Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO （x = 0.01, 0.02, 0.05, 0.10 and 0.20） diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction （XRD）, extended x-ray absorption fine structure （EXAFS） and superconducting quantum interference device （SQUID）. The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples （x 〈 0.05） exhibit clear hysteresis loops at 300K, and magnetization versus temperature from 5 K to 350K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.     

Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride

Zirconium nitride powders with rock salt structure （γ-ZrNx） are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride （h-BN） powders. The products are analysed by x-ray diffraction （XRD）, scanning electron microscopy （SEM）, and Raman spectroscopy （ITS）. The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN （a-BN） into Zr to form Zr（N） solid solution alloy, then the Zr（N） solid solution alloy decomposes into γf-ZrNx. No ZrB2 is observed in the as-milled samples or the samples annealed at 1050° C for 2h.     

Effect of Zn Interstitials on Enhancing Ultraviolet Emission of ZnO Films Deposited by MOCVD

ZnO films are grown on Si （111） substrates by a metal organic chemical vapor deposition method. Samples with different stoichiometric composition of Zn and 0 are obtained by varying Ⅱ/Ⅵ molar ratio between 3 and 1/3 in precursors. The x-ray photoelectron spectroscopy and photoluminescenee results show that the ultraviolet emission enhances with the increasing Zn/O composition ratio of the samples. It is suggested that the supertfluous Zn atoms pile up at interstitial positions to form Zn interstitial defects. The radiated recombination of the coupling of free exeitons with donor Zn interstitial enhances the ultraviolet emission of the samples.     

Structural and electronic characteristics of pure and doped ZnO varistors

We report here the structural,surface morphology,mechanical,and current-voltage characteristics of Zn1-xMxO ceramic samples with various x and M(0.00≤x≤0.20,M=Ni,Cu).It is found that the considered dopants do not influence the well-known peaks related to the wurtzite structure of ZnO ceramics,while the shapes and the sizes of grains are clearly affected.The average crystalline diameters deduced from the SEM micrographs are between 2.06 μm and 4.8 μm for all samples.The oxygen element ratio is increased by both dopants.Interestingly,the potential barrier can be formed by adding Cu up to 0.20,while it is completely deformed by 0.025 Ni addition.The breakdown field can be enhanced up to 4138 V/cm by 0.025 Cu addition,followed by a decrease with further increase of Cu up to 0.20.On the other hand,a gradual decrease in Vickers microhardness is reported for both dopants,and the values in the Ni samples are higher compared to those in the Cu samples.The electrical conductivity is generally improved by Ni,while the addition of Cu improves it only in the over doped region(≥0.10).These results are discussed in terms of the differences of valency and ferromagnetic ordering.     

Effect of milling atmosphere on structural and magnetic properties of Ni-Zn ferrite nanocrystalline

Powder mixtures of Zn, Ni O, and Fe2O3 are mechanically alloyed by high energy ball milling to produce Ni–Zn ferrite with a nominal composition of Ni0.36Zn0.64Fe2O4. The effects of milling atmospheres(argon, air, and oxygen),milling time(from 0 to 30 h) and heat treatment are studied. The products are characterized using x-ray diffractometry,field emission scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy, and transmitted electron microscopy. The results indicate that the desired ferrite is not produced during the milling in the samples milled under either air or oxygen atmospheres. In those samples milled under argon, however, Zn/Ni O/Fe2O3 reacts with a solid-state diffusion mode to produce Ni–Zn ferrite nanocrystalline in a size of 8 nm after 30-h-milling. The average crystallite sizes decrease to 9 nm and 10 nm in 30-h-milling samples under air and oxygen atmospheres, respectively. Annealing the 30-h-milling samples at 600°C for 2 h leads to the formation of a single phase of Ni–Zn ferrite, an increase of crystallite size, and a reduction of internal lattice strain. Finally, the effects of the milling atmosphere and heating temperature on the magnetic properties of the 30-h-milling samples are investigated.     

Characterization and Magnetic Properties of Nickel Ferrite Nanoparticles Prepared by Ball Milling Technique

Nickel ferrite nanoparicles with various grain sizes are synthesized using annealing treatment followed by ball milling of its bulk component materials. Commercially available nickel and iron oxide powders are first mixed, and then annealed at 1100~C in an oxygen environment furnace and for 3h. The samples are then milled for different times in an SPEX mill. X-ray diffraction pattern indicates that in this stage the sample is single phase. The average grain size is estimated by scanning electron microscopy （SEM） and x-ray diffraction techniques. Magnetic behavior of the sample at room temperatm＇e is studied using a superconducting quantum interference device （SQUID）. The Curie temperature of the powders is measured by an LCR meter unit. The x-ray diffraction patterns clearly indicate that increasing the milling time leads to a decrease in the grain size and consequently leads to a decrease in the saturation magnetization as well as the Curie temperatures. This result is attributed to the spin-glass-like surface layer on the nanocrystalline nickel ferrite with a ferrimagnetically aligned core.     

