Co doping enhanced giant magnetocaloric effect in Mn1-xCoxAs films epitaxied on GaAs (001)

A giant magnetocaloric effect was found in series of Mn1-xCoxAs films epitaxied on GaAs (001). The maximum magnetic entropy change caused by a magnetic field of 4 T is as large as 25 J/kg K around room temperature, which is about twice the value of pure MnAs film. The observed small thermal hysteresis is more suitable for practical application. Growing of layered Mn1-xCoxAs films with Co concentration changing gradually may draw layered active magnetic regenerator refrigerators closer to practical application. Our experimental result may provide the possibility for the combination of magnetocaloric effect and microelectronic circuitry.

Structural and magnetic properties of insulating Zn1-xCoxO thin films

Cobalt-doped ZnO (Zn1-xCoxO) thin films were fabricated by reactive magnetron cosputtering. The processing conditions were carefully designed to avoid the occurrence of Co precipitations. The films are c-axis oriented, and the solubility limit of Co in ZnO is less than 17%, determined by x-ray diffraction. X-ray photoemission spectroscopy measurements show Co ions have a chemical valance of 2+. In this paper, hysteresis loops were clearly observed for Zn1-xCoxO films at room temperature. The coercive field, as well as saturation magnetization per Co atom, decreases with increasing Co content, within the range of 0.07

High-field ferromagnetic resonance in fine particles

We investigate high-field ferromagnetic resonance of superparamagnetic particles with uniaxial anisotropy, In this case, since the field is large enough to saturate the magnetization, the thermal orientational fluctuations of the magnetic moment of the particle are negligible. Thus, we derive the dynamic susceptibility of the system on the basis of an independent particle model. High-field ferromagnetic resonance has been performed on fine cobalt particles, The analysis of the spectra obtained at different frequencies allows us to estimate the effective magnetic anisotropy, the gyromagnetic ratio, and the transverse relaxation time. (C) 1998 Elsevier Science B.V. All rights reserved.

铁氰化钴修饰电极的制备、表征及电催化性能的研究

多核过渡金属铁氰化物修饰电极的研究在电催化、电色显示器件、能量存储、离子识别等方面具有重要的意义。本文采用了电化学的循环伏安法（CV）、旋转圆盘电极技术（RDE）、傅立叶变换红外光谱法（FTIR）和扫描电子显微镜技术（SEM），系统研究了一类铁氰化物—铁氰化钴修饰玻碳（CoHCF／GC）电极的制备方法及其对神经递质多巴胺（DA）的电催化作用以及修饰溶液中一种阳离子表面活性剂十六烷基三甲基溴化铵（CTAB）的加入对所制备的CoHCF／GC电极的电化学性能和电催化性能的影响。主要内容和结论如下：1．改进了CoHCF／GC电极的制备方法。新方法具有简单易行、不受修饰物浓度干扰、修饰时间短、修饰量易于控制等优点，且制备的CoHCF粒径小、结构明确、电化学性能稳定。2．研究了CoHCF／GC电极对DA氧化的电催化性能。采用RDE测定了CoHCF／GC电极对DA的电催化氧化的动力学参数。结果表明用新方法制备的修饰电极对DA氧化有更好的电催化性能。3．研究了在修饰溶液中加入临界浓度的CTAB对所制备的CoHCF／GC电极的影响。CV的研究结果表明，CTAB的加入，基本不影响CoHCF／GC的氧化还原峰峰电位，却使其氧化还原峰峰电流明显增大，且使所制得CoHCF／GC电极对DA氧化表现出不同的电催化行为。FTIR研究表明，CTAB不吸附在电极表面，不改变COHCF膜的化学组成和结构。SEM研究结果表明，CTAB的加入使所制备的CoHCF膜长得更快，更厚，使CoHCF粒子长得更大。因此，可得出结论为CoHCF粒子的大小是影响其对DA不同的电催化行为的主要因素。4．用RDE技术测定了有、无CTAB情况下制备的CoHCF／GC电极对DA的电催化反应的动力学参数。结果表明，有CTAB时制备的CoHCF／GC电极对DA的表观动力学常数kΓ比没有时的要大。

