A. I. Schindler

Specific heat, electrical resistance, and other properties of superconducting Pd-H alloys

Abstract Specific heat and electrical resistance measurements have been carried out in the ∼1.2 to 4.2 K range for a series of Pd-H alloys having H/Pd atomic ratios in the ∼0.82 to 0.88 range. Superconductive transitions in the resistance were observed; the specific heat data exhibited broad, yet very pronounced peaks with characteristics that depended strongly upon hydrogen concentration. A fairly successful theoretical fitting of the specific heat data was carried out on the basis of the assumption that Pd-H alloys are simply inhomogeneous BCS superconductors. Data for the dependence of the superconducting transition temperature upon hydrogen concentration were derived from both the specific heat and electrical resistance measurements. An interesting scaling feature of the specific heat data is discussed. A search for a possible interrelationship between the superconductive properties and an exothermic process which occurs in Pd-H alloys yielded negative results. A marked suppression of superconductive effects was observed in Pd-H alloys based on one particular Pd ingot; this was probably due to Fe impurities.

Magnetostrictive properties of Hox Tb1 −x Fe2 intermetallic compounds

The magnetostriction of several polycrystalline compounds in the Hox Tb1−x Fe2 series have been measured from 77–300°K using magnetic fields up to 17.5 kOe. The extrapolated saturation magnetostriction at room temperature varies linearly with x between 80 × 10−6 for HoFe2 and 1720 × 10−6 for TbFe2, in good agreement with the single‐ion model. At the composition x =0.85, where the lowest‐order anisotropy constant K1 is very small, the saturation magnetostriction is 325 × 10−6, suggesting that this material might be useful in magnetostrictive device applications.

Residual Resistance Ratio of Pure Iron

The resistance at zero magnetic induction of iron specimens of varying impurity content has been determined by extrapolating the longitudinal magnetoresistance data taken at magnetic fields above saturation. These resistance values may then be used to determine a modified residual resistance ratio which is shown to be a more appropriate index of purity for ferromagnetic metals than that obtained with resistances measured in zero applied magnetic field.

Specific heat of superconducting PdH alloys

Abstract The low temperature specific heat of a series of superconducting Pd-H alloys ( H Pd ∼ 0.83 5 –0.87 5 ) has been measured; broad yet very pronounced peaks whose widths are a function of hydrogen concentration are observed. An interesting scaling feature and a theoretical fitting of the data are discussed. A few results of some associated electrical resistance measurements are mentioned, and the possible role of Fe impurities in suppressing superconductivity in PdH alloys is considered.

Investigations of the Effects of Neutron and He3 Irradiation on the Magnetic Properties of Permalloy Thin Films

Measurements have been made of the changes in the magnetic anisotropy energy and hysteresis loop parameters of evaporated Permalloy thin films resulting from 2.2‐MeV He3 irradiation and from reactor neutron irradiation. Only minor changes were observed following an irradiation of 1.6×1014 He3 particles/cm2. However, a total irradiation of either approximately 1017 neutrons/cm2, or 8 to 9×1016 He3 particles/cm2, yielded the following general results: Regardless of the magnetic state of the sample, the uniaxial anisotropy energy Ku increased (up to 200%) when irradiations took place in zero applied field. Similar increases were noted as a result of either neutron irradiation with a saturating magnetic field along the easy axis or He3 irradiation with a saturating magnetic field either parallel or perpendicular to the easy axis. A decrease of ∼50% in Ku resulted from neutron irradiation when the saturating magnetic field was applied perpendicular to the easy axis. Rotation of the easy axis, up to 78°, occurred for either neutron or He3 irradiation with a saturating magnetic field applied perpendicular to the easy axis. The more significant hysteresis loop modifications observed were the development of constricted loops (measured in the hard direction) for most of the samples irradiated in zero applied field, and decreases in the coercive force up to 28% (measured in the easy direction) of most of the irradiated samples.Measurements have been made of the changes in the magnetic anisotropy energy and hysteresis loop parameters of evaporated Permalloy thin films resulting from 2.2‐MeV He3 irradiation and from reactor neutron irradiation. Only minor changes were observed following an irradiation of 1.6×1014 He3 particles/cm2. However, a total irradiation of either approximately 1017 neutrons/cm2, or 8 to 9×1016 He3 particles/cm2, yielded the following general results: Regardless of the magnetic state of the sample, the uniaxial anisotropy energy Ku increased (up to 200%) when irradiations took place in zero applied field. Similar increases were noted as a result of either neutron irradiation with a saturating magnetic field along the easy axis or He3 irradiation with a saturating magnetic field either parallel or perpendicular to the easy axis. A decrease of ∼50% in Ku resulted from neutron irradiation when the saturating magnetic field was applied perpendicular to the easy axis. Rotation of the easy axis, up to 78°, occurr...

