C. P. Bean

Eddy‐Current Method for Measuring the Resistivity of Metals

A method for measuring the resistivity of metallic specimens is described. The measurement is made by noting the rate of decay of flux from a bar situated in an external magnetic field that has been rapidly reduce to zero. The method is suitable for specimens greater than 5×10−3 cm in diameter. For a specimen 1 cm in diameter, resistivities from 10−11 to 10−3 ohm‐cm may be measured with an error of less than three percent. The method requires no contact to the specimen, and local values of resistivity may be measured. Several applications are described.

Electrokinetic measurements with submicron particles and pores by the resistive pulse technique

Abstract Electrophoretic velocity measurements on individual colloidal particles above about 60 nm in diameter may be made at a rapid rate under a range of conditions. The observations employ an instrument, the Nanopar analyzer, that is based on the resistive-pulse technique of the Coulter Counter and that uses single submicron diameter pores made by the Nuclepore (etched particle track) process. The analyzer can also measure the electroosmotic velocities of fluids in pores, the dimensions of pores and particles, and particle concentrations. The size and electrophoretic velocity of each particle passing through the analyzing pore are simultaneously measurable. Experiments on latex spheres and T-even bacteriophages demonstrate these capabilities and reveal the potential for two-parameter characterization.

Magnetic Granulometry and Super‐Paramagnetism

The magnetic technique of particle size measurement based on the work of Elmore consists of a comparison of a modified Langevin curve with the observed magnetization curve for an appropriate system of ferromagnetic particles. The average magnetic moment per particle is obtained from this analysis, and the average particle size is thus determined. The prediction by Neel of thermal relaxation in single‐domain ferromagnetic particles broadens the range of applicability of the granulometry method, with special emphasis on the size range from 20 A to about 100 A. A possible objection to the method, arising from evidence for an abnormal temperature dependence of the saturation magnetization of very small magnetic particles, is re‐examined experimentally and found to be invalid. Several examples are presented that demonstrate the predicted thermal relaxation and that point to a wide utilization of the granulometry technique.

A MODEL FOR MAGNETIC INSTABILITIES IN HARD SUPERCONDUCTORS: THE ADIABATIC CRITICAL STATE.

The isothermal critical‐state model of hard superconductors is extended to include the effects of heating when the applied field is changed suddenly and magnetic flux enters adiabatically into the bulk. We consider the following specific situation. A semi‐infinite slab of superconductor is cooled in a magnetic field lying in its surface plane. Next, the external field is raised isothermally by an amount Hs. This excess field decreases linearly to a depth δ= 10Hs/4πJc from the surface. Finally, the field is raised by an infinitesimal amount ΔH in a time short compared to the thermal diffusion time and long compared to the electromagnetic diffusion time. Each element of volume exposed to the changing field receives a thermal impulse proportional to the local‐flux‐change times Jc. This thermal impulse, in turn, lowers the critical current and allows more flux to penetrate. We find that if Hs exceeds some critical value Hfj, then the isothermal critical state is not the only allowed state of the superconducto...

Etching of Submicron Pores in Irradiated Mica

As discovered by Price and Walker, small uniform pores may be created in muscovite by etching in HF thin samples that have been subjected to fission particle irradiation. The process of pore growth is followed by monitoring the conductance across a thin sample as the etching proceeds. For irradiation with 252Cf the tracks quickly etch to a radius of 33 A‐the region of primary damage. Further radial etching in the undamaged material is slow but increases to a fixed rate as the radius increases. This radius dependence of etching is interpreted by a kinetic analog of the Kelvin equation for vapor pressure over curved surfaces. With suitable assumptions on the mechanism of attack, the surface energy of the muscovite‐solution interface is calculated to be about 300 ergs/cm2.

Influence of Pulsed Magnetic Fields on the Reversal of Magnetization in Square‐Loop Metallic Tapes

Reversal of magnetization in square‐loop metallic tapes is described in terms of the domain geometry and the dependence of this geometry upon the applied magnetic field wave shape. For a steady field whose amplitude is slightly greater than the coercive force, the reversal is accomplished by growth of a single axial domain originating at the thin tape edge. For steady fields of much greater amplitude, the reversal domain grows in from all tape surfaces. Because of the finite nucleation time required to establish the reversal domain, short duration, high amplitude field pulses can be used to investigate the mechanism of nucleation of the high field reversal domain configuration. A model which accounts for the nucleation process is suggested. The compatability of this model with various experimental results is presented. From these results an understanding of the high field reversal‐domain configuration and the kinetics of the nucleation and growth of these domains in lamellar materials is obtained. An auxi...

Nucleation of Ferromagnetic Domains in Iron Whiskers

Nucleation fields up to 483 oe have been experimentally observed for the reversal of magnetization in regions of nearly perfect iron whiskers about 5 μ thick. This is near the theoretical limit 2K1/Ms ( = 560 oe  for iron) for the coercive force resulting from magnetocrystalline anisotropy. In addition, conspicuous growth defects on the surface are observed to be sites of easy nucleation. A field of about one oersted, if applied to the damaged end of an iron whisker of this size, suffices to reverse the magnetization by domain wall motion.

Rotational Hysteresis Loss in High‐Field Superconductors

The rotational hysteresis loss in hard type II superconductors is calculated using the critical state model. For materials that can be characterized by a magnetic field‐independent critical current density Jc the surface rotational loss is W(rot)s=[5/4π(6)]1/2(H03/Jc) erg/cm2−cycle. In this expression H0 is the amplitude in gauss of a rotating field applied parallel to the surface of a semi‐infinite slab while Jc is measured in amperes per square centimeter. This loss is 3.84 times the equivalent loss for alternating fields. In addition, it is shown that by measurement of the hysteresis loss or hysteresis torque as a function of field, one may calculate the field dependence of the critical current.

Magnetization of a Dilute Suspension of a Multidomain Ferromagnetic

The observed magnetization curve of a dilute suspension of carbonyl iron powder, type E, is shown to be closely that one would derive from consideration of a dilute assembly of randomly oriented, single‐crystal, multidomain spheres. The deviations are in the directions expected from the effects of stress and non‐spherical clumping. The effects of packing and the applicability of various laws of approach to saturation are discussed.

Effect of Thermal‐Neutron Irradiation on the Superconducting Properties of Nb3Al and V3Si Doped with Fissionable Impurities

The superconducting critical temperatures and current carrying capacities of Nb3Al and V3Si doped with small amounts of uranium and boron and subsequently irradiated with thermal neutrons are reported. While the critical temperatures are substantially unaffected, the critical current densities are dramatically increased by the internal fission of uranium in both materials. Samples of Nb3Al containing 0.321 at.% uranium and V3Si containing 0.19 at.% uranium irradiated with 1.7×1018 neutrons/cm2 give superconducting critical current densities over 106 A/cm2 at 30 kOe. The boron‐doped samples, however, showed no effects arising from fission of boron. Two new experimental techniques have been used in this investigation. First, uranium and boron analyses as well as information on homogeneity were determined by use of the Price‐Walker nuclear track detectors. Secondly, the critical current measurements were made on 50‐mg samples of 70‐μ powders by a technique that measures the generation of odd‐harmonic voltage...