Hongmei Qiu

Ferromagnetism analysis of Mn-doped CuO thin films

Mn (6.6–29.8%)-doped CuO thin film fabricated on a thermally oxidized silicon substrate by radio-frequency magnetron sputtering has been reported. The films were structurally characterized using x-ray diffraction with Rietveld refinement. The analysis indicates that Mn uniformly substituted at the Cu position in the CuO lattice. 5% cation vacancies were detected at the Cu sites and are supposed to be responsible for the p-type electrical conduction of Cu1−xMnxO films. No evidence for large scale Mn aggregation was found in the composition range analyzed. The origin of ferromagnetism was analyzed in the context of competition among several interactions among Mn and Cu ions. A chain model was developed to simulate the ferromagnetic behavior with the random Mn distribution in the samples. The consistency between simulation and experiment strongly indicates that the ferromagnetism mainly arises from the super-exchange interactions of Mn–O–Cu–O–Mn coupling in the [] chain and Mn–O–Mn coupling contributes to the antiferromagnetism.

STRUCTURAL, MAGNETIC AND DIELECTRIC PROPERTIES OF Fe-DOPED BaTiO3 SOLIDS

The structural, ferroelectric and magnetic properties of bulk perovskite Fe-doped BaTiO3 (BFTO) prepared by standard solid-state reaction have been investigated. X-ray diffraction (XRD) identifies the tetragonal structure of BFTO samples. Rietveld refinements of XRD data indicates that the doping ions led to ab-plane expansion and out-of-ab-plane shrinkage of the BFTO phases. X-ray photoelectron spectroscopy (XPS) measurements for the prepared samples reveals that Fe3+ and Fe4+ ions replaces Ti4+ ions in the crystal lattice to form single-phase BFTO solids. The results of the temperature-dependent dielectric properties and magnetic hysteresis loops for the BFTO solids show simultaneously the ferroelectric order and ferromagnetic order at room temperature. The doping of magnetic element Fe brings about ferromagnetic order for the samples, and the measured magnetic moment for each Fe atom increases from 0.70 μB to 1.55 μB in BFTO samples. The origin of ferromagnetism of the BFTO samples should be attributed to the double exchange interactions of Fe3+–O2–Fe4+ ions.

Metal-insulator transition in ferromagnetic Mn-doped CuO thin films

The ac susceptibility, electrical properties, and magnetotransport properties of the as-prepared MnxCu1−xO thin films (x=7%–29%) were studied in the temperature range of 2–300 K. The susceptibility measurement shows that ferromagnetic transition takes place for the samples with x≥14% below 100 K. The transport properties show the transition from variable-range-hopping to metalliclike behavior at the same transition temperature as the paramagnetism to ferromagnetism transition. The metal-insulator transition is a characteristic feature of magnetic ordering in this material.

Controlled synthesis, phase formation, growth mechanism, and magnetic properties of 3-D CoNi alloy microstructures composed of nanorods

3-D flower like CoNi alloys nanostructures composed of nanorods have been synthesized by template free hydrothermal method at relatively low temperature (120 °C). The detailed characterizations confirm the formation of good crystalline fcc CoNi alloy, average crystallite size of 18.8 ± 1.0 nm, lattice parameter of 3.531 ± 0.01 A, and the nearly equiatomic composition (Co50Ni50). Highly uniform flower like nano structures are built up with nanorod of diameter about 100 nm and length in range of 200–400 nm. The nanorods (building blocks of flower) have single crystalline nature with [111] preferred growth direction. The concentration of NaOH plays a vital role in formation of alloys and high concentration promotes the formation of CoNi alloy at low temperature. The concentration of NaOH also affects the morphology remarkably by changing the growth/reaction rate of CoNi nanostructures and results in hollow spheres to nanoplate flower of CoNi alloys. Based on the evolution of the morphology of the products, a step wise growth mechanism is rationally proposed for flower like nanostructures by considering the effects of kinetic parameters on growth. Magnetic measurements show Co50Ni50 flower like nanostructure have high saturation magnetization, coercivity, remanent magnetization, and high effective anisotropy constant of value 101.3 emu g−1, 210.5 Oe, 16.2 emu g−1, and 4.457 × 104 J m−3 respectively. The enhancements of coercivity and effective anisotropy constant are attributed to nanoscale effects such as shape/surface anisotropy.

