We combine ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy and ${}^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering (NRIXS) on nanoscale polycrystalline $[\mathrm{bcc}\text{−}{}^{57}\mathrm{Fe}/\mathrm{MgO}]$ multilayers with various Fe-layer thicknesses and layer-resolved density-functional-theory (DFT)-based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure (where ML denotes monolayer) to unravel the interface-related atomic vibrational properties of a multilayer system. Being consistent in theory and experiment, we observe enhanced hyperfine magnetic fields $B_{\text{hf}}$ in the multilayers as compared to $B_{\text{hf}}$ in bulk bcc Fe; this effect is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement.

We investigated the magnetic properties of the ilmenite-type iridate $\mathrm{CdIr}{\mathrm{O}}_3$ with a honeycomb lattice formed by $\mathrm{I}{\mathrm{r}}^{4+}$ ions prepared via a solid-state metathesis. The magnetization measurements made using the powder sample reveal a large effective magnetic moment and a fairly strong antiferromagnetic interaction, indicating a deviation from the Kitaev model. Considering the relationship between magnetism and crystal structure in $\mathrm{CdIr}{\mathrm{O}}_3$, and comparing it with the other ilmenite-type iridates $\mathrm{ZnIr}{\mathrm{O}}_3$ and $\mathrm{MgIr}{\mathrm{O}}_3$, we conclude that insulating $\mathrm{CdIr}{\mathrm{O}}_3$ cannot be described as a $J_{\mathrm{eff}}=1/2$ Mott state owing to a metathetically stabilized large trigonal distortion of $\mathrm{Ir}{\mathrm{O}}_6$ octahedra.

It was previously shown experimentally that the ionic liquid (IL) system based on 1-ethyl-3-methylimidazolium dicyanamide $\left(\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]\right)$ outperforms $N$-butyl-$N$-methylpyrrolidinium dicyanamide $\left(\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]\right)$ as an electrolyte in a rechargeable zinc battery. Specifically, $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ requires a lower overpotential to initiate electrodeposition of zinc, enables a higher zinc deposition peak current density, and supports a greater number of charge-discharge cycles. In the present work, a set of molecular dynamics simulations of two IL systems (i.e., IL + 9 mol % Zn + 3 wt % water) reveals that these differences could be related to the overscreening and crowding phenomena that exist in these IL systems. Furthermore, the work reveals the most likely mechanism by which water acts as an electrochemical catalyst in transferring metal cations to a negatively charged electrode. This transfer is a necessary step for subsequent metal deposition, as required for a rechargeable battery system, for example. In particular, the results of the simulations reveal three primary findings. First, it is shown that water activates the transfer of zinc onto the negative electrodes by temporarily disrupting zinc's [dca] coordination shell while zinc is located within the interfacial region; second, it is shown that the transfer of zinc is obviously greater in the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ system than in the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system, which is consistent with experimental measurements; and third, when the effects of crowding are largely eliminated in the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system (i.e., when the layering instability and the anion layer collapse in the anodic interfacial region are avoided by the use of a smaller electrode charge), the zinc transfer dynamics are completely changed and the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system starts providing a more favorable environment for zinc transfers. These findings present a significant advance in the understanding of the electrode-electrolyte interface in ILs.

Neutron diffraction measurements were performed under high magnetic fields up to 17 T in a multiferroic ${\mathrm{BiFeO}}_3$ single crystal, in which an intermediate magnetic (IM) phase has been found between the cycloid and canted antiferromagnetic phases [S. Kawachi et al., Phys. Rev. Mater. 1, 024408 (2017)]. We clearly found that the incommensurate magnetic peaks, which split perpendicular to the magnetic field in the cycloid phase, rotate by 90 deg to align parallel to the field in the IM phase. The magnetic structure in the IM phase can be best described by an antiferromagnetic cone (AF cone) structure. The transition from the cycloid to AF cone is of first order and the direction of the magnetic wave vector and the easy plane of the cycloidal component are rotated by 90 deg without changing the cycloidal modulation period, whereas the transition from the AF cone to canted antiferromagnetic phase is gradual and the cone angle becomes smaller gradually without changing the modulation period. Interestingly, the cycloidal component as well as the cone angle in the IM phase shows a large hysteresis between the field increasing and decreasing processes. This result, combined with the magnetostriction with a large hysteresis previously reported in the IM phase, suggests a strong magnetoelastic coupling.

