J.W. Chen

Superparamagnetic behavior of antiferromagnetic CuO nanoparticles

We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at T/sub B/=135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H=100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below T/sub B/ it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated Cu/sup 2+/ions at the surface of the particles.

Evolution of size, morphology, and magnetic properties of CuO nanoparticles by thermal annealing

Cupric oxide (CuO) nanoparticles with different morphologies were synthesized by thermal annealing of the copper hydroxide at various temperatures. Significant changes in both the particle size and the morphology with the annealing temperature (TA) were observed. The average particle size (d) increases from 13 to 33 nm and the morphology varies from ellipsoidal to rodlike as TA increases from 150 to 550 °C. The formation of these morphologies is explained in terms of the variation in the interplanar H-bonds breaking rate with different temperatures. The magnetic measurements reveal the presence of weak ferromagnetic interaction and the blocking behavior in these nanoparticles. The magnetic field dependence of the superparamagnetic blocking temperature (TB) follows the Brown equation. In addition, the linear variation in zero field cooled susceptibility with particle size is consistent with the predictions of Neel model for the uncompensated spins. These surface spins are responsible for the observed anoma...

Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

Abstract The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La1−xCdxMnO3 (0⩽x⩽0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (Tp1), which is close to paramagnetic to ferromagnetic transition (Tc) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (TP2) below Tp1. This transition becomes more prominent and the transition temperature (Tp2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.

Granular Fe–Pb–O films with large tunneling magnetoresistance

A method of manufacturing granular Fe–Pb–O films is described. The temperature dependence of resistivity in these samples exhibits a semiconductorlike behavior indicating that the electronic transport takes place via a tunneling process. The magnetoresistance ratio has been found to be about 10% at room temperature. The enhancement of the tunneling magnetoresistance effect is believed to be due to spin-dependent tunneling through the tunneling barrier containing magnetic ferrite formed by PbO and α-Fe2O3.

Microstructural Evolution of Nanocrystalline Magnetite Synthesized by Electrocoagulation

Nanocrystalline magnetite powders were synthesized by an electrocoagulation technique, in which an electric current was passed across two plate electrodes of carbon steel immersed in NaCl(aq) electrolyte, and the microstructure of the oxide powder was found to evolve in roughly three stages. The first stage involves formation and growth of severely defective colloidal crystallites. This is followe db y agglomeration among the oxide crystallites to form mesoporous agglomerates containing predominantly inter-crystallite pores, and the average crystallite size was found to reach a plateau. Finally, coarsening of the crystallites within the agglomerates leads to another rapid increase in crystallite size and reduction in pore opening. The synthesized powders typically showed a saturation magnetization of ∼75 emu/g and a coercivity Hc of ∼118 Oe. A mechanism involving competition between nucleation and growth of free colloids and coarsening of the skeletal framework was proposed to explain the temporary level-off in crystallite size during the synthesis.

Dielectric studies of Fe Doped SmCrO3 Perovskites

Polycrystalline Fe doped SmCrO3 perovskite ceramics were prepared by sol-gel route and characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), dielectric and Impedance measurements. Single-phase perovskite Fe doped SmCrO3 obtained has been confirmed from the XRD pattern with orthorhombic structure. The dielectric properties of perovskite Fe doped SmCrO3 ceramics were studied in the frequency range of 100Hz – 1MHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant value of ~ 104 at room temperature. The response is similar to that observed for relaxor-ferroelectrics. To get the detailed information, IS data analysis was performed.

Superconductivity in the Ni-based ternary compound La6Ni2Sn

Abstract We have investigated the transport and magnetic properties of the ternary compound La 6 Ni 2 Sn by means of the ac electrical resistivity ρ , dc magnetic susceptibility χ dc , and specific heat C ( T ) measurements. Bulk superconductivity was observed in this compound with a superconducting transition temperature of ∼2.25 K as inferred from a sudden drop to zero value in the ρ ( T ) curve, the diamagnetic behavior in the χ dc ( T ) curve, and a jump in the C ( T ) curve. Type II superconductivity can also be inferred from the isothermal M – H curve measured at T =2 K. The initial slope of the upper critical magnetic field −(d H c2 /d T ) T c0 has a value of 4000 Oe/K, from which a zero-temperature superconducting coherence length ξ 0 =230 A can be obtained. Analysis using the specific heat data yields a Debye temperature Θ D =135 K for this compound.

Preparation and characterization of perovskite Cu doped LaFeO3 semiconductor ceramics

Orthorhombic structured polycrystalline Cu doped LaFeO3 perovskite ceramics were prepared by sol-gel route and characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopic (FT-IR) techniques. The complex dielectric properties for these ceramic samples investigated as function of temperature, in the range 80K – 300K, and frequency, in the range 20 Hz -1MHz. These samples exhibiting colossal dielectric constant value of 104 near room temperature at low frequencies. The response is similar to that observed for relaxor ferroelectrics. Impedance spectroscopy (IS) data analysis indicates the ceramics to be electrically heterogeneous semiconductors consisting of semiconducting grains with dielectric constant below 100. We conclude, therefore that grain boundary effects is the primary source for the high effective dielectric constant in Cu doped LaFeO3 ceramics.

Synthesis and dielectric properties of Fe doped SmCrO3semiconductor ceramics

Polycrystalline Fe doped SmCrO 3 perovskite ceramics were characterized by X-ray diffraction (XRD). The dielectric properties of perovskite Fe doped SmCrO 3 ceramics were investigated in the frequency range of 100Hz - 1MHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant value of ∼ 104 at room temperature. The response is similar to that observed for relaxorferroelectrics. To get the detailed information IS data analysis was performed.

Synthesis and dielectric properties of Fe doped SmCrO[sub 3] semiconductor ceramics

Polycrystalline Fe doped SmCrO 3 perovskite ceramics were characterized by X-ray diffraction (XRD). The dielectric properties of perovskite Fe doped SmCrO 3 ceramics were investigated in the frequency range of 100Hz - 1MHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant value of ∼ 104 at room temperature. The response is similar to that observed for relaxorferroelectrics. To get the detailed information IS data analysis was performed.