John Q. Xiao

Giant negative magnetoresistance in granular ferromagnetic systems (invited)

Giant negative magnetoresistance (GMR) has been observed in a number of granular ferromagnetic systems [Co–Ag, Co–Cu, Fe–Cu, Fe–Ag, and (Fe–Ni)–Ag] with effect sizes as much as 85% at 5 K and 25% at 300 K. It is shown that the GMR is isotropic and is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities. The essential contribution to the resistivity is ρm[1−F(M/Ms)], where F(M/Ms) measures the spin disorder from ferromagnetic alignment and ρm is the magnetic resistivity that defines the size of the GMR. The magnitude of GMR is affected by the size and density of the magnetic entities which can be controlled by varying the composition and the process conditions. When the composition is varied, the maximum GMR is realized in systems with magnetic constituents of about 25%.

Magnetic properties and giant magnetoresistance of granular permalloy in silver

We report the first results of magnetic properties and giant magnetoresistance of granular permalloy in a metallic matrix. These new materials have resistivities comparable to those of the ferromagnetic alloys currently used in magnetoresistive devices, but yield significantly larger magnetoresistance effects that are nearly isotropic.

Radio frequency reactive sputtered iron nitrides using ammonia gas: Structure and magnetic properties

Iron nitrides of γ’‐Fe4N, e‐Fe2‐3N, and ζ‐Fe2N have been fabricated using rf sputtering with reactive NH3 gas, which offers significant advantages over the conventional N2 gas. All but ζ‐Fe2N nitride films have relative high saturation moments with a large in‐plane anisotropy. The Mossbauer results are consistent with structural analyses and those from bulk samples.

Effect of magnetic field on the superparamagnetic relaxation in granular Co-Ag samples

In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking temperature TB− were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of TB− with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density” Ku yielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials.

Domain Structures in Magnetoresistive Granular Metals

We have imaged the magnetic domain structure of several heterogeneous CoxAg1−x alloys by using scanning electron microscopy with polarization analysis. These images show that extended domain structures exist in both the as‐deposited samples and in samples annealed at moderate temperatures. This suggests that a significant fraction of the cobalt in these materials does not contribute to the giant magnetoresistance. Only those samples annealed at 600u2009°C and containing less than 40% cobalt by volume show no domain structure.

Observation of perpendicular anisotropy in granular magnetic solids

Granular Co‐Ag materials with low Co contents exhibit giant magnetoresistance and single‐domain magnetic properties. In this paper, we discuss unusual hysteresis behavior that we have observed in Co‐rich samples, where the magnetization is approximately linear with the applied field up to saturation, and the coercivity and remnant magnetization are both nearly zero. We attribute this unusual behavior to a perpendicular anisotropy that depends on the nanostructure in the phase‐segregated materials. These results are corroborated by magnetic domain imaging using scanning electron microscopy with polarization analysis.

Enhanced mechanical and magnetic properties of granular metal thin films

The mechanical properties of Ag‐Al2O3 and Ni‐Al2O3 granular metal films, as well as the magnetic behavior of the nickel‐based films, is presented. Enhancements in the mechanical properties of these materials, as measured by low load indentation techniques, were observed for a metal volume fraction p of about 0.55. This volume fraction corresponded to the percolation threshold pc as determined from the magnetic measurements. A peak in the compliance as a function of p was observed, similar to elastic anomalies reported for small bilayer period superlattices. A discontinuity in the rate of change of hardness as a function of p was also observed, and believed to result from a change in deformation mechanism at pc.

Nanostructured CoCu powders via a chemical route

Abstract Nanostructured Coue5f8Cu powders were synthesized by coprecipitation of constituent metallic salts using aqueous sodium borohydride. As-synthesized powders showed a fcc structure and an amorphous phase. The amount of amorphous phase was found to increase with the ratio of Co/Cu. Upon annealing the as-synthesizedpowdersphaseseparated into nanocrystalline fcc Co and fcc Cu. Coercivity increased with annealing temperature and reached a maximum value at about 620 Oe, exhibiting the behavior of magnetic granular solids.

Chemical precipitation and properties of nanocrystalline FeCu alloy and composite powders

Abstract Nanocrystalline Feue5f8Cu alloys and composite powders have been fabricated by reducing ferrous and cupric salts with sodium borohydride in aqueous solutions. Depending on the relative iron concentration, either fcc alloys or phase separated composites were precipitated from the solutions. The alloys were magnetically soft and the composites had coercivities as large as 400 Oe.

Structural studies and magnetic properties of Fe/Ag superlattices

Fe(110)/Ag(111) superlattices with Ag layer thicknesses of 30 or 60 A and various Fe layer thicknesses have been fabricated by high‐rate sputtering and studied by x‐ray diffraction magnetometries, and Mossbauer spectroscopy. A slightly enhanced magnetic moment and hyperfine field, and a reduced Tc have been observed in samples with thin Fe layers. The magnetization exhibits a BT3/2 dependence with very large values of B.