Fe离子注入ZnO生成超顺磁纳米颗粒

过渡族元素掺杂ZnO生成稀磁半导体, 成为近期国际材料科学研究的热点. 在本文中, 研究Fe离子注入ZnO单晶的结构和磁性变化, 目标是建立磁性和结构的对应关系, 澄清铁磁性的来源. 采用卢瑟福背散射/沟道技术 (RBS/Channelling)、同步辐射X射线衍射 (SR-XRD)和超导量子干涉仪 (SQUID), 研究注入温度和退火对样品的晶格损伤、结构及磁性的影响. 研究表明: 样品注入区损伤随注入温度升高而降低; 低温253 K注入样品中, SR-XRD未检测到新相, Fe离子分布于Zn位, ZnO (0002) 峰右侧肩峰可能属于Zn_1-xFe_xO, 5 K下测试样品不具有铁磁性; 623 K注入和823 K真空退火 (253 K注入) 样品中形成α和γ相金属Fe, 5 K下样品具有明显的剩磁和矫顽力, 零场冷却和场冷却 (ZFC/FC) 曲线和300 K下的磁滞回线显示纳米Fe颗粒具有超顺磁性. Fe离子注入ZnO的磁性源于第二相α-Fe和γ-Fe. 关键词： 离子注入 ZnO 同步辐射X射线衍射 超顺磁性     

Magnetoquantum de Haas-van Alphen oscillations in spin-split two-dimensional Fermi liquid

Theory of magnetoquantum oscillations with spin-split structure in strongly anisotropic (two-dimensional (2D)) metal is developed in the formalism of level approach. Parametric method for exact calculation of oscillations wave forms and amplitudes, developed earlier for spin degenerate levels is generalized on a 2D electron system with spin-split levels. General results are proved: 1) proportionality relation between magnetization and chemical potential oscillations accounting for spin-split energy levels and magnetic field unperturbed levels (states of reservoir), 2) basic equation for chemical potential oscillations invariant to various models of 2D and 1D energy bands (intersecting or overlapping) and localized states. Equilibrium transfer of carriers between overlapping 2D and 1D bands, characterizing the band structure of organic quasi 2D metals, is considered. Transfer parameter, calculated in this model to be of the order of unity, confirms the fact that the wave form of oscillations in organic metals should be quasisymmetric up to ultralow temperature. Presented theory accounts for spin-split magnetization oscillations at magnetic field directions tilted relative to the anisotropic axis of a metal. Theoretical results are compared with available experimental data on organic quasi-2D metal α-(BEDT-TTF)₂KHg(SNC)₄ explaining the appearance of clear split structure under the kink magnetic field and absence above by the corresponding change in the electron g-factor rather than cyclotron mass. Received 20 December 2000 and Received in final form 13 July 2001     

Superparamagnetic relaxation Mössbauer spectra by phonon–magnon interaction using the multilevel method

The formation of local moments and the effect of charge carrriers in dilute magnetic semiconductors can be well understood using local probe techniques like Mossbauer Spectroscopy. We report here on Mossbauer studies in the systems Fe_0.008Ge_{1 ? x} D _x (D = As, Bi), Fe_0.008Ge_{1 ? x} In _x , and Fe_0.008Ge_{1 ? x} Sn _x . At room temperature magnetic interactions were observed for donor (D) impurities at the Fe site in the Fe_0.008Ge system. No such magnetic ordering was observed for acceptor (In) or neutral (Sn) impurities.     

Deuterium isotope effects on iron core formation in ferritin

We present comparative Mössbauer investigations of nanosized FeOOH and FeOOD biomineral phases nucleated within the 7-nm diameter cavity of horse-spleen apoferritin in order to assess deuterium isotopic effects on nanoscale, bioinorganic lattice structures with extended hydrogen bond networks. Differences in magnetic anisotropy energy, packing density and degree of crystallinity in the resulting iron oxo-hydroxide nanophases obtained via D₂O (heavy water) vs. H₂O (light water) solution chemistry are noted. These observations point to the possibility of stabilizing new thermodynamic states in the solid-state by utilizing isotope effects, with important implications for new synthetic pathways to novel nano materials.     

