E.J. McNiff

Anomalous properties of magnetic nanoparticles

Abstract Nanoparticles of ferrimagnetic NiFe2O4 and antiferromagnetic NiO exhibit a variety of anomalous magnetic properties. The lower coordination of surface spins is responsible in both cases for the observed behavior. This conclusion is supported by calculations of field-dependent spin distributions in these nanoparticles.

Upper critical fields of cubic and tetragonal single crystal and polycrystalline Nb3Sn in DC fields to 30 tesla

Abstract Measurements of the upper critical field, H c2 (T), in single crystal Nb 3 Sn were extended to 30 tesla (300 kG) with dc fields produced by a Hybrid magnet. Observations of H c2 (T) were made for materials which remain in the cubic (c) phase and those which show a martensitic transformation at the tetragonal (t) phase. H c2 (T) measurements of Nb 3 Sn for a pure crystal for which the de Haasvan Alphen (DHVA) effect was observed and for polycrystalline (t) phase and (c) phase materials are also reported. Measured values of H c2 (4.2 K) and calculated values of H c2 (0) are: 1) along the [100] direction for our earlier Nb 3 Sn, H c2 (4.2 K) = 26 T for the (c) phase and 21.5 T for the (t) phase; H c2(0) = 29T for the (c) phase and 24 T for the (t) phase; 2) along the [100] direction for the DHVA material H c2 (4.2 K) = 18 T and H c2 (0) = 20 T; 3) for polycrystalline Nb 3 Sn (t) phase material H c2 (4.2 K) = 23 T and H c2 (0) = 25 T and for (c) phase material H c2 (4.2 K) = 26 T and H c2 (0) = 29 T. The values of (dH c2 /dT) T=Tc vary from 2.4T/K for the highest H c2 (T) material to 1.6T/K for the DHVA material. The anisotropy for various Nb 3 Sn single crystal materials is small and independent of temperature from T c to 0.1 T c . δT c between the (c) and (t) phase is c2 (T) with theory are obtained assuming a dirty or clean Type II superconductor with no Pauli paramagnetic limiting. Experimental details and strong-coupling effects are discussed. When strong-coupling is included, the effects of any paramagnetic limiting would be small and not detectable within our present experimental error. Brief comments also are made concerning H c2 of V 3 Si.

Improvements of ’’insitu’’ multifilamentary Nb3Sn superconducting wires

’’In situ’’ multifilamentary Nb3Sn wires with improved high‐field properties are reported. An overall critical current density greater than 104 A/cm3 is achieved at 14 T for a Cu–36 wt%–Nb–20 wt% Sn material. The results are comparable to the best reported commercial multifilamentary Nb3Sn materials.

Mechanical properties of in situ multifilamentary Nb3Sn superconducting wires

The mechanical properties of in situ Cu–36 wt% Nb–x wt% Sn superconducting wires are presented where 5<x<20 wt% Sn. For high x, no degradation of critical current Jc is observed for stresses σ up to 700 MPa (∼100 ksi) and a maximum Jc occurs at σ≃450 MPa. Strains of ∼0.6–0.9% are measured at 300 and 77 K for 450 MPa, and ∼1.5–2% for 700 MPa. Cyclic stress data are consistent with a prestress model for composite materials.

Mechanical properties of in situ multifilamentary Nb/sub 3/Sn superconducting wires

The mechanical properties of in situ Cu–36 wt% Nb–x wt% Sn superconducting wires are presented where 5<x<20 wt% Sn. For high x, no degradation of critical current Jc is observed for stresses σ up to 700 MPa (∼100 ksi) and a maximum Jc occurs at σ≃450 MPa. Strains of ∼0.6–0.9% are measured at 300 and 77 K for 450 MPa, and ∼1.5–2% for 700 MPa. Cyclic stress data are consistent with a prestress model for composite materials.

Fabrication on a laboratory scale and mechanical properties of Cu-Nb-Sn multifilamentary superconducting composite wires produced by cold powder metallurgy processing

Superconducting Cu‐Nb‐Sn multifilamentary composites are fabricated inexpensively on a laboratory scale. Small (40 μm) particles of Cu and Nb are compacted, placed in a suitable external jacket for containment, then elongated at room temperature to form a multifilamentary circular wire. Processing yields a multifilamentary Cu‐Nb‐Sn superconductor with high overall critical current densities Jc at high magnetic fields. Measurements of the mechanical properties show no degradation of Jc for strains greater than 1% for composite made with a large areal reduction ratio.

