Temperature dependent energy-dispersive X-ray diffraction and magnetic study of Fe/Al interface

In situ temperature dependent energy-dispersive structural and magnetic study of electron beam evaporated Fe/Al multilayer sample (MLS) has been investigated. The structural studies show the formation of an intermixed FeAl transition layer of a few nanometers thick at the interface during deposition, which on annealing at 300 °C transforms to B2FeAl intermetallic phase. Magnetization decreases with increase in temperature and drops to minimum above 300 °C due to increase in anti-ferromagnetic interlayer coupling and formation of nonmagnetic FeAl phase at the interface. The Curie temperature (T_c) is found to be 288 °C and is much less than that of bulk bcc Fe.     

Transmission electron microscopy of Co2(Cr1−xFex)Al sputtered films and their magnetic tunneling junctions

The microstructures of Co₂FeAl and Co₂(Cr_0.4Fe_0.6)Al sputtered films and of their magnetic tunnel junctions (MTJs) have been investigated to discuss the possible reasons for an unexpectedly low tunneling magnetoresistance (TMR). The structure of the Co₂FeAl film changed from B2 to L2₁ with increasing substrate temperature, while that of the Co₂(Cr_0.4Fe_0.6)Al film remained B2 up to 500 °C. The thermodynamically predicted phase separation was not observed in the films. The low TMR values obtained from the MTJs using the Co₂FeAl and Co₂(Cr_0.4Fe_0.6)Al films are attributed to the low-spin polarization expected from the low degree of order in these films. The TMR values depend sensitively on the interfacial structure of the tunnel junctions when the degree of order of the film is low.     

Investigation of Fe/Al interface as a function of annealing temperature using XPS

This article describes the systemic investigation of the interface chemical and electronic properties of ultrathin Fe/Al multilayer structure (MLS) as a function of annealing temperature. For this purpose electron beam evaporated [Fe/Al]_×15 ML samples have been prepared under ultrahigh vacuum conditions. The chemical and electronic information of the interfaces at different depth has been obtained from XPS technique.The core level study show a gradual change in the nature of the electronic bonding at the interface as a result of annealing. In particular, the MLS annealed at 200 °C and 400 °C clearly show shifts in the binding energy position of Fe-2p_3/2 core line towards higher energy and Al-2p_3/2 core line towards lower energy side as compared to as-deposited sample, suggesting the formation of FeAl alloy phase at the interface. Another important finding with annealing is that the intensity of peak corresponding to pure Al-2p increases and that of Fe-2p decreases as compared to as-deposited case. The increase in intensity of Al-2p core line suggests the migration of Al atoms towards the surface owing to annealing induced inter-diffusion. The corresponding valence band spectra show appreciable changes in the Fe-3d as well as Al-3s density of states due to strong hybridization of sp-d states at the Fermi level as a result of charge transfer and also provide strong evidence for FeAl alloy formation.     

Theory of order-disorder and order-order transformation in binary alloys with B.C.C. lattice—III.: Atomic ordering in Fe-Al system

On the basis of the general theory developed with the account of a four-particle correlation of atoms in the tetrahedron cluster sites the atomic ordering is considered in the Fe-Al system with 0–50 at % Al. The calculated equilibrium diagram of the phases Fe₃Al(DO₃), FeAl(B2) and short-range order phases α(A2) are in agreement with the diagram proposed by Rudman. The temperature dependences of short- and long-range order parameters in the case of Fe₃Al and Fe₁₃Al₃ compositions are discussed.     

Photoabsorption of alloys of Al with transition metals V,Fe, Ni and Cu and Pr near the AlL2,3-edge

The onset of Al 2p transitions of VAl₃, FeAl, NiAl, NiAl₃, CuAl₂, PrAl₂ and the disordered alloys VAl (16 at % Al, 28%, 41%), FeAl (11%) is shifted up to 1.1 eV. New pronounced structure develops close to the onset which for NiAl agrees with a density of states calculation by Connolly and Johnson.     

