Density Of Antiphase Boundaries Research Articles

Структурные домены аустенита нестехиометрических сплавов Гейслера на основе Ni-Mn-In

The relationship between antiphase boundaries (APBs), antiphase domains (APDs) and the functional properties of alloys of the Ni-Mn-In system has been studied. The density of APBs was controlled using various thermal treatments. The fine domain structure was observed by transmission electron microscopy (TEM), high-angle dark-field scanning transmission electron microscopy (HAADF-STEM), and electron backscatter diffraction (EBSD). The results obtained are discussed in the context of the possibility of modeling and designing materials with specified characteristics and properties.

Regulation of antiphase boundary density in Fe3O4 thin films and its effect on the electrical and magnetic properties

Antiphase boundaries (APBs) widely exist in Fe3O4 thin films and affect significantly their electrical and magnetic properties. Here, effects of APBs on the electrical and magnetic properties of Fe3O4 thin films have been quantitatively investigated by a combined study of pulsed laser deposition, transmission electron microscopy (TEM) and first-principles calculations. It was revealed that the substrates affected greatly the density of APBs in Fe3O4 thin films but did not change the atomic structures of APBs. The APB density in the Fe3O4 (111) thin film grown on the MgO (111) substrate was one order of magnitude larger than that on the Al2O3 (0001) substrate. Atomic-scale TEM characterizations revealed that the APBs in the Fe3O4 thin films on both substrates had three types of atomic structures. Due to the higher APB density in the Fe3O4/MgO thin film, the electrical resistivity at 300 K was two orders of magnitude larger than that of the Fe3O4/Al2O3 thin film. Meanwhile, the saturation magnetization and the coercivity of the Fe3O4/MgO thin film were reduced by 86 % and 35 %, respectively. First-principles calculations suggested that the antiferromagnetic coupling was easily formed at all the three types of APBs, which led to the decrease of spin polarization and the increase of electrical resistance of the Fe3O4 thin films.

Creep performance in a CoNi-based single crystal superalloy with super-high γ′ volume fraction at 760 °C and equivalent high stress

The creep performance at low temperature and high stress is one crucial property for the single crystal superalloys widely used for turbine blades. In this study, the creep behavior of a CoNi-based single crystal superalloy with super-high γ′ volume fraction (Vγ′ = ∼90%) was investigated at 760 °C/550 MPa based on microstructural (γ and γ′ phases) and deformation substructural (dislocations, anti-phase boundaries (APBs) and stacking faults (SFs)) analyses to reveal its creep mechanism under low temperature/high stress conditions. The super-high Vγ′ contributes to the rapid transformation of creep mechanisms from the matrix dislocation movement in the initial decelerating stage to the APB-coupled dislocation pairs and their evolving non-planar SF ribbons inside γʹ precipitates during the accelerating stage. In the following second decelerating stage, the interactions between SFs and APBs improve the deformation resistance of γ′ precipitates and decrease the creep rate. Subsequently, the high density of APBs is believed to play a significant role in microstructural stability and promotes the accumulation of matrix dislocations, which is responsible for the steady and final accelerating stages. Appropriate Vγ′ and increasing the APB energy are assumed to improve the creep resistance of CoNi-based single crystal superalloys by enhancing the strengthening effect of dislocation accumulation in γ matrix and critical shear stress of γ′ phase.

Atomic-scale investigation of γ-Ga2O3 deposited on MgAl2O4 and its relationship with β-Ga2O3

