Le Duc Tung

Magnetic properties of ultrafine cobalt ferrite particles

We have studied magnetic properties of a diluted system of ultrafine cobalt ferrite nanoparticles (d∼3.3 nm). From the peak of the zero-field-cooled measurements, we obtained the blocking temperature TB of about 90.5 K and it is virtually independent of the applied magnetic field up to 5 kOe. At the superparamagnetic region T>TB, the system follows the modified Curie-law variation of the magnetic susceptibility χ=χo+C/T. We observed that the saturation magnetization follows a spin-wavelike temperature dependence at temperature above 10 K. In spite of the cubic structure for cobalt ferrite, at 2 K, the reduced remanence Mr/Ms is equal to 0.46 which is close to the theoretical value of 0.5 expected for noninteracting uniaxial single-domain particles with the easy axis randomly oriented. From the ac susceptibility measurements at different frequencies, we obtained a linear dependence of the logarithm of the experimental time window τex as function of inverse blocking temperature (1/TB). The fitting results i...

Extraordinary Hall effect and ferromagnetism in Fe-doped reduced rutile

Room-temperature ferromagnetism is observed in reduced rutile TiO2−δ by Fe doping. The epitaxial films grown by pulsed-laser deposition are carefully examined by x-ray diffraction, transmission electron microscopy, and magnetic and transport measurements. The films exhibit the extraordinary Hall-effect and thin-film magnetic shape anisotropy. The magnetic moments and anticipated Curie temperatures of the films rule out Fe particles, iron oxides, and Ti–Fe oxides as possible sources for the observed magnetic signals. The carriers of the Fe-doped reduced rutile are p-type, with a carrier density of 1×1022/cm3. This room-temperature dilute magnetic semiconductor should find potential applications in spintronics.

Room-temperature ferromagnetism in manganese doped reduced rutile titanium dioxide thin films

MnxTi1−xO2−δ (x=0.02–0.12) thin films grown on α-Al2O3 substrates by pulsed-laser deposition have been investigated. X-ray diffraction and transmission electron microscopy results indicate that the films are single phase and reduced rutile-type. Superconducting quantum interference device magnetometer measurements show the films are ferromagnetic at room temperature with nonzero coercivity up to 170 Oe. The saturation magnetization of the reduced films is as high as 0.83 μB per Mn atom at room temperature. The temperature dependence of the resistivity shows semiconducting behavior with p-type carriers. The nature of the p-type conduction and its significance to the ferromagnetism are discussed.

Ferromagnetism and transport properties of Fe-doped reduced-rutile TiO2−δ thin films

We have investigated FexTi1−xO2−δ (x=0.02, 0.06, and 0.08) thin films grown on α-Al2O3 substrates by pulsed-laser deposition. X-ray diffraction results indicate that the films are single phase and of reduced-rutile type. Detailed microstructural observations reveal no measurable magnetic impurities in the films. Vibrating sample magnetometer measurements show the films are ferromagnetic at room temperature with coercivity ranging from 340 to 770 Oe. The temperature dependence of the resistivity shows nearly metallic behavior at room temperature but semiconducting behavior at lower temperatures. The extraordinary Hall effect with coercivities similar to those in magnetic hysteresis curves was observed at room temperature. The carriers are p type with a carrier density of about 1022/cm3.

Ultrafine NiFe2O4 powder fabricated from reverse microemulsion process

NiFe2O4 ultrafine powder with high crystallinity has been prepared through a reverse microemulsion route. The composition in starting solution was optimized, and the resulting NiFe2O4 was formed at temperature of around 550–600 °C, which is much lower than that observed from the solid-state reaction. Magnetic investigation indicates that samples are soft-magnetic materials with low coercivity and with the saturation magnetization close to the bulk value of Ni ferrite.

Multifunctional Co–C nanocomposite thin films

Abstract Nanocomposite cobalt–carbon (Co–C) thin films with a Co concentration ranging from 26 to 63 at.% were synthesized using a dual electron beam evaporation system. Transmission electron microscopy observations showed that the as-deposited films were nanocomposites consisting of nanocrystalline Co particles encapsulated in an amorphous C matrix. The spherical Co nanoparticles had a grain size between 2 and 5 nm depending on Co concentration. Magnetic properties were measured using a superconducting quantum interference devices magnetometer, which inferred that the Co–C films with a high Co content had a high saturation magnetization and low coercivity. Co–C films with a low Co content exhibited coercivity only at low temperature (10 K). The (magneto) transport properties of the Co–C films were also investigated. Additionally, nanomechanical and tribological properties were evaluated by nanoindentation and pin-on-disc experiments, respectively. Addition of C into the films was found to significantly improve the mechanical and tribological properties. Nanocomposite Co–C films can meet several requirements (electromagnetic, mechanical, tribological, etc.) and thus are multifunctional and have potential for demanding technological applications (such as microdevices and sensors).

