Ab initio calculation of the BCC Fe-Al-Mo (Iron-Aluminum-Molybdenum) phase diagram: Implications for the nature of the tau(2) phase

The metastable phase diagram of the BCC-based ordering equilibria in the Fe-Al-Mo system has been calculated via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. Four isothermal sections at 1750 K, 2000 K, 2250 K and 2500 K are calculated and correlated with recently published experimental data on the system. The results confirm that the critical temperature for the order-disorder equilibrium between Fe(3)Al-D0(3) and FeAl-B2 is increased by Mo additions, while the critical temperature for the FeAl-B2/A2 equilibrium is kept approximately invariant with increasing Mo contents. The stabilization of the Al-rich A2 phase in equilibrium with overstoichiometric B2-(Fe,Mo)Al is also consistent with the attribution of the A2 structure to the tau(2) phase, stable at high temperatures in overstoichiometric B2-FeAl. (C) 2009 Elsevier Ltd. All rights reserved.

重离子辐照引起的金属/金属界面混合与相变研究

本论文用重离子在室温下辐照Fe/Cu(Si/Fe(10nm)/[Cu(2.5nm)/Fe(2.5nm)]8/Cu(5n m)、Si/Fe(10nm)/[Cu(4nm)/Fe(4nm)]5(8)/Cu(4nm)、Si/[Fe(10nm)/Cu(10nm)]5)和Fe/Nb(Si /[Fe(10nm)/Nb(4nm)/Fe(4nm)/Nb(4nm)]2/[Fe(4nm)/Nb(4nm)]4))金属多层膜样品，然后利用X射线衍射谱、俄歇电子元素深度剖析谱、透射电子显微镜和振动样品磁强计对样品进行分析。主要研究界面原子混合、相变现象及其与离子辐照参数之间的关系。 1. 室温下用400 keV Xe离子辐照Fe/Cu(Si/Fe(10nm)/[Cu(2.5nm)/Fe(2.5nm)]8/Cu (5nm)、Si/Fe(10nm)/[Cu(4nm)/Fe(4nm)]5/Cu(4nm))和Fe/Nb多层膜。结果显示：随着辐照量的增加，离子辐照引起了Fe/Cu多层膜中Fe与Cu原子的混合和Cu基fcc固溶体和Fe基bcc固溶体地出现；而在Fe/Nb多层膜中，离子辐照引起Fe与Nb原子的混合和FeNb固溶体和非晶态FeNb合金相的出现。随着样品的结构变化，样品的磁滞回线也发生了变化。 2. 室温下用2 MeV Xe离子辐照Fe/Cu和Fe/Nb多层膜。结果显示：随着辐照量的增加，首先发生Fe、Cu原子的偏析和界面锐化，接着发生混合，辐照量较大时形成Cu基fcc固溶体和Fe基bcc固溶体；而在Fe/Nb多层膜中，低辐照量辐照引起多层膜的界面锐化，高辐照量辐照引起Fe与Nb原子混合和FeNb固溶体和非晶态FeNb合金相的出现。随着样品的结构变化，样品的磁滞回线也发生了变化。 3. 室温下用2.03 GeV Kr离子辐照Fe/Cu和Fe/Nb多层膜。结果显示：对于Si/Fe(10nm)/[Cu(4nm)/Fe(4nm)]5/Cu(4nm)多层膜，辐照量为1.0×1013ions/cm2时，发生Fe、Cu原子的偏析和界面锐化；对于Si/[Fe(10nm)/Cu(10nm)]5多层膜，随着辐照量的增加，首先发生Fe、Cu原子的偏析和界面锐化，接着发生混合；对于Fe/Nb多层膜，低辐照量辐照引起多层膜的界面锐化，高辐照量辐照引起Fe与Nb原子混合。随着样品的结构变化，多层膜样品的磁滞回线也发生了变化。 4. 室温下用1.08 GeV Kr离子辐照Fe/Cu和Fe/Nb多层膜。结果显示：在Si/Fe(10nm)/[Cu(4nm)/Fe(4nm)]5/Cu(4nm)、Si/[Fe(10nm)/Cu(10nm)]5和Fe/Nb多层膜中，辐照引起Fe层与Cu（Nb）层混合。随着样品的结构变化，多层膜样品的磁滞回线也发生了变化。 最后，对重离子辐照引起多层膜界面原子混合及相变的机理进行了探讨

Synthesis and Characterization of Magnetic Nanoparticles with High Magnetization and Good Oxidation Resistibility