FeAl(B2) 合金La, Ac, Sc 和 Y 元素微合金化的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 研究了稀土元素(La, Ac, Sc 和 Y) 微合金化对FeAl (B2) 有序金属间化合物合金晶体结构、 弹性和电子性能的影响. 计算结果表明: 稀土元素Y 易于取代Fe位, 而Sc, La和Ac易于取代Al位, 其中Ac元素的加入使晶格点阵发生最大的变形. 弹性性能的计算表明La, Ac, Sc 和 Y 元素的加入可以改善FeAl (B2) 的塑性, 其中Fe₇Al₈Sc具有最好的塑性和硬度. 稀土元素对合金性能的影响, 主要是由于稀土原子的加入改变了Fe和Al电子之间的杂化作用. 计算结果与已有的试验结果和理论结果相一致.     

Vacancies effect on the mechanical properties in B2 FeAl intermetallic by the first-principles study

ABSTRACT

The geometric structures, electronic and mechanical properties of the high vacancy concentration intermetallic FeAl (experimental value: 3.3 at.% at 1451?K) were investigated by first-principles calculations based on density functional theory. The FeAl structures of different vacancy concentration with minimum energy were addressed, which shows that vacancies of iron (V_Fe) are more favourable and tend to gather together. For mechanical properties, both Young's modulus and elastic constants show an overall downward trend as vacancy concentration increases, but increase abnormally with the vacancy concentration ranging from 3.7 at.% to 5.6 at.%. All can be explained by the strength of Al–Fe bond, in other words, the Al–Fe interaction. Interestingly enough, intermetallic FeAl shows a transfer from the brittle manner to ductile manner, which also behaves as an important feature of FeAl in experiments. All the mechanical properties agree well with experimental data, indicating the reasonable vacancy model of FeAl intermetallic.     

退火诱导机械合金化Fe42.5Al42.5Ti5B10合金的结构演变与晶粒生长动力学

利用XRD和TEM方法研究Fe_42.5Al_42.5Ti₅B₁₀合金在机械合金化及等温热处理过程中的结构演变及晶粒生长动力学,讨论了机械合金化合成机理和热处理过程中的晶粒生长机理.结果表明,球磨过程中Al,Ti,B原子向Fe晶格中扩散,形成Fe(Al,Ti,B)固溶体.机械合金化合成Fe(Al,Ti,B)遵循连续扩散混合机理.球磨50h后,金属Fe,Al,Ti,B已完全合金化,球磨终产物为纳米晶Fe(Al,Ti,B).球 关键词： XRD TEM 42.5Al_42.5Ti₅B₁₀合金')" href="#">Fe_42.5Al_42.5Ti₅B₁₀合金 机械合金化     

Hyperfine interactions in Zr(Fe1−x Al x )2 measured forx≤0.20 by TDPAC and Mössbauer spectroscopy

Mössbauer (⁵⁷Fe) and TDPAC spectroscopy (¹⁸¹Hf) have been used to study quasibinary compounds Zr(Fe_1?x Al _x )₂ forx≤0.20. It has been found that the dependence of the mean values of the hyperfine magnetic field, quadrupole splitting and isomer shift on the Al concentrationx is strong. The dependence of the hyperfine magnetic field on the number of Al atoms as nearest and next-nearest neighbours of⁵⁷Fe has been established. The TDPAC results also indicate a dependence of the hyperfine field on¹⁸¹Ta on Al concentration.     

Spray pyrolised Al/ZnO: Al heterojunction: Effect of Al on junction properties

Al/ZnO: Al heterojunction was fabricated by depositing ZnO: Al film on Al substrate by spray pyrolysis technique at 220 °C substrate temperature. XRD, SEM and EDAX techniques were used to study the properties of thin films. Heterojunction properties were studied by I–V and C–V measurements. The fabricated Al/ZnO: Al junctions were rectifying in character. The room temperature ideality factors of Al/ZnO: Al junctions are found to vary from 2.56 to 5.45. The reverse saturation currents are 5.21 × 10⁻⁹, 1.35 × 10⁻⁶, 1.99 × 10⁻⁶, 9.99 × 10⁻⁷ and 1.02 × 10⁻⁷ A for Al/ZnO: Al junctions. Junction forward current depends on doping concentrations and temperature, whereas reverse saturation current remains independent for Al concentration. The built-in-potential calculated from capacitance for Al/ZnO: Al junctions are 2.74, 2.60, 2.0, 2.50 and 2.43 V corresponding to 1, 2, 3, 4 and 5 mol% of Al. X-ray diffraction study confirmed that the films are polycrystalline, orientated in (0 0 2) plane. Scanning electron microscopy study confirmed circular ring patterns with inside ribbon type structure for Al doped ZnO films.     

