General Gradient Research Articles

Structural and Electromechanical Properties of Sr-Substituted Barium Titanate (BST) as Potential Material for High Performance Electroacoustic Devices

First-principles density functional calculations have been performed to study the electromechanical properties, i.e., elastic, dielectric, piezoelectric, and coupling factors, as well as the crystal structure for Ba1―xSrxTiO3 (BST) with 0≤ x ≤1 in the paraelectric and ferroelectric crystallographic phases. The equilibrium ground state structural parameters are calculated using the new Wu–Cohen Generalized Gradient Approximation (GGA―WC) in excellent agreement with the experimental data. The lattice stabilities in Ba1―xSrxTiO3 crystals are analysed from the doped formation energy and phonon dispersions. The mechanical quantities, such as the elastic constants (C11, C12, C44, C13, C33 and C66), mechanical moduli (Young modulus Y, Shear modulus G, Compressibility B), Poisson’s ratio and elastic anisotropy are investigated using the GGA―WC and GGA―PBEsol exchange functionals. The mechanical moduli (B, G and Y) are found to increase by increasing Sr content leads to enhance stiffness and hardness of BST systems. We find that both the para- and ferroelectric phases of all considered Ba1―xSrxTiO3 are elastically stable. The 3D surface anisotropy visualizations show that BST is a pure isotropic material at x∼ 0.5. It was found that Debye temperature (ΘD), milting temperature (TM) and acoustic wave velocities increase as the Sr content increases. The ferroelectric Ba0.75Sr0.25TiO3 (BST) exhibited high piezoelectric coefficients [d15 = 121.4 pC/N, d31= -20.5 pC/N, d33 = 89.3 pC/N], and large dielectric tensors [ε11 = 517.2, ε33 = 31.3], compared to pure BaTiO3 [d15 = 74.2 pC/N, d31= -14.1 pC/N, d33 = 65.0 pC/N; ε11 = 334.7, ε33 = 22.6]. Enhancement in electromechanical coupling factor and bulk acoustic velocity of ferroelectric BST is achieved when the Sr content is added. Ba1―xSrxTiO3 (BST) are extremely interest candidates to be used for the development of high performance electroacoustic devices such as acoustic sensors and resonators.

Gradient electrodeposition enables high-throughput fabrication and screening of alloy anodes for high-energy lithium-ion batteries

Alloy anodes provide high capacity for high-energy lithium-ion batteries. Multi-element alloys require high-throughput fabrication technologies to facilitate the screening of alloy composition, morphologies, and structures. Here, we report a gradient electrodeposition method to prepare Sn–Co–Sb alloy anodes with varied Sn:Co:Sb ratios and demonstrate the effectiveness of gradient electrodeposition on alloy anode development. Using this technology, we can vary each metal element to form a gradient distribution in one direction by tilting the sample alternatively during each electrodeposition. Such gradient electrodeposition realizes the complex composition of Sn–Co–Sb alloys in one large sample, enabling high-throughput fabrication simultaneously. After annealing, the obtained Sn–Co–Sb alloy forms varied phases such as Sn, SnSb, and CoSn2. It is noted that the elemental ratio has a significant influence on the microstructures and electrochemical performances of the deposited Sn–Co–Sb alloy. The Sn–Co–Sb alloy with a ratio of 71.3:12.8:15.9 delivers a high reversible capacity of 671.8 mAh g−1 and simultaneously shows excellent cyclability, which can be attributed to the optimal morphology structure and Sn:Co:Sb ratio. The optimized alloy can maintain a high capacity without sacrificing cyclability, which was usually limited by strain accumulation caused by high capacity. This work reports a general gradient electrodeposition technology for high-throughput screening of alloy anodes, which can also be applied to other alloy applications.

Perceptual variability: Implications for learning and generalization.

The generalization of learned behavior has been extensively investigated, but accounting for variance in generalized responding remains a challenge. Based on recent advances, we demonstrate that the inclusion of perceptual measures in generalization research may lead to a better understanding of both intra- and interindividual differences in generalization. We explore various ways through which perceptual variability can influence generalized responding. We investigate its impact on the ability to discriminate between stimuli and how similarity between stimuli may be variable, rather than fixed, because of it. Subsequently, we argue that perceptual variations can yield different learning experiences and that interindividual differences in generalized responding may be understood from this perspective. Finally, we point to the role of memory and decision-making within this context. Throughout this paper, we argue that accounting for perception in current generalization protocols will improve the precision of obtained generalization gradients and the ability to infer latent mechanisms. This can inspire future attempts to use generalization gradients as a (clinical) predictor or to relate them to individual traits and neural correlates and, ultimately, may lead to new theoretical and clinical insights.

