Xieqiu Zhang

Giant Hall effect in superparamagnetic granular films

A comprehensive review of the giant Hall effect (GHE) is presented, with emphasis on novel experimental data obtained in Ni–SiO2 and Co–SiO2 films prepared by co-sputtering. GHE is observed close to and on both sides of the metal–insulator transition. From the point of view of microscopic conduction mechanisms, this means a crossover from metallic conductivity with weak localization to tunneling, or hopping, between separate granules across insulating barriers. Magnetic percolation is also interrupted at this concentration of metal, leading to superparamagnetic behavior of the composite and blocking phenomena. Temperature dependencies of magnetization and extraordinary Hall coefficient in the composites near the critical concentration are compared. In single phase magnetic metals and alloys, the extraordinary Hall is believed to be directly proportional to the total magnetization, due to side jumps or skew scattering. In a metal–insulator composite, only those electrons traveling in conduction critical paths can contribute to the Hall signal, thus only magnetization of the material belonging to these paths is important in the Hall measurements. Comparison with the magnetic results leads to new possibilities in understanding both the electronic and magnetic properties of granular nanocomposites. r 2003 Elsevier Science B.V. All rights reserved.

Giant Hall effect in Co-SiO2 nanocomposites

Measurements of both the ordinary and extraordinary components of the Hall effect in co-sputtered granular magnetic Co-SiO2 nanocomposites are presented. The experiments were done in the temperature range 5 - 300 K, and fields up to 7 T. Both components show a large enhancement when the metal volume fraction is reduced to the metal-insulator transition. However the enhancement of the ordinary Hall effect is much weaker than that of the extraordinary one. We discuss the implications of this observation for understanding of the giant Hall effect.

Magnetic relaxation of diluted and self-assembled cobalt nanocrystals

We have studied the magnetic relaxation of monodispersed 4 nm cubic e-cobalt nanocrystals in both randomly oriented and pre-aligned assemblies.The blocking temperature TB, for the closely packed Co nanocrystal assemblies, is 30% higher than that of the highly diluted and well-dispersed Co nanocrystal-organic composites.This increase is attributed to the strong magnetic dipole interaction induced from the close packing of the nanocrystals.It is found that the frequency-dependent susceptibility data, obtained from the diluted samples, can be fitted to the half-circle Argand Diagrams, indicating a single barrier (or very narrow energy distribution) of the nanocrystals.This agrees well with the physical observation from TEM that the nanocrystals are monodispersed.The long time magnetic relaxation measurements reveal that energy barrier distribution in a pre-aligned nanocrystal assembly is significantly different from that in a randomly oriented one. r 2002 Elsevier Science B.V. All rights reserved.

Growth and magnetism of Ni films on GaAs(001)

Abstract Epitaxial growth of Ni thin films on GaAs(0xa00xa01) and their magnetic property have been investigated using reflection high energy electron diffraction and magneto-optical Kerr effect. The result shows that a body-centered-cubic (BCC) phase of Ni which does not exist in nature can grow epitaxially up to 2.5xa0nm on the GaAs(0xa00xa01) substrate, and it is determined to be ferromagnetic showing a four-fold in-plane magnetic anisotropy with the easy axes along the 〈1xa00xa00〉 directions.

Ordinary and extraordinary giant Hall effects in Co–SiO2 granular films

Abstract Magnetization, resistance and Hall effect are studied in granular magnetic Co–SiO2 nanocomposites in the temperature range 5–300xa0K and fields up to 6xa0T. Relative contributions from spin-independent and spin-dependent processes to the giant Hall effect near the metal–insulator transition are analysed.

Assembling and disassembling Ag clusters on Si(111)-(7×7) by vertical atomic manipulation.

Atomic manipulation has been rarely used in the studies of complex structures and a low temperature requirement usually limits its application. Herein we have demonstrated a vertical manipulation technique to reproducibly and reversibly manipulating Ag atoms on an Si(111)-(7×7) surface by a scanning tunneling microscope tip at room temperature. Simple and complex Ag nanoclusters were assembled and disassembled with a precise control of single Ag atoms, which provided critical information on the size of these nanoclusters. The manipulation showed the growth processes of these Ag clusters and even partly unveiled their atomic structures. This technique can form a fundamental basis for further studies of the Ag/Si(111)-(7×7) system and for fabricating functional nanodevices in various metal-semiconductor systems.

Size-dependent superconducting state of individual nanosized Pb islands grown on Si(111) by tunneling spectroscopy

By measuring the temperature-dependent tunneling spectroscopy of a set of flat-top Pb islands from 3.2 to 15 K, the limiting size of a nine-monolayer-thick Pb island with superconductivity above 3.2 K was determined to be ∼ 30 nm(2), in good agreement with the Anderson criterion. Further analysis indicates that the zero-temperature energy gap decreases significantly faster than the transition temperature when the Pb island size approaches this limit. This leads to a decrease of 2Δ(0)/k(B)T(C) from 4.5 to 3.3, thus showing that the Pb island superconductors undergo a change from strong to weak electron-phonon coupling.