S. Foner

Versatile and Sensitive Vibrating‐Sample Magnetometer

A vibrating‐sample magnetometer, which measures the magnetic moment of a sample when it is vibrated perpendicularly to a uniform magnetizing field, is described. With this instrument, changes as small as 10−5 to 10−6 emu have been detected, and a stability of one part in 104 has been attained. In addition to permitting convenient measurements in the usual laboratory electromagnet, this instrument eliminates or minimizes many sources of error found in other methods. It is simple, inexpensive, and versatile, yet permits precision magnetic moment measurements to be made in a uniform magnetizing field as a function of temperature, magnetizing field, and crystallographic orientation. The mechanical design and detailed operating characteristics are presented. Applications and limitations of the method are outlined.

Surface spin disorder in ferrite nanoparticles (invited)

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in ...

600 kG superconductors

Abstract Upper critical fields, H C2 ( T ), for Sn-, SnAl-, and Pb- molybdenum sulfide superconducting materials were measured in dc fields to 215 kG and pulsed fields to 495 kG. For the highest T c (14.4 K) Pb compounds H c2 (4.2K) is approximately 510 kG and extrapolates to H C 2 (0)=600 kG. These are the highest critical fields reported for any superconductor.

Upper critical fields of high-temperature superconducting Nb1−y(Al1−xGex)y and Nb3Al: Measurements of Hc2 > 400 kG at 4.2°K

Abstract Pulsed field measurements of the upper critical fields at 4.2°K, H c2 (4.2°K), in Nb 1− y (Al 1− x Ge x ) y and Nb 3 Al are approximately 410 kG and 295 kG respectively. The data are consistent with complete suppression of Pauli paramagnetic limiting.

High‐field critical current in in situ multifilamentary Cu‐Sn‐Nb alloys

High‐field properties of improved in situ Cu‐Sn‐Nb alloys are presented. Values of critical current densities of 104 A/cm2 and a resistivity of less than 2×10−13 Ω cm are observed at 12 T. Critical current densities for various in situ alloys are presented for fields to 18 T.

Detection coil, sensitivity function, and sample geometry effects for vibrating sample magnetometers

A general description based on the sensitivity function is presented for the effects of detection coil geometry and sample geometry on the output of a vibrating sample magnetometer (VSM). This function gives the VSM output versus position of a vibrating dipole. Calculations of the sensitivity function for axial and transverse detection coil configurations are presented using exact results and approximations including a spherical harmonic expansion. For axial, thick, rectangular cross‐section coils, the design yielding the output with a minimum sensitivity to sample position, sample shape, and sample size is presented. For the transverse geometry the small coil approximation is used. Various designs yielding maximum output and output insensitive to variations of sample geometry are reviewed. The signal‐to‐noise for various coil configurations is also discussed. Corrections for the VSM output are calculated for ’’large’’ samples with regular geometries (thin rod, circular cylinder, cube, and rectangular par...

68.4‐T‐long pulse magnet: Test of high strength microcomposite Cu/Nb conductor

A multilayer wire‐wound pulsed field magnet was fabricated using a new metal‐matrix microcomposite Cu/Nb conductor. The 0.5‐in.‐i.d. magnet used square cross‐section Cu/Nb wire, precompressed in a hardened steel container and cooled to 77 K, to generate 68.4±1 T for a 5.6‐ms half‐period. The maximum field was generated using a 100‐kJ, 4‐kV capacitor bank of the Francis Bitter National Magnet Laboratory pulsed field facility. The Cu/Nb composite had an ultimate tensile strength of 180 ksi at 77 K and 140 ksi at 293 K. These new microcomposites have exceptional strength, electrical, and thermomechanical properties for very high stress applications.

Phase formation sequence for the reaction of multilayer thin films of Nb/Al

We have investigated the phase formation sequence for the reaction of Nb and Al in multilayer films using cross‐sectional transmission electron microscopy and x‐ray diffraction. NbAl3 is the first intermetallic phase to form. Contrary to previous reports, we find evidence from cross‐sectional transmission electron microscopy that the sigma phase, Nb2Al, is not bypassed in the reaction sequence. Instead, its formation is concurrent with the formation of the superconducting A15 phase, Nb3Al. However, depending on the periodicity and the composition of the film, the Nb2Al phase can be consumed by the Nb3Al phase for long annealing times. The significance of this phase formation sequence to powder metallurgically processed magnet wire is discussed.

Fabrication of multifilamentary Nb-Al by a powder metallurgy process

Fabrication of multifilamentary Nb-Al superconducting wires with high overall critical current densities at high fields is discussed. Powder metallurgy processed materials have been made with low reaction temperatures and with J c > 104A/cm2at 14 T. The effects of variations of Nb and Al powder size, composition, reduction ratio, reaction temperature, and reaction time on J c vs appiied field are presented. All the Nb-Al materials show good strain tolerance at high J c and high field. The results also show that scaleup is promising.

Coils for the Production of High‐Intensity Pulsed Magnetic Fields

The problem of producing extremely high magnetic fields is briefly reviewed in the light of modern technique. The design, performance, and application of a pulsed‐field system capable of more than 750 000 gauss at room temperature is described. The coil comprises a suitably supported, machined, beryllium‐copper, helix having an inside diameter of 316 in. and a length of about ½ in.; it is connected directly to a 2000 μf, 3 kv bank of surge capacitors by means of a triggered‐spark gap. The discharge is oscillatory with a half‐period of 120 μsec. Detailed design data and performance characteristics are presented for a large range of similarly constructed coils which afford increased volume and field uniformity at a sacrifice in field intensity. A coil providing transverse access to the field and one suitable for operation in liquid helium are also described. Characteristics of less durable coils constructed of a single strip of conductor are discussed. Brief comments on applications to a broad range of soli...