The magnetic damping of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Permalloy, Py) thin films is studied via frequency-dependent ferromagnetic resonance (FMR) experiments. The thickness of the Py films is kept constant and they are all protected from oxidation by a 5-nm-thick Al cap layer. To separate the Py film from the oxidic sapphire substrates a systematic variation of the thickness of an additional Al spacer layer was carried out. Py sandwiched in Al exhibits a low, purely Gilbert-like magnetic damping when the Al spacer layer thickness is kept below 3 nm. Above this thickness the magnetic damping is strongly increased because of a pronounced two-magnon contribution. A detailed investigation of the temperature dependence as well as full angular dependence of the FMR allows for correlating the magnetic properties of the Py with the microscopic structural properties of the films as studied by x-ray reflectivity and transmission electron microscopy. It turns out that the detrimental two-magnon processes are activated by an island-like growth of the Al spacer layers, which leads to rough, wavy interfaces with characteristic length scales of the order of ten nanometers. Nevertheless, for Al spacer layers with a thickness below 3 nm, a low, purely Gilbert-like magnetic damping can be observed.

We report the magnetic properties of Fe-substituted ${\mathrm{Mn}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{NiGe}$ by detailed magnetization measurements. Weak first-order phase transition along with kinetic-arrest-like behavior is confirmed based on the bifurcation between zero-field cooling and field-cooled warming magnetization curves, thermal hysteresis between field-cooled cooling and field-cooled warming curves, and an unusual nature of virgin M-H curve lying outside the hysteresis loop. At 2 K, it exhibits spontaneous exchange bias (SEB) of $H_{\mathrm{EB}}\sim 132$ Oe, which is larger than the conventional exchange bias in a cooling field of 80 kOe. The constructed temperature-field T-H phase diagram illustrates the diminishing magnetic phase coexistence (first-order) region and increase of ferromagnetic phase with the increase of magnetic field strength by avoiding tricriticality. SEB in the present alloy is reported to be due to the formation of local-domain interfaces of long-range ferromagnetic and spin-glass phases. In addition, the relatively smaller $H_{\mathrm{SEB}}$ value is because of weak coupling between the magnetic phases at the interface. The present study stimulates interest in revisiting materials exhibiting coexisting magnetic phases in the quest for the exchange bias effect.

${\mathrm{ErAl}}_3$ can form in either a trigonal ($\alpha$) or cubic ($\beta$) polymorph and this paper investigates the physical properties of these polymorphs through characterizations of single crystals grown in an aluminum flux. We demonstrate that polymorph selection can be achieved based on the nominal composition of the crystal growth. Magnetic measurements confirm that both $\beta \text{−}{\mathrm{ErAl}}_3$ and $\alpha \text{−}{\mathrm{ErAl}}_3$ order antiferromagnetically at low temperatures. $\beta \text{−}{\mathrm{ErAl}}_3$ undergoes antiferromagnetic ordering at a Néel temperature $T_N=5.1\mathrm{K}$, and the transition is suppressed continually with applied field. $\alpha \text{−}{\mathrm{ErAl}}_3$ displays more complex behavior, with successive magnetic transitions at $T_N=5.7\mathrm{K}$ and $T_2=4.6\mathrm{K}$ for zero field, where heat capacity and dilatometry measurements evidence that these transitions are second and first order, respectively. Under magnetic field, strong anisotropy is revealed in $\alpha \text{−}{\mathrm{ErAl}}_3$, with several steplike metamagnetic transitions observed below $T_2$ for $H\parallel \mathrm{c}$. These transitions produce sequential magnetization plateaus near one-half of the apparent saturation magnetization. The electrical resistivity of $\alpha \text{−}{\mathrm{ErAl}}_3$ is strongly coupled to its magnetism. At $T=2\mathrm{K}$, we observe a positive magnetoresistance reaching 60%, with distinct anomalies at the metamagnetic transitions. The results are summarized in $H\text{−}T$ phase diagrams that demonstrate complex magnetic behavior for $\alpha \text{−}{\mathrm{ErAl}}_3$, suggesting an important role of competing interactions in this metallic system that possesses characteristics of Ising physics.

Potassium tantalate ($\mathrm{KTa}{\mathrm{O}}_3$) single crystals annealed in hydrogen exhibit room-temperature persistent photoconductivity. The oxygen-poor annealing environment creates oxygen vacancies that are substituted with hydrogen. A photon of energy near the indirect bandgap (3.65 eV) can excite this defect, resulting in free electrons in the conduction band. This causes an increase in n-type conductivity of 4 orders of magnitude with a recovery time of 2–4 years. Hydrogen leaves the O site and forms an O–H bond that is observed by IR spectroscopy. The microscopic origin of the defect model is corroborated by hybrid density functional theory calculations.