NMR study of the spin polarization in dilute Al-Mn alloys

The spatial dependence of the spin density oscillations σ(r) around Mn impurities in Al-Mn dilute alloys has been studied by ²⁷Al four pulse NMR experiments. The concentration dependence of the magnetic broadening $\text{ΔH}{}^{\mathrm{i}}\text{1}\text{2}$ of the ²⁷Al line is found slower than linear. This result, together with the temperature dependence of the magnetic broadening as measured by CW NMR, demonstrates both the preasymptotic depression of σ(r) in Al-Mn and the long range damping of σ(r) associated with the finite mean free path at high T.     

Influence of Al concentration on electrical, structural and optical properties of Al-As codoped p-ZnO thin films

We report the influence of Al concentration on electrical, structural, optical and morphological properties of Al-As codoped p-ZnO thin films using RF magnetron sputtering. Al-As codoped p-ZnO films with different Al concentrations were fabricated using As back diffusion from the GaAs substrate and sputtering Al₂O₃ mixed ZnO targets (1, 2 and 4 at%). The grown films were investigated by Hall effect measurement, X-ray diffraction (XRD), electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and atomic force microscopy (AFM) to study the electrical, structural, optical and morphological properties of the films. From the XRD, it was observed that both full-width at half-maximum (FWHM) and c-axis lattice constant have similar trends with respect to Al concentration. Hall measurements showed that the hole concentration increases as the Al concentration increases from 10¹⁵ to 10²⁰ cm⁻³. The increase in hole concentration upon codoping was supported by the red shift in the near-band-edge (NBE) emission observed from room temperature PL spectra. The proposed p-type mechanism due to As_Zn-2V_Zn complex was confirmed by low temperature PL and XPS analysis. The low FWHM, resistivity and peak-to-valley roughness observed by XRD, Hall measurement and AFM, respectively, suggest that 1 at% Al-doped ZnO:As film is the best codoped film.     

NMR study of pure and doped β-LiAl

The NMR spin-lattice relaxation time, T_I, has been measured as a function of temperature for both ⁷Li and ²⁷Al in pure and doped β-LiAl alloys. Compositions with ⁷Li concentration in the range 48.3–54.5% and doping in the form Li₅₀Al_50?xM_x, where M = Ag or In, were studied. The relaxation rates T₁^?1 for the ²⁷Li and the ²⁷Al resonances were found to be peaked functions of temperature with the maxima for ⁷Li appearing at composition dependent temperatures. The ²⁷Al maxima always appeared at a lower temperature, independent of composition, and the ²⁷Al maximum relaxation rate was a strong function of composition in contrast with ⁷Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the ⁷Li and coupling of the ²⁷Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.     

A Mössbauer-effect study of the isomer shifts of iron in nickel aluminides

The room-temperature isomer shifts of iron in nickel aluminides have been measured. A single Mössbauer-resonance line is observed for alloys of composition (NiAl)_100-cFe_c, 0 ? c ? 5 at. %, and Al₅₀Ni^50-cFec, 2 ? c ? 50 at. %. The isomer shift for dilute Fe concentration in stoichiometric NiAl is compared with that for FeAl at stoichiometric composition.     

Onset of magnetism in concentrated ternary alloys II: Laves phase compounds A(Fe1-xBx)2 (A = Y,Zr, U; B = Mn,Co and Al)

The onset of magnetism as a function of concentration and the magnetic properties of nine ternary Laves phase systems A(Fe_1-xB_x)₂ (A=Y, Zr, U; B=Mn, Co and Al) are discussed in terms of homogeneous and heterogeneous models based on the SEW model and the Landau theory of phase transition of second order. A significant influence of the nomagnetic elements Y, Zr and U upon the Fe magnetic moment substituted by Mn, Co and Al is observed. Low temperature freezing phenomena affect the magnetization process around the critical concentration x_c in Y and Zr compounds while they seem to be of minor importance for the U systems. Comparing the magnetic properties of these nine systems implies that the magnetization process becomes more homogeneous in the sequence of Y-, Zr- and U(Fe, B)₂ compounds. This - together with volume considerations and the different magnetovolume effects observed as well as susceptibility measurements lead to the suggestion that the Fe moment tends to become more delocalized as one proceeds from Y to U compounds as a consequence of the growing extent to which Y, Zr, U d-electrons are hybridized with the 3d electrons. Furthermore, U can be regarded as tetravalent in these Laves phase compounds, except UAl₂, in which presumably an admixture of U³⁺ and U⁴⁺ occurs.     