Magnetic nanoparticles attract increasing attention because of their current and potential biomedical applications, such as, magnetically targeted and controlled drug delivery, magnetic hyperthermia and magnetic extraction. Increased magnetization can lead to improved performance in targeting and retention in drug delivery and a higher efficiency in biomaterials extraction. We reported an approach to synthesize iron contained magnetic nanoparticles with high magnetization and good oxidation resistibility by pyrolysis of iron pentacarbonyl (Fe(CO)[subscript 5]) in methane (CH[subscript 4]). Using the high reactivity of Fe nanoparticles, decomposition of CH[subscript 4] on the Fe nanoparticles leads to the formation of nanocrystalline iron carbides at a temperature below 260°C. Structural investigation indicated that the as-synthesized nanoparticles contained crystalline bcc Fe, iron carbides and spinel iron oxide. The Mössbauer and DSC results testified that the as-synthesized nanoparticle contained three crystalline iron carbide phases, which converted to Fe[subscript 3]C after a heat treatment. Surface analysis suggested that the as-synthesized and subsequently heated iron-iron carbide particles were coated by iron oxide, which originated from oxidization of surface Fe atoms. The heat-treated nanoparticles exhibited a magnetization of 160 emu/g, which is two times of that of currently used spinel iron oxide nanoparticles. After heating in an acidic solution with a pH value of 5 at 60°C for 20 h, the nanoparticles retained 90 percentage of the magnetization.

Orbital-polarization terms: From a phenomenological to a first-principles description of orbital magnetism in density-functional theory

Phenomenological orbital-polarizition (OP) terms have been repeatedly introduced in the single-particle equations of spin-density-functional theory, in order to improve the description of orbital magnetic moments in systems containing transition metal ions. Here we show that these ad hoc corrections can be interpreted as approximations to the exchange-correlation vector potential A(xc) of current-density functional theory (CDFT). This connection provides additional information on both approaches: phenomenological OP terms are connected to first-principles theory, leading to a rationale for their empirical success and a reassessment of their limitations and the approximations made in their derivation. Conversely, the connection of OP terms with CDFT leads to a set of simple approximations to the CDFT potential A(xc), with a number of desirable features that are absent from electron-gas-based functionals. (C) 2008 Wiley Periodicals, Inc.

Spinpolarisierte Rastertunnelmikroskopie magnetischer Nanostrukturen am Beispiel von bcc-Co/Fe(110), Fe/Mo(110) und Kupfer-Phthalocyanin/Fe(110)

In dieser Arbeit werden, nach einer Einführung in die spinpolarisierte Rastertunnelmikroskopie und -spektroskopie als experimentelle Methode zur Untersuchung magnetischer Nanostrukturen, Ergebnisse zur spinpolarisierten elektronischen Struktur in Abhängigkeit von der Kristallstruktur am Beispiel ultradünner Co-Schichten sowie in Abhängigkeit von der Magnetisierungsrichtung für ultradünne Fe-Schichten vorgestellt. Hochaufgelöste Messungen zeigen die ortsabhängige Spinpolarisation auf einem einzelnen Kupfer-Phthalocyanin Molekül. rnrnKobalt wurde durch pseudomorphes Wachstum auf den (110)-Oberflächen der kubisch raumzentrierten Metalle Chrom und Eisen deponiert. Im Unterschied zu früheren Berichten in der Literatur lassen sich nur zwei Lagen Co in der kubisch raumzentrierten (bcc) Ordnung stabilisieren. Die bcc-Co Schichten auf der Fe(110)-Oberfläche zeigen keine Anzeichen von epitaktischen Verzerrungen. rnDickere Schichten rekonstruieren in eine dicht gepackte Struktur (hcp/fcc). Durch die bcc Ordnung wird die Spinpolarisation von Kobalt auf P=62% erhöht (hcp-Co: P=45%). rnrnDie temperaturabhängige Spinreorientierung (SRT) ultradünner Filme Fe/Mo(110) wurde mit spinpolarisierter Spektroskopie untersucht. Eine Neuausrichtung der Magnetisierung aus der senkrechten [110]-Achse in die in der Ebene liegenden [001]-Achse wird bei T=(13,2+-0,5)K festgestellt, wobei es sich um einen diskontinuierlichen Reorientierungsübergang handelt, d.h. die freie Energie weist innerhalb eines bestimmten Temperaturbereichs gleichzeitig zwei Minima auf. Weiterhin wird in der Mono- und Doppellage Fe/Mo(110 eine Abhängigkeit der elektronischen Struktur von der Ausrichtung der magnetisch leichten Achse und von der Magnetisierung beobachtet. rnrnDie Untersuchung des spinpolarisierten Ladungstransports durch ein Kupfer-Phthalocyanin-Molekül auf der Fe/Mo(110) Oberfläche liefert einen wesentlichen Beitrag zum Verständnis des Spintransports an der Grenzfläche zwischen Metall und organischem Molekül. Die HOMO-LUMO-Energielücke des freien Moleküls wird durch die Wechselwirkung mit der Metalloberfläche mit Grenzflächenzuständen gefüllt. Diese Zustände reduzieren die Spinpolarisation des durch das Molekül fließenden Tunnelstroms durch einen zusätzlichen unpolarisierten Strombeitrag um einen Faktor zwei. Spinpolarisierte hybridisierte Grenzflächenzustände mit größerem Abstand zur Fermi-Energie führen in Abhängigkeit von der Position auf dem Molekül zu weiteren Beiträgen zur effektiven Spinpolarisation. Diese Untersuchungen belegen die Möglichkeit einer effektiven Spininjektion in organische Halbleiter und damit das Potential dieser Materialien für die weitere Entwicklung von Spintronik-Bauteilen.

