Formation Of Antiphase Boundaries Research Articles

Modelling ternary effects on antiphase boundary energy of Ni3Al

The shearing of ordered γ′ precipitates by matrix dislocations results in the formation of antiphase boundaries (APB) in Ni-base superalloys. The APB energy is an important source of order-strengthening in disk and blade alloys where Ti and Ta substitute for Al in γ′. While the importance of APB energy is well-acknowledged, the effect of alloying on APB energy is not fully understood. In the present study, the effect of Ti and Ta additions on the {111} and {010} APB energies was probed via electronic structure calculations. Results suggest that at low levels of Ti/Ta, APB energies on either plane increases with alloying. However, at higher Ti/Ta levels, the APB energies decrease with alloying. These trends understood by accounting for nearest neighbour violations about the APB and additionally, invoking the effect of precipitate composition on the energy penalty of the violations. We propose an Environment Dependent Nearest Neighbour Bond (EDNNB) model that predicts APB energies that are in close agreement to calculated values.

Defect Structure of Y-Doped Ceria on Different Length Scales

An exhaustive structural investigation of a Y-doped ceria (Ce1–xYxO2–x/2) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO2- and Y2O3-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y2O3. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level.

Influence of cooling rate on iron loss behavior in 6.5 wt% grain-oriented silicon steel

The influence of cooling rate on iron loss behaviors has been investigated in 6.5wt% grain-oriented silicon steels. To fabricate 6.5wt% grain-oriented silicon steels and control the microstructure, 3.0wt% grain-oriented silicon steels covered with SiO2 materials were annealed at 1200°C and then cooled to room temperature using oil quenching, air cooling, or furnace cooling. The magnetic loss of furnace-cooled samples was reduced by 25% compared with oil-quenched samples due to lower anomalous and hysteresis losses. Microstructural analysis showed that these loss behaviors were strongly related to the formation and growth of ordered phases, i.e., B2 and D03. These correlations could be ascribed to the formation of antiphase boundaries, which acted as pinning sites of the magnetic domain walls.

Role of epitaxial strain on the magnetic structure of Fe-doped CoFe2O4

The magnetic structure of Fe-doped CoFe2O4 (Co1−xFe2+xO4) grown on MgO (001) and SrTiO3 (001) substrates is studied with superconducting quantum interference device magnetometry and soft x-ray magnetic spectroscopies. X-ray and electron diffraction show that the choice of substrate has large effects on the strain, crystal structure and surface morphology of Co1−xFe2+xO4 thin films. Samples grown on MgO have small, coherent strains and surfaces that are nearly atomically flat, whereas films grown on SrTiO3 have large tensile strains and surfaces terminated with islands, which indicate the presence of a large density of misfit dislocations. These differences in structural properties correlate with the large differences seen in the magnetic structure; samples grown on SrTiO3 have larger magnetic moments and increased anisotropies compared to those grown on MgO. Most strikingly, the large magnetic spin and orbital moments found in the films grown on SrTiO3 suggest a suppression of anti-phase boundary formation, which we attribute to the large compressive lattice mismatch and the formation of misfit dislocations during the film growth in order to relieve the epitaxial strain. This results in the films grown on SrTiO3 having magnetic properties that are more similar to bulk Co1−xFe2+xO4 than those grown on MgO, demonstrating that epitaxial strain can result in large changes in the magnetic structure of Co1−xFe2+xO4.

Strong enhancement of the critical current at the antiferromagnetic transition in ErNi2B2C single crystals

We report on transport and magnetization measurements of the critical current density Jc in ErNi2B2C single crystals that show strongly enhanced vortex pinning at the Neel temperature TN and low applied fields. The height of the observed Jc peak decreases with increasing magnetic field in clear contrast with that of the peak effect found at the upper critical field. We also performed the first angular transport measurements of Jc ever conducted on this compound. They reveal the correlated nature of this pinning enhancement, which we attribute to the formation of antiphase boundaries at TN.

