Role of boron diffusion in CoFeB/MgO magnetic tunnel junctions

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeBmagnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

Fracture in metallic glasses: mechanics and mechanisms

Significant progress in understanding the mechanical behavior of metallic glasses (MGs) was made over the past decade, particularly on mechanisms of plastic deformation. However, recent research thrust has been on exploring the mechanics and physics of fracture. MGs can be very brittle with K-Ic values similar to silicate glasses and ceramics or very tough with K-Ic akin to high toughness crystalline metals. Even the tough MGs can become brittle with structural relaxation following annealing at temperatures close to glass transition temperature (T-g). Detailed experimental studies coupled with complementary numerical simulations of the recent past have provided insights on the micromechanisms of failure as well as nature of crack tip fields, and established the governing fracture criteria for ductile and brittle glasses. In this paper, the above advances are reviewed and outstanding issues in the context of fracture of amorphous alloys that need to be resolved are identified.

Thermal Response on the Microstructure and Texture of ECAP and Cold-Rolled Pure Magnesium

This paper deals with dynamic recrystallization (DRX), static recrystallization, and grain growth phenomena of pure magnesium after equal channel angular pressing (ECAP) by route A and B-C at 523 K (250 A degrees C) followed by 80 pct cold rolling. The ECAP-deformed and the subsequently rolled samples were annealed at 373 K and 773 K (100 A degrees C and 500 A degrees C). The associated changes in the microstructure and texture were studied using electron back-scattered diffraction. ECAP produced an average grain size of 12 to 18 A mu m with B and C-2 fiber textures. Subsequent rolling led to an average grain size 8 to 10 A mu m with basal texture fiber parallel to ND. There was no noticeable increase in the average grain size on annealing at 373 K (100 A degrees C). However, significant increase in the average grain size occurred at 773 K (500 A degrees C). The occurrence of different DRX mechanisms was detected: discontinuous dynamic recrystallization was attributed to basal slip activity and continuous dynamic recovery and recrystallization to prismatic/pyramidal slip systems. Only continuous static recrystallization could be observed on annealing.

Nonequilibrium Microstructures for Ag-Ni Nanowires

This work illustrates that a variety of nanowire microstructures can be obtained either by controlling the nanowire formation kinetics or by suitable thermal processing of as-deposited nanowires with nonequilibrium metastable microstructure. In the present work, 200-nm diameter Ag-Ni nanowires with similar compositions, but with significantly different microstructures, were electrodeposited. A 15 mA deposition current produced nanowires in which Ag-rich crystalline nanoparticles were embedded in a Ni-rich amorphous matrix. A 3 mA deposition current produced nanowires in which an Ag-rich crystalline phase formed a backbone-like configuration in the axial region of the nanowire, whereas the peripheral region contained Ni-rich nanocrystalline and amorphous phases. Isothermal annealing of the nanowires illustrated a phase evolution pathway that was extremely sensitive to the initial nanowire microstructure.

Effect of sputtering parameters on the structure, microstructure and magnetic properties of Tb-Fe films

The effect of sputtering parameters such as gas pressure and power on the structure, microstructure and magnetic properties of sputtered Tb-Fe thin films was investigated. X-ray diffraction and transmission electron microscopy studies showed that all the films were amorphous in nature irrespective of the sputtering parameters. A fine island kind of morphology was observed at low sputtering power whereas large clusters were seen at higher sputtering power. While the composition of Tb-Fe films remained constant with increasing sputtering power, the magnetic behaviour was found to change from superparamagnetic to ferromagnetic. On the other hand, the increase in argon gas pressure was found to deplete the iron concentration in Tb-Fe thin films, which in turn reduced the anisotropy and Curie temperature. Annealing of the films at 773 K did not result in any crystallization and the magnetic properties were also found to remain unchanged. (C) 2015 Elsevier B.V. All rights reserved.

Effect of processing route on phase stability in equiatomic multicomponent Ti20Fe20Ni20Co20Cu20 high entropy alloy

The evolution of microstructure and phase formation in equiatomic Ti20Fe20Ni20Co20Cu20 high entropy alloy synthesised by conventional arc melting followed with suction casting and ball milling with spark plasma sintering route is distinctly different. The cast microstructure exhibits one body centre cubic and two face centre cubic high entropy phases based on titanium, cobalt and copper respectively along with a eutectic containing Ti2Ni type Laves phase. On the contrary, spinodal decomposed microstructure consisting of cobalt and copper solid solution is obtained in the sintered sample. However, long term annealing of cast sample at 950 degrees C reveals a eutectoid transformation with different phases than the cast sample. The aforementioned observations are discussed using CALPHAD thermodynamical approach and available literature.