Computational study of atomic mobility for bcc phase in Ti-Al-Fe system

Experimental diffusion data were critically assessed to develop the atomic mobility for the bcc phase of the Ti–Al–Fe system by using the DICTRA software. Good agreements were obtained from comprehensive comparisons made between the calculated and the experimental diffusion coefficients. The developed atomic mobility was then validated by well predicting the interdiffusion behavior observed from the diffusion-couple experiments in available literature.

Microstructural Evolution during Laser Resolidification of Fe-18 At. Pct Ge Alloy

The technique of laser resolidification has been used to study the rapid solidification behavior of concentrated Fe-18 at. pct Ge alloy. The microstructural evolution has been studied as a function of scanning rate of laser beam. Scanning electron microscopy (SEM) reveals the formation of a two-layer (designated as "A" and "B") microstructure in the remelted pool. The A layer shows a band consisting of a network of interconnected channels and walls, quite similar to cell walls. The B layer shows dendritic growth. Transmission electron microscopic observations reveal the formation of bcc alpha-FeGe in the B layer. Laser melting has been found to play an important role in formation of the A layer. Microstructural evolution in B has been analyzed using the competitive growth criterion, and formation of bcc alpha-FeGe has been rationalized in the remelted layers.

High-Temperature Deformation Processing Map Approach for Obtaining the Desired Microstructure in a Multi-component (Ni-Ti-Cu-Fe) Alloy

An equiatomic NiTiCuFe multi-component alloy with simple body-centered cubic (bcc) and face-centered cubic solid-solution phases in the microstructure was processed by vacuum induction melting furnace under dynamic Ar atmosphere. High-temperature uniaxial compression experiments were conducted on it in the temperature range of 1073 K to 1303 K (800 degrees C to 1030 degrees C) and strain rate range of 10(-3) to 10(-1) s(-1). The data generated were analyzed with the aid of the dynamic materials model through which power dissipation efficiency and instability maps were generated so as to identify the governing deformation mechanisms that are operative in different temperature-strain rate regimes with the aid of complementary microstructural analysis of the deformed specimens. Results indicate that the stable domain for the high temperature deformation of the multi-component alloy occurs in the temperature range of 1173 K to 1303 K (900 degrees C to 1030 degrees C) and (epsilon) over dot range of 10(-3) to 10(-1.2) s(-1), and the deformation is unstable at T = 1073 K to 1153 K (800 degrees C to 880 degrees C) and (epsilon) over dot = 10(-3) to 10(-1.4) s(-1) as well as T = 1223 K to 1293 K (950 degrees C to 1020 degrees C) and (epsilon) over dot = 10(-1.4) to 10(-1) s(-1), with adiabatic shear banding, localized plastic flow, or cracking being the unstable mechanisms. A constitutive equation that describes the flow stress of NiTiCuFe multi-component alloy as a function of strain rate and deformation temperature was also determined. (C) The Minerals, Metals & Materials Society and ASM International 2015

Strain and magnetic anisotropy of as-grown and annealed Fe films on c(4x4) reconstructed GaAs (001) surface

Fe films with the different thicknesses were grown on c(4x4) reconstructed GaAs (001) surfaces at low temperature by molecular-beam epitaxy. Well-ordered bcc structural Fe epitaxial films are confirmed by x-ray diffraction patterns and high-resolution cross-sectional transmission electron microscopy images. A large lattice expansion perpendicular to the surface in Fe film is observed. In-plane uniaxial magnetic anisotropy is determined by the difference between magnetizing energy along [110] and [110] directions, and the constant of interfacial uniaxial magnetic anisotropy is calculated to be 1.02x10(-4) J m(-2). We also find that magnetic anisotropy is not obviously influenced after in situ annealing, but in-plane strain is completely changed.

Modification of Fe/Cu multilayers under 400 keV Xe20+ irradiation

Two kinds of Fe/Cu multilayers with different modulation wavelength were deposited on cleaved Si(100) substrates and then irradiated at room temperature using 400 keV Xe20+ in a wide range of irradiation fluences. As a comparison, thermal annealing at 300-900 degrees C was also carried out in vacuum. Then the samples were analyzed by XRD and the evolution of crystallite structures induced by irradiation was investigated. The obtained XRD patterns showed that, with increase of the irradiation fluence, the peaks of Fe became weaker, the peaks related to Cu-based fcc solid solution and Fe-based bcc solid solution phase became visible and the former became strong gradually. This implied that the intermixing at the Fe/Cu interface induced by ion irradiation resulted in the formation of the new phases which could not be achieved by thermal annealing. The possible intermixing mechanism of Fe/Cu multilayers induced by energetic ion irradiation was briefly discussed.

