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XAS can be a very reliable tool for studying phase evolution and dynamic diffusion in intermetallic Ni/Bi system.

Spin-singlet Cooper pairs convert to spin-triplet Cooper pairs on passing through a magnetically noncollinear structure at a superconductor(S)/ferromagnet(F) interface. In this context, the generation of triplet supercurrents through intrinsic ferromagnetic domain walls, which are naturally occurring noncollinear magnetic features, was proposed theoretically in the past decade. However, an experimental demonstration has been lacking in the literature, particularly because of the difficulty in accessing a single domain wall, which is typically buried between two domains in a ferromagnetic material. By patterning a ferromagnetic nanoconstriction, we have been able to realize a nanoscale S/F/S planar junction, where a single domain wall (pinned at the nanoconstriction) acts as a Josephson barrier. In this geometry, we are able to show the predicted long-range triplet supercurrent across a ferromagnetic barrier exceeding 70 nm. Using this technique, we have demonstrated a ferromagnetic planar nano-SQUID device consisting of two Nb/Ni/Nb spin-triplet Josephson junctions.

The notion of competing ferromagnetic (F) and superconducting (S) orders in F/S hybrid structures was transformed by the first realization of ferromagnetic Josephson π-junctions and the almost simultaneous prediction of a possibility of spin–triplet correlations in such structures, almost two decades back. Such hybrid structures in various configurations are now studied as rich sources of emergent states and new effects. Unlike the spin singlet Cooper pairs, the spin triplet Cooper pairs are much less affected by the exchange field of a ferromagnet and, therefore, immediately finds interest in the field of spintronics. Theoretically, it has been shown that the basic protocol for spin–singlet to spin–triplet supercurrent conversion is the presence of magnetic non-collinearity at the superconductor–ferromagnet interface. Therefore, almost all experiments in this direction have utilized transport measurements on F/S systems with artificial magnetic non-collinearity formed by combination of several ferromagnetic layers next to the superconducting layer. Here we highlight two aspects of studying these heterostructures. Firstly we show that natural magnetic inhomogeneities, found in domain walls of ferromagnets, can also be used to achieve singlet–triplet conversion, instead of artificial magnetic inhomoheneities. This possibility was explored via transport measurements in nano-scale planar Nb–Ni–Nb junctions and nano-SQUIDs, where a domain wall was pinned at the Josephson junction barrier. By this method we were able to show Josephson coupling across about 70 nm of strong ferromagnetic planar barrier. Secondly we show that spin–triplet correlations at the F/S interface are robust enough to be probed by the diamagnetic screening currents at the interface. This was probed by studying the change in sperconducting transition temperature of Nb/Co/Py/Nb multilayers in presence of small in-plane magnetic field. The Co/Py combination, which is a soft-hard type magnetic exchange spring, worked as magnetic inhomogeneity for triplet generation at the interface of the superconducting Nb. These observations may promote new experiments in the field of superconducting-spintronics.

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report, we use the resistance of the domain wall as a probe to find indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic domain walls. Here we examine the domain wall induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 6um wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak near the coercive field, where the domain wall density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of domain walls becomes maximum. This observation may be a possible signature of magnetic domain wall induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic domain walls.

We present an analysis of electron recoils in cryogenic germanium detectors operated during the SuperCDMS Soudan experiment. The data are used to set new constraints on the axioelectric coupling of axionlike particles and the kinetic mixing parameter of dark photons, assuming the respective species constitutes all of the galactic dark matter. This study covers the mass range from 40 eV/c2 to 500 keV/c2 for both candidates, excluding previously untested parameter space for masses below ∼1 keV/c2. For the kinetic mixing of dark photons, values below 10−15 are reached for particle masses around 100 eV/c2; for the axioelectric coupling of axionlike particles, values below 10−12 are reached for particles with masses in the range of a few-hundred eV/c2.

The superconducting properties of the intermetallic compound NiBi3 have been studied mostly in bulk crystals. In this report we have studied several aspects of NiBi3 thin films, spontaneously formed at the interface of Ni and Bi multilayers. We have observed superconducting transition around 3.9 K, close to the bulk transition temperature of 4.1 K. Due to the diffusive mechanism of formation of NiBi3, additional phenomena such as magnetic (Nickel) impurity induced Kondo effect and self organized Turing pattern formation also come into picture. From X-ray diffraction and Magneto-resistance measurements we have established the presence of atomic Ni impurities in the films. Kondo minima were found to scale with varied concentration of Ni impurities in a series of Ni-Bi multilayer films (with fixed Ni thickness and varied Bi thickness). Similarly, saturation magnetization and superconducting transition temperature were also found to follow the concentration of Ni impurities. In addition, we have also observed signatures of Turing patterns, from magnetic force microscopy, naturally formed with time without any external heat treatment.