Microstructure and magnetic properties of magnetite thin films prepared by reactive sputtering

Highly oriented magnetite (Fe3O4) thin films have been produced by reactive sputtering in a mixture of hydrogen and argon. While different phases can be achieved by varying the ratio between hydrogen and argon, single phase magnetite films can be achieved with hydrogen concentration γ=0.75%–1%. For the sample grown at γ=1.0%, a Verwey transition at about 111K can be seen from the temperature dependence of the resistivity, which is confirmed in the magnetization measurements. Maximum magnetoresistance (MR) of about 13.8% is observed just about the Verwey transition at T=115K. MR results also suggest strong coupling among Fe3O4 nanoparticles originated from the Ruderman-Kittel-Kasuya-Yosida exchange interaction and dipolar interaction, which requires high order terms of (M∕Ms)2 to explain the MR behaviors. However, with the fields applied perpendicular to the plane, MR exhibits a distinct behavior. The MR values under the condition of low fields seem to show a linear relationship with ∣M∕MS∣.

Magnetic and structural characterizations of Heusler Ni2FeGa nanoparticles

This work describes the chemical preparation, structural characterization and magnetic properties of Heusler Ni2FeGa nanoparticles. The nanoparticles were synthesized by reducing a methanol solution mixture of NiCl2 6H2O, Fe(NO3)3 9H2O and Ga(NO3) xH2O, and using SBA-15 as template. The obtained nanoparticles were investigated by means of x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. The structure of the prepared nanoparticles is Heusler L21 phase with a second γ phase, and the ratio of the two components could be adjusted by the concentration of SBA in the precursor solution. The prepared nanoscale Ni2FeGa heusler alloys with a tensile strain of 0.56% present ferromagnetism at room-temperature and its Curie temperature exceeds 340 K.

Influence of sintering temperature on the thermoelectric properties of Ba8Ga16Si30 clathrate treated by spark plasma sintering

A series of Ba8Ga16Si30 clathrate samples were prepared by arc melting, ball milling, acid washing, and spark plasma sintering (SPS). X-ray diffraction analysis revealed that the lattice of the Ba8Ga16Si30 samples expanded as the SPS temperature was increased from 400 to 750°C. Lattice contraction recurred when the SPS temperature was further increased in the range of 750–1000°C. This phenomenon can be explained by the variation of Ga content in the lattice. The thermoelectric figure of the merit ZT value of clathrates increased with the increase in SPS temperature and reached a maximum when the sample was subjected to SPS at 800°C. A further increase in SPS temperature did not contribute to the improvement of ZT. The variation of the lattice parameter a vs. SPS temperature T was similar to the variation observed in the ZT-T curve.

Fabrication and magnetic properties of ferromagnetic Co50Fe50 nanorings

In this paper, ferromagnetic Co50Fe50 nanorings were fabricated by template-assisted sputtering deposition. The obtained Co50Fe50 nanorings have a good dimensional uniformity, with outer diameter of 360 nm, width of 50 nm and thickness of 40 nm. The magnetic properties of Co50Fe50 nanorings were investigated by micromagnetic simulations and vibrating sample magnetometer. We observed two parallel magnetic switching processes in the micromagnetic simulations of nanorings, the vortex formation process and onion rotation process, respectively. Quantitative analysis on the basis of experimental results indicates that 43.7% of the Co50Fe50 nanorings undergo the vortex formation process and others undergo the onion rotation process.

Facile Synthesis, Structural and Magnetic Properties of Hierarchical Tree-Leaf Like FeCo Alloy Microstructures

[Rafique, M. Yasir; Lqbal, M. Zubair; Javed, Qurat-ul-ain; Qiu, Hongmei; Guo Zhengang] Univ Sci & Technol Beijing, Dept Phys, Beijing 100083, Peoples R China. [Pan, Liqing] China Three Gorges Univ, Coll Sci, Yichang 443002, Peoples R China. [Pan, Liqing] China Three Gorges Univ, Inst New Energy, Yichang 443002, Peoples R China. [Farooq, M. Hassan] Univ Sci & Technol Beijing, Sch Mat Sci & Engn, Beijing 100083, Peoples R China. [Iqbal, M. Azhar] Univ Punjab, Dept Phys, Lahore 54590, Pakistan.