N. Chau

Crystallization process and magnetic properties of amorphous iron oxide nanoparticles

This paper studied the crystallization process, phase transition and magnetic properties of amorphous iron oxide nanoparticles prepared by the microwave heating technique. Thermal analysis and magnetodynamics studies revealed many interesting aspects of the amorphous iron oxide nanoparticles. The as-prepared sample was amorphous. Crystallization of the maghemite ?-Fe2O3 (with an activation energy of 0.71?eV) and the hematite ?-Fe2O3 (with an activation energy of 0.97?eV) phase occurred at around 300??C and 350??C, respectively. A transition from the maghemite to the hematite occurred at 500??C with an activation energy of 1.32?eV. A study of the temperature dependence of magnetization supported the crystallization and the phase transformation. Raman shift at 660?cm?1 and absorption band in the infrared spectra at 690?cm?1 showed the presence of disorder in the hematite phase on the nanoscale which is supposed to be the origin of the ferromagnetic behaviour of that antiferromagnetic phase.

Preparation of magnetic nanoparticles embedded in polystyrene microspheres

Superparamagnetic particles are widely used for biological applications such as cell separation. The size of the particles is normally in the range of 10 – 20 nm which is much smaller than the size of a cell. Therefore small particles create small force which is not strong enough to separate the cells from solution. Superparamagnetic nanoparticles embedded in Polystyrene microspheres (magnetic beads) are very useful for cell separation. Magnetic beads have been prepared by solvent evaporation of an emulsion. The beads with size of 0.2 μm – 1.0 μm have a saturation magnetization of 10 – 25 emu/g. The change of the amount of surfactants, volatile solvent, magnetic particles resulted to the change of size, magnetic properties of the magnetic beads.

Arsenic removal from water by magnetic Fe1− x Co x Fe2O4 and Fe1− y Ni y Fe2O4 nanoparticles

This article studies the effects of Co and Ni replacement in Fe1− x Co x Fe2O4 and Fe1− y Ni y Fe2O4 (x, y = 0, 0.05, 0.1, 0.2, 0.5) nanoparticles, pH, weight of nanoparticles/mL of water, and time of stirring on the arsenic removal ability. The results showed that a small amount (0.25 g L−1) of Fe3O4 nanoparticles after stirring time of 3 min can reduce the arsenic concentration from 0.1 to 0.01 mg L−1. The removal was also affected by the pH of the water. Absorption of arsenic by nanoparticles was effective when pH was smaller than seven and reduced with the increase of pH. At pH of 13, there was a strong release of arsenic ions from arsenic-absorbed nanoparticles back to water. The time of stirring was studied from 1 min to 2 h and the optimal time was about few minutes. Co and Nis presence was reported to keep saturation magnetisation stable under working conditions. For Co replacement, absorption does not change significantly when x ≤ 0.1 and slightly reduces when x > 0.1. The presence of Ni improved the absorption in most cases.

Current-driven channel switching and colossal positive magnetoresistance in the manganite-based structure

The transport and magnetotransport properties of a newly fabricated tunnel structure manganite/depletion layer/manganese silicide have been studied in the current-in-plane (CIP) geometry. A manganite depletion layer in the structure forms a potential barrier sandwiched between two conducting layers, one of manganite and the other of manganese silicide. The voltage–current characteristics of the structure are nonlinear due to switching conducting channels from an upper manganite film to a bottom, more conductive MnSi layer with an increase in the current applied to the structure. Bias current assists tunnelling of a carrier across the depletion layer; thus, a low-resistance contact between the current-carrying electrodes and the bottom layer is established. Below 30u2009K, both conducting layers are in the ferromagnetic state (magnetic tunnel junction), which allows control of the resistance of the tunnel junction and, consequently, switching of the conducting channels by the magnetic field. This provides a fundamentally new mechanism of magnetoresistance (MR) implementation in the magnetic layered structure with CIP geometry. MR of the structure under study depends on the bias current and can reach values greater than 400% in a magnetic field lower than 1u2009kOe. A positive MR value is related to peculiarities of the spin-polarized electronic structures of manganites and manganese silicides.