EFFECT OF IRRADIATION ON MAGNETIC PROPERTIES OF Fe-Ni ALLOYS

Hysteresis loop measurements were conducted on several commercial iron‐nickel alloys during neutron irradiations at 90°K and at temperatures up to 175°C, in both zero and saturating magnetic fields. Isochronal annealing tests were also performed on the alloys after irradiation. The low‐temperature irradiation results indicated that magnetic properties are affected not by the point defects themselves but by their motion and eventual disposition. The radiation‐induced changes appeared similar in many ways to those observed in these alloys upon magnetic annealing.Experiments on the alloys at temperatures from 70° to 175°C in a saturating magnetic field showed that the activation energy of the radiation‐induced process was 0.3 eV. In all cases the radiation‐induced changes were annealed out at temperatures below the usual ordering temperature for Ni3Fe. This recovery process appeared to be dependent upon the state of the magnetization of the sample.

Effect of Neutron Irradiation of the Magnetic Properties and Degree of Order of Magnetic Metal Alloys

Sixty‐cycle sine current hysteresis loops have been made on thermally treated Permalloy type material both prior to and following neutron irradiation. In all cases the hysteresis loops of the irradiated alloys resembled the constricted loops characteristic of partially ordered Ni3Fe. The effect of the sample temperature during irradiation on the magnetic properties suggests that ordering results from diffusion of vacancies.

Effect of Applying a Magnetic Field During Neutron Irradiation on the Magnetic Properties of Fe-Ni Alloys

To test the similarity between thermally induced ordering and irradiation induced ordering, a series of commercial Fe-Ni samples have been irradiated in the presence of a saturating magnetic field. Square hysteresis loops were found for all the samples irradiated in this manner. Such results are consistent with the proposal that directional short range ordering and consequent uniaxial anisotropy has been created by the neutron irradiation.

Electrical Resistivity of Nickel‐Palladium Alloys

The concentration dependence of the residual resistivity of Ni–Pd alloys might be expected to obey a simple Nordheim law since the number of holes in the d band and the number of electrons in the s band are approximately independent of composition. Measurements have indicated, however, that the maximum resistivity occurs at 70 atomic percent Pd instead of 50%. This behavior can be explained by the spin dependence of electron scattering processes associated with the fact that the alloys with greater than 3% Ni are ferromagnetic. Quantitative fit of the data is possible if it is assumed that the density of states at the Fermi energy in the d band is approximately proportional to the number of holes, rather than to the cube root of this number. Data on the resistivity and paramagnetic susceptibility of Ag–Pd alloys support this hypothesis. The relative strength of s‐d scattering to s‐s scattering must be assumed to be about 5:1.

Magnetic properties of iron‐implanted graphite

We have measured the magnetic properties of highly oriented pyrolytic graphite (HOPG) implanted with fluences of 25‐keV iron atoms ranging from 1016 to 1017 atoms/cm2. The lowest fluence specimen was paramagnetic down to 2 K, with evidence for clusters of only a few spins, while the highest fluence specimen was clearly ferromagnetic, with magnetization curves resembling those of a set of randomly oriented soft magnetic planes. The critical fluence for formation of a ferromagnetic state appears to be between 1 and 3×1016 atoms/cm2 at 25 keV. These results can be qualitatively understood based on the critical density for percolation of near neighbor exchange interactions.