Tuning the growth orientation of epitaxial films by interface chemistry

The support of epitaxial films frequently determines their crystallographic orientation, which is of crucial importance for their properties. We report a novel way to alter the film orientation without changing the substrate. We show for the growth of CoO on the Ir(100) surface that, while the oxide grows in (111) orientation on the bare substrate, the orientation switches to (100) by introducing a single (or a few) monolayer(s) of Co between the oxide and substrate. This tunability of the orientation of epitaxial films by the appropriate choice of interface chemistry most likely is a general feature.     

Tuneable magnetic properties of hydrothermally synthesised core/shell CoFe2O4/NiFe2O4 and NiFe2O4/CoFe2O4 nanoparticles

The aim of this study was to prepare a novel targeting nano drug delivery system of 2-methoxyestradiol (2-ME) based on the folic acid-modified bovine serum albumin, in order to improve the clinical application disadvantages and antitumor effect of 2-ME. In this study, 2-methoxyestradiol-loaded albumin nanoparticles (2-ME-BSANPs) were prepared by desolvation method, and then the activated folic acid was conjugated to 2-ME-BSANPs by covalent attachment (2-ME-FA-BSANPs). The size and zeta potential of 2-ME-FA-BSANPs were about 208.8 ± 5.1 nm and ?32.70 ± 1.01 mV, respectively. 2-ME loading efficiency and loading amount of the nanoparticles were 80.49 ± 3.80 and 10.25 ± 1.59 %, respectively. SEM images indicated that 2-ME-FA-BSANPs were of a round shape, similar uniform size, and smooth surface. Studies on drug release indicated that 2-ME-FA-BSANPs had the properties of sustained and controlled release, which provided them with the ability to fight continually against cancer cells. Internalization analysis demonstrated that 2-ME-FA-BSANPs-targeting drug delivery system could get efficiently transferred into the cells through the folic acid-mediated endocytosis, leading to higher apoptosis and affording higher antitumor efficacy against SMMC-7721 cells in vitro compared with 2-ME alone. Furthermore, the cell-cycle arrest of 2-ME-FA-BSANPs on the SMMC-7721 cells occurred at G2/M phase, and 2-ME-FA-BSANPs did not change the inhibition of the tumor mechanisms of 2-ME. Based on these results, it was concluded that albumin nanoparticles could be the promising nano carrier for 2-ME, and 2-ME-FA-BSANPs-targeting drug delivery system may be promising candidate for providing high treatment efficacy with minimal side effects in future cancer therapy.     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.     

Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe₂O₄ spinel oxides. Mössbauer spectra for NiFe₂O₄ at various temperatures (79 ≤?T?≤ 900 K) are well fitted by two sextets associated with ⁵⁷Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0)[{1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.     

Raman study of NiFe2O4 nanoparticles,bulk and films: effect of laser power

Nanoparticles, bulk and thin films of NiFe₂ O₄ compounds are studied by micro‐Raman spectroscopy. The effect of varying the incident laser power up to 40 mW was studied in all forms of the samples. The spectra showed a large magnitude of red shift and line broadening as a result of high incident laser power. It is shown that the inverse spinel structure remains robust, and no trace of laser‐induced oxidation was observed. The low‐temperature study of the bulk and nanoparticles has also been carried out for elucidating thermal effects due to the high incident laser power. The rise in temperature for maximum incident laser power of 40 mW was estimated to be ∼625 °C. Copyright © 2010 John Wiley & Sons, Ltd.     

NiFe2O4纳米粉末的制备及磁性研究

以硝酸铁、硝酸镍以及柠檬酸为原料,采用凝胶-热分解法制备了N iFe2O4纳米粉末。利用X射线衍射确定了粉体的相结构、比表面积和晶格常数,扫描电子显微镜(SEM)观察了颗粒的形貌,振动样品磁强计(VSM)测量样品的磁性能。结果表明:所制备的样品均为尖晶石结构,颗粒粒径为36nm~68nm,且颗粒的粒径随着热处理温度的升高而增大,样品的比饱和磁化强度最大可达54.63 emu/g。同时,文章也对反应的动力学原理进行了研究,得出N iFe2O4纳米颗粒形成的活化能为15.8kJ/mol。     