Tandem p–n junctions have been synthesized in ZnO nanorods (NRs) by implantation of 50 and 350 keV O+ ions. Conducting-AFM measurements reveal the formation of Schottky like junctions between AFM tip and NRs. Photoluminescence measurements demonstrate the recovery of near band edge (NBE) emission upon annealing, while an additional dominant deep level emission is also observed. De-convolution analysis shows that these peaks originate from Oxygen interstitials ( O i ) and may contribute to p-type conductivity in ZnO NRs. Such implanted NRs, may be suitable for green emissions and p-type conductivity.

In this report, we demonstrate significantly enhanced field emission properties of ZnO nanotapers achieved via a corrugated decoration of Au. Field emission experiments on these Au-decorated ZnO nanotapers showed emission current densities comparable to the best results in the literature. Au decoration of 5 nm also reduced the effective turn-on field to ∼0.54 V/μm, compared to the as grown ZnO nanotapers, which showed a turn-on field of ∼1.1 V/μm. Tunneling atomic force microscopy measurements revealed a very uniform spatial emission profile in the 5 nm Au decorated nanotapers, which is a basic requirement for any large scale application. We believe that metal induced mid-gap states formed at the ZnO–Au interface are responsible for the observed low turn-on field because such interface states are known to reduce the effective work function. A direct measurement of effective work function using Kelvin probe force microscopy indeed showed more than 1.1 eV drop in the case of 5 nm Au decorated ZnO nanotapers compared to the pristine nanotapers, supporting the above argument.

The proposed Mitchell Institute Neutrino Experiment at Reactor (MINER) experiment at the Nuclear Science Center at Texas A&M University will search for coherent elastic neutrino-nucleus scattering within close proximity (about 2 m) of a 1 MW TRIGA nuclear reactor core using low threshold, cryogenic germanium and silicon detectors. Given the Standard Model cross section of the scattering process and the proposed experimental proximity to the reactor, as many as 5–20 events/kg/day are expected. We discuss the status of preliminary measurements to characterize the main backgrounds for the proposed experiment. Both in situ measurements at the experimental site and simulations using the MCNP and GEANT4 codes are described. A strategy for monitoring backgrounds during data taking is briefly discussed.

We report the synthesis of a unique zinc oxide nanorod structure in which an amorphous ZnO layer is sandwiched between two identical crystalline segments of ZnO. A simple hydrothermal reaction method was used for this purpose, which allowed us to tune the amorphous and crystalline sections of the nanorods via reaction temperature. A systematic study of the morphology and dimensions of the nanorods grown under various conditions was performed using a combination of scanning and transmission electron microscopy. Transmission electron microscopy (TEM) clearly showed an amorphous separation between the two crystalline segments. UV-vis absorption spectroscopy of the twin nanorods (TNRs) showed a redshift in the optical band gap as a function of the growth duration, indicating slightly stressed growth of the crystalline segments. For a longer growth duration, as the amorphous gap starts to get bridged by crystalline growth, redshift in optical band gap becomes constant. This confirms a true mechanical gap between the two crystalline segments of the nanorods. Temperature dependent photoluminescence (PL) spectra of the TNRs showed a variation in free exciton (FX) emission energy, which fitted very well to a model incorporating lattice dilation in addition to the standard electron-phonon interactions. At low temperatures (below ∼180 K) we observed the appearance of visible emission peaks due to localization of defect levels. A loss in the near band edge emission intensity was observed at low temperatures, commensurate with the appearance of defect emission in the visible range.

Over the last decade it has been shown that magnetic non-collinearity at a s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bi-layer structure can function as a field tunable non-collinear magnetic structure which may offer magnetic-field tuneability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields., 12 pages, 4 figures

This corrects the article DOI: 10.1103/PhysRevLett.121.051301.