Effect of Al substitution for B on magnetic and structural properties of Co-based melt-spun ribbons

This paper reports structural and magnetic properties of rapidly quenched Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al_x (x=0, 1, 2, 3) amorphous ribbons prepared by the single roller melt spinning process. Thermal analysis of the ribbons shows that the replacement of B by Al causes a decrease in the crystallization temperature. Structural studies of the samples have been carried out by transmission electron microscopy and X-ray diffraction. With optimum amount of Al in the alloy, the as-cast material has better soft magnetic properties. The highest maximum permeability (3.55×10⁵), saturation magnetization (523.7 mT) and the lowest coercivity (0.8174 A/m) were obtained in the sample with x=2.     

Enhancement of room temperature ferromagnetism of Fe-doped ZnO epitaxial thin films with Al co-doping

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering. The Al doped optimized samples with carrier concentration n_c∼8.0×10²⁰ cm⁻³ show about 3 times enhanced saturation magnetization (0.58 μ_B/Fe²⁺) than the one with n_c∼3.0×10²⁰ cm⁻³ (0.18 μ_B/Fe²⁺). A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of donors have been found as is expected for carrier-induced room temperature ferromagnetism. The transport mechanism of the electrons in all the DMS films at low temperature range has been identified with the Efros's variable range hopping due to the electron-electron Coulomb interaction.     

Electrical behaviour of lateral Al/n-GaN/Al structures

The electrical behaviour of lateral Al/n-GaN/Al structures has been studied by current-voltage measurements between a large pad with an area of 22 mm² and small contacts with different areas in the range of 0.01-1 mm². The results indicated that near room temperature the current was limited by the GaN layer exhibiting linear I-V characteristics for large contacts around 1 mm², while it was contact limited for small contacts around 0.1 mm² and below. This indicates that the same metal contact can behave as ohmic or rectifying depending on the contact area and so on the ratio of contact resistance to the series resistance of the structure.Near liquid nitrogen temperature, the current through the lateral Al/n-GaN/Al structures was limited by space charges. The Al/n-GaN contacts exhibited a very low Schottky barrier height below or around 0.2 eV. A new possible mechanism responsible for the temperature dependence of the ideality factor is proposed.     

Current transport mechanism in Al/Si3N4/p-Si (MIS) Schottky barrier diodes at low temperatures

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-insulator-semiconductor (Al/Si₃N₄/p-Si) Schottky barrier diodes (SBDs) were measured in the temperature range of 80-300 K. By using the thermionic emission (TE) theory, the zero-bias barrier height Φ_B0 calculated from I-V characteristics was found to increase with increasing temperature. Such temperature dependence is an obvious disagreement with the negative temperature coefficient of the barrier height calculated from C-V characteristics. Also, the ideality factor decreases with increasing temperature, and especially the activation energy plot is nonlinear at low temperatures. Such behaviour is attributed to Schottky barrier inhomogeneties by assuming a Gaussian distribution of barrier heights (BHs) at interface. We attempted to draw a Φ_B0 versus q/2kT plot to obtain evidence of a Gaussian distribution of the BHs, and the values of Φ_Bo = 0.826 eV and α_o = 0.091 V for the mean barrier height and standard deviation at zero-bias, respectively, have been obtained from this plot. Thus, a modified ln(I_o/T²) − q²σ_o²/2(kT)² versus q/kT plot gives Φ_B0 and Richardson constant A^* as 0.820 eV and 30.273 A/cm² K², respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 30.273 A/cm² K² is very close to the theoretical value of 32 A/cm² K² for p-type Si. Hence, it has been concluded that the temperature dependence of the forward I-V characteristics of the Al/Si₃N₄/p-Si Schottky barrier diodes can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. In addition, the temperature dependence of energy distribution of interface state density (N_SS) profiles was determined from the forward I-V measurements by taking into account the bias dependence of the effective barrier height and ideality factor.     

Multiple roles of bathocuproine employed as a buffer-layer in organic light-emitting diodes

Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/Alq₃ (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm²). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al³⁺ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al)_x(BCP)_y]^3x+/Al has formed under electric field and the new cation [(Al)_x(BCP)_y]^3x+ at the BCP/Al interface has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (<1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting.