Scanning Transmission Electron Microscopy Characterization Research Articles

Picometer-Scale Atomic Shifts Governing Subdisordered Structures in Diamond.

Diamond is considered the most promising next-generation semiconductor material due to its excellent physical characteristics. It has been more than three decades since the discovery of a special structure named n-diamond. However, despite extensive efforts, its crystallographic structure and properties are still unclear. Here, we show that subdisordered structures in diamond provide an explanation for the structural feature of n-diamond. Monocrystalline diamond with subdisordered structures is synthesized via the chemical vapor deposition method. Atomic-resolution scanning transmission electron microscopy characterizations combined with the picometer-precision peak finder technology and diffraction simulations reveal that picometer-scale shifts of atoms within cells of diamond govern the subdisordered structures. First-principles calculations indicate that the bandgap of diamond decreases rapidly with increasing shifting distance, in accordance with experimental results. These findings clarify the crystallographic structure and electronic properties of n-diamond and provide new insights into the bandgap adjustment in diamond.

High-Performance Broadband Image Sensing Photodetector Based on MnTe/WS2 van der Waals Epitaxial Heterostructures.

Two-dimensional transition metal dichalcogenide (TMDC) heterostructure is receiving considerable attention due to its novel electronic, optoelectronic, and spintronic devices with design-oriented and functional features. However, direct design and synthesis of high-quality TMDC/MnTe heterostructures remain difficult, which severely impede further investigations of semiconductor/magnetic semiconductor devices. Herein, the synthesis of high-quality vertically stacked WS2/MnTe heterostructures is realized via a two-step chemical vapor deposition method. Raman, photoluminescence, and scanning transmission electron microscopy characterizations reveal the high-quality and atomically sharp interfaces of the WS2/MnTe heterostructure. WS2/MnTe-based van der Waals field effect transistors demonstrate high rectification behavior with rectification ratio up to 106, as well as a typical p-n electrical transport characteristic. Notably, the fabricated WS2/MnTe photodetector exhibits sensitive and broadband photoresponse ranging from UV to NIR with a maximum responsivity of 1.2 × 103 A/W, a high external quantum efficiency of 2.7 × 105%, and fast photoresponse time of ∼50 ms. Moreover, WS2/MnTe heterostructure photodetectors possess a broadband image sensing capability at room temperature, suggesting potential applications in next-generation high-performance and broadband image sensing photodetectors.

Seeing is believing: Correlating optoelectronic functionality with atomic scale imaging of single semiconductor nanocrystals.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical-structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam-scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor-metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical-structural correlation method for numerous scenarios and timely questions in the field of QD research.

Microstructural and Hall–Petch Analysis of Additively Manufactured Ferritic Alloy Using 2507 Duplex Stainless Steel Powder

The powder bed fusion–laser beam (PBF-LB) process, a method of additive manufacturing (AM), was used to print duplex stainless steel (DSS) using commercial-grade 2507 powders. While conventionally processed DSS has a two-phase microstructure consisting of 50% austenite and 50% ferrite, the PBF-LB-printed 2507 alloy was nearly 100% ferrite. Optimal processing conditions that minimized porosity were determined to be 290 W laser power and 1000 mm/s scan speed, and grain size, texture, and phases were characterized as a function of laser power and scan speed. Grain size increased with increasing laser power but decreased with increasing scan speed. A <100> texture diminished with increasing scan speed from 1000 mm/s to 1400 mm/s. No austenite phase was detected. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) characterization revealed nanoscale chromium nitride precipitates in the ferritic matrix (incoherent hexagonal close-packed (HCP) precipitates at grain boundaries and coherent body-centered cubic (BCC) precipitates within the grains) and a high density of tangled dislocations. Tensile tests of as-printed alloys showed a yield strength of 570 MPa, an ultimate tensile strength of 756 MPa, and an elongation to failure of 10%. The tensile properties were analyzed based on the observed microstructure considering grain size, nanoscale precipitates, and the high density of dislocations.