Origin of Room-Temperature Ferromagnetism for Cobalt-Doped ZnO Diluted Magnetic Semiconductor

The pure single phase of Zn0.95 Co0.05 O bulks is successfully prepared by solid-state reaction method. The effects of annealing atmosphere on room-temperature ferromagnetic behaviour for the Zn0.95 Co0.05 O bulks are investigated. The results show that the air-annealed samples has similar weak ferromagnetic behaviour with the as-sintered samples, but the obvious ferromagnetic behaviour is observed for the samples annealed in vacuum or Ar/H2 gas, indicating that the strong ferromagnetism is associated with high oxygen vacancies density. High saturation magnetization Ms = 0.73μg/Co and coercivity Hc = 233.8 Oe are obtained for the Ar/H2 annealed samples with pure single phase structure. The enhanced room-temperature ferromagnetic behaviour is also found in the samples with high carrier concentration controlled by doping interstitials Zn （Zni）.     

Lanthanum-doped BiaTi3O12 ceramics prepared by high-pressure technique

We present an effective way in this paper to increase the density of lanthanum doped bismuth titanate ceramics, Bi4-xLaxTi3O12 （BLT）, thereby significantly improving the performance of the BLT ceramics. Dense BLT ceramicses, Bi4-xLaxTi3O12 （x = 0.25, 0.5, 0.75, 1.0）, are prepared by using nanocrystalline powders fabricated by a -gel method and high-pressure technique. The microstructures of the BLT ceramicses prepared separately by conventional-pressure and high-pressure techniques are investigated by using x-ray diffraction and transmission electron microscope. The influence of La-doping on the densification of bismuth titanate ceramics is investigated. The experimental results indicate that the phase compositions of all samples with various lanthanum dopings sintered at 900℃ possess layer- structure of Bi4Ti3O12. The green compacts are pressed under 2.5 GPa, 3.0 GPa, 3.5 GPa and 4.0 GPa, separately. It is found that the density of BLT ceramics is significantly increased due to the decreasing of porosity in the green compacts by high-pressure process.     

L-Shell X-Ray Yields and Production Cross Sections of Zr and Mo Bombarded by Slow Highly Charged Ar16+ Ions

The L-shell x-ray yields of Zr and Mo bombarded by slow Ar16＋ ions are measured. The energy of the Ar^16＋ ions ranges from about 150 keV to 350 keV. The L-shell x-ray production cross sections of Zr and Mo are extracted from these yields data. The explanation of these experimental results is in the framework of the adiabatic direct- ionization and the binding energy modified BEA approximation. We consider, in the slow asymmetric collisions such as Ar and Mo/Zr, the transient united atoms （UA） are formed during the ion-surface interaction and the direct-ionization is the main mechanism for the inner-shell vacancy production. Generally, the theoretical results are in good agreement with the experimental data.     

Magnetic properties of Sn-substituted Ni-Zn ferrites synthesized from nano-sized powders of NiO,ZnO, Fe_2O_3, and SnO_2

A series of Ni_(0.6-x/2)Zn_(0.4-x/2)Sn_xFe_2O_4(x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.3)(NZSFO) ferrite composities have been synthesized from nano powders using a standard solid state reaction technique. The spinel cubic structure of the investigated samples has been confirmed by x-ray diffraction(XRD). The magnetic properties such as saturation magnetization(M_s),remanent magnetization(M_r), coercive field(H_c), and Bohr magneton(μ) are calculated from the hysteresis loops. The value of M_s is found to decrease with increasing Sn content in the samples. This change is successfully explained by the variation of A-B interaction strength due to Sn substitution in different sites. The compositional stability and quality of the prepared ferrite composites have also been endorsed by the fairly constant initial permeability(μ') over a wide range of frequency. The decreasing trend of μ' with increasing Sn content has been observed. Curie temperature TChas been found to increase with the increase in Sn content. A wide spread frequency utility zone indicates that the NZSFO can be considered as a good candidate for use in broadband pulse transformers and wide band read-write heads for video recording. The composition of x = 0.05 shows unusual results and the possible reason is also mentioned with the established formalism.