The advent of machine learning in materials science opens the way for exciting and ambitious simulations of large systems and long time scales with the accuracy of ab initio calculations. Recently, several pretrained universal machine learned interatomic potentials (UPMLIPs) have been published, i.e., potentials distributed with a single set of weights trained to target systems across a very wide range of chemistries and atomic arrangements. These potentials raise the hope of reducing the computational cost and methodological complexity of performing simulations compared to models that require for-purpose training. However, the application of these models needs critical evaluation to assess their usability across material types and properties. In this work, we investigate the application of the following UPMLIPs: MACE, CHGNET, and M3GNET to the context of alloy theory. We calculate the mixing enthalpies and volumes of 21 binary alloy systems and compare the results with DFT calculations to assess the performance of these potentials over different properties and types of materials. We find that the small relative energies necessary to correctly predict mixing energies are generally not reproduced by these methods with sufficient accuracy to describe correct mixing behaviors. However, the performance can be significantly improved by supplementing the training data with relevant training data. The potentials can also be used to partially accelerate these calculations by replacing the ab initio structural relaxation step.

We explored ${\mathrm{Mn}}_2\mathrm{VAl}$ (MVA) Heusler alloy thin films to investigate their potential as a material with negative spin polarization. Density-of-states calculations showed that MVA has a gap in the majority-spin state at the Fermi energy, leading to negative spin polarization. Negative spin polarization was found to be higher in the $L{2}_1$-ordered state than that in the $B2$-ordered state. ${\mathrm{V}}_{\mathrm{Mn}}$ antisites (V atoms occupying Mn sites) introduced an in-gap state that significantly reduced negative spin polarization, while ${\mathrm{Mn}}_{\mathrm{V}}$ and ${\mathrm{Al}}_{\mathrm{V}}$ antisites preserved the energy gap and thus weakly affected negative spin polarization. High-quality MVA films were fabricated via magnetron sputtering at elevated temperatures, achieving the $B2$ and $L{2}_1$ order parameters of 0.88 and 0.5 at $600{}^{\circ }\mathrm{C}$ in the stoichiometric composition, respectively. The comparison of stoichiometric and various off-stoichiometric samples revealed that the Mn-rich and Al-rich compositions showed an improved ordering and a smaller number of ${\mathrm{V}}_{\mathrm{Mn}}$ antisites. The benefit of using off-stoichiometric compositions was further highlighted by the obtained negative magnetoresistance, which was measured via epitaxial MVA/Ag spacer/CoFe current-perpendicular-to-plane giant magnetoresistance devices. Devices with the MVA composition of ${\mathrm{Mn}}_{2.2}{\mathrm{V}}_{0.6}{\mathrm{Al}}_{1.2}$ showed a very large negative magnetoresistance of −4.4%, indicating high negative spin polarization in MVA. Furthermore, highly efficient spin-transfer torque generation, with a torque direction opposite to that seen in positive spin polarization materials, was demonstrated via spin injection from MVA.

Recently, GeSe was proposed as a promising candidate for solar cells because of its excellent material properties. Its favorable photovoltaic performance has been attributed to its defect tolerance. However, a rigorous study of the defect-assisted nonradiative recombination in GeSe is still lacking. Here, we employ first-principles calculations to systematically assess the defect-assisted nonradiative recombination in GeSe. We find that $V_{\mathrm{Ge}}$, ${\mathrm{Se}}_{\mathrm{Ge}}$, and ${\mathrm{Ge}}_i$ are all energetically favorable and possess multiple charge-state transition levels deep in the band gap. Nevertheless, they are not strong nonradiative recombination centers. Counterintuitively, the presence of additional charge-state transitions does not necessarily enhance, but may even suppress the total recombination rate. Our study provides compelling computational evidence for the potential of GeSe as a promising material for solar cells, and unveils the nonradiative recombination mechanism with multiple charge-state transitions in general.

This paper investigates the thermochemistry of the heterogeneous Al/CuO thermite reaction through density functional theory calculations. We examine the interactions of atomic Al, Cu, O, as well as ${\mathrm{O}}_2$, AlO, ${\mathrm{Al}}_2\mathrm{O}, \mathrm{Al}{\mathrm{O}}_2, {\mathrm{Al}}_2{\mathrm{O}}_2$ molecular species, with Al(111), Cu(111), and ${\mathrm{Al}}_2{\mathrm{O}}_3$ ($\gamma$ and amorphous) surfaces, all of which being condensed phase products during the thermite reaction. Al(111) exhibits a very high reactivity, characterized by adsorption energies ranging from 3 to 5.3 eV for atomic Al, Cu, O, and from 4 to 9.5 eV for all molecular species. This reactivity is associated to barrierless molecular decomposition, followed by the spatial spreading of adsorbate species across the surface facilitated by hot adatom migration processes. The ${\mathrm{Al}}_2{\mathrm{O}}_3$ surface also exhibits extremely high reactivity, with adsorption energies of 4.5 and 9.4 eV for atomic Cu and Al, respectively. Additionally, adsorption energies range from 7 to 15 eV for condensation of ${\mathrm{Al}}_x{\mathrm{O}}_y$ suboxides. Al-rich suboxides, namely ${\mathrm{Al}}_2\mathrm{O}$ and ${\mathrm{Al}}_2{\mathrm{O}}_2$, show the greatest adsorption energy with 15.05 eV for ${\mathrm{Al}}_2\mathrm{O}$, against 6.52 eV for $\mathrm{Al}{\mathrm{O}}_2$. In contrast, O and ${\mathrm{O}}_2$ exhibit no reactivity on ${\mathrm{Al}}_2{\mathrm{O}}_3$ surfaces exhibiting oxidation states being superior or equal to ${\mathrm{Al}}^{III}$. Finally, Cu(111) surface exhibits much lower reactivity compared to Al(111) and ${\mathrm{Al}}_2{\mathrm{O}}_3$, with adsorption energies ranging from 2 to 3.5 eV for Al, O, and Cu atoms, which effect is even more pronounced at high temperature due to entropic effects. Although energetic, molecular ${\mathrm{Al}}_x{\mathrm{O}}_y$ suboxides show nondissociative adsorption on Cu(111). These findings point to different modes of oxide nucleation on these surfaces, pleading for planar nucleation and growth onto Al(111), while being more localised onto Cu(111). They renew our understanding of the thermite reaction chemistry, quantitatively differentiating the various type of heterogeneous reactions, the effect of rising the temperature, and their implication on the overall reaction. They also provide valuable data for higher-level diphasic simulations of the computational fluid dynamics, aiming to achieve predictive capability.