Nominally phase-pure γ-Ga2O3 was deposited on (100) MgAl2O4 within a narrow temperature window centered at ∼470 °C using metal-organic chemical vapor deposition. The film deposited at 440 °C exhibited either poor crystallization or an amorphous structure; the film grown at 500 °C contained both β-Ga2O3 and γ-Ga2O3. A nominally phase-pure β-Ga2O3 film was obtained at 530 °C. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the γ-Ga2O3 film grown at 470 °C revealed a high density of antiphase boundaries. A planar defect model developed for γ-Al2O3 was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of γ-Ga2O3. The presence of the 180° rotational domains and 90° rotational domains of β-Ga2O3 inclusions within the γ-Ga2O3 matrix is discussed within the context of a comprehensive investigation of the epitaxial relationship between those two phases in the as-grown film at 470 °C and the same film annealed at 600 °C. The results led to the hypotheses that (i) incorporation of certain dopants, including Si, Ge, Sn, Mg, Al, and Sc, into β-Ga2O3 locally stabilizes the “γ-phase” and (ii) the site preference(s) for these dopants promotes the formation of “γ-phase” and/or γ-Ga2O3 solid solutions. However, in the absence of such dopants, pure γ-Ga2O3 remains the least stable Ga2O3 polymorph, as indicated by its very narrow growth window, lower growth temperatures relative to other Ga2O3 polymorphs, and the largest calculated difference in Helmholtz free energy per formula unit between γ-Ga2O3 and β-Ga2O3 than all other polymorphs.

Suppression of anti-phase boundary defects in Mn-Al-Ti permanent magnets

Permanent magnets (PMs) based on manganese show significant potential for applications in electric motors and devices as an alternative to Rare-earth PMs (REPMs). The metastable ferromagnetic τ phase of the Mn-Al system has magnetic performance between the REPM Nd-Fe-B magnets and the lower performance ferrite magnets. However, the maximum performance in Mn-Al PMs has not reached its theoretical limit, in part due to challenges in controlling crystalline defects such as anti-phase boundaries (APBs). APBs act as nucleation sites for domain reversal in Mn-Al and negatively affect magnetization by interrupting the L10 ordering, placing Mn atoms into AFM-coupled Mn-Mn configurations. In this study, Ab-initio modeling was used to screen ternary elements based on their affinity to segregate to the APB and on their preference for FM configuration. The ternary element addition of 1 at.% Ti lowered the density of APBs as observed in a transmission electron microscope. The 1 at.% Ti addition was observed to improve (BH)max over the base alloy, by preserving Hci and improving Mr. It also significantly increased the fraction of twin boundaries in the τ phase. AFM behavior that was observed in the base alloy during TC heating experiments (Néel behavior) and high-field VSM measurements (spin-flop behavior) was effectively suppressed with the addition of Ti. Additionally, the Ti addition thermally stabilized Hci at 550 °C, improving τ phase processability. Other candidate elements, Cr, V, and Zr, were modeled to have similar potential for minimizing APBs when added to Mn-Al.

Influence of the Cooling Rate on Austenite Ordering and Martensite Transformation in a Non-Stoichiometric Alloy Based on Ni-Mn-In

The effect of the melt cooling rate on the atomic ordering of austenite and, as a consequence, on the martensitic transformation of a nonstoichiometric alloy of the Ni-Mn-In system has been studied. In situ TEM observations revealed differences in the mechanism of phase transformations of the alloy subjected to different cooling conditions. It is shown that during quenching a high density of antiphase boundaries (APB) is formed and the alloy is in the austenite–martensitic (10M and 14M) state up to a temperature of 120 K. In a slowly cooled alloy, a lower APB density is observed, and a two-stage transformation, L21/B2 → 10M → 14M, occurs in the range of 150–120 K.

High-temperature cyclic oxidation of micro- and nano-ZrO2–25wt.%CeO2–2.5wt.%Y2O3 thermal barrier coatings at 1300 °C

In this study, the high-temperature cyclic oxidation resistance of micro- and nano-ZrO2–25wt.%CeO2–2.5wt.%Y2O3 (CYSZ) thermal barrier coatings (TBCs) was investigated. The TBCs were applied on IN738LC/CoNiCrAlY using the atmospheric plasma spray (APS) technique and subjected to high-temperature cyclic oxidation through 5-h cycles at 1300 °C. Various characterization techniques, including X-ray diffraction (XRD), Raman spectroscopy, nanoindentation, and field-emission scanning electron microscopy (FESEM), were used to assess the TBCs. Results indicated that annealing tetragonal zirconia at 1300 °C can generate antiphase boundary defects. The Young's modulus and hardness of nano- and micro-CYSZ TBCs decrease with the increase of antiphase boundary density. XRD and Raman results, the phase transformation behavior of tetragonal zirconia can be correlated to antiphase boundary density. Furthermore, the study showed that the nano-CSZ TBC has better oxidation resistance than the micro-CYSZ TBC at 1300 °C.

