Determination of the magnetic penetration depth in a superconducting Pb film

peer reviewedBy means of scanning Hall probe microscopy technique we accurately map the magnetic field pattern produced by Meissner screening currents in a thin superconducting Pb stripe. The obtained field profile allows us to quantitatively estimate the Pearl length Λ without the need of pre-calibrating the Hall sensor. This fact contrasts with the information acquired through the spatial field dependence of an individual flux quantum where the scanning height and the magnetic penetration depth combine in a single inseparable parameter. The derived London penetration depth λL coincides with the values previously reported for bulk Pb once the kinetic suppression of the order parameter is properly taken into account

Determination of the magnetic penetration depth in a superconducting Pb film

Under the terms of the Creative Commons Attribution (CC BY) license to their work.By means of scanning Hall probe microscopy technique, we accurately map the magnetic field pattern produced by Meissner screening currents in a thin superconducting Pb stripe. The obtained field profile allows us to quantitatively estimate the Pearl length ¿ without the need of pre-calibrating the Hall sensor. This fact contrasts with the information acquired through the spatial field dependence of an individual flux quantum where the scanning height and the magnetic penetration depth combine in a single inseparable parameter. The derived London penetration depth λL coincides with the values previously reported for bulk Pb once the kinetic suppression of the order parameter is properly taken into account.This work was partially supported by the Fonds de la Recherche Scientique-FNRS, the Methusalem Funding of the Flemish Government, the Fund for Scientic Research-Flanders (FWO-Vlaanderen), and the ARC Grant 13/18-08 for Concerted Research Actions, financed by the Wallonia-Brussels Federation. J.B. acknowledges support from FRS-FNRS (Research Fellowship). J.V.d.V. acknowledges support from FWO-Vl. The work of A.V.S. was partially supported by “Crédit de demurrage,” U.Lg.Peer Reviewe

Enhanced spin signal in nonlocal devices based on a ferromagnetic CoFeAl alloy

The Creative Commons Attribution 3.0 Unported License to their work.We systematically study the nonlocal spin signal in lateral spin valves based on CoFeAl injectors and detectors and compare the results with identically fabricated devices based on CoFe. The devices are fabricated by electron beam evaporation at room temperature. We observe a > 10-fold enhancement of the spin signal in the CoFeAl devices. We explain this increase as due to the formation of a highly spin-polarized Co2FeAl Heusler compound with large resistivity. These results suggest that Heusler compounds are promising candidates as spin polarized electrodes in lateral spin devices for future spintronic applications.We acknowledge the financial support from the Spanish Ministerio de Ciencia e Innovación, MICINN (MAT2010-18065, FIS2009-06671-E, and GICSERV program “Access to ICTS integrated nano- and microelectronics cleanroom”). J.V.d.V. acknowledges the support from FWO-VL.Peer Reviewe

Metastable states and hidden phase slips in nanobridge SQUIDs

We fabricated an asymmetric nanoscale SQUID consisting of one nanobridge weak link and one Dayem bridge weak link. The current phase relation of these particular weak links is characterized by multivaluedness and linearity. While the latter is responsible for a particular magnetic field dependence of the critical current (so-called vorticity diamonds), the former enables the possibility of different vorticity states (phase winding numbers) existing at one magnetic field value. In experiments the observed critical current value is stochastic in nature, does not necessarily coincide with the current associated with the lowest energy state and critically depends on the measurement conditions. In this work, we unravel the origin of the observed metastability as a result of the phase dynamics happening during the freezing process and while sweeping the current. Moreover, we employ special measurement protocols to prepare the desired vorticity state and identify the (hidden) phase slip dynamics ruling the detected state of these nanodevices. In order to gain insights into the dynamics of the condensate and, more specifically the hidden phase slips, we performed time-dependent Ginzburg-Landau simulations.Comment: 10 pages, 4 figures, 1 supplementary vide

Al3_3Sc thin films for advanced interconnect applications

Al$_x$Sc$_{1-x}$ thin films have been studied with compositions around Al$_3$Sc ($x = 0.75$) for potential interconnect metallization applications. As-deposited 25 nm films were x-ray amorphous but crystallized at 190{\deg}C with a recrystallization observed at 440{\deg}C. After annealing at 500{\deg}C, 24 nm thick stoichiometric Al$_3$Sc showed a resistivity of 12.6 ${\mu}{\Omega}$cm, limited by a combination of grain boundary and point defect (disorder) scattering. Together with ab initio calculations that found a mean free path of the charge carriers of 7 nm for stoichiometric Al$_3$Sc, these results indicate that Al$_3$Sc bears promise for future interconnect metallization schemes. Challenges remain in minimizing the formation of secondary phases as well as in the control of the non-stoichiometric surface oxidation and interfacial reaction with the underlying dielectrics.Comment: 15 pages, 4 figure

