M. P. Mathur

Thermal and elastic properties of silicate oxyapatite crystals

The elastic constants, thermal expansion, thermal conductivity, and heat capacity are reported for a new class of laser hosts‐the silicate oxyapatites. For Ca2La8(SiO4)6O2 the five independent elastic constants obtained from sound velocity measurements are C11=1.71, C12=0.62, C33=1.71, C44=0.52, and C13=0.39×1012 dyn/cm2, the thermal expansion coefficients at 300 K are 8.9×10−6 and 6.6×10−6 K−1 for the [1010] and [0001] directions, respectively, and the room‐temperature conductivity is about 0.019 W cm−1 K−1. Sound velocity and thermal expansion data suggest a Debye temperature between 400 and 500 K.

Magnetic Studies of the Alloy System SnTe–MnTe

Magnetic ordering and superconductivity have been investigated in the system Sn0.97‐xMnxTe for 0.00001≤x≤0.16 over the temperature range 0.02≤T≤300°K. Reciprocal Hall coefficient, 1/Re, for all these samples was ∼1021 cm−3. Samples with x>0.005 showed ferromagnetic ordering with the Curie temperatures varying linearly with x. For 0.0001<x<0.005, neither magnetic ordering nor superconductivity were observed above 0.02°K. In samples with x<0.0001 superconductivity has been observed with the transition temperatures being depressed below that for Sn0.97Te (Tc≃0.20°K). Measurements have also been made on samples with constant manganese concentration and varying carrier density. Heat capacity measurements between 2°–30°K showed anomalies attributable to the magnetic coupling of the Mn ions. These results will be discussed in terms of current long‐range indirect‐exchange interaction theories.

Study of Magnetic Interactions in (SnTe–MnTe) and (SnTe–EuTe) Systems

As reported earlier the alloy system Sn0.97−xMnxTe orders ferromagnetically with Curie temperatures varying linearly with x. The present work has extended this and includes: (i) More superconductivity results which indicate that for low carrier densities (∼1021 cm−3), the paramagnetic impurity influences Tc very drastically, e.g., 100‐ppm Mn destroys the superconductivity completely. However, if the carrier density is increased to 4×1021 cm−3, the transition temperature decreases gradually with increasing Mn content and superconductivity persists even for x=0.001. In both cases, however, the transition is broadened probably due the presence of the internal fields. (ii) Electrical resistivity measurements made on these samples (0.00001<x<0.10) between 4° and 100°K, show a characteristic metallic behavior but with no maxima or minima appearing in the residual part. (iii) A study of s‐f interactions by means of Eu doping for comparison with s‐d interaction results (Mn doped). Eu‐doped samples are single phas...

Lower critical field measurements in NbN bulk and thin films

Low‐field magnetization measurements have been made at 4.2°K on thin‐film and bulk NbN samples using a vibrating‐sample Foner magnetometer. These data can be used to calculate the upper critical field, Hc2, without having to resort to resistivity data, which, particularly in the case of NbN, yield very anomalous results. In the present work the experimental value of Hc1 is obtained from the magnetization curves and Hc2 is then calculated using the GLAG equations. These calculations and the significance of the Pauli spin paramagnetism and spin‐orbit scattering in these materials are discussed.

Flux pinning in Nb 3 Sn multifilamentary conductors

Magnetization measurements on diffused reacted samples were made at several temperatures T, between 4.2K and T c and magnetic fields H up to 75 kOe. The samples were distinguished from each other by different heat treatments and diameter sizes ranges from 200-500 μm. From these measurements J c (H,T) and F p (H,T) curves were derived where F p is the flux pinning force density. For all samples, F p (H,T)/F pmax (T) vs H/H c2 needs to be described by two functions, one for T 8k. This is in contrast to other materials where flux pinning is usually described by a single function.

Scaling laws for flux pinning in Nb and NbTi multifilamentary conductors

Magnetization measurements M(H,T) are made on several commercially drawn Nb–Cu and NbTi–Cu multifilamentary conductors as a function of applied magnetic field and temperature. The samples are distinguished by their filament sizes or the amount of cold work. From these measurements the pinning force Fp(H,T) and the critical current characteristics Jc(H) are deduced for all samples. Using this information Kramers scaling laws for flux pinning in multifilamentary conductors were tested. It is found that the model with some reservations seems to account for most of the observed behavior in the Nb–Cu and NbTi–Cu systems.

Superconductivity as a function of carrier density and magnetic spin concentration in the SnTe-MnTe system☆

Abstract Superconductivity results on self doped as-cast tin telluride show a strong particle size dependence of the superconducting critical temperature ( T c , °K). The T c of the powdered as-cast material (size ⩽ 100μ) approaches the T c of the pressed and sintered samples. A similar but larger effect was observed in Sn 0·87 Ag 0·10 Te, where the T c of the as-cast material (1·8°K) decreased to about 1°K on grinding (size ⩽ 100μ). Pressing and sintering the same material did not increase the T c Complete T c data on the as-cast Sn 0·97− x Ag x Te samples with 0·01 T c of Sn 0·97− x−y Ag x Mn y Te samples was also studied by changing the parameters and the results are discussed on the basis of prevalent theories.

Anisotropy of the critical current density in the NbTi filaments of round composite superconductors

Magnetization measurements have been performed on several multifilament NbTi wires in transverse and longitudinal magnetic fields. The ratio of circumferential and axial critical current densities for the individual filaments is deduced from the measurements and appears to be an increasing function of the magnitude of the magnetic field and the amount of cold work in the filaments. The results suggest that filaments may be less stable in longitudinal magnetic fields than calculations utilizing standard critical current data would indicate. Transverse swept field magnetization data confirm the theory of coupling in the composite.

Invar Behavior in Antiferromagnetic Fe–Mn–(Ni) Alloys

Occurrence of the Invar anomaly in thermal expansion characteristics of antiferromagnetic Fe–Mn–(Ni) alloys having constant Mn/Fe atomic ratios of ⅔ has been investigated. Linear spontaneous magnetostriction, Neel temperature, and inflection temperature were found to be well described by electron concentration; Ni substitution decreased all of these parameters. Such results are shown to be consistent with Zeners Invar theory based on competing exchange interactions.

Critical currents of superconducting conductors measured by a new technique

A magnetic torque technique for measuring critical current densities is described. Results obtained on Nb–Cu and (Nb–Ti)–Cu composite conductors by this technique are presented and compared with direct transport current measurements.