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Using scanning tunneling microscopy, we have conducted an in situ study of the room-temperature growth of Ag structures on a Si(111)-(7 × 7) surface. By observing the STM images at various Ag cover...

Giant magnetoresistance was found in DC magnetron sputtering Fe/ITO multilayers. The magnetic properties, electrical properties and magnetoresistance were investigated. A critical temperature is found around 50 K where the temperature dependence of resistivity and magnetoresistance ratio exhibit an abruptly change. The temperature dependence of resistance is found to obey Mott's 1/4 law for low temperature. The max magnetoresistance ratio of 2.0% and 6.7% is found at room temperature and 12.5 K, respectively. The increase of magnetoresistance ratio at low temperature is due to the decrease of spin-mixing effect.

Large negative magnetoresistance (MR) ( | Δ R / R | = | [ R ( H ) - R max ] / R max | > 50 % ) has been observed in the broken cold-pressed CrO2 powder samples near the room temperature, which is ascribed to the magnetic field-induced mechanical contact variation. This large, room-temperature negative MR might easily lead to a wrong conclusion that the spin-polarization is still very high at high temperatures. This observation may also point to the possibility to develop field sensors based on the mechanical MR.

Resistivity of cold-pressed CrO 2 powder has been measured over the temperature range of 2 to 350 K with a magnetic field up of to 50 kOe. High resolution transmission electron microscopy (HRTEM) images show that the CrO 2 particles are needle-shaped with an aspect ratio of 7:1 and have an average length of 200 nm, and are covered with an ultra-thin (∼2.5 nm) native Cr 2 O 3 layer. Temperature dependent conductivity can be well described by the expression exp[-(Δ/T) 1/2 ] below 55 K, a feature of electron tunneling, and by the higher-order hopping for temperature above 55 K. The magnetoresistance, MR = [ρ(max) - ρ(H)]/ρ(max), reaches ∼33.5% at 2 K and decreases to nearly zero at room temperature. Temperature dependent saturation magnetization follows well the relation M(r)=M s (1-BT 3/2 ) below 240 K. The spin-wave stiffness constant, D, is estimated to be -91 meV A 2 . By fitting the specific heat measured in the temperature range of 1.8-34 K by the relation C =γT + βT 3 +β 1 T 5 +αT 3/2 , we obtained the electronic specific heat coefficient γ=1.7 mJ/mol K 2 , this corresponds to a density of states at the Fermi level of 0.73 states/eV, which is in good accordance with the theoretical value of 0.69 states/eV.

TM x –(SiO 2 ) 1− x (TM=Fe, Co, Ni) thin films were prepared in a wide concentration range (0.35 ⩽ x ⩽1). Structure was studied with transmission electron microscopy (TEM), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Magnetic and magnetotransport properties were investigated by means of magnetization and Hall effect measurements. TEM images display nanometric spherical structures embedded in a SiO 2 amorphous matrix, with typical sizes increasing from 3 to 5 nm when TM volume concentration x is increased. SAXS measurements indicate a complex structure formed by nanosized objects. XRD measurements show that the structure is composed by amorphous SiO 2 and TM crystallites. Slightly above the percolation threshold all samples display giant Hall effect. The observed magnetic properties are dependent on x , and display an evolution resulting from the progressive increase of the mean particle size.

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.

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.

In situ measurements of resistance were made on cosputtered Co/sub 0.35/[SiO/sub 2/]/sub 0.65/ granular films during annealing. The aim is to control the thermal treatment parameters and map the microstructural changes of the samples, with the respective magnetotransport response. Results of transmission electron microscopy, dc magnetization, and temperature dependence of resistivity, after annealing, show a clear evolution in the nanostructure of the samples, with increasing average Co grain sizes and wider dispersion.

Epitaxial growth of Ni thin films on GaAs(0 0 1) 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 2.5 nm on the GaAs(0 0 1) substrate, and it is determined to be ferromagnetic showing a four-fold in-plane magnetic anisotropy with the easy axes along the 〈1 0 0〉 directions.

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

A cluster consisting of as few as three Ag atoms deposited on $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface was found to behave as a strong electric rectifier, with a rectification ratio as large as $\ensuremath{\sim}150$. Based on first-principles calculations, we found that the wave functions of electronic states at different energies spread out from the surface to different distances. We demonstrated by theoretical simulations and experiments that the observed rectification is a result of coupling the scanning tunneling microscope tip with selective wave functions of the cluster.

Using scanning tunneling spectroscopy, we have investigated the local electronic property change upon single Co atom adsorption on Pb nanoislands and/or films grown on a $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface. The quantum well states formed on the clean Pb film were found to be locally destroyed by the adsorbed single Co atom. Moreover, the differential conductance $dI/dV$ curves, exhibiting an asymmetric line shape with a dip and a hump, respectively, below and above the Fermi level, demonstrated a Kondo effect from the adsorbed Co atom on the Pb film. The similar line shapes of the $dI/dV$ spectra and the similar Kondo temperatures for Co atoms adsorbed on Pb island areas of different Pb thicknesses further showed that the different densities of states at the Fermi level originated from the quantum well states do not play a significant role, possibly due to the destruction of the quantum well states upon the Co atom adsorption.

Using scanning tunneling microscopy and spectroscopy and first principles calculations, we have systematically studied the morphological and electronic structures of a ${\mathrm{C}}_{60}$ monolayer on Ag(100). Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim ${\mathrm{C}}_{60}$ molecules, one a monomer and the other a dimer. With distinctive electronic properties and distinctive charge transfers in the differently adsorbed ${\mathrm{C}}_{60}$ molecules, this ${\mathrm{C}}_{60}$ monolayer is highly inhomogeneous and aperiodic.

Using scanning tunneling microscopy and first-principles density functional calculations, we have investigated systematically the initial stages of single Ge atoms adsorbed on a $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. When the deposition is at an elevated temperature of $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, single Ge atoms are found to substitute for the Si adatoms randomly. When the deposition is at room temperature, single Ge atoms do not replace the Si adatoms but move frequently within half unit cells. When the room temperature prepared sample is imaged at $78\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the Ge atoms are observed to either adsorb at the stable high coordination sites near the corner Si adatoms or hop among the high coordination sites near the center Si adatoms. The adsorption sites of Ge atoms at high coordination positions have been determined by first-principles calculations and comparisons with measured scanning tunneling microscopic images.