General gradient delay correction method in bipolar multispoke RF pulses using trim blips.

To correct with gradient trim blips for gradient delays in bipolar-spoke RF pulses in slice-selective and slab-selective excitations, compatible with tilted acquisitions and anisotropic delays. The effect of small gradient delays with respect to RF pulses results in a dephasing of the second RF spoke, proportional to the slab-selection gradient amplitude and the distance of the slice center from the magnet isocenter. Accordingly, adding a trim blip along the corresponding logical gradient axis between the two spokes compensates for the same dephasing, and therefore cancels the gradient delay effects, regardless of position and orientation. Gradient delays on different axes were first measured on a 7T system based on an imaging method. Parallel transmission universal bipolar spokes were designed offline to mitigate the RF field inhomogeneity problem in the human brain. Trim blips were used to compensate for the known delays, which was validated with flip angle and temporal SNR measurements on two different volunteers at 7 T. Pulses corrected with trim blips greatly reduced gradient delay effects. Acquisitions made with corrected and noncorrected pulses showed good fidelity with simulations. Unlike time or phase-shifting approaches, trim blip-based methods apply to all possible bipolar spoke scenarios such as slice excitations, slab excitations, and anisotropy in the gradient delays.

Modelling generalisation gradients as augmented Gaussian functions.

Studying generalisation of associative learning requires analysis of response gradients measured over a continuous stimulus dimension. In human studies, there is often a high degree of individual variation in the gradients, making it difficult to draw conclusions about group-level trends with traditional statistical methods. Here, we demonstrate a novel method of analysing generalisation gradients based on hierarchical Bayesian curve-fitting. This method involves fitting an augmented (asymmetrical) Gaussian function to individual gradients and estimating its parameters in a hierarchical Bayesian framework. We show how the posteriors can be used to characterise group differences in generalisation and how classic generalisation phenomena such as peak shift and area shift can be measured and inferred. Estimation of descriptive parameters can provide a detailed and informative way of analysing human generalisation gradients.

Investigation of structural and magnetoelectronic properties of new half‐metallic Heusler alloys Ru2VGexSb1−x (x = 0, 0.5 and 1): A density functional theory study

AbstractWe performed an ab initio study using a method named linearized augmented plane wave with a full potential (FP‐LAPW) based on the density functional theory. We predicted the physical properties of Ru2VGexSb1−x (x = 0, 0.5 and 1) Heusler alloys in L21 structure. We computed the magnetic and structural properties using the general gradient approximation. The modified Becke‐Johnson scheme was used to study the electronic structure of these compounds. The obtained results show that the lattice constants and the spin magnetic moments are in favorable agreement compared with theoretical values and experimental data. The computed densities of state (DOSs) of these compounds indicate a half‐metallic behavior with a real gap for the ternary materials, which gives perfect spin polarization, while for the quaternary one, the DOS indicate a nearly half‐metallic character with a pseudogap in the minority spin close to the Fermi level EF.

Luminescence dynamics of MgGa2O4 prepared by solution combustion synthesis

Magnesium gallate (MgGa2O4) was synthesized by solution combustion synthesis method at 500 °C. The X-ray diffraction (XRD) pattern indicates that the prepared compound crystalized into a single phase, and a porous morphology associated with combustion was observed from the image of the scanning electron microscopy (SEM). A diffuse reflectance spectrum was recorded using the UV–Vis–NIR absorption spectrometer, which revealed the band-to-band absorption edge, from which the band gap of the material was approximated using the Kubelka-Munk relation. The photoluminescence (PL) emission displayed a broad blue emission from 350 to 600 nm, when the sample was excited with a 235 nm beam. The thermoluminescence (TL) spectroscopy was used to prove that the associated kinetics in trapping and de-trapping of electrons by the electron trapping centres were of general order. The corresponding depths of the dominant electron trapping centres were approximated to 0.65 ± 0.01eV to 1.23 ± 0.01eV. Using the general gradient approximation (GGA-DFT) as implemented within the CASTEP code, we found that the MgGa2O4 compound has energy levels close to the conduction band, attributed to oxygen vacancies.

