Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

We have investigated the electrical transport properties of 8 nm thick La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5 micrometer-wide bridges by Ar-ion etching. We find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and non-linear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. We find colossal magnetoresistance properties typical for strained LCMO on STO but no signature of non-linear effects (so-called electroresistance) connected to electronic inhomogeneites. In the metallic state below 150 K, the highest current densities lead to heating effects and non-linear I-V characteristics.Comment: 3 pages, 5 figure

Vortex dynamics in superconducting channels with periodic constrictions

Vortices confined to superconducting easy flow channels with periodic constrictions exhibit reversible oscillations in the critical current at which vortices begin moving as the external magnetic field is varied. This commensurability scales with the channel shape and arrangement, although screening effects play an important role. For large magnetic fields, some of the vortices become pinned outside of the channels, leading to magnetic hysteresis in the critical current. Some channel configurations also exhibit a dynamical hysteresis in the flux-flow regime near the matching fields

Inhomogeneous superconductivity induced in a weak ferromagnet

Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g. in the so-called pi-coupling in S/F/S junctions. In principle, the inhomogeneous state can be induced at T_c as function of the F-layer thickness d_F in S/F bilayers and multilayers, which should result in a dip-like characteristic of T_c(d_F). We show the results of measurements on the S/F system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T_c(d_F) data.Comment: 4 pages, 4 figures. To be publishedin Physica C (proceedings of the Second Euroconference on Vortex Matter in Superconductors, Crete, 2001

Picovoltmeter for probing vortex dynamics in a single weak-pinning Corbino channel

We have developed a picovoltmeter using a Nb dc Superconducting QUantum Interference Device (SQUID) for measuring the flux-flow voltage from a small number of vortices moving through a submicron weak-pinning superconducting channel. We have applied this picovoltmeter to measure the vortex response in a single channel arranged in a circle on a Corbino disk geometry. The circular channel allows the vortices to follow closed orbits without encountering any sample edges, thus eliminating the influence of entry barriers.Comment: 4 pages, 3 figures, submitted to Review of Scientific Instrument

Depairing currents in superconducting films of Nb and amorphous MoGe

We report on measuring the depairing current J_{dp} in thin superconducting films as a function of temperature. The main difficulties in such measurements are that heating has to be avoided, either due to contacts, or to vortex flow. The latter is almost unavoidable since the sample cross-section is usually larger than the superconducting coherence length \xi_s and the magnetic field penetration depth \lambda_s. On the other hand, vortex flow is helpful since it homogenizes the distribution of the current across the sample. We used a pulsed current method, which allows to overcome the difficulties caused by dissipation and measured the depairing current in films of thin polycrystalline Nb (low \lambda_s, low specific resistance \rho) and amorphous Mo_{0.7}Ge_{0.3} (high \lambda_s, high \rho), structured in the shape of bridges of various width. The experimental values of J_{dp} for different bridge dimensions are compared with theoretical predictions by Kupriyanov and Lukichev for dirty limit superconductors. For the smallest samples we find a very good agreement with theory, over essentially the whole temperature interval below the superconducting critical temperature.Comment: 5 pages, 6 figure

Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet

In (S/F) hybrids the suppression of superconductivity by the exchange field h_ex of the ferromagnet can be partially lifted when different directions of h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer geometries where the magnetization directions of the two F-layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F-layer yield a similar effect. We study the transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples. Comparison of domain wall width \delta_w to the temperature dependent superconductor coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the Cooper pairs sample a large range of different magnetization directions.Comment: 4 pages, 5 figure