We perform a systematic study of thermal resistance and conductance of tilt grain boundaries (GBs) in Si using classical molecular dynamics. The GBs studied are naturally divided into three groups according to the structural units forming the GB core. We find that, within each group, the GB thermal conductivity strongly correlates with the excess GB energy. All three groups predict nearly the same GB conductivity extrapolated to the high-energy limit. This limiting value is close to the thermal conductivity of amorphous Si, suggesting similar heat transport mechanisms. While the lattice thermal conductivity decreases with temperature, the GB conductivity slightly increases. However, at high temperatures it turns over and starts decreasing if the GB structure undergoes a premelting transformation. Analysis of vibrational spectra of GBs resolved along different directions sheds light on the mechanisms of their thermal resistance. The existence of alternating tensile and compressive atomic environments in the GB core gives rise to localized vibrational modes, frequency gaps creating acoustic mismatch with lattice phonons, and anharmonic vibrations of loosely bound atoms residing in open atomic environments.

The influence of hydrogen on dislocation junctions has been analyzed by incorporating a hydrogen-dependent core force into nodal-based discrete dislocation plasticity simulations. Hydrogen reduces the core energy of dislocations, which reduces the magnitude of the dislocation core force. We refer to this as hydrogen core force shielding, as it is analogous to hydrogen elastic shielding but occurs at much lower hydrogen concentrations. The dislocation core energy change due to hydrogen was calibrated at the atomic scale, accounting for the nonlinear interatomic interactions at the dislocation core, giving the model a sound physical basis. Hydrogen was found to strengthen binary junctions and promote the nucleation of dislocations from triple junctions. Simulations of microcantilever bend tests showed hydrogen core force shielding reduced the yield stress followed by increased strain hardening due to junction strengthening. These simulations demonstrate hydrogen effects at a small bulk hydrogen concentration, 10 appm, realistic for body-centered cubic iron, allowing micromechanical tests on hydrogen charged samples to be simulated.

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect ${\mathrm{V}}_{\mathrm{Ni}}$ (nickel vacancy) in the wide-gap $p$-type semiconductor nickel oxide (NiO). ${\mathrm{V}}_{\mathrm{Ni}}$ can easily be introduced into NiO thin films by offering high oxygen reactivity during film growth, rendering nonstoichiometric semiconducting material. However, exposure to lower oxygen reactivity after growth, e.g., in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration returns to its thermodynamic equilibrium value. In this study we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420 K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature DC conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile ${\mathrm{V}}_{\mathrm{Ni}}$ species can be indirectly observed through the occurrence of electrode polarization characteristics.

Three-dimensional (3D) electronic nanomaterials are less explored than their counterparts in the lower dimensions because of the limited techniques for the preparation of high-quality materials. Here we report characterization of and quantum transport measurement on 3D metalattices of Pt grown in the voids of self-assembled templates of silica nanospheres by confined chemical fluid deposition. These Pt metalattices featuring an unprecedentedly large surface area to volume ratio were found to show positive magnetoresistance (MR) at low temperatures, changing from positive to negative in low magnetic fields within a narrow temperature window as the temperature was raised. The low-field MR was attributed to the effect of quantum interference of electronic waves in the diffusive regime in three dimensions, processes known as weak localization and antilocalization. We argue that the presence of the large surface area to volume ratio results in such a strong enhancement in the electron-phonon scatterings that a change in the sign in the MR is enabled.

We report the magnetic phase diagram of ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{PSe}}_3$, which represents a random magnet system of two antiferromagnetic systems with mixed spin, mixed spin anisotropies, mixed nearest-neighbor magnetic interactions, and mixed periodicities in their respective antiferromagnetic structure. Bulk samples of ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{PSe}}_3$ have been prepared and characterized phase pure by powder x-ray and neutron diffraction and x-ray fluorescence. Nature and extent of magnetically ordered state has been established using powder neutron diffraction, dc magnetic susceptibility, and heat capacity. Long-range magnetic ordering exists between $x=0.0$ and 0.25 (${\mathrm{MnPSe}}_3$ type) and between $x=0.875$ and 1 (${\mathrm{FePSe}}_3$ type). A short-range magnetic order with the existence of both ${\mathrm{MnPSe}}_3$- and ${\mathrm{FePSe}}_3$-type nanoclusters has been established between $x=0.25$ and 0.875. Irreversibility in dc magnetization measurements, also characterized by isothermal and thermoremanent magnetization measurements, suggest similarities to magnetic nanoparticles where uncompensated surface spins result in diverging thermoremanent and isothermal remanent magnetization responses, further reinforcing existence of magnetic nanoclusters or domains. A spin-glass state, observed in analogous ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{PS}}_3$, has been ruled out, and formation of nanoclusters exhibiting both ordering types results from unusually high anisotropy values. The effect of ligand contributions to the spin-orbit interactions has been suggested as a possible explanation for high $D$ values in these compounds.

We present a detailed analysis of the structure dependence of the lattice thermal conductivity of ${\mathrm{SrTiO}}_3$. We have used both ab initio molecular dynamic simulations and density functional theory calculations to decouple the effect of different structural distortions on the thermal conductivity. We have identified two main mechanisms for tuning the thermal conductivity when a distortion is applied. First, the modification of the acoustic-modes energy dispersion when a change in the lattice parameters is imposed and second, the low energy polar modes. In particular and counterintuitively, we have found that an increase in the angle of the oxygen octahedral rotations increases the thermal conductivity due to its coupling to these polar modes.

Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, $\mathrm{N}{\mathrm{i}}_{95}\mathrm{P}{\mathrm{t}}_5$ exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is ∼12%. In contrast, the magnitude of the AEE signals is moderate in $\mathrm{N}{\mathrm{i}}_{95}\mathrm{P}{\mathrm{t}}_5$ but largest in $\mathrm{N}{\mathrm{i}}_{75}\mathrm{P}{\mathrm{t}}_{25}$ and $\mathrm{N}{\mathrm{i}}_{50}\mathrm{F}{\mathrm{e}}_{50}$. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.

To advance the use of thermally activated magnetic materials in device applications it is necessary to examine their behavior on the localized scale operando conditions. Equiatomic FeRh undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase above room temperature (∼350–380 K), and hence is considered a very desirable material for the next generation of nanomagnetic or spintronic devices. For this to be realized, we must fully understand the intricate details of the AF to FM transition and associated FM domain growth on the scale of their operation. Here we combine in situ heating with a comprehensive suite of advanced transmission electron microscopy techniques to investigate directly the magnetostructural transition in nanoscale FeRh thin films. Differential phase contrast imaging visualizes the stages of FM domain growth in both cross-sectional and planar FeRh thin films as a function of temperature. Small surface FM signals are also detected due to interfacial strain with the MgO substrate and Fe deficiency after HF etching of the substrate, providing a directional bias for FM domain growth. Our work provides high resolution imaging and quantitative measurements throughout the transition, which were previously inaccessible, and offers fundamental insight into their potential use in magnetic devices.

Perovskite ${\mathrm{PbRuO}}_3$, which has been synthesized only in bulk polycrystalline form under high pressure, is an orthorhombic Pbnm at room temperature, exhibiting structural phase transition to Imma at around 90 K. This structural transition is accompanied with an orbital ordering and antiferromagnetic spin fluctuation. Here, we report the fabrication of single crystalline perovskite ${\mathrm{PbRuO}}_3$ thin films on various substrates. Magnetotransport measurements reveal that the films have metallic electronic ground state without any clear electronic transitions reported in previous literature. Evidenced by a shift of transition temperature, suppression of magnetic spin fluctuation due to epitaxial strain is implicated. Nonlinear Hall effect is also observed with indiscernible hysteresis, plausibly originating from antiferromagnetic spins, yet multiband effect cannot be completely ruled out.

Ab initio-calculated ideal strength and toughness describe the upper limits for mechanical properties attainable in real systems and can, therefore, be used in selection criteria for materials design. We employ density-functional ab initio molecular dynamics (AIMD) to investigate the mechanical properties of defect-free rocksalt-structure (B1) TiN and $\text{B}1\mathrm{T}{\mathrm{i}}_{1-x}\mathrm{A}{\mathrm{l}}_x\mathrm{N}(x=0.25$, 0.5, 0.75) solid solutions subject to [001], [110], and [111] tensile deformation at room temperature. We determine the alloys’ ideal strength and toughness, elastic responses, and ability to plastically deform up to fracture as a function of the Al content. Overall, TiN exhibits greater ideal moduli of resilience and tensile strengths than (Ti,Al)N solid solutions. Nevertheless, AIMD modeling shows that, irrespective of the strain direction, the binary compound systematically fractures by brittle cleavage at its yield point. The simulations also indicate that $\mathrm{T}{\mathrm{i}}_{0.5}\mathrm{A}{\mathrm{l}}_{0.5}\mathrm{N}$ and $\mathrm{T}{\mathrm{i}}_{0.25}\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{N}$ solid solutions are inherently more resistant to fracture and possess much greater toughness than TiN due to the activation of local structural transformations (primarily of B1 → wurtzite type) beyond the elastic-response regime. In sharp contrast, (Ti,Al)N alloys with 25% Al exhibit similar brittleness as TiN. The results of this work are examples of the limitations of elasticity-based criteria for prediction of strength, brittleness, ductility, and toughness in materials able to undergo phase transitions with loading. Comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions that are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms.

We use density functional theory calculations to study the electronic structure of epitaxial (111) interfaces of the topological crystalline insulators SnSe and SnTe with the magnetic insulator EuS and the nonmagnetic insulator CaTe, respectively. We consider both interface slab models with a vacuum region and periodic heterostructures without vacuum. We find that gaps of 21 meV at the $\mathrm{\Gamma }$ point and 9 meV at the $M$ point arise in the topological state at the SnSe/EuS interface, due to the magnetic proximity effect, which breaks the time reversal symmetry. The surface state at $\mathrm{\Gamma }$ is shifted below the Fermi level by 88 meV and the surface state at $M$ is shifted above the Fermi level by 47 meV, due to band bending at the interface. By comparison, the topological state at the interface of SnTe/CaTe is unperturbed by the presence of nonmagnetic CaTe.