Stability and Instability in Saddle Point Dynamics—Part I

We consider the problem of convergence to a saddle point of a concave-convex function via gradient dynamics. Since first introduced by Arrow, Hurwicz, and Uzawa, such dynamics have been extensively used in diverse areas; there are, however, features that render their analysis nontrivial. These include the lack of convergence guarantees when the concave-convex function considered does not satisfy additional strictness properties and also the nonsmoothness of subgradient dynamics. Our aim in this two-part article is to provide an explicit characterization to the asymptotic behavior of general gradient and subgradient dynamics applied to a general concave-convex function in C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We show that despite the nonlinearity and nonsmoothness of these dynamics, their ω-limit set is comprised of trajectories that solve only explicit linear ODEs characterized within this article. More precisely, in part I, an exact characterization is provided to the asymptotic behavior of unconstrained gradient dynamics. We also show that when convergence to a saddle point is not guaranteed, then the system behavior can be problematic, with arbitrarily small noise leading to an unbounded second moment for the magnitude of the state vector. In part II, we consider a general class of subgradient dynamics that restrict trajectories in an arbitrary convex domain, and show that when an equilibrium point exists, the limiting trajectories belong to a class of dynamics characterized in part I as linear ODEs. These results are used to formulate corresponding convergence criteria and are demonstrated with examples.

Study on the influence of an underground low-light environment on human safety behavior

Objective. This study aimed to research the impact that a coal mine lighting environment has on human safety behavior in an underground coal mine. Methods. We built a coal mine lighting simulation experiment system including both general lighting and local lighting divided into six different illumination gradients (0, 10, 30, 50, 75 and 100 lx) to test and analyze the effects of general illumination gradients on human fatigue, attention, reaction and eye–hand coordination with SPSS version 22.0. Results. Illuminance has a significant effect on human fatigue, attention, reaction ability and eye–hand coordination ability. Specifically, human fatigue is negatively correlated and the other indicators are positively correlated with the increase of illuminance. Notably, attention distribution ability seems to be most significantly influenced by illuminance according to this study, followed by human fatigue, reaction and eye–hand coordination ability. Conclusion. The results of the study indicate that illumination of the general lighting environment is expected to be controlled at a gradient of at least 50–75 lx or above in the coal mine environment where there is both general lighting and local lighting to reduce the incidence of accidents.

Magnetic and optical properties of perovskite-graphene nanocomposites LaFeO3-rGO: Experimental and DFT calculations

We present an experimental and theoretical study of the perovskite-graphene nanocomposites LaFeO3 – rGO, where we demonstrate a easy way to prepare this compound using citrate auto-compulsion method, starting from corresponding metal nitrate and graphene oxide solution. The Hummer’s method was used to prepare graphene oxide by chemical exfoliation of graphite. The structural characterization has been performed using XRD, FT-IR and scanning electron microscope (SEM) to analyze the morphology of the sample, FT-IR, whereas the magnetic properties has been studied using VSM measurements. Furthermore, ab-initio calculations has been performed based on the Density Functional Theory (DFT), where for a better description of the electronic and magnetic properties, the Hubbard correction was considered over the General Gradient Approximation (GGA + U).

On the strain gradient elasticity theory for isotropic materials

Classical elasticity mechanics fails to explain size-dependent phenomenon, the strain gradient elasticity theories including the couple stress theory, the simplified strain gradient elasticity theory and the general strain gradient elasticity theory are used to describe size effects. In this paper, the relations among the couple stress theory, the simplified strain gradient elasticity theory and the general strain gradient elasticity theory are derived. The general strain gradient elasticity theory contains the effects of all strain gradients while the couple stress theory and the simplified strain gradient elasticity theory include effects of only part of the strain gradients, thus, the general strain gradient elasticity theory can reduce to the couple stress theory, the simplified strain gradient elasticity theory when some strain gradients are neglected. Subsequently, a reasoning comparison with examples is performed to discuss the differences between the general strain gradient elasticity theory and the reduced theories in describing the size effects: one is the static and dynamic responses of a microbeam and the other is the tensile of a microbar. In the beam bending and vibration example, the material length-scale parameters and the deflections predicted by the reduced theories are larger while the natural frequency is smaller than that of the general strain gradient elasticity theory. The reduced theories underestimate the size-dependent response of a bending beam. In the bar tensile example, the couple stress theory fails to predict the size-dependent tensile behaviour. This illustrates clearly that compared with the reduced theories, the general strain gradient elasticity theory is capable to describe the size effects more appropriately.