On the formation of antiphase boundaries in Fe4Al13 intermetallics during a high temperature treatment

In this paper, we report atomic scale observations and formation mechanisms of a high-density of antiphase boundaries (APBs) within an ultra-fine-grained Fe4Al13 intermetallic layer at an Al/steel interface after a heat treatment at 596 °C. The results reveal that the APBs are formed by nucleation and the glide of partial dislocations with Burgers vector of b/3[010] (b = 12.47 Å). The intensive activation of APBs locally transforms the Fe4Al13 structure from the quasicrystal approximant structure to a quasicrystal. Very few stacking faults and nanotwins are observed indicating that the formation of planar defects is mainly driven by this transformation. This new insight on the formation of high density of APBs could possibly lead to an improvement in toughness by increasing the strength/ductility balance of this intermetallic.

Magnetoresistance of Antiphase Boundaries in Sr2FeMoO6−δ

Antiphase boundaries (APB) in strontium ferromolybdate induce antiferromagnetically coupled spins of the Fe ions at the boundary affecting both the magnetic properties and the electron transport across the boundary. Herein, first, 3D presentation of the APB in strontium ferromolybdate is presented. For the first time, the weighted antiferromagnetic (AF) modulus, the bulk ferrimagnetic exchange stiffness constant, and the AF exchange stiffness constant across the APB of strontium ferromolybdate are estimated. This is done by analyzing carefully selected experimental data of the magnetoresistance (MR). The boundary width and boundary energy are calculated by means of Kittel's theory of the structure of ferromagnetic domains in films and small particles. The evaluation of these parameters deepens the understanding of MR in strontium ferromolybdate ceramic and thin‐film samples possessing a high density of APBs.

(Invited) A Comparative Study of Li and Na Intercalation Compounds: Cation Ordering, Diffusion and Novel Redox Mechanisms

Li and Na intercalation compounds exhibit a wide variety of electronic, structural and phase transformation phenomena. Very different phases and electronic properties can be realized within the same layered transition metal oxide by simply changing the guest cation from Li to Na. For example, Na intercalation compounds exhibit many more ordering reactions and structural phase transformations than their Li counterparts. In this talk we will focus on layered Li and Na intercalation compounds that start out as O3. The larger size of Na leads to the stabilization of P3 upon Na extraction. Our first-principles statistical mechanics studies have revealed that the P3 host tends to stabilize an intriguing family of Na-vacancy orderings that consist of well-ordered domains separated by anti-phase boundaries. The family of ordered phases forms a devil’s staircase whereby the concentration can be varied by changing the density of anti-phase boundaries. A systematic study of diffusion mechanisms in the devil’s staircase of ordered P3 phases shows that Na transport is mediated by the migration of anti-phase boundaries rather than conventional cation-vacancy exchanges as occurs in Li-intercalation compounds.We will also discuss a variety of intriguing redox mechanisms in Li and Na layered intercalation compounds that arise either from p-bonded complexes involving oxygen and transition metals or complexes made of metal-metal bonds. An important example is Na2Mn3O7, which can be charged in a Na-ion battery in spite of the fact that Mn has a formal oxidation state of 4+. The ‘excess’ capacity is enabled by a p-redox centered that is delocalized over a ring of six Mn p-bonded to six O surrounding a Mn vacancy. Similar molecular-like complexes that can host redox centers are also present in Mo containing layered Li-intercalation compounds.

Competing magnetic interactions and emergent phase diagrams in double perovskite Y2NixCo1−xMnO6

