We report the appearance of spontaneous vortices in lead superconducting films with embedded magnetic nanoparticles and a temperature induced phase transition between different vortex phases. Unlike common vortices in superconductors, the vortex phase appears in the absence of applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. Transport measurements reveal dynamical phase transitions that depend on temperature (T) and applied field (H) and support the obtained (H-T) phase diagram. PACS numbers: 74.25.Dw, 74.25.Fy, 74.81.Bd The interplay between of superconductivity (SC) and ferromagnetism (FM) has been attracting the attention of the scientific community since the discovery of superconductivity [1]. Recently, there has been a resurgence of this interest due to new phenomena: for example, the increase of the critical current J c in hybrid systems containing sub-micron ferromagnetic particles on top of type II superconductors [2, 3] and the coexistence of superconductivity with long-range magnetic order in magnetic superconductors [4]. In a SC-FM hybrid, the magnets strongly affect the properties of the superconductor, leading to a change in the critical temperature T c and critical current J c . Also, they give an opportunity to observe new phenomena such as domain-wall superconductivity [5] and hysteresis pinning effect [6].In order to study the characteristics of the novel spontaneous vortex phases that arise from the interaction between vortices and embedded magnetic nanoparticles, we fabricated a hybrid system that consists of a 100 nm lead film (Pb) containing homogeneously distributed single domain cobalt (Co) particles (mean diameter about 4.5 nm) with randomly oriented magnetization.The samples are produced by the so-called inertgas(Ar) aggregation method with an Ar pressure of about 10 −1 mbar. It is a co-deposition of Pb and well-defined Co clusters directly onto a sapphire substrate without buffer and capping layer. The Ar is absorbed by a cryopump at the other end of the cluster chamber and only well-defined Co clusters can enter the main chamber. The substrate is mounted on a coldfinger of a rotatable 4 He cryostat and is cooled to ∼ 40 K during the deposition. The samples are deposited at low temperature in order to get high quality Pb films. The angle between the matrix and the cluster beams was 45 o . Due to the different beam directions, samples with different Co volume fraction can be made within one preparation. The de-position rates are controlled by three quartz balances in order to monitor the deposition rate at different positions of the substrate. Ag contacts for transport measurements are pre-deposited and connected to a multi-channel automatic measurement system. The typical dimensions of the sample were 10mm×3mm×100nm. After deposition, transport properties in both zero and non-zero magnetic field were investigated in-situ...
In the present work we have studied the effects of nitrogen addition on the equilibrium of the volume fraction ratio austenite/ferrite and on the structural properties of weldments produced by gas tungsten arc welding (GTAW). The welded joints in tubes UNS S32707 of hyper-duplex steel were produced using argon and nitrogen gas welding, with nitrogen added in ratios of 1.5, 2.5, 3.5, 4.5, and 5.5%. Microstructural characterization of this material under the different processing condition was conducted by means of optical microscopy, scanning electron microscopy, local chemical analysis by X-ray energy dispersive spectroscopy, electron backscatter diffraction Vickers microhardness test, and digital image processing. All welding conditions resulted in welded joints with equiaxial grains microstructure containing austenite both at the boundaries and in the ferrite matrix. Microhardness measurements did not show significant variation in relation to the base metal. The use of welding gases with higher percentages of nitrogen resulted in increases in the austenite volume fraction in the order of 39 to 57% when considering the nitrogen content ranging from 1.5 to 5.5%.
Different methods of producing nanostructured materials at the laboratory scale have been studied using a variety of physical and chemical techniques, though the challenge here is the homogeneous distribution of the elements which also depends on the precursor elements. This work thus focused on the micro-analytical characterization of Cu–Ni–Co metallic nanoparticles produced by an alternative chemical route aiming to produce solid solution nanoparticles. This method was based on two steps: the co-formation of oxides by nitrates’ decomposition followed by their hydrogen reduction. Based on the initial composition of precursor nitrates, three homogeneous ternaries of the Ni, Cu and Co final alloy products were pre-established. Thus, the compositions in %wt of the synthesized alloy particles studied in this work are 24Cu–64Ni–12Co, 12Cu–64Ni–24Co and 10Cu–80Ni–10Co. Both precursor oxides and metallic powders were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM/EDS) and transmission electron microscopy (TEM). The results show that the synthesis procedure was successful since it produced a homogeneous material distributed in different particle sizes depending on the temperature applied in the reducing process. The final composition of the metallic product was consistent with what was theoretically expected. Resulting from reduction at the lower temperature of 300 ∘C, the main powder product consisted of particles with a spheroidal and eventually facetted morphology of 50 nm on average, which shared the same FCC crystal structure. Particles smaller than 100 nm in the Cu–Ni–Co alloy agglomerates were also observed. At a higher reduction temperature, the ternary powder developed robust particles of 1 micron in size, which are, in fact, the result of the coarsening of several nanoparticles.
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