Characterization of anti-phase boundaries in hetero-epitaxial polar-on-nonpolar semiconductor films by optical second-harmonic generation

Compound semiconductor layers (e.g., GaAs) grown on elemental semiconductor substrates (e.g., Si, Ge) are vulnerable to formation of anti-phase boundary (APB) defects. We show that optical second-harmonic generation (SHG) signals from APB-rich epi-layers are orders of magnitude weaker than from APB-free samples. Moreover, scanning SHG images of APB-rich layers reveal microstructure lacking in APB-free layers. We attribute these findings to the sign reversal of the second-order nonlinear optical susceptibility χijk(2) between neighbouring anti-phase domains within the incident laser spot. In contrast, SHG is insensitive to threading dislocations. Thus, SHG can identify APBs selectively and non-invasively for advanced MOSFET device applications.

Effect of geometry on the magnetic properties of CoFe2O4–PbTiO3 multiferroic composites

In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.

Effect of annealing time on structural and magnetic properties of laser ablated oriented Fe3O4 thin films deposited on Si(100)

We have fabricated ∼143 nm Fe3O4 thin films on Si(100) substrates at 450°C and then annealed them at the same temperature for 30, 60 and 90 min under a vacuum of 10 − 6 torr with pulsed laser deposition. We studied the effects on the structural and magnetic properties of Fe3O4 thin films. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). XRD studies showed pure single phase spinel cubic structure of Fe3O4 with a preferential [111] orientation, independent of substrate orientation at 90 min annealing. Higher magnetization was obtained up to 60 min annealing due to Fe phase but at 90 min, we obtained reduced magnetization of 335 emu/cc. This is attributed to the formation of antiphase boundaries between substrate and film.

Growth and optical properties of axial hybrid III–V/silicon nanowires

Hybrid silicon nanowires with an integrated light-emitting segment can significantly advance nanoelectronics and nanophotonics. They would combine transport and optical characteristics in a nanoscale device, which can operate in the fundamental single-electron and single-photon regime. III-V materials, such as direct bandgap gallium arsenide, are excellent candidates for such optical segments. However, interfacing them with silicon during crystal growth is a major challenge, because of the lattice mismatch, different expansion coefficients and the formation of antiphase boundaries. Here we demonstrate a silicon nanowire with an integrated gallium-arsenide segment. We precisely control the catalyst composition and surface chemistry to obtain dislocation-free interfaces. The integration of gallium arsenide of high optical quality with silicon is enabled by short gallium phosphide buffers. We anticipate that such hybrid silicon/III-V nanowires open practical routes for quantum information devices, where for instance electronic and photonic quantum bits are manipulated in a III-V segment and stored in a silicon section.

Selective Area Growth of InP on On-Axis Si(001) Substrates with Low Antiphase Boundary Formation

We discuss the selective epitaxial growth of InP on patterned Si (001) substrates with Shallow Trench Isolation using a thin Ge buffer to facilitate InP nucleation. The main focus is the defect formation during epitaxial growth and to develop solutions to reduce defect density so that device-quality III-V virtual substrates can be realized on large-scale Si substrates. We compare the InP growth on on-axis and off-axis Si substrates. In the case of off-axis wafers, the formation of stacking faults / twins cannot be avoided, at least not at one of the four side-walls of the Shallow Trench Isolation. The formation of antiphase domain boundaries is reduced (but not yet completely eliminated) by engineering the local Ge surface profile. Further, the high density of Ge surface steps promotes step-flow growth mode instead of 3D growth during the growth of the InP seed layer. Finally, high aspect ratios (>2) allow to confine threading dislocations in the bottom of the trench.

(Invited) Selective Area Growth of InP on On-Axis Si(001) Substrates with Low Antiphase Boundary Formation

We discuss the selective epitaxial growth of InP on patterned Si (001) substrates with Shallow Trench Isolation using a thin Ge buffer to facilitate InP nucleation. The main focus is the defect formation mechanism during epitaxial growth and to develop solutions to reduce defect density so that device-quality III-V virtual substrates can be realized on large-scale Si substrates. We compare the InP growth on on-axis and off-axis Si substrates. In the case of off-axis wafers, the formation of stacking faults / twins cannot be avoided, at least not at one of the four STI side-walls. The formation of antiphase domain boundaries is reduced (but not yet completely eliminated) by engineering the local Ge surface profile. Further, the high density of Ge surface steps promotes step-flow growth mode instead of 3D growth during the growth of the InP seed layer. Finally, high aspect ratios (>2) allow to confine threading dislocations in the bottom of the trench.

