T. H. Tiefel

High TC superconductors—composite wire fabrication

Commercially useful, bulk superconductors typically require stabilization using a normal metal cladding for reasons of electrical, thermal, and mechanical protection and, in general, need to be drawn into fine fibers and wound into a magnet configuration. The recent discovery of high‐TC superconductor materials such as Ba2YCu3O7 stimulated worldwide interest in the subject, however, with much concern about fabricability of such brittle ceramic materials into desirable fine wire geometry. In this letter, we report preliminary success in the fabrication of fine‐wire, composite superconductors consisting of a high‐conductivity normal metal shell such as Ag or Cu/Ni/Au and a superconducting core of Ba2YCu3O7 oxide. The wire is would into a coil, and then heat treated to produce the desired chemistry in a dense structure. The resistivity of the composite wire is measured to be zero at ≊90 K (in zero field) with a zero‐field critical density of ≊175 A/cm2. Microscopy and x‐ray analysis show that the superconduc...

High frequency properties of Fe–Cr–Ta–N soft magnetic films

High frequency magnetic properties of a new, soft magnetic Fe–Cr–Ta–N alloy film have been investigated. Thin films with a composition of Fe-4.6% Cr-0.2% Ta-7.4% N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc as low as 1.2 Oe (95.3 A/m) and 4πMs∼20 kG. The easy-axis M–H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–100 Oe (1.59–7.95 kA/m). By virtue of their high 4πMs and relatively high Ha, these soft magnetic films exhibit high permeability and low loss in the GHz frequency range with the undesirable ferromagnetic resonance interference suppressed to beyond 2 GHz.

Effect of Y sub 2 BaCuO sub 5 inclusions on flux pinning in YBa sub 2 Cu sub 3 O sub 7 minus. delta

The effect of Y{sub 2}BaCuO{sub 5} (211) inclusions on flux pinning in the YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (123) superconductor has been investigated. Intragranular {ital J}{sub {ital c}} has been measured on samples containing various amounts of dispersed (211) particles (0%--44%) with various particle sizes (1.3--10.4 {mu}m). The results clearly show that the presence of (211) particles (at least down to the micron size level) neither significantly enhances the flux pinning nor deteriorates {ital J}{sub {ital c}}. The observed insensitivity of {ital J}{sub {ital c}} to the variation in the total particle surface area indicates that the likelihood of a substantial interface-pinning effect is minimal. While the (211) inclusions appear to assist in the peritectic reactions resulting in better microstructural perfection during melt processing, particle size needs to be controlled to a much finer scale in order for them to serve as efficient pinning centers.The effect of Y2BaCuO5 (211) inclusions on flux pinning in the YBa2Cu3O7−δ (123) superconductor has been investigated. Intragranular Jc has been measured on samples containing various amounts of dispersed (211) particles (0%–44%) with various particle sizes (1.3–10.4 μm). The results clearly show that the presence of (211) particles (at least down to the micron size level) neither significantly enhances the flux pinning nor deteriorates Jc. The observed insensitivity of Jc to the variation in the total particle surface area indicates that the likelihood of a substantial interface‐pinning effect is minimal. While the (211) inclusions appear to assist in the peritectic reactions resulting in better microstructural perfection during melt processing, particle size needs to be controlled to a much finer scale in order for them to serve as efficient pinning centers.

Fe-Cr-N soft magnetic thin films

Soft magnetic properties of new Fe-Cr-N and Fe-Cr-Ta-N alloy films have been investigated. Thin films with compositions in the range of Fe-2∼8% Cr-0∼1% Ta-5∼15% N (in at.u2009%) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc∼1–2 Oe (79.4–158.8 A/m) and 4πMs∼15–20 kG. The easy-axis M-H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–60 Oe (1.59–4.77 kA/m). The combination of high 4πMs and relatively high Ha in these films is conducive to the suppression of the undesirable ferromagnetic resonance (FMR) interference up to the GHz frequency range.

Fabrication and properties of high-T/sub c/ superconducting wires

The discovery of high‐temperature superconductivity in perovskite‐related oxides has generated an enormous amount of research activity and development effort toward applications. Commercially useful bulk superconductors typically require stabilization using a normal metal cladding for reasons of electrical, thermal, and mechanical protection, and in general need to be fabricated into fine fibers and wound into a solenoid configuration. The YBa2Cu3O7−δ type compound is a ceramic material which is mechanically hard and brittle, and is difficult to fabricate into fine wires. However, this difficulty has been overcome by several fabrication methods such as powder‐in‐tube method (metal‐clad composite), powder‐coating method (metal‐core composite), and molten oxide processing method. The effect of various processing steps on the structure and superconducting properties such as transition temperature and critical current density will be discussed. Some experiments to raise the high field critical current through...

Fe-Cr-Hf-N and Fe-Cr-Ta-N Soft Magnetic Thin Films

We have investigated the soft magnetic properties of new Fe-Cr-Hf-N and Fe-Cr-Ta-N alloy films. Thin films with compositions in the range of Fe-2/spl sim/8%Cr-0/spl sim/1%Hf (or Ta)-5/spl sim/10%N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post-deposition heat treatment, e.g., H/sub c//spl sim/1-2 Oe (79.4-158.8 A/m) and 4/spl pi/M/sub s//spl sim/15-20 kG. The magnetic properties are highly anisotropic in the film plane as the easy-axis M-H loop is square and the hard-axis loop is linear and closed, with the anisotropy field H/sub a/=20-60 Oe (1.59-4.77 kA/m). The films exhibit excellent high frequency behavior even in the GHz frequency range which is attributed to the combination of high 4/spl pi/M/sub s/ and relatively high H/sub a/.

Magnetoresistance in cosputtered Cu–Ni–Fe films (abstract)

The magnetoresistance effect, especially the giant magnetoresistance (GMR) effect, has received much attention in recent years. In this study, we discuss the magnetoresistance behavior in Cu–Ni–Fe thin films with Cu content varied from 40 to 90 at.%, prepared by the cosputtering of both Cu and Fe50Ni50 targets. Films with low Cu content, for example, Cu50Ni25Fe25 and Cu40Ni30Fe30, exhibit a mixed behavior of GMR and anisotropic magnetoresistance (AMR). The electrical resistivity of these films substantially increases once the field is applied due to the anisotropic magnetoresistance contribution, and then decreases again at higher fields, which is believed to be related to the giant magnetoresistance effect. As a result of a compromise between both the GMR and the AMR effects, the MR ratios of these low Cu content films are only minus 1%–2% both at room temperature and at 4.2 K. However, it is found that the giant magnetoresistance contribution dominates magnetoresistance behavior in films with Cu content...