Estudo da evolução estrutural induzida por irradiação iônica em multicamadas de Fe/sub 30/Co/sub 70/Cu

Neste trabalho, relata-se o comportamento estrutural de multicamadas de Fe30Co70/Cu, quando submetidas a irradiações com íons de He+ e Kr+ e tratamentos térmicos. As multicamadas foram confeccionadas à temperatura ambiente, numa câmara de ultra-alto vácuo por evaporação alternada de uma liga Fe30Co70 e Cu. As energias das irradiações foram escolhidas de tal forma que os íons atravessassem a multicamada, ficando alojados no substrato de Si. As doses foram calculadas para que os dois tipos de íons depositassem a mesma energia total na multicamada. Após a confecção dos filmes, os mesmos foram submetidos a tratamentos térmicos de 10 minutos em temperaturas variando de 350 a 450 oC. Este processo quebra a estrutura de camadas e forma uma liga heterogênea de grãos magnéticos embebidos numa matriz metálica de Cu. Para investigar a evolução das propriedades estruturais das amostras utilizou-se difração, refletividade e espectroscopia de absorção de raios-X. Os resultados da refletividade de raios-X mostraram claramente que uma estrutura de multicamadas é formada após a evaporação, fato que é comprovado pelo pico de Bragg observado nas curvas de refletividade. As irradiações com He+ e Kr+ não destroem a estrutura de multicamadas, ao contrário, elas provocam um alisamento de todas as interfaces. Pelas medidas de difração de raios-X verificou-se que as irradiações resultam no aumento de cristalinidade e no crescimento de tamanho dos cristalitos do sistema. A irradiação com He+ não provoca mudanças significativas na multicamada, mas a irradiação com Kr+ induz uma transformação de fase de bcc para fcc/hcp nas camadas de Fe30Co70. Esta nova fase induzida pela irradiação com Kr+ é metaestável, pois um simples tratamento térmico de 450 oC durante 10 minutos destrói a nova estrutura fcc/hcp, fazendo com que a liga retorne a estrutura bcc. As medidas de absorção de raios-X estão em perfeito acordo com as medidas de difração e refletividade, mostrando que as irradiações promovem uma melhora na ordem cristalográfica. De acordo com o que foi observado das medidas, a irradiação com He+ não provoca nenhum efeito significativo na estrutura da liga de FeCo. Contudo, a irradiação com Kr+ induz uma transformação de fase nesta liga de bcc para uma estrutura fcc e/ou hcp. Tal transformação é induzida pela irradiação com Kr+ porque estes íons transferem mais energia, em colisões nucleares, do que os íons de He+. Além disso, estes íons provocam um grande número de cascatas de colisões, fazendo com que os átomos contidos dentro do volume destas cascatas tornem-se móveis. Desta forma, os átomos de Fe e Co se rearranjam em uma estrutura fcc, que é imposta pelas camadas de Cu adjacentes. Embora a estrutura fcc seja muito mais provável, também é possivel que a estrutura induzida seja hcp.

Structural, morphological and magnetic characterization of electrodeposited Co-Fe-W alloys

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Synthesis and Raman spectroscopic study of Mg/Al,Fe hydrotalcites with variable cationic ratios

Hydrotalcites of formula Mg6 (Fe,Al)2(OH)16(CO3).4H2O formed by intercalation with the carbonate anion as a function of divalent/trivalent cationic ratio have been successfully synthesised. The XRD patterns show variation in the d-spacing attributed to the size of the cation. Raman and infrared bands in the OH stretching region are assigned to (a) brucite layer OH stretching vibrations (b) water stretching bands and (c) water strongly hydrogen bonded to the carbonate anion. Multiple (CO3)2- symmetric stretching bands suggest that different types of (CO3)2- exist in the hydrotalcite interlayer. Increasing the cation ratio (Mg/Al,Fe) resulted in an increase in the combined intensity of the 2 Raman bands at around 3600 cm-1, attributed to Mg-OH stretching modes, and a shift of the overall band profile to higher wavenumbers. These observations are believed to be a result of the increase in magnesium in the structure. Raman spectroscopy shows a reduction in the symmetry of the carbonate, leading to the conclusion that the anions are bonded to the brucite-like hydroxyl surface and to the water in the interlayer. Water bending modes are identified in the infrared spectra at positions greater than 1630 cm-1, indicating the water is strongly hydrogen bonded to both the interlayer anions and the brucite-like surface.