentials. These pinning potentials are induced by the array of Ni triangles. In one configuration (longitudinal effect) the driven force is applied perpendicular to the triangle reflection symmetry axis; in the second one (transverse effect) the input current is injected parallel to the triangle reflection symmetry axis. In the framework of the rocking ratchet mechanism, the appropriate Langevin equation allows us to model the experimental data, taking into account the vortex-vortex interaction.

Rocking ratchet induced by pure magnetic potentials with broken reflection symmetry

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.

Enhancement of synchronized vortex lattice motion in hybrid magnetic/amorphous superconducting nanostructures

Superconducting a-Mo_(3)Si and Nb films have been grown on arrays of Ni nanodots. We have studied the vortex lattice dynamics close to critical temperatures. Different vortex lattice configurations are obtained with the same array unit cell. These different vortex lattices occur at matching conditions between the vortex lattice and the array unit cell. The interplay between the random intrinsic pinning of the superconducting films and the periodic pinning of the array govern the vortex lattice configurations. Different vortex lattice configurations and enhancement of synchronized vortex lattice motion are obtained by increasing the periodic pinning strength and decreasing the random pinning strength.

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.”

Fabrication and Properties of Longitudinal and Transverse Current Rectifier Devices Based on Superconducting Films With Arrays of Nanodefects

Superconducting rectifiers have been fabricated by electron beam lithography, sputtering and ion etching techniques. The rectifiers are made growing Nb films on top of arrays of submicrometric Ni triangles. In this device, in the superconducting mixed state, injecting an input AC current yields an output DC voltage. This effect is due to the motion of the vortex lattice on periodic asymmetric pinning potentials. The vortex lattice dynamics follows ratchet effect behavior. The device shows two ratchet effects: Longitudinal and transverse. In the longitudinal ratchet effect the DC signal polarity could be switched increasing the applied magnetic field strength. Otherwise, the sign of the transverse effect does not change increasing the applied magnetic field.

Injection-Detection Experiments in All Aluminum 1-D Imaging Spectrometers Based on Superconducting Tunnel Junctions

We report on a class of low temperature radiation detectors based on superconducting tunnel junctions (STJs) in which the incoming radiation is absorbed in a long superconducting strip while the readout operation occurs at the two ends of the strip, where two STJs are laterally positioned. These Distributed Read-Out Imaging Devices, or DROIDs, provide spectroscopy, 1-D imaging, single-photon sensitivity, and high quantum efficiency, all in one device. Typically these devices are realized by using Tantalum for the absorber strip and Aluminum for the two STJs. In this way the quasi-particles are created in the Tantalum and subsequently trapped in the Aluminum. As illustrated here, it is possible to fabricate a DROID using a single superconducting material. This choice gives up the trapping effect but has the advantage of eliminating the interface between different superconducting materials. Such a device combines the best quality STJs, large diffusion and lifetime values, with low energy gap for improved energy and position resolution. We report on measurements of current injection done on prototype devices, which demonstrates that STJs can serve as quasi-particle sinks and facilitate charge division in DROIDs. For sufficiently high tunneling rates, DROIDs based on a single material may be able to obtain performances comparable to DROIDs based on two materials.