过渡金属烯烃聚合催化剂的研究

本论文合成、表征了一系列以铁、钻、镍、铬为中心离子的非茂过渡金属配合物，研究了这些配合物催化烯烃聚合的反应行为。主要工作和结论如下：1，合成、表征了一系列苯环邻、对位含取代基的毗咤双亚胺铁、钻乙烯聚合催化剂。在改性甲基铝氧烷（MMAO）的活化下，这些配合物可高效催化乙烯聚合。在优化条件下，研究了邻、对位取代基电子效应对催化活性的影响。对于铁催化剂来说，不仅邻位取代基体积的大小对乙烯的插入及所得聚合物的分子量有微调作用，对位取代基的电子效应也对催化活性和聚合物分子量影响也很大。2．我们在单核毗咤双亚胺铁催化剂的基础上，设计合成了环状三核铁催化剂。这种环状三核铁催化剂的活性中心位于环状结构的内部。在有机铝的活化下，可高效催化乙烯聚合。新的催化剂展示出更长的催化剂寿命，与单核铁催化剂相比，性能上得到非常大的改善，不但能防止催化剂分子间的失活，还可有效抑制链转移反应的发生。3．在MMAO的活化下，吡啶双亚胺铁可催化（甲基）丙烯酸酷的聚合。催化MMA的聚合时，得到低分子量、窄分布的间规PMMA；催化BMA的聚合时，活性很高，得到无规、低分子量的PBMA；催化丙烯酸酷聚合时，不仅活性高，而且易得到高分子量的聚合物。催化活性、聚合物产率、立构规整度、分子量及分子量分布都会受到反应参数、催化剂结构及助催化剂的性质的影响。4．合成、表征了一系列带有不同取代基的份二亚胺镍催化剂。在MMAO的活化下，这些镍配合物可以高效催化乙烯聚合。不仅邻位取代基体积的大小对乙烯的插入及所得聚合物的分子量有微调作用，对位取代基的电子效应也对催化活性、聚合物分子量及其支化度也有很大的影响。5．合成、表征了一系列新型二苯硫醚双亚胺铬催化剂。在MMAO的活化下，这些铬配合物可常温催化乙烯聚合，生成具有宽分布的聚乙烯。配体结构以及聚合条件对催化剂的活性及所得聚合物的性质有很大影响。

INTERFACIAL REACTIONS IN THE CO/SI/GAAS AND SI/CO/GAAS SYSTEMS

The interfacial reactions between thin films of cobalt and silicon and (100)-oriented GaAs substrates in two configurations, Co/Si/GaAs and Si/Co/GaAs, were studied using a variety of techniques including Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy. The annealing conditions were 200, 300, 400, 600-degrees-C for 30 min, and rapid thermal annealing for 15 s. It was found that Si layer in the Co/Si/GaAs system acts as a barrier at the interface between Co and GaAs when annealed up to 600-degrees-C. The interfacial reaction between Co and Si is faster than that between Co and GaAs in the system of Si/Co/GaAs. The sequence of compound formation for the two metallizations studied (Co/Si/GaAs and Si/Co/GaAs) depends strongly on the sample configuration as well as the layer thickness of Si and Co (Co/Si atomic ratio). From our results, it is promising to utilize Co/Si/GaAs multilayer film structure to make a CoSi2/GaAs contact, and this CoSi2 may offer an alternative to the commonly used W silicides as improved gate metallurgies in self-aligned metal-semiconductor field effect transistor (MESFET) technologies.

Deviation from the stoichiometric composition of single-crystal 'Er2Co17'

The deviation from the stoichiometric composition of single-crystal 'Er2Co17' has been determined by theoretical analysis. It is found that the composition of this single-crystal 'Er2Co17' is rich in cobalt, and its real composition is suggested to be Er2-deltaCo17+2 delta (delta = 0.14) on the basis of a comparison of calculations based on the single-ion model with a series of experiments. The values of the Er-Co exchange field H-ex and the crystalline-electric-field (CEF) parameters A(n)(m) at the rare-earth (R) site in the 'Er2Co17' compound are also evaluated at the same time. The experiments provide the following data: the temperature dependence of the spontaneous magnetization of the compounds and the normalized magnetic moment of the Er ion, the magnetization curves dong the crystallographic axes at 4.2 K and 200 K, and the temperature dependence of the magnetization along the crystallographic axes in a field of 4 T.

High-field ferromagnetic resonance in fine particles

We investigate high-field ferromagnetic resonance of superparamagnetic particles with uniaxial anisotropy, In this case, since the field is large enough to saturate the magnetization, the thermal orientational fluctuations of the magnetic moment of the particle are negligible. Thus, we derive the dynamic susceptibility of the system on the basis of an independent particle model. High-field ferromagnetic resonance has been performed on fine cobalt particles, The analysis of the spectra obtained at different frequencies allows us to estimate the effective magnetic anisotropy, the gyromagnetic ratio, and the transverse relaxation time. (C) 1998 Elsevier Science B.V. All rights reserved.