Switching of current channels and new mechanism of magnetoresistance in a tunneling structure

We have experimentally studied the transport properties of a planar La0.7Sr0.3MnO3 (LSMO)/Mn-depleted LSMO/MnSi tunneling structure, in which the Mn-depleted LSMO layer plays the role of a potential barrier between the conducting layers of LSMO and MnSi. The measurements were performed in geometry with the current direction parallel to the planes of interfaces in the tunneling structure. It is established that the structure exhibits a nonlinear current-voltage characteristic and possesses a positive magnetoresistance, the value of which depends on the tunneling current. It is suggested that specific features of the transport properties of this structure are related to the phenomenon of current channel switching between the conducting layers. The switching mechanism is based on the dependence of the resistance of the tunneling junction between the conducting layers on the bias voltage and the applied magnetic field.

Using Cross-layer Heuristic and Network Coding to Improve Throughput in Multicast Wireless Mesh Networks

Wireless mesh networks (WMNs) receive much research interests because of their reliability, scalability and low cost. Obtaining high-throughput for multicast applications (e.g. video streaming broadcast) in WMN is challenging due to the interference and the change of channel quality. Cross-layer design and network coding are approaches which have been recently received considerable attention for high-throughput problem in wireless networks. In this paper, we propose an approach namely CLNC (Cross-Layer Network Coding) which is a combination of the above approaches to improve throughput in multicast wireless mesh networks. Our simulation results show that when the number of receivers is high CLNCs throughput is higher at least 30% than that of known methods such as AODV, DSDV and DSR and higher than that of MAODV. Moreover, PDR (Packet Delivery Ration) of CLNC is higher than that of MAODV and DSDV.

Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets

Nanocomposite hard magnetic materials (Nd,Dy)4.5Fe77.5B18 (No. 1) and (Nd,Dy)4.5Fe76B18Nb1.2Cu0.3 (No. 2) have been prepared by crystallizing amorphous ribbons, fabricated by single roll melt-spinning. The evolution of a multiphase structure was monitored by an x-ray diffractometer and by thermomagnetic measurement. We observed that, at annealing temperatures below 670??C, there is crystallization of soft phase Fe3B and a small amount of hard phase Nd2Fe14B. At annealing temperatures above 670??C, crystallization of ?-Fe and probably Dy2Fe14B phases with large magnetocrystalline anisotropy led to a drastic enhancement in the hard magnetic properties of the materials. The maximum value of HC is found to be 4.2?kOe for sample No. 1. For sample No. 2, with co-doping of Nb and Cu, nanostructure refinement yields a strong enhancement in exchange coupling between the component phases. Thereby, we obtained high reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max value up to 16.2?MGOe.

Influence of Cr Doping on the Critical Behavior of Amorphous Alloy Ribbons Fe 78– x Cr x Si 4 Nb 5 B 12 Cu 1

Though many previous works focused on studying Cr-doped Fe-Si-Nb-B-Cu amorphous alloys, magnetic-interaction mechanisms in these materials have not been carefully investigated yet. Dealing with these issues, we have prepared the amorphous alloy ribbons Fe_78-xCr_xSi₄Nb₅B₁₂Cu₁ with x= 1, 3, and 6, and then studied their magnetic and critical properties. Magnetization versus temperature and magnetic-field measurements, MHT, performed on a vibrating sample magnetometer reveal that the Cr-content increase in Fe_78-xCr_xSi₄Nb₅B₁₂Cu₁ reduces the T_C from 430 K for x= 1 to about 322 K (for x= 6). This indicates the decline of ferromagnetic (FM) exchange interactions between Fe atoms when there is the presence of Cr atoms. We have also analyzed the M(H) data at the temperatures in the vicinity of the T_C using the modified Arrott plot method and the scaling hypothesis, and determined the values of the critical exponents β = 0.367-0.376 and γ = 1.315-1.338. These values are close to those expected for the 3-D Heisenberg model with β = 0.365 and γ = 1.336, proving the existence of short-range FM order in the amorphous alloy ribbons.

Current channel switching in the manganite-based multilayer structure

The transport properties of the structure La0.7Sr0.3MnO3/depleted manganite layer/MnSi have been studied. The depleted manganite layer serves as a potential barrier between the ferromagnetic conducting La0.7Sr0.3MnO3 and MnSi layers by forming a magnetic tunnel junction. The study in the CIP (current-in-plane) geometry has revealed the effect of current channel switching between the manganite layer and the manganese silicide layer with higher conductivity. The effect is controlled by bias current, magnetic field, and optical radiation. Such switching is responsible for the features of the transport properties and the magnetoresistive and photovoltaic effects in the structure.