We present a systematic study of the magnetization reversal of nickel and permalloy micro-stripes with Neel and Bloch domain walls using Kerr microscopy. Magnetic field driven domain propagation was observed from higher width to lower width stripes for magnetic fields applied along the length of micro-stripes. Stripe like domains were observed with nucleation starting in lower width region followed by their propagation to higher width regions for magnetic fields applied along the width of micro-stripes. The comparison of magnetization reversal in Bloch and N\'eel domain wall regime showed higher domain wall density in Bloch regime for both nickel and permalloy stripes., Comment: 13 pages, 5 figures

We present a systematic study of the magnetic domain wall induced modulation of superconducting transition temperature (Tc) in Nb/Ni bilayer stripes. By varying the thickness of the Ni layer from 20 nm to 100 nm we have been able to measure the low field Tc-H phase diagram spanning the Neel domain wall and Bloch domain wall range of thicknesses. Micromagnetic simulations and magnetic force microscopy measurements confirmed a stronger out-of-plane stray field in the Bloch domain walls compared to the Neel walls. A suppression in Tc was observed in the magnetization reversal region of the Ni film, the magnitude of which followed linearly to the strength of the out-of-plane stray field due to the domain walls. The magnitude of the stray field was quantified by comparing the Tc of the suppressed region of H-Tc phase diagrams with the unaffected part of the H-Tc curve. With Bloch domain walls a change in Tc of more than 60 mK was observed which is much more compared to the earlier reports. We believe that the narrow stripe geometry of the bilayers and the transverse external field maximized the effect of the domain walls in the Ni layer on the overlying superconducting film, leading to a larger change in Tc. This observation may be useful for domain wall controlled switching devices in superconducting spintronics., Comment: 13 pages, 4 figures

We have studied the structural phase transformation in thin Co films using moderate energy Au ion irradiation. We found that local energy transfer along the track of the ion beam is an effective method for transforming fcc phase of Co into technologically appealing hcp phase. As a function of the irradiation fluence, a gradual crossover to the hcp phase was observed in X-ray diffraction data. Such phase transition was explained on the basis of evolution of grains under increasing ion fluence. Bloch-Gruneisen simulations to the four contact resistivity data showed a decrease in Debye temperature with evolution of hcp-phase Cobalt, consistent with the reduced modal freedom for phonons in the hcp structure compared to the fcc structure.

Two segments of horizontally grown crystalline ZnO nanorods (NRs) connected with an amorphous layer have been successfully and reproducibly synthesized using one-step hydrothermal technique by controlling the growth rate. The confocal photoluminescence (PL) imaging and spectroscopy of twin ZnO NRs at different temperatures shows intense red emission with comparably week UV emission. The strong red emission from the twin NRs is a consequence of structural imperfections. Both UV and red bands showed signatures of strong temperature dependent exciton-phonon scattering. Using the intensity ratio of the UV and red emissions, we show that the individual ZnO NRs can be used as highly sensitive cryogenic temperature sensors below ∼175 K.

There is a growing consensus through various experimental and theoretical studies that gold can exhibit magnetic properties at low dimensions in contrast to its well-known diamagnetic nature when in bulk form. Although theoretical simulation studies show that bare gold nanoclusters can be intrinsically magnetic, experimental reports to prove this theory are scarcely available since most of the studies are based on functionalized gold nanoparticles. In this article, we report unusual ferromagnetic behaviour that is observed in embedded non-functionalized Au nanoparticles using superconducting quantum interference device (SQUID) magnetometry. These nanoparticles are obtained after irradiating thermally deposited Au/Bi double bilayer films using 1.5 MeV Au ions at different fluences from 5 × 1014 to 1 × 1016 ions per cm2. A detailed study of cross-sectional high resolution transmission electron microscopy (X-HRTEM) results for the irradiated sample confirms the presence of embedded Au nanoparticles with an average size of 2.61 nm. The unusual ferromagnetic behaviour is attributed to these embedded Au nanoparticles that were formed after ion irradiation.

Vertically aligned ZnO nanorods grown by a wet chemical method were implanted with O + ions with three different ion fluences: ( Φ ) = 5 × 10 14, 5 × 10 15, and 5 × 10 16 ions/cm 2. It is observed that the concentration of Oxygen vacancies (O V) introduced by implantation first increases from 25.94 % to 54.76 % with increasing Φ and decreases beyond Φ = 5 × 10 15 ions/cm 2. We attribute this to the knocking out of oxygen atoms from the host matrix, which gets saturated due to the presence of an ample amount of O + ions inside the host matrix beyond Φ = 5 × 10 15 ions/cm 2 and further confirmed by extended x-ray absorption fine structure measurements. Therefore, the abundant O V becomes more delocalized followed by overlapping with the maxima of valence bands resulting in the narrowing of the bandgap of ∼ 0.4 eV. The appearance of an additional Raman peak at ∼ 575 cm − 1 in Raman spectra further confirmed the presence of impurity states. It is evident that at a fixed J = 100 μA/cm 2, the turn-on field increases from 3.61 V/ μm to 6.61 V/ μm with increasing Φ, and as a result of this, the field-enhancement factor ( β) decreases. We attribute this increase in turn-on field as a consequence of charge trapping in deep-level states created by O V.