A Systematic Search for the Geometric Structures, Stabilities and Electronic Properties of Bimetallic CsAu(0,±1)n Clusters

The geometric structures, relative stabilities, growth-pattern behaviors, and electronic properties of neutral and charged CsAun(0,±1) clusters have been systematically investigated with the general gradient approximation (GGA) exchange correlation density functional theory (TPSSTPSS) and a relativistic effective core potential, together with a vibrational frequency anslysis. The optimized geometric structures show that the ground-state structures have three-dimensional (3D) structures from n = 4 and that the doped Cs atoms tend to locate at the symmetry and surface position. One gold-atom-capped CsAun−1(0,±1) structure for CsAun(0,±1) clusters with increasing number of Au atoms is the dominant growth pattern. The total energies and the averaged atomic binding energies exhibit a sequence of CsAun− > CsAun > CsAun+ , which indicates that the stability of anionic CsAun− clusters is higher than that of neutral CsAun clusters and that the CsAun clusters is more stable than the cationic CsAun+ clusters. The relative stabilities are examined based on the analysis of the averaged atomic binding energy and the second-order difference energy, and the results indicate that the CsAu3, CsAu6+ and CsAu2− clusters are the most stable clusters. Based on the analysis of the frontier molecular orbitals, we find that the CsAun(0,±1) (n = 1 − 10) clusters can more easily acquire electrons and that the loss of electrons is difficult. The highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps exhibit significant even-odd oscillation behavior. The higher gaps are exhibited by neutral CsAun clusters with odd numbers of gold atoms and ionic CsAun±1 clusters with even numbers of gold atoms. Especially, the CsAu3, CsAu6+ and CsAu6− clusters have the highest HOMO-LUMO gaps, which proves again that CsAu3, CsAu6+ and CsAu2− clusters are the most stable and the magic structures. Because of the electronegativity of Au is higher than that of the Cs atom, the charge transfers from the Cs atoms to the Aun frames in the CsAun+1 clusters. Furthermore, we have calculated the dissociation energies and found that the CsAun+ clusters have lower dissociation energies. This indicates that CsAun+ clusters are easy to dissociate and less stable. The dissociation energies of the CsAun and the CsAun− clusters exhibit a generally opposite odd-even behavior.

Learning to predict pain: differences in people with persistent neck pain and pain-free controls.

BackgroundLearning to predict threatening events enables an organism to engage in protective behavior and prevent harm. Failure to differentiate between cues that truly predict danger and those that do not, however, may lead to indiscriminate fear and avoidance behaviors, which in turn may contribute to disability in people with persistent pain. We aimed to test whether people with persistent neck pain exhibit contingency learning deficits in predicting pain relative to pain-free, gender-and age-matched controls.MethodWe developed a differential predictive learning task with a neck pain-relevant scenario. During the acquisition phase, images displaying two distinct neck positions were presented and participants were asked to predict whether these neck positions would lead to pain in a fictive patient with persistent neck pain (see fictive patient scenario details in Appendix A). After participants gave their pain-expectancy judgment in the hypothetical scenario, the verbal outcome (PAIN or NO PAIN) was shown on the screen. One image (CS+) was followed by the outcome “PAIN”, while another image (CS−) was followed by the outcome “NO PAIN”. During the generalization phase, novel but related images depicting neck positions along a continuum between the CS+ and CS− images (generalization stimuli; GSs) were introduced to assess the generalization of acquired predictive learning to the novel images; the GSs were always followed by the verbal outcome “NOTES UNREADABLE” to prevent extinction learning. Finally, an extinction phase was included in which all images were followed by “NO PAIN” assessing the persistence of pain-expectancy judgments following disconfirming information.ResultsDifferential pain-expectancy learning was reduced in people with neck pain relative to controls, resulting from patients giving significantly lower pain-expectancy judgments for the CS+, and significantly higher pain-expectancy judgments for the CS−. People with neck pain also demonstrated flatter generalization gradients relative to controls. No differences in extinction were noted.DiscussionThe results support the hypothesis that people with persistent neck pain exhibit reduced differential pain-expectancy learning and flatter generalization gradients, reflecting deficits in predictive learning. Contrary to our hypothesis, no differences in extinction were found. These findings may be relevant to understanding behavioral aspects of chronic pain.