Thermal faceting on the ductility-dip cracking fracture surfaces of nickel-based alloys – Occurrence, characterization, and implications for the cracking mechanism

Ductility-dip cracking (DDC) fracture surfaces containing thermal faceting (TF) are documented in this study. Research began with the discovery of TF on the DDC fracture surface in the weld metal of high-chromium nickel (Ni)-based filler metal 52M, a valuable alloy to the nuclear industry. Fracture surfaces were generated using strain-to-fracture (STF) and simulated strain ratcheting (SSR) testing on a Gleeble® thermomechanical simulator. Initial fractography performed by scanning electron microscopy (SEM) revealed faceting with preferred grain orientations and faceting interacting with void nucleation near grain boundary (GB) triple points. Internal voids were exposed via metallography, where TF was also observed on the free surface. A thin orientation- and site-specific foil was extracted from the faceted fracture surface using focused ion beam (FIB) nanofabrication. Subsequent scanning transmission electron microscopy (STEM) characterization down to atomic-resolution revealed the facets have perfect crystallographic orientations parallel to close-packed directions. Sharp edges and transitions between individual facets on the scale of individual atomic columns and atomic planes were observed. An analysis of the deformation microstructure and the chemical composition underneath the faceted surface showed a low dislocation density and no evidence of localized segregation or diffusion, respectively. These findings have led to the development of a hypothesis related to the interdependence of TF and DDC and support initial claims regarding TF formation in DDC voids at elevated temperatures.

Adjustment of a novel β′-type precipitate in a creep-aged Mg−15 wt%Gd alloy

A Mg−15 wt%Gd binary alloy was prepared by hot-extrusion and then creep-aged at 200 °C under a constant stress of 200 MPa in this work. Compared with the traditional aging, creep aging clearly accelerated the age-hardening response, as the aging time for the peak yield strength was shorten from 72h to ∼16h. In the peak-creep-aged sample, a novel precipitation structure, named as β′H herein, was observed as the dominant precipitate. Interestingly, these dense β′H precipitates preferred to distributing along a certain direction which generally deviated from the extrusion direction (ED) by 54–67°. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization revealed that this novel β′H precipitate is composed of distorted Gd-rich hexagonal units, with the chemical formula of Mg7Gd2 and having a monoclinic structure, a = 0.671 nm, b = 1.727 nm, c = 0.521 nm and β = 100.7°. Density functional theory (DFT) calculations suggest that the applied stress during creep aging could provide an effective tensile stress for the motion of the surrounding Mg atoms when forming the β′H structure.

Laser liquid ablation for silver nanoparticles synthesis and conjugation with hydroxychloroquine for Covid-19 treatment

Synthesis, characterization, and anchorage studies of pristine silver nanoparticles were achieved. That would later be followed by the conjugation of the synthesized silver nanoparticles with hydroxychloroquine, a compound that is commonly identified as active against plasmodium, a malaria-causing agent. The synthesis of the pristine silver nanoparticles was achieved through laser ablation, a physical method. To achieve different concentrations and sizes of the nanoparticles, various parameters such as time and pulse repetition frequencies were varied. The base medium of the ablation was distilled water of 10 ml for every process of ablation. The laser ablation technique used proved effective in the production of silver nanoparticles spherical in shape and of the size range between 16 nm and 30 nm as deduced from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) characterization techniques. EDS analysis confirmed the presence of AgNPs in the synthesized solution of silver nanoparticles, while the UV–Vis results showed that the absorption peak of AgNPs occurred at 403 nm. Through these findings, the use of laser ablation, a physical technique in the synthesis of AgNPs, is shown to be cost-effective and environmentally friendly with the properties of the nanoparticles proving to be effective for biomedical applications later in this work.

Patterning two‐dimensional semiconductors with thermal etching

AbstractThe controllable synthesis of complicated nanostructures in advanced two‐dimensional (2D) semiconductors, such as periodic regular hole arrays, is essential and remains immature. Here, we report a green, facile, highly controlled synthetic method to efficiently pattern 2D semiconductors, such as periodic regular hexagonal‐shaped hole arrays (HHA), in 2D‐TMDs. Combining the production of artificial defect arrays through laser irradiation with anisotropic annealing etching, we created HHA with different arrangements, controlled hole sizes, and densities in bilayer WS2. Atomic force microscopy (AFM), Raman, photoluminescence (PL), and scanning transmission electron microscopy (STEM) characterization show that the 2D semiconductors have high quality with atomical clean and sharp edges as well as undamaged crystals in the unetched region. Furthermore, other nanostructures, such as nanoribbons and periodic regular triangular‐shaped 2D‐TMD arrays, can be fabricated. This kind of 2D semiconductors fabrication strategy is general and can be extended to a series of 2D materials. Density functional theory (DFT) calculations show that one WS2 molecule from the edges of the laser‐irradiated holed region exhibits a robust etching activation, making selective etching at the artificial defects and the fabrication of regular 2D semiconductors possible.image

Ab Initio Simulations of Tritium Diffusion in Al-Rich Iron Aluminide Coating Phases