The Cryogenic Dark Matter Search (CDMS II) experiment aims to detect dark matter particles that elastically scatter from nuclei in semiconductor detectors. The resulting nuclear-recoil energy depositions are detected by ionization and phonon sensors. Neutrons produce a similar spectrum of low-energy nuclear recoils in such detectors, while most other backgrounds produce electron recoils. The absolute energy scale for nuclear recoils is necessary to interpret results correctly. The energy scale can be determined in CDMS II silicon detectors using neutrons incident from a broad-spectrum $^{252}$Cf source, taking advantage of a prominent resonance in the neutron elastic scattering cross section of silicon at a recoil (neutron) energy near 20 (182) keV. Results indicate that the phonon collection efficiency for nuclear recoils is $4.8^{+0.7}_{-0.9}$% lower than for electron recoils of the same energy. Comparisons of the ionization signals for nuclear recoils to those measured previously by other groups at higher electric fields indicate that the ionization collection efficiency for CDMS II silicon detectors operated at $\sim$4 V/cm is consistent with 100% for nuclear recoils below 20 keV and gradually decreases for larger energies to $\sim$75% at 100 keV. The impact of these measurements on previously published CDMS II silicon results is small., Comment: 22 pages, 17 figures, 1 table, 1 appendix

The Super Cryogenic Dark Matter Search experiment (SuperCDMS) at the Soudan Underground Laboratory studied energy loss associated with Frenkel defect formation in germanium crystals at mK temperatures using in situ $^{210}$Pb sources. We examine the spectrum of $^{206}$Pb nuclear recoils near its expected 103 keV endpoint energy and determine an energy loss of $\left(6.08\pm0.18\right)$ %, which we attribute to defect formation. From this result and using TRIM simulations, we extract the first experimentally determined average displacement threshold energy of $\left(19.7^{+0.6}_{-0.5}\right)$ eV for germanium. This has implications for the analysis thresholds of future germanium-based dark matter searches., 4 Figures, Two data files, One data release description file

Future direct searches for low-mass dark matter particles with germanium detectors, such as SuperCDMS SNOLAB, are expected to be limited by backgrounds from radioactive isotopes activated by cosmogenic radiation inside the germanium. There are limited experimental data available to constrain production rates and a large spread of theoretical predictions. We examine the calculation of expected production rates, and analyze data from the second run of the CDMS low ionization threshold experiment (CDMSlite) to estimate the rates for several isotopes. We model the measured CDMSlite spectrum and fit for contributions from tritium and other isotopes. Using the knowledge of the detector history, these results are converted to cosmogenic production rates at sea level. The production rates in atoms/(kg$\cdot$day) are 74$\pm$9 for $^3$H, 1.5$\pm$0.7 for $^{55}$Fe, 17$\pm$5 for $^{65}$Zn, and 30$\pm$18 for $^{68}$Ge., 14 pages, 10 figures, 5 tables. v5 contains the extended data release (and documentation) of the CDMSlite Run 2 data as ancillary files

We present the first limits on inelastic electron-scattering dark matter and dark photon absorption using a prototype SuperCDMS detector having a charge resolution of 0.1 electron-hole pairs (CDMS HVeV, a 0.93 gram CDMS HV device). These electron-recoil limits significantly improve experimental constraints on dark matter particles with masses as low as 1 MeV/$\mathrm{c^2}$. We demonstrate a sensitivity to dark photons competitive with other leading approaches but using substantially less exposure (0.49 gram days). These results demonstrate the scientific potential of phonon-mediated semiconductor detectors that are sensitive to single electronic excitations., 6 pages + title and references, 6 figures, includes erratum submitted to PRL and data release

We report the result of a blinded search for weakly interacting massive particles (WIMPs) using the majority of the SuperCDMS Soudan data set. With an exposure of 1690 kg d, a single candidate event is observed, consistent with expected backgrounds. This analysis (combined with previous Ge results) sets an upper limit on the spin-independent WIMP–nucleon cross section of 1.4×10−44 (1.0×10−44) cm2 at 46 GeV/c2. These results set the strongest limits for WIMP–germanium-nucleus interactions for masses >12 GeV/c2.