Modelling spatiotemporal trends in range shifts of marine commercial fish species driven by climate change surrounding the Antarctic Peninsula

In recent decades, the relationships between species distributional shifts and climate change have been investigated at various geographic scales, yet there is still a gap in understanding the impacts of climate change on marine commercial fish species surrounding the Antarctic Peninsula. The dynamic bioclimate envelope model (DBEM) is a mechanistic model that encompass species distribution model and population dynamic model approaches to project the spatiotemporal change of marine commercial fish species driven by various climate change scenarios in the Southern Ocean. This paper focuses on the spatiotemporal changes of marine commercial fish species surrounding the Antarctic Peninsula under a high emissions scenario (RCP8.5) and a low emissions scenario (RCP2.6) from 1970 to 2060 following three different Earth System Models (ESMs), namely, the GFDL-ESM 2G, IPSL-CM5A-MR and MPI-ESM-MR. Results reveal that: i) The general latitudinal gradient patterns in species richness shifts poleward associated with a global abundance decrease ii) The Spp. richness in Eastern Antarctic Peninsula (EAP) is higher than in the Western Antarctic Peninsula (WAP) at the same latitude (>65°S latitude). iii) The reasons are that the krill-dependent predators in WAP could face a higher risk of depletion than that in EAP due to ocean warming and anthropogenic activities.

Avoiding Based on Shades of Gray: Generalization of Pain-Related Avoidance Behavior to Novel Contexts

Avoidance behavior is protective, yet in the absence of genuine bodily threat, it may become disabling. Therefore, we investigated whether avoidance generalizes to novel safe contexts based on the similarity with the acquisition context.Healthy participants performed arm movements using a robotic arm to reach a target. Three trajectories (T1-3) led to the target. During acquisition, a painful stimulus could be partly/completely prevented by performing more effortful trajectories (ie, longer and more force needed), T2/T3, in the pain-avoidance context (eg, black background); in the yoked context (eg, white background), the same reinforcement schedule was applied irrespective of the chosen trajectories. Generalization of avoidance was tested in 2 novel contexts (eg, shades of gray backgrounds). We assessed self-reported pain-expectancy and pain-related fear for all trajectories, and avoidance behavior (ie, maximal deviation from T1).Results confirm that fear and expectancy ratings reflect the response-outcome contingencies and differential learning selectively generalized to the novel context resembling the original pain-avoidance context. Furthermore, a linear trend in avoidance behavior across contexts emerged, which is indicative of a generalization gradient. Participants avoided more in the context resembling the original pain-avoidance context than in the one resembling the yoked context, but this effect was not statistically significant. PerspectivePerspective: We demonstrated acquisition of pain-related avoidance behavior in a within-subjects design, showing modulation of pain-related fear and pain-expectancy by context and providing limited evidence that avoidance selectively generalizes to novel, similar contexts. These results provide insight regarding the underlying mechanisms of the spreading of protective behavior in chronic pain patients.

Boron Impurity Effect on the Structural, Elastic, and Electronic Properties of Titanium Carbide

Atomic, structural, and elastic properties of titanium carbide with the boron impurity have been studied in the framework of the density functional theory in the general gradient approximation, by using the software ABINIT. The calculations of the total energy of a TiC supercell with boron impurity atoms showed that the latter do not tend to form clusters in titanium carbide. The equilibrium distances between the adjacent planes of titanium atoms were found to increase in the presence of the boron impurity. The electronic spectra of TiC supercells with various numbers and positions of boron impurity atoms are analyzed. The presence of boron impurity atoms is found to result in the appearance of a subband of their electron states, which is located between the local electronic spectra of the 2s and 2p carbon states by about 0.24 Hartree below the Fermi level. The coordination positions of boron impurity atoms affect only the shape and half-width of their electronic subband. An insignificant increase in the electron density of states just below the Fermi level also takes place. The bulk modulus of a titanium carbide supercell with boron impurity atoms is calculated and analyzed.

Arbitrarily high-order linear energy stable schemes for gradient flow models

We present a paradigm for developing arbitrarily high order, linear, unconditionally energy stable numerical algorithms for general gradient flow models. We apply the energy quadratization (EQ) technique to reformulate the gradient flow model into an equivalent one with a quadratic free energy and a modified mobility. For any positive integer k>0 and tn=nΔt, where Δt is the time step size, we linearize the EQ-reformulated model in (tn,tn+1] through an extrapolation from numerical solutions already obtained in [0,tn] so that the approximation error is in the order of O(Δtk). Then we employ an s-stage algebraically stable Runge-Kutta method to discretize the linearized model in (tn,tn+1]. For the spatial discretization, we use the Fourier pseudo-spectral method to match the order of accuracy in time. The resulting fully discrete scheme is linear, unconditionally energy stable, uniquely solvable, and can reach arbitrarily high order. Furthermore, we present a family of linear schemes based on prediction-correction methods to complement the new linear schemes. Some benchmark numerical examples are given to demonstrate the accuracy and efficiency of the schemes.