Surface coatings of steels used in extreme conditions and corrosive environments generally aim to provide protection and increased durability. In the case of tritium-producing burnable absorber rods (TPBARs) used in nuclear reactors, a 316 stainless steel has been coated with Al. Scanning transmission electron microscopy (STEM) characterization of the coating found three Al-rich (>60 atom % Al) iron aluminide alloys identified as hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. Density functional theory simulations using nudged elastic band have been performed to investigate the diffusion of interstitial tritium in each Al-rich iron aluminide phase. While FeNiAl5 and Fe4Al13 can be viewed as the stacking of two layers, the structural peculiarity of Fe2Al5 is that channels of variable Al vacancy content are present along the c-axis. Therefore, three stoichiometries for Fe2Alx phase, namely, Fe2Al4, Fe2Al5, and Fe2Al6, have been considered to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Altogether, we found that at 600 K, tritium diffusion decreases from a faster rate in the channels of Fe2Alx phases (DT ≤ 10–11 m2·s–1) to Fe4Al13 (DT ≈ 10–12 m2·s–1), and finally in FeNiAl5 (DT ≈ 10–13 m2·s–1). We also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO2 (DT ≈ 10–14 m2·s–1) but slightly slower than in 316 stainless steel (DT ≈ 10–10 m2·s–1).

Effect of in situ and ex situ magnetic field on the microstructural transformation of the thermally reduced graphene oxide

The control over microstructural characteristics of graphene oxide (GO) is one of the most serious issues in the domain of graphene synthesis as this affects the graphene’s properties, and functionality. In this study, the primary objective is electrochemical synthesis graphene in the presence of magnetic field that is applied externally. During the synthesis process, the magnetic field was applied in a direction that was perpendicular to the applied potential. This causes the electrolyte to spin flow around the cell. Subsequently, the goal is to provide a comparative analysis between the microstructural characteristics of graphene that has been synthesized in situ and ex situ magnetic field. The cylindrical graphite was used as an anode, and a carbon electrode that had been recovered from a waste dry cell battery was used as a cathode. The pre-oxidized graphite was sonicated (synthesized under magnetic field, and without magnetic field) in sterilized water for 10[Formula: see text]min with a probe-type sonicator and thermally reduced at same temperature i.e., 850∘C followed by furnace cooling. The findings of the Raman spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) characterizations indicate that the magnetic flux that was applied has a significant influence on the surface height and roughness, microstructure, and surface state, a structural disorder in comparison to when there was no magnetic field applied to the thermally reduced graphene oxide (rGO). On the other side, from the data obtained by XRD and TGA analysis, the applied magnetic field seems to have very little effect on phase, lattice parameter and thermal stability.

Engineering antiferroelectric nucleation in ferroelectric films with enhanced piezoelectricity

Tailoring ferroelectric-antiferroelectric phase transitions provides strategic advantages for potential applications in digital displacement transducers and ultrathin ferroelectric capacitors. In particular, epitaxy of artificial superlattice has been widely used and proved to be capable of creating antiferroelectric-type characteristics within intrinsic ferroelectrics. Here, we have achieved dedicatedly control of ferroelectric-antiferroelectric phase transition by introducing 2 unit-cells antiferroelectric PbZrO3 layers into the intrinsic ferroelectric BiFeO3 films, with the ultrathin PbZrO3 layers acting as the structural spacer to bridge the robust nucleation of striping phase in BiFeO3 layers. Aberration-corrected scanning transmission electron microscopy characterization revealed that the BiFeO3 layers experienced a transition to the antiferroelectric-like polymorph via interface-transferred lattice distortion of alternate cation displacement, thereby forming the periodic ferroelectric-antiferroelectric nanoscale laminas. Further piezoresponse force measurements gave the direct evidences that the modulated superlattice exhibited improved piezoelectric performance compared with typical rhombohedral BiFeO3 films. This work paves the way for engineering polarity-mismatched superlattices with enhanced piezoelectric response and enriches the fundamental of structure-functionality correlations in oxide heterosystems.

On the phase transformation pathway during localized grain boundary oxidation in an Fe-10 at% Cr alloy at 200ºC

We have performed short-term, controlled oxidation experiments of sputter deposited Fe-10 at% Cr alloy films at 200ºC. Nanoscale structural characterization by four-dimensional scanning transmission electron microscopy (4d-STEM) and chemical characterization by atom probe tomography (APT) have provided insights on the phase transformation pathway followed during localized, inward-progressing oxidation of grain boundaries at this low temperature. Direct nucleation of a metastable spinel-structured oxide, with suppression of the corundum-structured Cr2O3 has been observed even at the early stages. A ‘partitioning-free oxide nucleation mechanism’ under conditions of extremely restricted kinetics is proposed to explain the non-equilibrium transformation pathway.