LiMn2O4 films grown by pulsed-laser deposition

LiMn₂O₄ films have been deposited onto silicon wafer by pulsed-laser deposition (PLD) technique in order to test their reliability as cathode materials in rechargeable lithium microbatteries. The film formation has been studied as a function of the preparation conditions, i.e., composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Depending on the conditions of deposition, Mn₂O₃ was present as an impurity phase. When deposited onto silicon substrate maintained at 300 °C in an oxygen pressure of 100 mTorr from the target LiMn₂O₄+15 % Li₂O, the PLD films are well-textured with crystallite size of 300 nm. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by x-ray diffraction and Raman scattering measurements. Surface morphologies of layers were investigated by SEM. The cells Li//LiMn₂O₄ have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the range 3.0–4.2 V. The voltage profiles show the two expected steps for Li_xMn₂O₄ with a specific capacity as high as 120 mC/cm² μm. The chemical diffusion coefficients for the Li_xMn₂O₄ thin films appear to be in the range of 10⁻¹¹-10⁻¹² cm²/s. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

A study of formation process of NiFe2O4 and ZnFe2O4 prepared bij oxalate coprecipitation

MFe₂(C₂O₄)₃*6H₂O (M=Ni, Zn) powders were prepared by oxalate coprecipitation. Using Mössbauer spectroscopy, XRD, DTA and TEM, the thermal decomposition of MFe₂(C₂O₄)₃*6H₂O and the formation process of NiFe₂O₄ and ZnFe₂O₄ were investigated.     

The study of transition on NiFe2O4 nanoparticles prepared by co-precipitation/calcination

In this study, the NiFe₂O₄ nanoparticles have been prepared by co-precipitation and calcination process. Using a vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometer of X-ray (EDX), and X-ray photoelectron spectroscopy (XPS), the samples obtained by co-precipitation and then by further calcination have been analyzed. The experimental results show that the precursor synthesized by co-precipitation is the composite of both amorphous FeOOH and Ni(OH)₂, but has no amorphous NiFe₂O₄. The results of both EDX and XPS revealed that the FeOOH species is wrapped up by Ni(OH)₂ species. In the calcination process, the amorphous composite is dehydrated and transformed gradually into crystalline NiFe₂O₄ nanoparticles, with the metal ions diffusing. The reaction is different from the one used to prepare other ferrite (e.g., CoFe₂O₄, MnFe₂O₄, Fe₃O₄, etc.) nanoparticles directly by co-precipitation. With increasing calcination temperature, the NiFe₂O₄ grains grow and the magnetization is enhanced.     

低温固相反应法制备的NiFe2O4纳米颗粒的结构与磁性

采用低温固相反应法制备了晶粒尺寸在8-47 nm之间的NiFe2O4纳米颗粒系列样品,用X射线衍射仪(XRD)、高分辨中子粉末衍射谱仪、振动样品磁强计和超导量子干涉仪等对样品的晶体结构、宏观磁性和纳米颗粒的表面各向异性进行了分析研究.XRD和中子衍射测量结果显示纳米颗粒的晶格常数略高于块体材料,样品的氧参量表明纳米颗粒的晶格畸变程度没有块体材料严重.相对块体材料,纳米颗粒具有较小的磁化强度、较大的矫顽力和各向异性能密度.纳米颗粒从多畴转变为单畴的临界尺寸约为40 nm,超顺磁性临界尺寸约为16 nm.     

红外光谱技术在纳米NiFe2O4制备过程中的应用

采用溶胶 凝胶法制备NiFe2 O4 纳米粉末 ,并经不同温度热处理 .测定了制备过程中各阶段的红外吸收光谱和不同温度处理样品的红外漫反射光谱 .结果表明 ,红外吸收光谱较好地反映了溶胶 凝胶法制备NiFe2 O4 纳米粉末过程中结构的变化 ,为确定热处理温度提供了实验依据 ,彻底消除有机物 ,热处理需在 40 0℃以上 ;红外漫反射谱可以较好地反映粉末的尺寸效应和形态效应 ,粉末粒径越小 ,漫反射函数 (K M)值越大 ;当粒径达到一定尺寸时 ,红外漫反射的尺寸效应消失     

NiFe2O4纳米线阵列的制备与磁性

在氧化铝模板的纳米孔洞中, 用电化学的方法沉积铁镍合金纳米线,经过550℃30h氧化处理, 成功制备出 NiFe2O4纳米线阵列. 分别用扫描电子显微镜 (SEM) 、透射电子显微镜 (TEM) 、x射线衍射仪 (XRD) 和振动样品磁场计 (VSM) 对样品的形貌、晶体结构和磁学性质进行了表征测试. SEM和TEM观察结果显示氧化铝模板的孔洞分布均匀,孔心距约为110nm; 纳米线的直径约为70nm. XRD显示纳米线阵列的物相结构为NiFe2O4; VSM测试结果表明,NiFe2O4纳米线阵列膜的易磁化方向垂直于膜面. 当垂直磁化时磁滞回线的矩形比约为0.5,矫顽力为41×103A/m,比氧化处理前的铁镍合金纳米线阵列都有显著提高.     

