H. Claus

Atomic short-range order in oxygen deficient YBa2Cu3O7−σ

Abstract Low field magnetization measurements are presented for a set of oxygen deficient single crystals of YBa 2 Cu 3 O 7−σ . It is demonstrated that the superconducting transition temperature T c not only depends on the oxygen concentration but also on the degree of oxygen vacancy ordering of the samples. Various degrees of ordering were obtained by annealing crystals with reduced oxygen stoichiometries at various temperatures below 350°C and fast quenching them to cryogenic temperatures. Depending on oxygen stoichiometry and annealing temperature, changes in T c of up to 27 K are observed. The observed annealing and quenching behavior is explained in terms of the kinetics of vacancy short-range order in the Cu-O chains. One major conclusion is that the so-called “60 K plateau” in the plot of T c versus σ is completely absent when the crystals are in their most disordered state, i.e., with the smallest degree of vacancy short-range order.

High critical current “weld” joints in textured YBa2Cu3Ox

Abstract A technique has been developed to “weld” parts made from textured high T c superconductors of the type RBa 2 Cu 3 O x (R=Y or rare earth element). The weld joint supports a supercurrent comparable to that carried by the joined parts. Welding is accomplished by using the textured parts to seed a spacer material which has a lower decomposition temperature than the material comprising the parts. In this paper, we demonstrate the technique by welding YBa 2 Cu 3 O x parts with a TmBa 2 Cu 3 O x spacer layer. The weld joints were evaluated with SEM, optical and magneto-optical imaging techniques.

Joining of melt-textured YBCO : a direct contact method.

We report a method for making weld joints, capable of transmitting high supercurrent densities, in bulk samples of melt-textured YBCO. The joining procedure is carried out in a flowing atmosphere of O2 to eliminate problems associated with nitrogen gas, which can become trapped in the joint. No filler or fluxing material is used. The method can be used to join large areas (several cm2) that are capable of transmitting supercurrent densities exceeding 104 A cm−2.

New observation of a phase boundary in oxygen deficient YBa2Cu3O7−δ single crystals

Abstract We have measured the superconducting transition temperature Tcof oxygen deficient single crystals of YBa2Cu3Ox, where x = 7−δ in the range 6.5 T ∗ . We associate T ∗ with the onset of a transition from a vacancy ordered low temperature (ortho II and/or ortho III) phase to a high temperature ortho I phase. The nature of the vacancy ordering below T ∗ varies with x. We believe that, upon heating above T ∗ , a signigicant concentration of chain fragments develops in normally vacant chains of the ordered structure. When the crystals were quenched from temperatures above T ∗ , and subsequently permitted to anneal at 51°C, two relaxation processes were found. The two processes have relaxation times differing by about two orders of magnitude. The faster relaxation process is associated with changes in vacancy short-range disorder. The slower process is attributed to restoration of long-range chain vacancy order as misplaced fragments anneal out. T ∗ is determined for different oxygen concentrations and compared to theoretical phase diagram calculations. Electron diffraction measurements on well annealed single crystals with stoichiometries x = 6.50 and 6.65 show that different chain ordered phases (primarily ortho II and ortho III, respectively) exist at each of these compositions. Chain vacancy order is also observed at temperatures above T ∗ .

Magnetism and atomic clustering in Au-Fe alloys

Abstract a.c. magnetic susceptibility measurements are used to investigate atomic clustering in Au-Fe alloys near 15 at. pct. Fe. The highest degree of disorder is obtained by quenching from 550C. Quenching from higher temperatures leads to inhomogeneous states. Changing the degree of clustering has qualitatively the same effect on the magnetic properties as changing the chemical concentration.

Magnetism in Ni-Cu alloys

Very low field (<1 Oe) magnetization measurements on Ni-Cu alloys are reported. The magnetic properties depend sensitively on the metallurgical state. For concentrations less than 45 at.% Ni, a spin glass-like or micto- magnetic freezing is observed, signaled by a cusp in the a.c. susceptibility. Above 45 at.% Ni the alloys are ferromagnetic.

Transport currents measured in ring samples: test of superconducting weld

Abstract The critical current densities in bulk melt-textured YBa2Cu3Ox and across superconducting “weld” joints are measured using scanning Hall probe measurements of the trapped magnetic field in ring samples. With this method, critical current densities are obtained without the use of electrical contacts. Large persistent currents are induced in ring samples at 77 K, after cooling in a 3 kG field. These currents can be determined from the magnetic field they produce. At 77 K a supercurrent exceeding 2000 A (about 104 A/cm2) was induced in a 2 cm diameter ring; this current produces a magnetic field exceeding 1.5 kG in the bore of the ring. We demonstrate that when a ring is cut, and the cut is repaired by a superconducting weld, the weld joint can transmit the same high supercurrent as the bulk.

Comparison of Au-free versus Au-doped YBa2Cu3O7−δ single crystals: Overdoping and oxygen mobility

Abstract By systematically changing the oxygen concentration of YBa2Cu3O7−δ single crystals grown in Au-crucibles, we demonstrate that the incorporated Au-impurities have only a minor effect on the maximum in Tc due to overdoping. On the other hand, we demonstrate that the Au-impurities significantly increase the oxygen mobility in the basal planes of oxygen deficient YBa2Cu3O7−δ single crystals.

The mictomagnetic transition in Cu-Mn☆

Abstract The isothermal magnetization processes in the alloy Cu81Mn19 are found to undergo a qualitative change, involving the sudden appearance of magnetic hysteresis, at the temperature (∼ 85°K) at which the initial susceptibility has a sharp maximum.

Electrical resistivity and initial susceptibility of weakly ferromagnetic VFe alloys

Abstract Electrical resistivity measurements between 1.5 and 300°K on VFe alloys indicate that the large magnetic clusters in these alloys are stable up to 300°K. The onset of ferromagnetic ordering causes a sharp peak in the initial susceptibility and a clear kink in the temperature coefficient of resistivity, dϱ/dT.