Study on properties and precipitation behavior of 6000 series alloys with high Mg/Si ratios and Cu contents

A combination of high Cu and excess Mg contents has previously shown excellent effects on suppressing the natural aging (NA) and enhancing the precipitation strengthening response during artificial aging (AA) of Al-Mg-Si-Cu alloys. In this work, the precipitation behaviors of a series of 6000 alloys with high Mg/Si ratios from 1.5 to 3.0 and high Cu content levels (0.7 and 1.0 wt%) under different aging conditions were studied by Vickers hardness measurements, tensile tests, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. During AA, an increased content of Cu and a reduced Mg/Si ratio can promote the precipitation of fine metastable phases, and the effect of the former is more significant. A Mg/Si ratio of 1.5 is found to be the optimum composition in this study, which produces the maximum precipitation hardening response. Besides, high Mg/Si ratio can enhance the precipitation of disordered QP2 phases, which is obviously different from the conventional applied alloys strengthened by the β” phase. Thus, the effects of chemistry on the complex precipitation behavior and the properties of Mg-rich high Cu alloys have been discussed, which will be beneficial to develop new Al-Mg-Si based alloys. • The activation of clustering and precipitation increases with the Cu content but decreases with the Mg/Si ratio. • The excess Mg and high Cu results in a moderate T4 temper strength due to the balance effect between the Mg and Cu. • The combination of high Cu and excess Mg can increase the age hardening response of these alloys except for Mg/Si ratio larger than 2.2.

Liquid cell scanning transmission electron microscopy characterization of combined chemical and electrochemical reduction of palladium

We systematically investigate the effect of the driving force on the growth of palladium nanoparticles by comparing electrochemical reduction of polyvinylpyrollidone-stabilized palladium nitrate solution with chemical reduction by ascorbic acid using in-situ liquid cell transmission electron microscopy. Electrochemical data is simultaneously collected while high spatial resolution is maintained. As a chemical reductant, ascorbic acid results in the formation of smaller, more tightly spaced palladium nanoparticles through increased nucleation. When present during electrochemical reduction of palladium, ascorbic acid reduces the degree to which dendrites form due to the growth of a more compact palladium layer. Because the nanoparticles formed during chemical reduction have diameters on the order of a nanometer and are invisible under full liquid conditions, we employ electrochemical water splitting to generate a gas bubble in order to observe the process in real time. This is a step towards real-time characterization of complex solution-phase growth in which multiple pathways exist for metals to reduce and combinations of additives interact to control size and shape.

Effect of Melt Pool Boundaries on Repassivation of Selective Laser Melted Stainless Steel

Recent studies have shown that melt pool boundaries (MPBs) in selective laser melted (SLM) stainless steels (SS) can serve as preferential sites for pit initiation and corrosion propagation. In this study, we utilize electrochemical polarization experiments combined with detailed microstructural analysis to elucidate the role played by MPBs on localized corrosion in chloride media. Potentiodynamic polarization of artificial 1-D pits constructed from SLM SS316L grown to sufficient depth showed a sudden current drop below the salt-film dissolution potential on the downward scan, which was followed by an active Tafel-like behavior until the zero-current potential (ZCP) was reached. The observed current drop marks a critical potential for pit stability and characterizes the partial repassivation of the pit, as the current density value after the drop was significantly greater than the reported current density at complete repassivation. When the same 1-D pit was polarized upwards from a potential below the ZCP, it showed characteristic re-activation before salt film formation. This behavior was unique to SLM SS316L and was not found in conventional wrought SS316L. Moreover, 1-D artificial pit study on SLM SS316L using a modified ASTM G192 sequence showed a similar partial repassivation behavior. Scanning electron microscopy of SLM SS316L pit bottom morphology after long-term potentiostatic polarization at a potential between the partial repassivation potential and the ZCP showed selective, crystallographic attack along MPBs. The morphology also revealed melt pool fall-out due to the continued attack of the MPBs. Scanning transmission electron microscopy characterization of the SLM 316L material revealed Cr and Mo depletion at the MPBs. The observation of passivating element depletion at MPBs is in line with calculations of solute partitioning during melt pool solidification for the material under study. Based on our results, we conclude that localized corrosion under conditions at the edge of passivity can lead to selective attack of Cr- and Mo-depleted MPBs. The implications of inter-melt-pool corrosion on component structural integrity and protective repassivation potentials will be discussed. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-4016 A

Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman) and microscopic (atomic force microscopy and scanning transmission electron microscopy) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that the BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (∼234 V) as compared to GaN (∼168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step toward bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

Electric Control of Exchange Bias at Room Temperature by Resistive Switching via Electrochemical Metallization.