High-Temperature Mössbauer Spectroscopy of Mechanically Milled NiFe2O4

Oxide spinels, in particular those containing iron, often exhibit technically important electrical- and magnetic-properties. We report here on X-ray powder diffraction and Mössbauer studies of nanostructured NiFe₂O₄ particles prepared by high-energy ball milling from bulk NiFe₂O₄, which is an inverse spinel. The Mössbauer spectra were recorded in situ at different temperatures in the range of 300–850 K. The Mössbauer spectra of the milled samples show a broad distribution of magnetic hyperfine fields together with a paramagnetic state at room temperature. Initially, at 700 K the spectrum is mainly paramagnetic, but during the process of annealing, magnetic sextets emerge. The treatment results in a significant change in the B/A area ratio of the ferrite. The Neél temperature of the samples is estimated from the B(T) relation to be in the range of 800–850 K.     

尖晶石型纳米铁酸镍的制备与磁性研究

以Fe（NO3）3·9H2O和Ni（NO3）2·6H2O为原料,用柠檬酸为还原剂,采用燃烧法制备了NiFe2O4纳米粉体,用X射线粉末衍射仪（XRD）、透射电子显微镜（TEM）、红外光谱（IR）和振动样品磁强计（VSM）等手段对样品进行了表征,结果表明,样品为立方晶系尖晶石铁酸镍纳米粉体,其粒径在20—40nm之间,具有超顺磁性,并探讨了焙烧温度、焙烧时间对样品磁性的影响。     

复合磁性纳米NiFe2O4的制备及光谱特性

用微乳液化学剪裁方法制备了明胶包裹的复合纳米NiFe2 O4 。将明胶和亚铁盐以及镍盐制成凝胶 ,使该凝胶状反应物在微乳液的胶束中剪裁成微粒 ,再被还原 化合 成核长大。生成的微粒处于明胶蛋白分子的包裹之中。用XRD ,TEM ,EDS ,IR等测试表明 :微粒为明胶包裹球形纳米微粒。微球的平均粒径为 10～10 0nm ,单个微粒的粒径 3 3～ 4 6nm。每个复合微球中约有 3～ 2 2个NiFe2 O4 粒子。NiFe2 O4 复合微粉的比饱和磁化强度σs=36 31× 10 3 4π(A·m- 1 ·g- 1 ) ,矫顽力Hc=6 75 0A·m- 1 ,剩磁Br=4 39× 10 3 4π(A·m- 1 ·g- 1 )。     

Improvement of high-frequency properties of Co_2FeSi Heusler films by ultrathin Ru underlayer

Heusler Co2FeSi films with a uniaxial magnetic anisotropy and high ferromagnetic resonance frequency fr were deposited by an oblique sputtering technique on Ru underlayers with various thicknesses tRufrom 0 nm to 5 nm.It is revealed that the Ru underlayers reduce the grain size of Co2FeSi,dramatically enhance the magnetic anisotropy field HK induced by the internal stress from 242 Oe(1 Oe=79.5775 A·m^-1)to 582 Oe with an increment ratio of 2.4,while a low damping coefficient remains.The result of damping implies that the continuous interface between Ru and Co2FeSi induces a large in-plane anisotropic field without introducing additional external damping.As a result,excellent high-frequency soft magnetic properties with fr up to 6.69 GHz are achieved.     

Synthesis of ZnAl2O4/α-Al2O3 complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnOfilm and α-Al2O3 substrate, ZnAl2O4/α-Al2O3 complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl2O4 changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM)images indicate that the surface morphology of ZnAl2O4 transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRDspectra present that ZnAl2O4 prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl2O4/α-Al2O3 substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes from c-axis single crystal to poly-crystal as ZnAl2O4 layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl2O4 layer can promote nucleation at initial stage of GaN growth, which leadsto the (0001)-oriented GaN film.     

Magnetic and Gas Sensing Property of Nanosized NiFe2O4 Powders Synthesized by Pulsed Wire Discharge

Nickel ferrite (NiFe₂O₄) powders were synthesized by pulsed wire discharge method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses show that only nickel ferrite spinel and no other phase was observed in the powders. Mean size of the obtained particles strongly depended on the oxygen pressure: the higher oxygen pressure corresponds to larger powder size, as determined by Brunauer–Emmet–Teller (BET). The room temperature saturation magnetization of the synthesized powders was 42–46 emu/g depending on the powder size. These powders also showed high chlorine sensitivity at about 280–360°C, and a good linear sensitivity with chlorine concentration.