We present a detailed study of thermal and electrical transport behavior of single crystal Titanium disulphide flakes, which belongs to the two dimensional, transition metal dichalcogenide class of materials. In-plane Seebeck effect measurements revealed a typical metal-like linear temperature dependence in the range of 85 - 285 K. Electrical transport measurements with in-plane current geometry exhibited a nearly Tlsupg2l/supg dependence of resistivity in the range of 42 - 300 K. However, transport measurements along the out-of-plane current geometry showed a transition in temperature dependence of resistivity from Tlsupg2l/supg to Tlsupg5l/supg beyond 200 K. Interestingly, Au ion-irradiated TiSlsubg2l/subg samples showed a similar Tlsupg5l/supg dependence of resistivity beyond 200 K, even in the current-in-plane geometry. Micro- Raman measurements were performed to study the phonon modes in both pristine and ion-irradiated TiSlsubg2l/subg crystals.

We demonstrate a metallic template assisted method for realizing out-of-plane magnetic anisotropy in electro-deposited Co and Ni films, using highly porous TiN films as templates. TiN films containing high aspect ratio sub-50 nm wide pillars were obtained by controlling film-morphology via gas pressure during sputter deposition of the films. The nanoscale separation between these pillars allowed us to electro-deposit magnetic films on the pillars resulting in high aspect ratio magnetic structures. Co and Ni films grown on such nanostructured TiN films showed a significant coercivity enhancement as compared to planar films. Angular dependence of coercive field and the remnance ratio (M r /M s ) reveals that, in the ultrathin limit, magnetic films coated onto the nano-pillars are akin to magnetic nanotubes.

Large-area ordered TiO2 nanostructural arrays with tunable shapes were fabricated by annealing the magnetron sputtered Ti on anodic aluminum oxide (AAO) templates in air at 400 °C for 6 h. The morphology of the sputtered TiO2 nanostructural film was controlled by adjusting the parameters of the porous AAO substrate. XRD measurement shows the annealed TiO2 nanotubular film is polycrystalline anatase phase without preferred orientation. Photoluminescence (PL) measurement reveals the TiO2 nanostructural film is an indirect bandgap semiconductor with oxygen vacancy defects. These nanostructural arrays may find potential applications in photocatalytic and photovoltaic devices.

Hysteretic effects are seen in the upper and lower threshold fields for the appearance of dendritic flux instabilities, first explained in Yurchenko et al. [Phys. Rev. B 76 (2007) 092504], in NbN-films. We have measured the threshold fields at increasing and decreasing applied fields at different temperatures and proposed a mechanism explaining how the hysteresis arises by analyzing the field profiles inside the sample.

We report a study of the switching behavior in the current–voltage characteristics of long (∼8 μm) Nb microbridges in the hysteretic regime. Microbridges of various widths, ranging from . 71 μ m to 3.7 μ m , were prepared via standard optical lithography and reactive plasma etching techniques and the switching behavior was studied in a temperature range of 0.8 T c ⩽ T ⩽ T c . The current driven switching from superconducting to normal state of the bridge was analyzed in the Ginzburg–Landau formalism while the resistive-to-superconducting switching was analyzed using the hotspot model.

We report fabrication and characterization of epitaxial NbN/TaN/NbN Josephson junctions grown by pulsed laser ablation. These SNS junctions can be used as elements of rapid-single-flux-quantum (RSFQ) logic, which is a promising technology for high speed digital electronic devices. The NbN/TaN/NbN trilayer films were prepared on a single crystal MgO substrate by pulsed laser ablation, and patterned into junctions using a novel process utilizing e-beam lithography, chemical vapor deposition and e-beam evaporation. The quality of junctions was tested by measuring the temperature dependence of the junctions' IcRn values, observed to be quite close to theoretical values.