Electric field control of exchange bias (EB) plays an important role in spintronics due to its attractive merit of lower energy consumption. Here, we propose a novel method for electrically tunable EB at room temperature in a device with the stack of Si/SiO2/Ta/Pt/Ag/Mn-doped ZnO (MZO)/Pt/FeMn/Co/ITO by resistive switching (RS) via electrochemical metallization (ECM). The device shows enhanced and weakened EB when set at high-resistance state (HRS) and low-resistance state (LRS), respectively. For the device at LRS, the aberration-corrected scanning transmission electron microscopy (STEM) characterizations unambiguously reveal that the Ag filaments grow initially from the Ag anode and then elongate toward the ITO cathode. It is inferred that at LRS, a small portion of Ag filaments have passed through the MZO and the intervening thin Pt layer and extended into the FeMn layer. After applying reverse voltage, these Ag filaments are electrochemically dissolved and ruptured near the MZO/Pt interface. This is considered to be the main mechanism responsible for RS and switchable EB as well. This work presents a new strategy for designing low-power, nonvolatile magnetoelectric random access memory devices.

Gallium doping-assisted giant photoluminescence enhancement of monolayer MoS2 grown by chemical vapor deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted enormous research interest owing to their unique photo-physics and excellent optoelectronic properties. However, the ubiquitous defects in 2D TMDCs greatly affect the optoelectronic properties of them. For example, the prototype molybdenum disulfide (MoS2) exhibits very poor photoluminescence (PL) due to the high defect density. Here, we report a defect repair strategy based on a facile one-step chemical vapor deposition method that achieves two orders of magnitude enhancement in photoluminescence (PL) and one order of magnitude prolonging in carrier lifetime. Interestingly, we can controllably synthesize Ga-doped samples with different morphologies by adjusting the ratio of precursors, and the PL intensities at the central and edge regions are quite different. Combined with scanning transmission electron microscopy characterization, we systematically elucidate this growth behavior and obtain a more precise defect repair strategy. This strategy of selectively repairing the defects of monolayer MoS2 by gallium doping to achieve significant enhancement of photoluminescence may provide a facile and feasible method for the regulation of optoelectronic properties of 2D materials.

Atomic layer deposited Al2O3 as a protective overlayer for focused ion beam preparation of plan-view STEM samples

Preservation of analyte integrity during focused ion beam (FIB) sample preparation is a significant challenge in the scanning transmission electron microscopy (STEM) characterization of plan-view samples with sensitive surface chemistries. This can preclude the characterization of atomic arrangements, nanoscale surface coverages, and distributions and morphologies of functional molecular materials composed of surface-immobilized metal nanoparticles, clusters or coordination complexes. This work demonstrates effective protection of Pt nanoparticle (NP) morphology through a plan-view FIB lift-out and thinning procedure by encapsulating the sample surface in an Al2O3 overlayer grown by atomic layer deposition (ALD). High-angle annular dark field (HAADF)-STEM analysis was used in concert with energy dispersive X-ray spectroscopy (EDS) to identify and image sub-10 nm features attributed to Pt and to evaluate the distribution of implanted Ga+ (derived from the FIB milling beam). ALD is a mild chemical vapor deposition (CVD) technique that has the capability to generate dense, pinhole-free films with tunable compositions and properties, making this ALD-FIB procedure applicable to many sample architectures for plan-view lamella preparation and STEM analysis.

Atomic origin of magnetic coupling of antiphase boundaries in magnetite thin films

Revealing the magnetic coupling nature of boundary defects is crucial for in-depth understanding of the behavior and properties of magnetic materials and devices. Here, magnetite (i.e., Fe3O4) thin films were grown epitaxially on (100) SrTiO3 single-crystal substrates by pulsed laser deposition. Atomic-scale scanning transmission electron microscopy characterizations reveal that three types of antiphase boundaries (APBs) are formed in the Fe3O4 thin film. They are the (100) APB that is formed on the (100) plane with a crystal translation of (1/4)a[011¯], the type I and type II (110) APBs that are both formed on the (110) plane with the same crystal translation of (1/4)a[101] but different terminated atomic planes. The type I (110) APB is terminated at the atomic plane with mixed tetrahedral- and octahedral-sites Fe atoms, the type II (110) APB is terminated at the octahedral-site Fe plane. First-principles calculations reveal that the (100) APB and the type I (110) APB tend to form the ferromagnetic coupling that will not decrease the spin polarization of Fe3O4 films, while the type II (110) APB prefers to form the antiferromagnetic coupling that will degrade the magnetic properties. The magnetic coupling modes of the APBs are closely related to the Fe-O-Fe bond angles across the boundaries.