We present a comparative study of double perovskites Y2NixCo1−xMnO6 for x = 1, 0.5, and 0. The polycrystalline samples of Y2NixCo1−xMnO6 with space group P21/n were synthesized via sol-gel technique. X-ray photoelectron spectroscopy (XPS) confirms the presence of majority Ni2+/Mn4+ in Y2NiMnO6 (YNMO) and a mixed valence state Ni2+/3+/Co2+/3+/Mn4+/3+ in co-doped Y2Ni0.5Co0.5MnO6 (YNCMO). The temperature dependent magnetization results suggest homogeneous substitution of Ni ions with Co ions with minimal contribution of 3 + cationic magnetic interactions in YNCMO. The superexchange ferromagnetic (FM) interaction has a major contribution to the magnetism of YNMO. The strength of antiferromagnetic (AFM) coupling resulting from the antiphase boundaries in Y2NixCo1−xMnO6 increases with the increase in the Co concentration. The AC susceptibility (AC-χ) study further validates the presence of higher density of antiphase boundaries, which is reflected by the dynamics of domain walls in YNCMO. The isothermal magnetic entropy change (ΔSM) as functions of temperature and magnetic field is exploited to assess the stabilization of different magnetic phases. The findings of ΔSM (T,µ0H) lead to the proposed new magnetic phase diagrams for Y2NixCo1−xMnO6 for x = 1 and 0.5, in comparison with the previously established phase diagram of Y2CoMnO6.

Theoretical and experimental study on epitaxial growth of antiphase boundary free GaAs on hydrogenated on-axis Si(001) surfaces

The formation of double-layer atomic steps on Si(001) surfaces is an efficient way to eliminate the antiphase boundaries (APBs) on GaAs/Si(001) interfaces. The surface energy of on-axis Si(001) surfaces with different atomic step structures was calculated and analyzed from the first principles. An optimal hydrogen-annealing process condition, the hydrogen pressure of 800 mbar and the annealing temperature of 800 °C for 10 min, was obtained experimentally. Under this annealing condition, a 420 nm APB-free GaAs epitaxial layer grown on on-axis Si(001) substrates was achieved by metal-organic chemical vapor deposition. The effect of the annealing temperature on the APB density of the GaAs/Si(001) samples was explained from the aspects of thermodynamics and kinetics. It is of great significance to optimize the growth conditions of APB-free III–V epitaxial materials on on-axis Si(001) surfaces for the large-scale silicon monolithic integration of optoelectronic devices.

Correlation between ordered solid solution and cellular structure of Sm2Co17 type magnets with high iron content

The ordering transformation of solid solution and the insufficient 1:5H boundary phase in the final magnets challenge the realization of high performance in Sm2Co17 magnets with high iron content. The investigation correlates the ordering degree of solid solutions to the cellular structure of the final magnets with the composition of Sm(CobalFexCu0.06Zr0.02)7.8 in which x ranges from 0.30 to 0.36. The ordering tendency of solid solution increases with increasing iron content. Rietveld structure refinement demonstrates that increasing iron content reduces the occupancy of Sm at Co-Co dumbbell sites in 2:17R, as well as Zr solubility on Sm 6c sites, leading to higher ordering degree and less Z planar phase formation separately. Antiphase boundaries (APBs) arising from the different 2:17R variants with the same or different directions were frequently observed in solid solutions, which facilitate the nucleation and formation of 1:5H phase during aging treatment. With increasing Fe content, the width of nanotwins increases and thus the APB density decreases in the solid solution. Furthermore, overlapping zones appear in solid solutions with higher Fe content, which will retain and enlarge as the twins grow during aging treatment, inhibiting the formation of 1:5H boundary phase. In addition, the density of Z phase declines and more defective platelet phases occur in x = 0.36 magnets, which depress the element diffusion and re-distribution during slow cooling. The above correlation between solid solution and cellular structure well explain the structure origin for the deterioration of magnetic performance with higher Fe content, which may provide guidance for further performance improvement in Sm2Co17 type magnets with higher iron content.

The role of antiphase domain boundary density on the surface roughness of GaSb epilayers grown on Si (001) substrates

Abstract GaSb epilayers were grown on nominal Si (001) substrates by molecular beam epitaxy, using AlSb nucleation layer (NL) as an interfacial misfit array. The effects of AlSb NL thickness, growth temperature and post annealing on the GaSb epilayers in terms of formation of antiphase boundaries (APBs) were monitored by scanning electron microscopy (SEM). APBs were identified via SEM following a defect sensitive etch solution treatment. SEM images of etched samples were used to calculate the APB densities and the results were correlated with the surface roughness of the samples. We found that the density of APBs together with the area peak-to-valley height has a role on the surface roughness of the GaSb epilayers grown on nominal Si (001) substrates.