An experimental study of proximity effect in La{0.67}Sr{0.33}MnO3 - YBa2Cu3O7 - La{0.67}Sr{0.33}MnO3 trilayers is reported. Transport measurements on these samples show clear oscillations in critical current (I_c) as the thickness of La{0.67}Sr{0.33}MnO3 layers (d_F) is scanned from ~ 50 A to ~ 1100 A. In the light of existing theories of ferromagnet-superconductor (FM-SC) heterostructures, this observation suggests a long range proximity effect in the manganite, modulated by it's weak exchange energy (~ 2 meV). The observed modulation of the magnetic coupling between the ferromagnetic LSMO layers as a function of d_F, also suggests an oscillatory behavior of the SC order parameter near the FM-SC interface., Comment: 4 pages, 4 Figures

Zinc oxide twin nanorods, with two identical crystalline sections connected by an amorphous layer, were reproducibly grown using a simple one-step hydrothermal technique. The thickness of the amorphous layer between the crystalline segments was tunable with growth parameters, as confirmed by high resolution transmission electron microscopy. The photoluminescence spectra of these twin nanorods exhibit strong near band edge emission in the UV range, with convoluted phonon sidebands. De-convolution analyses of these spectra showed that the amorphous interlayers act as effective phonon barriers beyond a certain thickness. Such oriented grown individual crystalline–amorphous–crystalline structures may be a suitable test system for fundamental studies of phonon tunneling in the nanostructure. While physical vapor deposition techniques are seriously constrained in realizing crystalline–amorphous–crystalline structures, our results show the viability of engineering embedded interfaces via chemical routes.

A highly sensitive ac susceptometer has been fabricated using a commercially available thin film Hall sensor of effective area 25×25 μm2. This instrument allows measurements of superfluid density and dynamical behavior of vortices in thin film superconductors over a broad range of frequency (10 Hz–100 kHz), temperature (4.2–300 K), and ac-field amplitude (2–150 Oe), and in static magnetic field as high as 5 T collinear with the ac field. The applicability of the susceptometer has been tested on Y1Ba2Cu3O7 films of thickness as small as ∼7 nm.

Epitaxial multilayers of YBa2Cu3O7-δ and P1Ba2Cu3O7-δ have been deposited on (100) cut SrTiO3 substrates using the technique of pulsed laser deposition. Standard ϑ-2ϑ X-ray diffraction measurements on the films showed excellent superlattice reflections. The mixed state of these superlattices has been probed through measurements of radio frequency penetration depth (λ) as a function of temperature (T), magnetic field (H) and it’s orientation (ϑ) with respect to the planes of the superlattices. These data reflect the two-dimensional nature of the mixed state in these systems.

We report a study of the structural and magnetic behavior of the topmost magnetic layer in a ferromagnet-nonmagnet (Co-Au) multilayer system. Glancing angle X-ray diffraction measurements performed on a series of multilayers showed a gradually increasing degree of amorphization of the topmost magnetic layer with increasing number of bilayers. Concurrently, the magnetic hardness and magneto-crystalline anisotropy of the top Co layer were found to decrease, as observed by magneto-optical Kerr effect measurements. This magnetic softening has been discussed in the light of Herzer's random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations., 5 pages, 6 figures

Tunnel junctions incorporating GdN ferromagnetic semiconductor barriers show a spin polarization exceeding 90% and a high conductance. These devices show an unusual low-bias conductance peak arising from a strong bias-dependence of the spin polarization. This originates from a strong magneto-electric coupling within a double Schottky barrier formed with the NbN electrodes.

Simultaneous occurrence of insulating and ferromagnetic behaviour in GdN makes it an ideal spin filtering material suitably compatible with other transition metal nitrides, such as NbN. Here we present the effect of spin-filtering on the normal state and superconducting state tunnelling current in lithographically patterned Josephson junctions. A large suppression of junction critical current is observed below Tc of NbN, from which we estimate a spin-filtering efficiency of ∼75% in these room temperature grown GdN layers. The temperature dependence ofsuper-current deviates from the usualAmbegaokar-Baratoff relation at low temperatures, roughly scaling with the magnetic coercivity of the GdN barrier. The critical current Fraunhofer pattern shows highly suppressed side lobes and possible signature of a second harmonic Josephson current. In the normal state conductance we observe possible signatures of spin accumulation effect, which leads to a decrease in conductance at low bias voltages.

We have studied the superconducting proximity effect in a series of e-beam evaporated Bi-Ni-Bi trilayers, where diffusion of Bi into Ni spontaneously formed superconducting NiBi3 layers at both Ni-Bi interfaces, effectively resulting in superconductor-ferromagnet-superconductor (S-F-S) trilayers. The thickness of top and bottom superconducting layers was found to be different with slightly different transition temperatures. Both resistive transition temperatures in the series of S-F-S trilayers showed 0-π crossover as a function of ferromagnetic Ni thickness. The zero bias conductance calculated from the in-plane current-voltage measurements also confirmed the 0-π crossovers. The possibility of proximity effect in the superconducting fluctuation regime, above transition temperature, was investigated via in-plane magneto-transport measurements at 4 K. We observed clear modulations in magneto-resistance (MR) and in low-field MR-hysteresis at 4 K, in contrast to their monotonic behavior at 10 K. Although the...

