Jianbiao Dai

Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.

Magnetic coupling induced increase in the blocking temperature of γ-Fe2O3 nanoparticles

In this article, we report the magnetic properties of surfactant coated γ-Fe2O3 nanoparticles which are pressed under different pressures. With increasing pressure, the sample volume decreases, density increases, and a 55% density change has been achieved. The blocking temperature is increased from 50 to 80 K. Analyzing the data of blocking temperature versus densities, which exhibits linear relationship, and comparing the magnetic properties, the increase in blocking temperature is understood in terms of increased magnetic interactions between neighboring nanoparticles, which is due to the reduced average interparticle distance by the applied pressure.

Current-controlled channel switching and magnetoresistance in an Fe3C island film supported on a Si substrate

A film of magnetic Fe3C islands separated by nanochannels of graphite was prepared with pulsed laser deposition on a Si substrate with a native SiO2 surface. When the temperature is increased above 250 K the resistance suddenly drops because electron conduction switches from the film to the Si inversion layer underneath. The film shows a negative magnetoresistance. The inversion layer exhibits a large positive magnetoresistance. The transition to the low resistance channel can be reversed by applying a large measuring current, making possible current-controlled switching between two types of electron magnetotransport at room temperature.

Thickness dependence of magnetic properties of granular thin films with interacting particles

The effect of film thickness on magnetic properties of Cu80Co20 granular alloy was studied. It was observed that the susceptibility peak temperature, TM, strongly increases with the film thickness, t, for t<100 nm. The long-range nature of this effect points to magnetic dipole interaction as responsible mechanism. This dependence of TM can be explained within the framework of Dormann’s theory of dipolar interaction between magnetic particles. The coercive field has different thickness dependence and it is related to formation of magnetic domain structure of Co particles in the granular alloy.

Magnetic Properties of Fe-Doped Rutile

Ball-milling method was applied to dissolve Fe into titanium dioxide (TiO2). X-ray diffraction indicated the starting anatase changed to a rutile-type structure with oxygen deficiency after ball milling. Transmission electron microscopy and X-ray absorption experiments were conducted to examine the possible existence of magnetic impurities in the ball-milled powders after they were leached in HCl solutions. Temperature dependence of the resistivity shows semiconducting behavior and the magnetic hysteresis loops at 5 and 300 K exhibit ferromagnetic characteristics. Fe-doped TiO2 films were also prepared by pulsed laser deposition. The magnetic properties of the films are discussed.

Low field intergranular tunneling effect in CrO2 nanoparticles and characterization of the barriers

The magnetoresistance (MR) and microstructures of half-metallic CrO2 nanoparticle systems were studied. Using field alignment, the needle-shaped CrO2 single domain particles were aligned in the same orientation. The MR of this structure showed a magnetic junction-like behavior with two well-separated peaks in the MR at coercivity fields and the MR of the aligned CrO2 particles reached >41% at a relatively low field of about 1000 Oe. The magnetotransport mechanism was analyzed in terms of spin dependent tunneling between CrO2 nanoparticles. Using transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy techniques, the intergranular tunneling barrier was characterized to be a very thin Cr2O3 interface layer between the CrO2 particles. Temperature dependence of MR and conductivity in cold-pressed CrO2 nanopowders were studied. The MR significantly decreased with increasing temperature and the spin independent hopping conduction is suggested to be responsible for the suppressi...

COLLOIDAL BISMUTH NANOPARTICLES: SYNTHESIS AND UV-VIS ABSORPTION

Quantum-confined Bi structures (quantum wells and quantum wires) are currently being studied for their varies novel properties. We now pursue the next level of quantum confinement in bismuth, quantum dots. In this work, we present a technique for the synthesis of bismuth nanoparticles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be formed with ultrafine particle in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band centered at ~268 nm.

Magnetic properties of (γ‐Fe2O3)80Ag20 nanocomposites prepared in reverse micelles

The magnetic properties of nanoparticles of γ‐Fe2O3 prepared by reverse micelles have been studied by dc magnetization, transverse ac susceptibility, and Mossbauer spectroscopy. The nanoparticles of γ‐Fe2O3 in the nanocomposite (γ‐Fe2O3)80Ag20 exhibit superparamagnetic behavior. The blocking temperatures determined by the three methods indicate the superparamagnetic nature of (γ‐Fe2O3)80Ag20 above 70–80 K and show correlation with measuring time. The average particle diameter obtained by transmission electron microscopy of the γ‐Fe2O3 particles is ∼10nm and that of the Ag particles is ∼20nm. The average particle size determined from the magnetic analyses for the γ‐Fe2O3 particles is ∼12nm. Mossbauer spectra obtained between 4.2 and 295 K clearly reveal the presence of superparamagnetic relaxation at temperatures above ∼80K. The Mossbauer spectra reveal at most 1% of paramagnetic Fe2+ ions in the 295-K spectrum.

Analysis of the thickness dependence of magnetic properties and interparticle interaction of granular films

In this paper, we study the interparticle interaction among the magnetic nanoparticles in granular films and how this interaction affects their magnetic properties. Attention has been paid to the thermal stability and coercivity of Cu/sub 80/Co/sub 20/ granular thin films with thickness from 7-400 nm. It is found that the blocking temperature and coercivity increase when the film thickness increases until the thickness is larger than a critical value. These effects have been analyzed due to the magnetic dipolar interaction. Our analysis shows that the interaction energy is a function of film thickness. The magnetic long-range order has been introduced in our model. Theoretical approaches of the thickness dependence of blocking temperature and coercivity have been given and they are in agreement with the experimental data.