A. Zieba

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...

On the magnetic properties of transition metal substituted MnAs

The properties of Mn1−tTt As phases (T: V, Cr, Fe or Co; 0<t

Superconducting magnet image effects observed with a vibrating sample magnetometer

= 0.10) are studied in external magnetic fields up to 250 kOe, and temperatures between 80 and 400 K. The results are presented and discussed in terms of phase diagrams comprising concentration, temperature, and magnetic field axes.

Geometrical equatorial aberrations in a Bragg- Brentano powder diffractometer with a linear position-sensitive detector

The distortion of the sample flux by a superconducting material produces a change of magnetometer output known as an image effect. Image effects for three Nb–Ti magnets were examined using a small current‐carrying coil as a source of a field‐independent and adjustable, controlled moment of a vibrating sample magnetometer. This allowed determination of the initial image effect (in zero field) and the field‐dependent image effect from zero to maximum magnetic field. The initial image effect (not detected by conventional calibration) can be large, and the field‐dependent part was 0.25–0.35 of initial image effect. The variation of the field‐dependent image effect on field sweep rate and magnet temperature, and the change of the spatial distribution of the detection coil sensitivity were also observed. The image effect is independent of sample moment and decreases rapidly with decreasing detection coil radius. The method of images was employed to calculate the image effect for perfect shielding as the functio...

Low frequency AC losses in multifilamentary superconductors up to 15 tesla

The use of a position-sensitive detector with a Bragg–Brentano diffractometer implies the registration of the diffracted beam at a finite distance from the true focusing point. This gives rise to an aberration which is characterized by the peak intensity profile I(y) and its parameters, the shift of the centre of gravity 〈y〉 and the profile variation W. These are calculated in terms of exact expressions and approximate results. The obtained closed formulae for 〈y〉 and W augment analogous calculations for other aberrations, tabulated by Parrish and Wilson in the International Tables for X-ray Crystallography (Vol. III, 1995). Calculation of the peak profiles is verified by experiment.

128-Channel Silicon Strip Detector Installed at a Powder Diffractometer

Low frequency (1 Hz) ac losses were measured in a variety of A15 superconducting wires having different fiber geometries. Field modulations of ≤ 1 tesla were superimposed on a fixed background field up to 15 tesla. Losses were measured for Nb 3 Sn in continuous fiber, modified jelly-roll, In Situ, and powder metallurgy processed materials, and for Nb 3 Al powder metallurgy processed materials. The results are compared with dc magnetization measurements. The losses are purely hysteretic at these low frequencies, scale with J c (above ∼ 3 tesla), and are reduced substantially by twisting for all the materials. The lowest losses are observed for the Nb 3 Al wires.

Effective exponents for de almeida-thouless line

Silicon strip detectors represent a new class of one-dimensional position-sensitive single photon counting devices. They allow a reduction of measurement time at the powder diffractometers by a factor up to 100 compared to instruments with a single counter, while maintaining comparable count statistics. Present work describes a 128-channel detector working with a standard diffractometer. The detector is 12.8 mm long and covers the angular range of 3.2 deg. We discuss the diffraction geometry in real and reciprocal space, the FWHM of diffraction peaks, and the background level. Measurements were made on standard samples and on complex samples of industrial importance (e. g., portland clinker). Applications of the detector to diffraction measurements of single crystals and thin films are discussed briefly.

Mn0.9Co0.1P in field parallel to hard direction: phase diagram and irreversibility of CONE phase

Abstract The experimental irreversibility line at the magnetic phase diagram H-T of spin glasses is often approximated using a power law Hα (TG-T)φ/2. The mean field theory of de Almeida and Thouless predicts φ=3 for temperatures T close to the spin-glass transition temperature TG . For a range of reduced temperature involved in an experiment one should use, however, effective exponents (φ>3) obtained by fitting the power law to the exact expression for de Almeida-Thouless line. These compare favorably with experimental values of φ obtained for different spin glasses. The increase of effective exponent up to φ=7 on approaching the spin-glass-ferromagnetism multicritical point is also considered.

Improved Performance Powder Metallurgy and In Situ Processed Multifilamentary Superconductors

Abstract A single crystal of Mn 0.9 Co 0.1 P, a homologue of MnP with disordered metal sublattice, has been studied by the ac susceptibility method in a steady field H . This report concerns H parallel to the orthorhombic a axis ( a > b > c ). The magnetic phase diagram is qualitatively similar to that of MnP, including the presence of a Lifshitz multicritical point ( T L = 98 K, H L = 42 kOe) at the confluence of the paramagnetic, ferromagnetic and modulated FAN phases. Contrary to pure MnP, irreversible behaviour was observed in the susceptibility of the modulated CONE phase. This phenomenon develops only for fields above 30 kOe, in contrast to the irreversibility of the FAN phase (reported previously for H ‖ b in the whole field range down to H = 0). New features of the presumably continuous CONE-FAN transition were also found.

Crystal and magnetic structure of NiMnGe

The powder metallurgy (P/M) process and the In Situ process are alternative technologies for producing fine fiber, multifilamentary, A15 superconductors with high overall critical current densities, Jc, at high fields and with good strain tolerance. Several papers were presented at the 1983 Applied Superconductivity Conference by our group.These papers, references cited therein, and related papers should be consulted for progress to that date. Because of space limitations, in this paper we will refer to these papers briefly, examine some fundamental limits to further improved performance, report on new developments in processing, outline recent results, and give some overall assessments of future directions. At this time it is clear that the performance of Nb3Sn can be improved within rather well defined limits, whereas the Jc of Nb3Al, which now surpasses Nb3Sn at high fields, is expected to show further substantial increases in performance. The discussion here is directed toward highest field applications.