Studies of the spin filter properties of tunnel barriers consisting of the insulating ferromagnets, EuO and EuS have been conducted for many years, but detailed investigation and application of their properties has been restricted by difficulties associated with growth and stoichiometry. We have recently demonstrated a new insulating ferromagnetic material GdN and shown that GdN barriers are strongly spin-filtering and can be incorporated into superconducting tunnel junctions. S/Insulator/S tunnel junctions enable detailed investigation of the magnitude of non-tunnel (leakage) currents and the optimisation of the tunnel barrier. We show that our insulating ferromagnet devices have a very low zero-bias leakage, but that behaviour for higher bias voltages is strongly non-ideal. We will discuss the potential for such devices to be used as spin-sources for both conventional and superconducting spintronics.

Measurements of the critical current density (Jc) and irreversibility field [Birr(T)] of epitaxial, c-axis-oriented EuBa2Cu3O7 (Eu-123) films are reported. The transition temperature (Tc), Jc, and Birr(T) of these pulsed-laser-deposited films depend critically on the growth temperature (TD), growth rate (Gr), and the oxygen partial pressure (pO2) during growth. As compared to YBa2Cu3O7, Eu-based films require lower Gr, higher pO2, and higher TD. These differences are attributed to the lower adatom mobility of Eu and its propensity for a 2+ oxidation state. The lattice mismatch between (100) LaAlO3 and the ab-plane of Eu-123 leads to an elongated c axis and some randomly oriented islands whose growth is truncated by epitaxial lateral overgrowth as the film thickness increases.

Magnetotransport measurements are performed over a broad range of temperature (T) and magnetic field (H) on highly degenerate n-type Zn(1-x)Co(x)O [Formula: see text] epitaxial films. The cobalt-free samples are characterized by a metallic resistivity ρ(T) down to 2 K, a negative and predominantly isotropic magnetoresistance (MR) and optical transmission above 85% in the visible range of the electromagnetic spectrum. X-ray diffraction measurements show that while for [Formula: see text], all cobalt atoms occupy the tetrahedral sites of the wurtzite structure of ZnO, a phase separation into CoO is seen for x0.2. In the solution phase, we do not observe any signatures of a spontaneous ordering of the cobalt spins despite a large concentration of mobile electrons (10(20) cm(-3)). The absence of anomalous Hall resistance is consistent with this observation. The carrier concentration (n) over the entire range of x remains above the Mott limit for the insulator-to-metal transition in a doped semiconductor. However, while the Co-free samples are metallic (T2 K), we see a resistivity (ρ) minimum followed by lnT divergence of ρ(T) at low temperatures with increasing x. The magnetoresistance of these samples is negative and predominantly isotropic. Moreover, the MR tends to follow a logH behaviour at high fields. These observations, including the Kondo-like minimum in the resistivity, suggest s-d exchange dominated transport in these dilute magnetic semiconductors.

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called π-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (I(F)) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current I(c), difficulties with the few known I(F) materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of I(c)is strongly modified by the I(F). In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the I(F) barrier can be modified by magnetic inhomogeneity in the barrier.

We report a study of the structure and magnetic behavior of polycrystalline GdN films grown at room temperature by reactive magnetron sputtering. By controlling the relative fraction of reactive species during film growth, we observe a continuous crossover from soft ferromagnetic films into relatively hard ferromagnetic films. While samples with a Curie temperature (${T}_{c}$) of less than $~60$ K showed low coercive fields, a significant increase in the hysteretic loss was observed for samples with ${T}_{c}\ensuremath{\gtrsim}$ 60 K. Accompanying the change in the magnetic behavior of the films, signatures of a secondary phase of GdN (GdN-II) were observed in x-ray diffraction measurements. Such dual-phase samples (with GdN and GdN-II) showed an exchange bias effect, which confirmed that the GdN-II phase was antiferromagnetic. The Curie temperatures of the dual-phase samples were found to be much higher than the reported value of ${T}_{c}$ for GdN. We believe that the origin of the antiferromagnetic phase and the enhanced ${T}_{c}$ of ferromagnetic GdN can be closely related to nitrogen vacancies in the samples. While the local strain induced by nitrogen vacancies can strengthen antiferromagnetic ordering in GdN-II, the change in carrier concentration due to the nitrogen vacancies strengthens the ferromagnetic ordering in the GdN phase. Hall effect measurements showed that transport properties of polycrystalline GdN films can be tuned from almost-insulating to semimetallic behavior by varying the amount of nitrogen in the samples. Amid a continuing debate on the origin of ferromagnetism in GdN, our data show considerable support for a carrier-mediated mechanism of ferromagnetism.