High critical currents in cold‐powder‐metallurgy‐processed superconducting Cu‐Nb‐Sn composites

Cold‐powder‐metallurgy‐processed superconducting Cu‐Nb‐Sn (discontinuous) multifilamentary composites have been fabricated. Overall critical current Jc comparable to the best in situ and commercial multifilamentary Nb3Sn (scaled for the same Nb content) have been achieved. Values of Jc approximately 105 A/cm2 at 12 T, 5×104 A/cm2 at 14 T, and 2×103 A/cm2 at 18 T are observed for a material with Cu–40 wt.% Nb–20 wt.% Sn with respect to Cu. The physical characteristics of the starting materials and some advantages of the cold‐powder‐metallurgy process are discussed.

Magnetization and magnetic suspension of YBa2Cu3Ox-AgO ceramic superconductors

Abstract The magnetic force which accounts for the newly-discovered suspension of a superconductor below a permanent magnet is determined by the magnetization of the superconductor and the magnetic-field gradient. Magnetization measurements were carried out on a series of YBa 2 Cu 3 O x -AgO ceramic superconductors, with T c ≈93 K. The samples were from the set of samples in which the magnetic-suspension phenomenon was first discovered. Magnetization data were taken at 4.2 and 77 K in magnetic fields up to 180 kOe. Hysteresis loops at low fields, up to 1.2 kOe, were also studied at 4.2, 77 and 87 to 88 K. The magnetization and hysteresis in most of the samples are among the largest observed to date in ceramic high- T c superconductors. In most of our samples, the remanent moment at 4.2 K is about 80 emu/g, and about 3 emu/g at 77 K. The large magnetization and hysteresis indicate the presence of strong pinning forces. The strong hysteresis at 77 K results in an appreciable positive magnetization, parallel to the field, when the field H is decreased from a finite value (above≈0.5 kOe). This positive magnetization increases with decreasing H . The positive magnetization can be produced by bringing a permanent magnet close to the superconductor, and then withdrawing it slowly. This leads to an attractive magnetic force between the superconductor and the permanent magnet. Calculations, based on a realistic model, show that at 77 K this magnetic attraction can be sufficiently strong to balance the gravitational force. As a result, the superconductor can be suspended below a permanent magnet. The expected damped oscillatory motion near the suspension point, following the application of a vertical impulse to the superconductor, is discussed. This motion is more complicated than that near the bottom of a conventional potential well. Some remaining problems associated with the magnetic-suspension phenomenon are outlined.

Magnetization steps due to pairs of distant-neighbor spins in Zn1-xCoxSe and Zn1-xCoxS

Magnetization steps (MSTs) were observed in Zn1-xCoxSe and Zn1-xCoxS using temperatures T < 1 K, steady magnetic fields up to 200 kOe, and pulsed fields up to ∼400 kOe. Unlike the MSTs observed earlier in other materials, these MSTs are due to pairs of distant-neighbor spins rather than to pairs of nearest-neighbor spins. For Zn1-xCoxSe the results give a second-neighbor exchange constant J2/kB = −3.04±0.1 K, and suggest a third-neighbor exchange constant J3/kB ≌ −0.8 K. An exchange constant J/kB = −2.25±0.2 K observed in Zn1-xCoxS is either J2 or J3.

Temperature anomalies observed in liquid 4He columns in magnetic fields with field–field‐gradient products >21 T2/cm

Previously reported temperature increases above 4.2u2009K in an immersion geometry in liquid 4He at fields >19 T have been verified and examined. Magnetic forces (B dB/dx) above a threshold level of ∼21 T2/cm were found to trap 4He bubbles near the center of the field. Significant heat input to the region (from conduction, radiation, etc.) can then raise the temperature by as much as several kelvins. Thus, measurements at very high B dB/dx should not depend on vapor pressure thermometry. To assure temperature accuracy and stability, the temperature should be measured directly at the sample position.