CoFe2O4 nanocrystalline powders prepared by citrate-gel methods: Synthesis, structure and magnetic properties

Nanocrystalline CoFe₂O₄ powders were prepared by decomposition of metal ion citrate precursors. Four samples were synthesized from precursor solutions having different pH values in the range <1–7.0. The powders were characterized by X-ray Diffraction, Thermogravimetry, Differential Thermal Analysis, N₂ physisorption and Transmission Electron Microscopy. Magnetic properties were explored by a SQUID magnetometer. Three out of the four samples, coming from solutions of pH 2, 4 and 7, were produced by an autocombustion reaction and are very similar as regards average size of the nanoparticles (about 20 nm), their morphology and the magnetic properties, while the fourth sample was produced by a slower thermal decomposition and is composed of smaller nanoparticles (about 10 nm).     

Hydrogen Treatment for Superparamagnetic VO2 Nanowires with Large Room‐Temperature Magnetoresistance

One‐dimensional (1D) transition metal oxide (TMO) nanostructures are actively pursued in spintronic devices owing to their nontrivial d electron magnetism and confined electron transport pathways. However, for TMOs, the realization of 1D structures with long‐range magnetic order to achieve a sensitive magnetoelectric response near room temperature has been a longstanding challenge. Herein, we exploit a chemical hydric effect to regulate the spin structure of 1D V–V atomic chains in monoclinic VO₂ nanowires. Hydrogen treatment introduced V³⁺ (3d²) ions into the 1D zigzag V–V chains, triggering the formation of ferromagnetically coupled V³⁺–V⁴⁺ dimers to produce 1D superparamagnetic chains and achieve large room‐temperature negative magnetoresistance (?23.9 %, 300 K, 0.5 T). This approach offers new opportunities to regulate the spin structure of 1D nanostructures to control the intrinsic magnetoelectric properties of spintronic materials.     

Inhibition of domain wall formation and its effects on the bulk magnetic properties of certain spinels

It is shown that hysteresis and susceptibility studies at various temperatures could provide an idea whether a magnetic sample contains multidomain, single-domain or superparamagnetic particles. Our results on titanomagnetites with those of others on cobalt substituted magnesium ferrites suggest that domain wall formation is inhibited in some of them whereby only single-domain or superparamagnetic particles occur irrespective of the physical grain size of such materials. At high concentration of titanium in titanomagnetites, the magnetic behaviour is similar to a spin glass, which we interpret as a transition of optimum single-domains going over to superparamagnetic state.     

Synthesis of NiFe2O4 Powders Well Defined in Size and Morphologies

The uniform NeFe2O4 powders with different particle size and morphologies (octahedral, cubic and spherical) have been prepared from different precursors via hydrothermal process. The nanocrystallines derived from precursor B in the weak alkali solution (pH≥10) are superparamagnetic.     

Magnetic properties of the cyclic spincluster Fe6:bicine

The magnetic properties of the cyclic compound [Fe₆(bicine)₆] LiClO₄ ^. 2MeOH are reported. The cluster Fe₆(bicine)₆ forms an antiferromagnetically coupled ring structure of Fe ^III ions. The magnetic susceptibility is measured between 2 and 300 K and yields the exchange coupling of J/k _B = - 27.5±0.5 K. The field dependence of the magnetic moment is studied at 3 and 6 K in magnetic fields up to 5 T. The zero-field splitting of the first excited spin states with S = 2 and 3 are determined by ESR at 94 GHz. The intra-molecular interactions of the Fe ^III ions are analyzed and the on-site anisotropy of the Fe ^III due to the ligand-configuration is determined to d /k _B = - 0.633±0.008K. Received 28 October 2002 / Received in final form 22 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: bernd@piobelix.physik.uni-karlsruhe.de     

溶剂化金属原子浸渍法制备的Ni-Ag/γ-Al2O3催化剂的结构表征

应用溶剂化金属原子浸渍法和普通浸渍法制备了三种不同摩尔比的γ－Ａｌ２Ｏ３负载Ｎｉ－Ａｇ双金属催化剂。ＸＲＤ和磁测定结果表明ＳＭＡＩ催化剂中Ｎｉ和Ａｇ的粒度均小于金属相同的ＣＩ催化剂。ＳＭＡＩ催化剂中Ｎｉ和Ａｇ未形成合金，而ＣＩ催化剂中Ｎｉ和Ａｇ形成了合金。