Synthesis, structure, and magnetic properties of ε-Co nanocrystalline thin films and annealing effects

Cobalt (Co) nanocrystalline thin films consisting of Co nanocrystals (NCs) embedded in an amorphous carbon matrix were directly synthesized at ambient temperature by codeposition of Co and carbon using high vacuum dual electron-beam evaporation. In the as-deposited films, Co was identified to have a cubic e-Co structure (with a controllable particle size). After annealing at 360 °C for 2 h, the Co NCs with a particle size of 5 nm in the as-deposited film were transformed into hexagonal α-Co (8 nm in size), whereas, the as-deposited Co NCs of 3 nm in size had a tendency to coalesce and interconnect to each other, forming a face-centered-cubic β-Co network. While the e-Co NCs of 3 nm in size showed superparamagnetism at room temperature, the e-Co NCs of 5 nm in size exhibited a ferromagnetic state due to coupling interaction or local interparticle connections. The saturation magnetization and coercivity substantially increased after annealing compared to the as-deposited films due to coarsening of Co NCs.

Synthesis and magnetic properties of CoPt–poly(methylmethacrylate) nanostructured composite material

We have prepared nanometer-sized CoPt particles dispersed in a poly(methyl methacrylate) (PMMA) matrix, as a novel nanostructured magnetic plastic, through a soft chemical processing route. In this work, CoPt nanoparticles were successfully synthesized from a solution phase reduction system in the presence of capping ligands and stabilizing agents at high temperature. The CoPt nanoparticles were annealed at 400 °C for 3 h, and were subsequently re-dispersed in methylmethacrylate (monomer). The polymerization was induced by a UV source and the hardness of final product was adjusted by varying the amount of monomeric cross-link agent. Annealed bare CoPt nanoparticles as a “core” material and CoPt–PMMA composite material were characterized by using energy dispersive spectroscopy, transmission electron microscopy, and x-ray diffraction, indicating that we are able to prepare CoPt nanoparticles with <10 nm in diameter (after annealing) by employing this high temperature colloidal processing method. Magnetic in...

Effects of rare-earth size on the electronic structure of La1−xLuxVO3

The electronic structure of La(1-x)Lu(x)VO(3)(x = 0, 0.2, 0.6 and 1) single crystals has been investigated using soft x-ray absorption spectroscopy, soft x-ray emission spectroscopy, and resonant soft x-ray inelastic scattering to study the effects of rare-earth size. The x-ray absorption and emission spectra at the O K-edge present a progressive evolution with R-site cation, in agreement with local spin density approximation calculations. This evolution with R, together with the temperature dependence of the O K-edge spectra, is attributed to changes in the crystal structure of La(1-x)Lu(x)VO(3). The crystal-field dd. excitations probed by resonant inelastic x-ray scattering at the V L(3)-edge exhibit an increase in energy and enhanced intensity with the decrease of R-site ionic radius, which is mainly attributed to the increased tilting magnitude of the VO(6) octahedra. Upon cooling to ~95 K, the dd* excitations are prominently enhanced in relative Intensity, in agreement with the formation of the Jahn.Teller distortion int he orbital ordering phase. Additionally, the dd* transitions of the mixed compounds are noticeably suppressed with respect to those of the pure compounds, possibly owing to the formation of C-type orbital ordering induced by large R-site size variances.

Low-energy V t2g orbital excitations in NdVO3

The electronic structure of NdVO(3) and YVO(3) has been investigated as a function of sample temperature using resonant inelastic soft x-ray scattering at the V L(3)-edge. Most of the observed spectral features are in good agreement with an atomic crystal-field multiplet model. However, a low energy feature is observed at ∼ 0.4 eV that cannot be explained by crystal-field arguments. The resonant behaviour of this feature establishes it as due to excitations of the V t(2g) states. Moreover, this feature exhibits a strong sample temperature dependence, reaching maximum intensity in the orbitally-ordered phase of NdVO(3), before becoming suppressed at low temperatures. This behaviour indicates that the origin of this feature is a collective orbital excitation, i.e. the bi-orbiton.