Stimulus discriminability and induction as independent components of generalization.

Generalization of learning can arise from 2 distinct sources: failure to discriminate a novel test stimulus from the trained stimulus and active extrapolation from the trained stimulus to the test stimulus despite them being discriminable. We investigated these 2 processes in a predictive learning task by testing stimulus discriminability (identification of the trained stimulus) as well as generalization of learning (outcome expectancy). Generalization gradients were broader for expectancy than for identification, in both single cue and differential (discrimination) designs, implying a substantial extrapolation component for the most dissimilar stimuli. The shapes of the expectancy gradients were strongly determined by the training design (single cue vs. differential) and by the rules inferred by participants (similarity vs. linear). By contrast, the identification gradients were unaffected by the training design or inferred rules and were equivalent for predictive and nonpredictive stimuli. These results indicate that perceptual discriminability plays a substantial role in generalization, but it is largely unaffected by associative learning. Instead, learning appears to impact on generalization via an independent extrapolation component which involves cognitive processes such as inductive reasoning. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Quantitative Structure-Activity Relationship Models for Predicting Inflammatory Potential of Metal Oxide Nanoparticles.

Background:Although substantial concerns about the inflammatory effects of engineered nanomaterial (ENM) have been raised, experimentally assessing toxicity of various ENMs is challenging and time-consuming. Alternatively, quantitative structure–activity relationship (QSAR) models have been employed to assess nanosafety. However, no previous attempt has been made to predict the inflammatory potential of ENMs.Objectives:By employing metal oxide nanoparticles (MeONPs) as a model ENM, we aimed to develop QSAR models for prediction of the inflammatory potential by their physicochemical properties.Methods:We built a comprehensive data set of 30 MeONPs to screen a proinflammatory cytokine interleukin (IL)-1 beta () release in THP-1 cell line. The in vitro hazard ranking was validated in mouse lungs by oropharyngeal instillation of six randomly selected MeONPs. We established QSAR models for prediction of MeONP-induced inflammatory potential via machine learning. The models were further validated against seven new MeONPs. Density functional theory (DFT) computations were exploited to decipher the key mechanisms driving inflammatory responses of MeONPs.Results:Seventeen out of 30 MeONPs induced excess production in THP-1 cells. In vivo disease outcomes were highly relevant to the in vitro data. QSAR models were developed for inflammatory potential, with predictive accuracy (ACC) exceeding 90%. The models were further validated experimentally against seven independent MeONPs (). DFT computations and experimental results further revealed the underlying mechanisms: MeONPs with metal electronegativity lower than 1.55 and positive were more likely to cause lysosomal damage and inflammation.Conclusions: released in THP-1 cells can be an index to rank the inflammatory potential of MeONPs. QSAR models based on were able to predict the inflammatory potential of MeONPs. Our approach overcame the challenge of time- and labor-consuming biological experiments and allowed for computational assessment of MeONP inflammatory potential by characterization of their physicochemical properties. https://doi.org/10.1289/EHP6508

Ab initio study on exchange integrals and magnetic anisotropy change of BaFe12−xScxO19 (x = 0, 0.5, 1, 1.5, 2)

We investigate the exchange integrals and magnetic anisotropy change in Sc substituted barium hexaferrite BaFe12−xScxO19 (x = 0, 0.5, 1, 1.5, 2) by density functional theory (DFT) calculations. The general gradient approximation (GGA) with on-site coulomb repulsion U (GGA + U) method was used to improve the description of the strongly correlated 3d electrons of Fe. Our calculations reveal that Sc3+ preferentially replace Fe3+ at the 4f2 and 2b sites successively. The total energies of 9 spin configurations are calculated to obtain 8 inter-sublattice exchange integrals for different values of x. Comparison of the exchange integrals between BaFe12O19 (BFO) and BaFe12−xScxO19 (x = 0.5, 1, 1.5, 2) (BFSO) shows that substituting Sc for Fe influences most interactions involving the 4f2 sites. The f1-f2 and f2-k interactions decreases as substitution increases, while the b-f2 interaction fluctuates significantly as x changes. Magnetic anisotropy change of Sc substituted M-type barium hexaferrite (Ba-M hexaferrite) has been studied by using the force theorem method. The results show that Sc3+ in its preferred 4f2 and 2b sites contribute to the decrease of the uniaxial magnetic crystal anisotropy. The magnetic anisotropy of BFSO transforms from uniaxial to planar with transition point in the range of 1 < x < 1.5.