Large Non-ergodic Magnetoelastic Damping in Ni–Mn–Ga Austenite

Resonant ultrasound spectroscopy was used to analyze magnetoelastic damping in Ni–Mn–Ga single crystals in austenite and premartensite phases. Crystals with different treatment were studied, exhibiting different density of antiphase boundaries (APBs), as confirmed by magnetic force microscopy, and different magnetic behavior. For a quenched single crystal with high density of APBs, extremely strong damping was observed in a broad temperature range in the austenite phase. It was shown that this damping is history-dependent, i.e., non-ergodic, appearing only during heating runs preceded by a premartensite → austenite transition. We suggest that this non-ergodicity results from the pinning of the fine magnetic domain structure on APBs and other defects.

Antiphase boundaries, magnetic domains, and magnetic vortices in Ni–Mn–Ga single crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni–Mn–Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments. The APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/µm), air quenching in a medium density (≈8/µm), and water quenching in a high density (≈15/µm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in close correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy decreased and became lower than the stray field energy. In addition, magnetic vortices appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.

Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)

Polycrystalline Fe3O4 films with the thickness of 27 nm were grown at different temperature (Ts) on Si (100) substrate by Laser-Molecular Beam Epitaxy to study the influence of density of antiphase boundaries (APBs) and the spin polarization of Fe3O4. Films prepared with Ts in the range of 500 °C - 650 °C were proved to be of good crystallization, preferred orientation, and desirable values of saturation magnetization (a little smaller one for film prepared at Ts = 500 °C). The lowest density of APBs was verified by the measured bulk like electron-phonon (s-p) coupling constant for film prepared at Ts = 550 °C, along with the highest value of spin polarization and the best crystalline quality as the most obvious inflection in the curves of lnρ vs. 1000/T around Verwey transition. This optimum value of Ts can be interpreted by the APBs thermally activated migration process with an activation energy. Films prepared with Ts = 500 °C, 600 °C, and 650 °C still have a reasonable value of the s-p coupling constant compared to recently reported results, while larger magnetoresistance accompanied by the superparamagnetic behavior was observed for films prepared at lower Ts (Ts = 400 °C and 450 °C).

3C-SiC Growth on Inverted Silicon Pyramids Patterned Substrate.

This work reports on the properties of cubic silicon carbide (3C-SiC) grown epitaxially on a patterned silicon substrate composed of squared inverted silicon pyramids (ISP). This compliant substrate prevents stacking faults, usually found at the SiC/Si interface, from reaching the surface. We investigated the effect of the size of the inverted pyramid on the epilayer quality. We noted that anti-phase boundaries (APBs) develop between adjacent faces of the pyramid and that the SiC/Si interfaces have the same polarity on both pyramid faces. The structure of the heterointerface was investigated. Moreover, due to the emergence of APB at the vertex of the pyramid, voids buried on the epilayer form. We demonstrated that careful control of the growth parameters allows modification of the height of the void and the density of APBs, improving SiC epitaxy quality.

Antiphase Boundaries, Magnetic Domains, and Magnetic Vortices in Ni-Mn-Ga Single Crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni-Mn-Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments and the APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/μm), air quenching in a medium density (≈8/μm), and water quenching in a high density (≈15/μm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in one-to-one correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy became lower than the stray field energy, and magnetic vortices additionally appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.

Formation of antiphase boundaries in CuFe2O4 films induced by rough MgAl2O4 (001) substrates

Antiphase boundaries in spinel ferrites have been considered as the functional elements with great potential for applications in spintronic devices. Engineering the antiphase boundaries is a big challenge in experiments. Here, we report on that a high density of antiphase boundaries in CuFe2O4 films have been prepared on rough surface of MgAl2O4 (001) substrates. By means of advanced electron microscopy these antiphase boundaries are found to be of {011}14〈011〉-type and bound by interfacial dislocations with Burgers vector of a/4〈011〉 that contribute to relaxation of in-plane and out-of-plane lattice strain in the heterostructure. Our results provide evidence that the rough substrate surface can be applied for the formation of antiphase boundaries in the CuFe2O4/MgAl2O4 system to tune the magnetic and electrical transport properties of epitaxial spinel ferrite films.