Selective Area Growth of InP on On-Axis Si(001) Substrates with Suppressed Antiphase Boundary Formation

not Available.

Effects of Anti-phase Boundary on the Iron Loss of Grain Oriented Silicon Steel

We present a systematic analysis of the iron loss behavior of grain oriented silicon steels containing different Si contents using transmission electron microscopy. When the silicon content changed in the range of 3–6.5 wt%, the iron loss showed a convex profile and the maximum iron loss was observed in 5.2 wt% silicon steel. This maximum iron loss should be ascribed to the formation of antiphase boundaries that acted as pinning centers in the magnetic domain wall.

Void Formation and Structure Change Induced by Heavy Ion Irradiation in GaSb and InSb

Void formation and structure change by heavy ion irradiation were investigated in GaSb and InSb thin films. The voids were formed after irradiation in both materials. The average diameter of the voids was about 15 nm in GaSb and 20 nm in InSb irradiated with 60 keV Sn þ ions to a fluence of 0:25 � 10 18 ions/m 2 at room temperature. The void size in InSb is larger than that in GaSb. The large void size is quantitatively explained by the amount of induced vacancies obtained by the SRIM code simulation. The Debye-Scherrer rings were observed in the SAED patterns on both materials. The structure changes into a polycrystal by ion irradiation. Additionally, the 200 superlattice reflections in the [001] net pattern were almost absent, and the streak pattern along the h110i direction was observed in InSb. It is considered that the anti phase domains of different lengths are formed by ion irradiation. Ion irradiation transforms the structure of InSb from chemical ordering to chemical disordering via the formation of anti phase boundaries. [doi:10.2320/matertrans.M2010037]

Strain relaxation inFe3O4/MgAl2O4heterostructures: Mechanism for formation of antiphase boundaries in an epitaxial system with identical symmetries of film and substrate

Strain relaxation studies in epitaxial magnetite, ${\text{Fe}}_{3}{\text{O}}_{4}$, thin films grown on ${\text{MgAl}}_{2}{\text{O}}_{4}$(100) substrates are reported. The study shows that the films were relaxed in line with the theoretical model prediction with a critical thickness, ${t}_{\text{c}}=5\text{ }\text{nm}$. Antiphase boundaries (APBs) are not expected to form in ${\text{Fe}}_{3}{\text{O}}_{4}$ films grown on ${\text{MgAl}}_{2}{\text{O}}_{4}$ substrates because both film and substrate have the same crystal symmetry. In contrast, our study reveals the formation of APBs within the ${\text{Fe}}_{3}{\text{O}}_{4}$ films. Our analysis shows that the APBs in a ${\text{Fe}}_{3}{\text{O}}_{4}/{\text{MgAl}}_{2}{\text{O}}_{4}$ heteroepitaxial system are formed by partial dislocations, which accommodate the misfit. This formation mechanism of APBs is fundamentally different from the one found in the ${\text{Fe}}_{3}{\text{O}}_{4}/\text{MgO}$ system, where APBs are formed as a consequence of equivalent nucleation sites on the MgO substrate separated by nontranslational vectors of the ${\text{Fe}}_{3}{\text{O}}_{4}$ lattice. The mechanism for the formation of antiphase boundaries through partial dislocations should be applicable to a wide range of epitaxial systems having identical symmetries of the film and the substrate and significant lattice mismatch.

