W. J. Carr

Conductivity, permeability, and dielectric constant in a multifilament superconductor

Results are given for the permeability, conductivity, and dielectric constant of a multifilament superconductor, treated as an anisotropic state of matter. The calculations are made for the limiting cases of strong and weak ac magnetic fields compared with the field required for complete penetration of the filaments. General expressions are given for the ac power loss.

Temperature Dependence of Magnetostriction and Anisotropy in MnBi

Measurements were made of the magnetostriction and anisotropy in oriented polycrystals of MnBi at various temperatures between 77–300°K. The magnetostriction constants in a four-constant expression have been evaluated as a function of temperature, and the first three anisotropy constants also have been estimated. At least at the lower temperatures, it is found that all three constants are necessary to fit the anisotropy data.

Energy Loss Resulting from Domain Wall Motion

At low frequencies the fraction of domain walls moving at a given instant is small; at higher frequencies all the walls move. Therefore, as the frequency approaches zero the eddy current constant becomes larger. The calculations of Pry and Bean are shown to pertain to the higher frequency case.It also is pointed out that the hysteresis loss should not be the lower limit of the loss vs frequency curve as usually measured.

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.

Alternating field loss in a multifilament superconducting wire for weak ac fields superposed on a constant bias

Expressions are given for the alternating field loss in a twisted filamentary superconductor as a function of frequency and magnetic field, for the case of small ac applied fields which do not completely penetrate the filaments. The ac field is superimposed on a fixed bias field. In this limit the effective diamagnetism of the filament leads to a permeability less than unity for the composite. Expressions are presented for both the hysteresis and the eddy current loss.

Propagation of magnetic domain walls by a self‐induced current distribution

Calculations are made for the motion of a domain wall surrounded by a perturbed region of current density resulting from the action of the magnetic field of the domain on a uniform impressed current density. The perturbed region follows the motion of the domain wall and produces a magnetic field which causes the domain to propagate. In particular, a ferromagnetic layer containing a cylindrical domain, with a Hall‐effect overlayer, is considered.

Temperature Dependence of Ferromagnetic Anisotropy

The temperature dependence of the crystalline anisotropy constants of iron, cobalt, and nickel is discussed.It is shown that Zeners result for iron (i.e., the first anisotropy constant varies as the tenth power of the magnetization) also may be derived from molecular field theory.In cobalt a satisfactory agreement with experiment is obtained by using Zeners results together with the postulate that the intrinsic anisotropy varies with thermal expansion in the manner recently calculated by the author.For nickel also the temperature dependence of K1 seems to require, in addition to the tenth power of the magnetization, a multiplicative factor that is linear in the temperature. No explanation has been found for this latter term.

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...

Macroscopic and microscopic models of in situ superconductors

It is argued that a macroscopic formalism similar to that for continuous filament material can be applied to in situ superconductors, providing an elongated volume element is used for averaging over the microscopic fields and currents. The difference between in situ and continuous filament materials is found in the constitutive equations. For an in situ tape these consist of a specification of three critical current densities for the macroscopic superconducting state, together with three conductivities for the nonsuperconducting state, and, under some conditions, three magnetization components. The interesting region of the nonsuperconducting state occurs when the material is macroscopically nonsuperconducting, but superconducting microscopically. A microscopic model based on the proximity effect is developed for computing the constitutive equations. For calculating the axial current density the model assumes that chains of filaments run through the conductor. When the superconducting proximity layers sur...

Explanation for the Low‐Temperature Behavior of the Anisotropy of Iron

The seemingly anomalous results obtained by Graham for the temperature dependence of magnetic anisotropy in iron are interpreted on the basis of Zeners theory, with the effect of thermal expansion included.