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low-and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields

Vortex dynamics and rectification effects in superconducting films with periodic asymmetric pinning

Controlled motion of nanoparticles has attracted recently a lot of attention in biology and physics [1]. In biology, typical examples are ion channels in cell membranes [2], biological motors [3]; in physics particle [4], charge [5] and vortex [6] ratchets, optical tweezers[7]. In spite of a broad variety, here the underlying principles of these effects have many remarkable similarities which make it possible to use new findings, discovered for a given type of the devices, for controlling the motion of nanoparticles in other devices in general. The idea of applying the ratchet mechanism to the case of vortices in type-II superconductors was introduced by Lee et al. [8]. Their theoretical research was invoked by the development of recent lithographic techniques, allowing to modulate material parameters on the scale of the coherence length ξ and the penetration length λ. This modulation creates a periodic pinning landscape for the present vortices, which depends on the properties of the periodic structuring. Breaking the spatial symmetry of the pinning potential results in a so-called ratchet potential capable of rectifying the motion of vortices. The tunability of these kind of systems makes it an ideal candidate to study the ratchet effect in general. Several kinds of pinning potentials were investigated [9-11], either magnetic or nonmagnetic. A detailed investigation of the effects of temperature and magnetic field on the ratchet effect will increase our knowledge of this kind of systems and pave the road towards new devices capable of manipulating vortex motion. [1] Special issue on Ratchets and Brownian Motors: Basic, Experiments and Applications, edited by H. Linke, Appl. Phys. A: Mater. Sci. Process. 75, 167 (2002). [2] R. D. Astumian, J. Phys. Chem. 100, 19075 (1996). [3] J. Howard, Nature 89, 561 (1997). [4] S. Matthias and F. Müller, Nature 424, 53 (2003). [5] H. Linke et al., Science 286, 2314 (1999). [6] J. E. Villegas et al., Science 302, 1188 (2003). [7] D.G. Grier, Nature 424, 810-816 (2003). [8] C.-S. Lee, B. Jankó, I. Derényi, and A.-L. Barabási, Nature 400, 337 (1999). [9] J. Van de Vondel et al., Phys. Rev. Lett. 94, 057003 (2005). [10] C.C. de Souza Silva, J. Van de Vondel, M. Morelle and V.V. Moshchalkov, Nature 440, 651 (2006). [11] C. C. de Souza Silva et al., Phys. Rev. Lett.status: publishe

Stroboscopic phenomena in superconductors with dynamic pinning landscape

Introducing artificial pinning centers is a well established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain without dissipation. In case of spatially periodic pinning, a clear enhancement of the superconducting critical current arises when commensurability between the vortex configurations and the pinning landscape occurs. With recent achievements in (ultrafast) optics and nanoengineered plasmonics it has become possible to exploit the interaction of light with superconductivity, and create not only spatially periodic imprints on the superconducting condensate, but also temporally periodic ones. Here we show that in the latter case, temporal matching phenomena develop, caused by stroboscopic commensurability between the characteristic frequency of the vortex motion under applied current and the frequency of the dynamic pinning. The matching resonances persist in a broad parameter space, including magnetic field, driving current, or material purity, giving rise to unusual features such as externally variable resistance/impedance and Shapiro steps in current-voltage characteristics. All features are tunable by the frequency of the dynamic pinning landscape. These findings open further exploration avenues for using flashing, spatially engineered, and/or mobile excitations on superconductors, permitting us to achieve advanced functionalities.Manipulation of quantum units of magnetic flux by confined ligh

Controlled electromigration protocol revised

Electromigration has evolved from an important cause of failure in electronic devices to an appealing method, capable of modifying the material properties and geometry of nanodevices. Although this technique has been successfully used by researchers to investigate low dimensional systems and nanoscale objects, its low controllability remains a serious limitation. This is in part due to the inherent stochastic nature of the process, but also due to the inappropriate identification of the relevant control parameters. In this study, we identify a suitable process variable and propose a novel control algorithm that enhances the controllability and, at the same time, minimizes the intervention of an operator. As a consequence, the algorithm facilitates the application of electromigration to systems that require exceptional control of, for example, the width of a narrow junction. It is demonstrated that the electromigration rate can be stabilized on pre-set values, which eventually defines the final geometry of the electromigrated structures