Magnetic and magneto-optical properties of heteroepitaxial magnetite thin films

The magnetic and magneto-optical properties of heteroepitaxial magnetite (Fe 3O 4) thin films were investigated. Of interest was the role of misfit strain on determining its magnetic properties. Epitaxial magnetite thin films were deposited by molecular beam epitaxy using molecular oxygen on (1 0 0) oriented SrTiO 3, BaTiO 3 and MgO. Polar spectroscopic magneto-optic Kerr effect (MOKE) measurements on the thin films over near infrared to visible spectral region revealed two dominant magneto-optical transitions. It is shown that the magnetic and magneto-optical properties depend strongly on misfit strain. The longitudinal MOKE saturation rotation angle decreased with the lattice misfit and was attributed to the formation of antiphase boundaries. Coercivity ranged from 19 to 39 mT and was proportional to the lattice misfit. Thin films were all magnetically isotropic within the plane. The magnetic domain size measured by magnetic force microscopy (MFM) decreased with misfit.

Structural and magnetic properties of magnetite-containing epitaxial iron oxide films grown on MgO(001) substrates

The structural and magnetic properties of three kinds of Fe oxide films—Fe3O4, a berthollide type of Fe oxide, and a composite of Fe3O4 and FeO—grown on MgO(001) substrates epitaxially at 423K by pulsed laser deposition were investigated by magnetoresistance measurement and grazing incidence x-ray diffraction. The Fe3O4 film had the largest negative magnetoresistance and the roughest film-substrate interface. The very close lattice matching of single-phase Fe3O4 and MgO facilitates the formation of antiphase boundaries due to natural growth defects and of a rough interface probably due to cation interdiffusion.

Ordered domain structures of nitrogen-doped GaInP layers grown by organometallic vapor phase epitaxy

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies have been performed on organometallic vapor phase epitaxial GaInP heterostructures grown on (001) GaAs singular and vicinal substrates to investigate nitrogen doping effect on the ordering and domain structures. TEM results show that well-defined order–disorder–order heterostructures are formed when nitrogen doping level is high. This indicates that nitrogen hinders the occurrence of ordering. For the singular samples, ordered domain structures are found to be dependent on the nitrogen doping level of the underlying layer, on which they are grown. The doping dependence of ordered structures and the formation of anti-phase boundaries are described based on surface undulations (i.e., hillocks) and step configuration.

Long-term phase instability in a water-quenched uranium alloy

The U–6 wt% Nb (U6Nb) alloy in the water-quenched (WQ) state has been in service for a number of years. Its long-term reliability is affected by the changes of the alloy microstructure and mechanical properties during service. In this paper, the water quenched U–6 wt% Nb (WQ-U6Nb) alloy in service for 15 years at ambient temperatures was studied using an analytical transmission electron microscopy (TEM) analysis. We found that the long-term natural aging resulted in a disorder–order phase transformation, leading to the formation of anti-phase boundaries (APBs). The newly found ordered phase was then identified by proposing two phase transform schemes, which were also discussed with regards to the potential subsequence of the microstructural evolution for the alloy in further service. The initial study also provides convincing evidence for the disorder–order transformation, which has been predicted by numerous studies to be a transient thermodynamic event before spinodal decomposition. This suggests that the long-term naturally aged WQ–U6Nb is a good model alloy to study thermodynamic and kinetic phenomena requiring chronic processes.

Revolutionary microstructure control with phase diagram evaluation for the design of E21 Co3AlC-based heat-resistant alloys

E21 Co3AlC might be used as a strengthener to develop new Co-based heat-resistant alloys. To establish the basis of microstructure control for further improving mechanical properties, phase equilibria in the CoAl-C ternary system were investigated and phase diagram information particularly related to E21 Co3AlC and liquid phase were evaluated and reconsidered. Single crystals of E21 Co3AlC have been grown successfully for the first time using optical floating zone (OFZ) melting. Extra ordering of carbon atoms taking place in E21 Co3AlC was confirmed using single crystals by transmission electron microscopy. As a result of this ordering, E21 ′ Co3AlC0.5 accompanies formation of antiphase boundaries. A wide variety of the two-phase Co3Alc/α(Co) microstructures were prepared by unidirectional solidification using OFZ.