A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3–5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude.
An AC magnetic field can rotate a vortex-molecule composed of two fractional vortices in a multiband type of multicomponent superconductor, where two components couple with each other through the intercomponent Josephson interaction. It may directly relate to an abnormal AC loss peak, which we have found recently in the multilayer cuprate supercondutor CuxBa2Ca2Cu3Oy (Cu-1223).
Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-m-period square array of 2.5-m-sized holes ͑antidots͒. Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling ͑ZFC͒ to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.
Because of the trouble of a chip of a digital-analog converter of a superconducting magnet controller, the magnitude of the DC applied field was indicated by 2 times larger than the real applied value. All of the magnitude of the applied field is a half of the described value. The conclusion and discussions are not altered by this correction.
A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.
The electrical transport properties of (BaCuO 2 ) 2 /(CaCuO 2 ) 2 superconducting artificial structures, grown by pulsed laser deposition stacking in sequence nonsuperconducting individual BaCuO 2 and CaCuO 2 layers, have been studied. Such artificially layered materials have T c values up to 80 K and show a number of unusual properties such as the relatively high normal state resistivity with a nonlinear dependence on temperature, the enhanced thermodynamic fluctuations, the relatively low value of the critical temperature with respect to the natural compounds with the same number of CuO 2 planes, and the same carrier concentration. It is demonstrated that such properties can be explained if the high level of structural disorder, typical of these artificial structures, is considered.
A controlled pinning change from the ab-plane dominant to the c-axis dominant has been achieved in a novel method of nanostructured YBa 2 Cu 3 O x (YBCO) growth. The method is a synchronous self-assembly of BaZrO 3 (BZO) and Ag-assisted YBCO nanothreads. The formation of entangled nanothreads increases the critical current density while keeping the critical temperature close to that in pure YBCO films. The nanothreads extend through the whole thickness of thick films, making the method suitable for increasing total critical current density per centimeter of width (I c−w ). Two growth mechanisms, the formation of BZO nanorods and YBCO nanocolumns, complement each other, form a coherent structure and produce samples with strong correlated pinning. In addition to the increase in I c−w , correlated pinning leads to an increase in vortex melting temperature in a wide range of magnetic fields. The films grown by this method have high I c−w both in low magnetic fields along the c axis and high magnetic fields in the ab plane of YBCO. Such a superconductor would be suitable for both cable and magnet applications.
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