We propose a mechanism responsible for the abrupt vanishing of the dendritic flux instability found in many superconducting films when an increasing magnetic field is applied. The onset of flux avalanches and the subsequent reentrance of stability in NbN films were investigated using magneto-optical imaging, and the threshold fields were measured as functions of critical current density ${j}_{c}$. The results are explained with excellent quantitative agreement by a thermomagnetic model published recently [D. V. Denisov et al., Phys. Rev. B 73, 014512 (2006)], showing that the reentrant stability is a direct consequence of a monotonously decreasing ${j}_{c}$ versus field.

We report the spontaneous formation of superconducting NiBi3 phase in thermally evaporated Ni-Bi bilayer films. High reaction-diffusion coefficient of Bi is believed to drive the formation of NiBi3 during the deposition of Bi on the Ni film. Cross sectional transmission electron microscopy and glancing incidence X-ray depth profiling confirmed the presence of NiBi3 throughout the top Bi layer. Superconducting transition at ∼3.9 K, close to the bulk value, was confirmed by transport and magnetization measurements. The bilayers were irradiated with varying fluence of 100 MeV Au ions to study the robustness of superconducting order in presence of large concentration of defects. Superconducting parameters of NiBi3, such as transition temperature and upper critical field, remained unchanged upto an ion dose of 1 × 1014 ions/cm2. The diffusive formation of NiBi3 in Ni opens the possibility of studying superconducting proximity effect at a truly clean superconductor-ferromagnet interface.

The normal-state transport and the dynamic vortex response in simple cubic (sc) and face-centered cubic (fcc) polymorphs of NbN synthesized as epitaxial films of thickness ${d}_{F}\ensuremath{\sim}2000\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ using the technique of pulsed laser ablation are reported. The nonequilibrium sc phase of NbN stabilizes on (100) MgO at elevated deposition temperatures ${T}_{D}$ $(g200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, and over a restricted range of nitrogen pressure $({p}_{{\mathrm{N}}_{2}})$. The normal-state resistivity ${\ensuremath{\rho}}_{N}(T)$, critical temperature $({T}_{c})$, critical current density $[{J}_{c}(T)]$ and the temperature at which ${J}_{c}$ goes to zero $({T}^{*})$ depend largely on the extent of nonstoichiometry rather than the crystal symmetry of these films. The divergence of ${\ensuremath{\rho}}_{N}(T)$ on cooling before the onset of ${T}_{c}$ is analyzed in the framework of a grain boundary model in which electron transmittivity $\ensuremath{\Gamma}$ across the grains is considered explicitly. The zero-field ${J}_{c}$ deduced from screening measurements follows a temperature dependence of the type ${J}_{c}(T)\ensuremath{\sim}{(1\ensuremath{-}T∕{T}_{c})}^{\ensuremath{\beta}}$, with $\ensuremath{\beta}$ increasing from 0.75 to 1.2 as the $d{\ensuremath{\rho}}_{N}(T)∕dT$ changes sign from negative to positive. The films with negative $d{\ensuremath{\rho}}_{N}(T)∕dT$ are disordered and possibly nonstoichiometric. For stoichiometric films, the temperature dependence of ${J}_{c}$ in the mixed state and the variation of the critical temperature ${T}^{*}$ as a function of applied dc field are consistent with the model of thermally activated flux creep.

Superconductivity in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ films of thickness ${d}_{\mathrm{Y}}l500\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ sandwiched between ferromagnetic and nonmagnetic epitaxial layers has been probed. The irreversibility temperature ${T}_{\mathrm{irr}(\mathrm{H})}$ in the vortex state maps onto a clean-limit pair-breaking temperature ${T}_{\mathrm{c}(\mathrm{H})}$, and a universal relation exists between the pair-breaking field ${H}_{\mathrm{pb}}$ and an effective thickness ${d}_{\mathrm{eff}}(l{d}_{\mathrm{Y}})$ of the superconductor. The vortex response in these two-dimensional (2D) films indicates a crossover temperature $T*$, which separates two regimes of flux motion controlled by formation of dislocations $(TgT*)$, and migration of preexisting dislocations and dislocation pairs $(TlT*)$ in the 2D flux line lattice.