The deciding role of texture on ductility in a Ce containing Mg alloy

This is the first successful attempt to produce Mg-Ce alloys of different texture through different processing routes while keeping the grain size and grain size distribution same. Tensile data shows that contribution of texture to ductility enhancement is primary and that of grain refinement is secondary. The texture resulting from multi-axial forging of extruded billets followed by annealing exhibits the highest ductility (similar to 40%) at room temperature. (C) 2015 Elsevier B.V. All rights reserved.

Studies on RF Magnetron Sputtered HfO2 Thin Films for Microelectronic Applications

We investigated the effect of oxygen flow rate during the reactive magnetron sputtering on the compositional, structural, optical and electrical properties of HfO2 films. We also studied the influence of annealing temperature on the structural and electrical properties of optimized HfO2 films of 25 to 30 nm thick. X-ray photoelectron study reveals that the films deposited at 15 SCCM of oxygen flow rate are stoichiometric and have an optical band gap of 5.86 eV. X-ray diffraction indicates that films without oxygen flow are amorphous, and beyond an oxygen flow rate of 5 SCCM exhibit polycrystalline monoclinic structure. At an annealing temperature of 600 degrees C, tetragonal phase was observed besides the monoclinic phase. The dielectric constant of 11 and low leakage currents of 1 x 10(-7) A/cm(2) were achieved for the stoichiometric films. As-deposited films show significant frequency dispersion due to the presence of defect states at the HfO2/Si interface, and it reduces after annealing.

In-doped multifilled n-type skutterudites with ZT=1.8

In this paper we maximize the thermoelectric (TE) figure of merit, ZT, of n-type skutterudites, (In,Sr,Ba,Yb)(y)Co4Sb12, via three different routes: (i) find the optimum fraction of In as fourth filler (ii) check the influence of powder particle, grain, and crystallite size on the TE properties and (iii) check thermal stability. Filled n-type (Sr, Ba, Yb)(y)Co4Sb12 was mixed in three different proportions with In0.4Co4Sb12, ball milled (regular or high-energy (HB) ball milling) and hot-pressed. Particle size analyses and SEM pictures of the broken surfaces of the hot pressed samples document that only HB produces uniform particles/grains with average crystallite sizes similar to 100 nm, proven by transmission electron microscopy. X-ray Rietveld refinements combined with EDX indicate that in all cases indium entered the icosahedral voids of the skutterudite. Temperature dependent physical properties of all three regularly ball-milled samples show that increasing In-content infers an increasing electrical resistivity, increasing Seebeck coefficient but a decreasing total thermal conductivity. Although ZT (823 K) is in the same range as for the sample without In, the ZT values in the whole temperature range are higher and consequently the TE-conversion efficiency, eta is at least 10% higher. Annealing the samples at 600 degrees C for three days shows minor changes in structure and thermoelectric properties, indicating TE stability. The HB sample, due to uniformly small particles, equally sized grains and crystallites, exhibits a high power factor (4.4 mW/m K-2 at 730 K) and a very low thermal conductivity leading to an outstanding high ZT = 1.8 at 823 K (eta(max) = 17.5%). (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A Simple Label Switching Algorithm for Semisupervised Structural SVMs

In structured output learning, obtaining labeled data for real-world applications is usually costly, while unlabeled examples are available in abundance. Semisupervised structured classification deals with a small number of labeled examples and a large number of unlabeled structured data. In this work, we consider semisupervised structural support vector machines with domain constraints. The optimization problem, which in general is not convex, contains the loss terms associated with the labeled and unlabeled examples, along with the domain constraints. We propose a simple optimization approach that alternates between solving a supervised learning problem and a constraint matching problem. Solving the constraint matching problem is difficult for structured prediction, and we propose an efficient and effective label switching method to solve it. The alternating optimization is carried out within a deterministic annealing framework, which helps in effective constraint matching and avoiding poor local minima, which are not very useful. The algorithm is simple and easy to implement. Further, it is suitable for any structured output learning problem where exact inference is available. Experiments on benchmark sequence labeling data sets and a natural language parsing data set show that the proposed approach, though simple, achieves comparable generalization performance.

Role of Fuel on Cation Disorder in Magnesium Aluminate (MgAl2O4) Spinel Prepared by Combustion Synthesis

Magnesium aluminate spinel (MgAl2O4) forms an interesting system having tetrahedral and octahedral voids filled with near similar sized divalent Mg2+ and trivalent Al3+ cations. Structural disorder (e.g., Mg-Al antisite defects) can be tuned by synthetic conditions. This study reports the evolution of Mg/Al disorder in MgAl2O4 prepared by combustion synthesis using different types of fuels. The effect of nature of fuel and the final calcination temperature (600 degrees C-900 degrees C for 9h) on degree of cation ordering has been investigated combining powder X-ray (XRD) and neutron (NPD) diffraction. The results indicate very high degree of inversion in the samples crystallized at low annealing temperature, which on further annealing reduces toward the thermodynamically stable values. Raman spectroscopy, probing MgO4, and AlO4 tetrahedral bonds, confirmed the results at a local level.