Significant advances have been made in predicting new topological materials using high-throughput empirical descriptors or symmetry-based indicators. To date, these approaches have been applied to materials in existing databases, and are severely limited to systems with well-defined symmetries, leaving a much larger materials space unexplored. Using tetradymites as a prototypical class of examples, we uncover a two-dimensional descriptor by applying an artificial intelligence (AI)-based approach for fast and reliable identification of the topological characters of a drastically expanded range of materials, without prior determination of their specific symmetries and detailed band structures. By leveraging this descriptor that contains only the atomic number and electronegativity of the constituent species, we have readily scanned a huge number of alloys in the tetradymite family. Strikingly, nearly half of them are identified to be topological insulators, revealing a much larger territory of the topological materials world. The present work also attests to the increasingly important role of such AI-based approaches in modern materials discovery.

We have used a combination of x-ray magnetic circular dichroism (XMCD) spectroscopy and polarized small-angle neutron scattering (P-SANS) to investigate the distribution of magnetization in heterogenous magnetic nanoparticles (NPs) consisting of a metallic Fe core/Fe oxide shell (CS NPS) or Fe core/partial void layer/oxide shell (CVS NPs). Fe $L_{2,3}$ XMCD spectra were analyzed with a combination of experimental metallic Fe XMCD spectra and calculated $L$-edge spectra for the Fe cations in the oxide shell. Analyses of the temperature-dependent spectra indicate a weak variation of the relative contribution of the metallic and oxide contributions for the CS NPs, and a somewhat larger contribution from the metallic Fe core near the blocking temperature $T_B$ of the CVS NP ensemble. The P-SANS data also indicate a larger variation in the magnetization of the CVS NPs near $T_B$. Modeling of the spin-dependent neutron scattering reveals large variations in the radial magnetization distribution, with a region of reversed magnetization adjacent to the metallic core. Interfacial roughness may play a role in the development of this magnetization profile.

Germanium-tin (GeSn) alloys have emerged as a promising material for future optoelectronics, energy harvesting, and nanoelectronics owing to their direct band gap and compatibility with existing Si-based electronics. Yet, their metastability poses significant challenges calling for in-depth investigations of their thermal behavior. With this perspective, this work addresses the interdiffusion processes throughout thermal annealing of pseudomorphic GeSn binary and SiGeSn ternary alloys. In both systems, the initially pseudomorphic layers are relaxed upon annealing exclusively via thermally induced diffusional mass transfer of Sn. Systematic postgrowth annealing experiments reveal enhanced Sn and Si diffusion regimes that manifest at temperatures below 650 °C. The amplified low-temperature diffusion and the observation of only subtle differences between binary and ternary hint at the unique metastability of the Si-Ge-Sn material system as the most important driving force for phase separation.

The crystal structures of the $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Ru}{\mathrm{O}}_3\left(0\le x\le 1\right)$ perovskites are investigated using both long-range and local structural probes. High-resolution synchrotron powder x-ray-diffraction characterization at ambient temperature shows that the materials have orthorhombic long-range crystal structures to high precision, and we support previous work showing that $\mathrm{C}{\mathrm{a}}^{2+}$ substitution for $\mathrm{S}{\mathrm{r}}^{2+}$ primarily changes the tilting of rigid corner-sharing $\mathrm{Ru}{\mathrm{O}}_6$ octahedra at their shared oxygen vertices. X-ray pair-distribution function analysis for $\mathrm{SrRu}{\mathrm{O}}_3,\mathrm{CaRu}{\mathrm{O}}_3$, and one intermediate composition show them to display monoclinic symmetry at the local level; when averaged over different domain orientations this yields orthorhombic symmetry in the long range, and no long-range or local phase transitions are observed between 80 and 300 K for materials with intermediate compositions. High-resolution transmission electron microscopy shows that the Sr/Ca distribution is random at the nanoscale. We plot magnetic characteristics such as the ferromagnetic $T\text{c}$, Curie-Weiss theta, effective moment, and ambient temperature susceptibility vs the octahedral tilt and unit-cell volume.

We perform a crystal structure search using Bayesian optimization and evaluate its efficiency with a varying parameter value in the descriptor. Applying the crystal structure search to crystalline silicon shows that the efficiency of the search depends heavily on the parameter value. We find that the efficiency is linked to the distribution of the descriptor. Therefore, we introduce an information measure of the distribution to estimate an appropriate parameter value for performing the crystal structure search efficiently. The measure can also be used to predetermine an appropriate parameter value. The validity of the measure is confirmed with its applications to silicon oxide and yttrium-cobalt alloy.