U. Atzmony

Magnetic properties of thin‐film Ag+iron‐oxide granular metals

The magnetic properties of iron‐oxide particulates dispersed in silver (prepared by rf diode sputtering silver and iron oxide over a wide composition range) have been investigated using the Mossbauer effect, electron microscopy, and vibrating sample magnetometry. The sputtered films are nanocrystalline mixtures of immiscible particles of the two constituents (even when only a very small amount of silver is present). At room temperature, the Mossbauer effect data show that for most compositions (even those with low silver content) the majority of the film is superparamagnetic, with the fraction of material in the superparamagnetic state increasing with increasing Ag content. Consistent with these results, very narrow hysteresis loops were observed. On decreasing the temperature to near 100 K, the fraction of material in the superparamagnetic state decreased and the magnetically ordered component increased. Electron diffraction patterns clearly showed the presence of polycrystalline Ag and Fe3 O4 in the fil...

Fourfold anisotropy of an electrodeposited Co/Cu compositionally modulated alloy

Abstract A compositionally modulated alloy of 3 nm layers each of Cu and Co was electrodeposited on a (100) oriented Cu single crystal substrate. Anisotropy with fourfold symmetry in the plane of the film was investigated using ferromagnetic resonance, vector VSM techniques, and measurements of magnetic viscosity. This anisotropy is thought to be the first such effect to be observed in multilayered samples produced by either sputtering or electrodeposition. The anisotropy energy, K 1 , is roughly consistent with values published for fcc cobalt.

Accommodation study of a nanograin iron powder

Accommodation (reptation) is a magnetizing process which occurs when the field is cycled between two extrema. The resulting minor loops do not close upon themselves, but rather approach a closed limit cycle with each application of the applied field extrema. This paper shows that there is significant accommodation in an evaporated nanograin iron powder. The shape of the accommodating loops depends on the applied field extrema.

Magnetization and magnetic aftereffect in textured Ni/Cu compositionally-modulated alloys

Abstract The magnetic properties of Ni/Cu compositionally-modulated alloys with [100], [110] and [111] textures were measured by magnetometry and ferromagnetic resonance. These alloys were found to exhibit a pronounced magnetic aftereffect.

Magnetic viscosity investigations of nanograin iron powder

Magnetic properties of three kinds of nanograin Fe powder were studied. The first two, designated as NM(Ar) and NM(N), were obtained by ball milling the iron powder under an argon or nitrogen atmosphere. The third, designated NN, was obtained by evaporation and condensation. They have grain sizes of 20, 7, and 60 nm, respectively. The particle sizes of the first two were in the micrometer range, whereas those of the NN were in the nanometer range. The NN powder exhibited a coercivity of 79 mT; its magnetic moment at saturation was lower than that of bulk iron. The coercivities of the NM(N) and NM(Ar) were much smaller and their magnetic moment values at high fields were closer to that of bulk iron (210 A⋅m2/kg). The NM(N) and NN samples exhibited marked time dependences of the magnetic moments following a relatively abrupt change of the applied field. The moment changes by more than 0.5% of the saturation moment within 50 min and continues to change for many hours. Much smaller effects were detected in th...

Constricted diamagnetic hysteresis loops observed for the high Tc superconductors

Magnetic hysteresis loops, ac susceptibility, and resistivity measurements have been made on a Ba2 YCu3 O7−x‐type high‐ Tc superconductor. The shape of the hysteresis loops well below Tc are reminiscent of constricted hysteresis loops observed in certain ferromagnetic materials which are usually associated with magnetic aftereffects. Similar dynamic effects, with a time constant on the order of 10 s at 40 K, are shown to be present in the superconducting material. This dynamic magnetic viscosity effect is in addition to the flux creep observed for longer time periods.

Fe Mössbauer effect in YxPr1−xBa2(Cu0.98Fe0.02)3O7

57Fe Mossbauer effect and magnetic‐susceptibility measurements were performed on Y0.2Pr0.8Ba2(Cu0.98Fe0.02)3O7−δ and Y0.8Pr0.2Ba2(Cu0.98Fe0.02)3O7−δ, where δ≊0. The insulating 80% Pr compound showed susceptibility anomalies and a large hyperfine field distribution at low temperatures in addition to a hyperfine field spectrum for Fe on the Cu(2) sites. The superconducting 20% Pr compound displayed none of these effects at any temperature. After examining the alternatives, it is postulated that magnetic ordering of the Pr ions and an enhanced rare‐earth transition‐metal interaction due to f‐electron admixture is responsible for these observations.

Magnetic behavior of both superconducting thin films and their deposition targets

The superconducting properties of the bulk oxides La1.85Sr0.15CuO4−x and Ba2YCu3O7−y were investigated through their magnetic behavior by vibrating‐sample magnetometry and complex ac susceptibility, and by dc resistivity and microwave response. These oxides were then used as targets in the preparation of thin films using a laser‐ablation technique. The superconducting properties of the films were established through their microwave response. The M‐H loops at low temperatures clearly establish the type‐II behavior of the Ba‐Y‐Cu‐O materials. The real part of the ac susceptibility showed the superconducting transition. Simultaneously, the imaginary part of the ac susceptibility showed the existence of nonsuperconducting portions in the samples which have a low enough resistivity to carry a significant amount of current. The novel method of microwave response was used to detect the superconductivity for both the bulk oxides and the thin films.

Is there a paramagnetic Meissner-Ochsenfeld effect?

We report on low-field measurements of the Meissner effect in a BYCO high-T/sub c/ superconductor with large pinning. The positive Meissner effect (PME) observed at low fields in a SQUID magnetometer is shown to be an artifact arising from the presence of a small field gradient. No PME is observed using an extraction technique to measure the magnetization.

Magnetic Aftereffect in Compositionally-Modulated Ni/Cu Multilayers Prepared by Electrodeposition and by Sputtering

A relaxation of the magnetization following a rapid change in the magnetic field has been found in a compositionally-modulated Ni/Cu multilayer produced by sputtering. This finding demonstrates that the relaxation previously found in compositionally-modulated alloys produced by electrodeposition is a property of the material and not an artifact of the electrodeposition process.