High performance sol-gel spin-coated titanium dioxide dielectric based MOS structures

High-kappa TiO2 thin films have been fabricated using cost effective sol-gel and spin-coating technique on p-Si (100) wafer. Plasma activation process was used for better adhesion between TiO2 films and Si. The influence of annealing temperature on the structure-electrical properties of titania films were investigated in detail. Both XRD and Raman studies indicate that the anatase phase crystallizes at 400 degrees C, retaining its structural integrity up to 1000 degrees C. The thickness of the deposited films did not vary significantly with the annealing temperature, although the refractive index and the RMS roughness enhanced considerably, accompanied by a decrease in porosity. For electrical measurements, the films were integrated in metal-oxide-semiconductor (MOS) structure. The electrical measurements evoke a temperature dependent dielectric constant with low leakage current density. The Capacitance-voltage (C-V) characteristics of the films annealed at 400 degrees C exhibited a high value of dielectric constant (similar to 34). Further, frequency dependent C-V measurements showed a huge dispersion in accumulation capacitance due to the presence of TiO2/Si interface states and dielectric polarization, was found to follow power law dependence on frequency (with exponent `s'=0.85). A low leakage current density of 3.6 x 10(-7) A/cm(2) at 1 V was observed for the films annealed at 600 degrees C. The results of structure-electrical properties suggest that the deposition of titania by wet chemical method is more attractive and cost-effective for production of high-kappa materials compared to other advanced deposition techniques such as sputtering, MBE, MOCVD and AID. The results also suggest that the high value of dielectric constant kappa obtained at low processing temperature expands its scope as a potential dielectric layer in MOS device technology. (C) 2015 Elsevier Ltd. All rights reserved.

Transport properties of solution processed Cu2SnS3/AZnO heterostructure for low cost photovoltaics

Cu2SnS3 thin films were deposited by a facile sot-gel technique followed by annealing. The annealed films were structurally characterized by grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). The crystal structure was found to be tetragonal with crystallite sizes of 2.4-3 nm. Texture coefficient calculations from the GIXRD revealed the preferential orientation of the film along the (112) plane. The morphological investigations of the films were carried out using field emission scanning electron microscopy (FESEM) and the composition using electron dispersive spectroscopy (EDS). The temperature dependent current, voltage characteristics of the Cu2SnS3/AZnO heterostructure were studied. The log I-log V plot exhibited three regions of different slopes showing linear ohmic behavior and non-linear behavior following the power law. The temperature dependent current voltage characteristics revealed the variation in ideality factor and barrier height with temperature. The Richardson constant was calculated and its deviation from the theoretical value revealed the inhomogeneity of the barrier heights. Transport characteristics were modeled using the thermionic emission model. The Gaussian distribution of barrier heights was applied and from the modified Richardson plot the value of the Richardson constant was found to be 47.18 A cm(-2) K-2. (c) 2015 Elsevier B.V. All rights reserved.

Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core-shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management.

Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization

Thin films of Cu2SnS3 (CTS) were deposited by the facile solution processed sol-gel route followed by a low-temperature annealing. The Cu-Sn-thiourea complex formation was analysed using Fourier Transform Infrared spectrophotometer (FTIR). The various phase transformations and the deposition temperature range for the initial precursor solution was determined using Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). X-Ray Diffraction (XRD) studies revealed the tetragonal phase formation of the CTS annealed films. Raman spectroscopy studies further confirmed the tetragonal phase formation and the absence of any deterioratory secondary phases. The morphological investigations and compositional analysis of the films were determined using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) respectively. Atomic Force Microscopy (AFM) was used to estimate the surface roughness of 1.3 nm. The absorption coefficient was found to be 10(4) cm(-1) and bandgap 1.3 eV which qualifies CTS to be a potential candidate for photovoltaic applications. The refractive index, extinction coefficient and relative permittivity of the film were measured by Spectroscopic ellipsometry. Hall effect measurements, indicated the p type nature of the films with a hole concentration of 2 x 10(18) cm(-3), electrical conductivity of 9 S/cm and a hole mobility of 29 cm(2)/V. The properties of CTS as deduced from the current study, present CTS as a potential absorber layer material for thin film solar cells. (C) 2015 Elsevier B.V. All rights reserved.