We use atomistic simulations to study the structure and shape of niobium precipitates with a body-centered cubic crystallographic structure in a zirconium matrix with a hexagonal close-packed structure. We consider a Pitsch-Schrader orientation relationship, which is observed experimentally for Nb precipitates appearing under irradiation in zirconium alloys. In this regard, we have developed a Zr-Nb empirical potential using the embedded atom method. This parametrization offers a good description of the interfaces between Nb precipitates and the Zr matrix. The potential includes all the necessary physical ingredients to account for the coherency strain between both phases. The atomistic simulations reveal a significant decrease in the precipitate formation energy when they become semicoherent, with enough misfit dislocations at the interface to eliminate the lattice misfit between the Nb precipitates and the Zr matrix. The “idealized” equilibrium shape that minimizes the formation energy of these semicoherent precipitates matches the experimental shape: platelets lying in the Zr basal planes with a reduced dimension along the $\left[0001\right]$ axis. Our simulations suggest that the platelet shape observed in zirconium for irradiation-induced Nb-enriched precipitates simply results from a minimization of their energy cost.

Efficient heuristics have predicted many functional materials such as high-temperature superconducting hydrides, while inorganic structural chemistry explains why and how the crystal structures are stabilized. Building upon the foundational insights of inorganic structural chemistry, we propose a mathematical programming formalism to search and systematize structural prototypes of crystals. By integrating empirical principles that govern the spatial arrangement of anions and their effects on cations, we define the objective functions: minimizing the unit cell volume and assigning discrete geometrical constraints including chemical bonds. This formulation introduces inequality constraints on interatomic distances derived from inorganic chemistry, substantially narrowing the search space for feasible structures. The method has been effectively applied to generate various oxide structures, including spinel, pyrochlore-$\alpha$, and ${\mathrm{K}}_2{\mathrm{NiF}}_4$, demonstrating its ability to explore candidate crystal structures of complex inorganic compounds.

In this study, ${\mathrm{YCrO}}_3$ and Ba and Ni co-doped ${\mathrm{YCrO}}_3$ ($\mathrm{B}{\mathrm{a}}_{0.05}{\mathrm{Y}}_{0.95}\mathrm{C}{\mathrm{r}}_{0.95}\mathrm{N}{\mathrm{i}}_{0.05}{\mathrm{O}}_3$) polycrystalline samples were prepared using the standard sol-gel method, and their structure, surface morphology, and electrical and magnetic properties were examined. The samples crystallize in an orthorhombic Pnma structure. Rietveld analysis of all x-ray diffraction patterns revealed a decrease in lattice constant from 5.5230(7) Å to 5.4991(4) Å and a crystallite size reduction from $\sim 68.64\left(4\right)\mathrm{nm}$ (${\mathrm{YCrO}}_3$, YCO) to $\sim 58.86\left(3\right)\mathrm{nm}$ ($\mathrm{B}{\mathrm{a}}_{0.05}{\mathrm{Y}}_{0.95}\mathrm{C}{\mathrm{r}}_{0.95}\mathrm{N}{\mathrm{i}}_{0.05}{\mathrm{O}}_3$, BYNCO). Furthermore, the deviation of the Cr-O1-Cr bond angle, octahedral distortion, off-center displacement of A-site ions, and rotation of ${\mathrm{CrO}}_6$ due to doping appear to strongly affect the magnetic and electric properties. The distortion parameter values increased for the doped sample compared to that of the pristine one. The samples exhibited paramagnetic behavior at 300 K (room temperature), with a paramagnetic-to-antiferromagnetic transition occurring at 145 K for YCO and at 130.9 K for BYNCO. The fitting of the magnetization vs field (MH) loop aids in quantifying the extent of weak ferromagnetism in the samples. The presence of negative magnetization has been attributed to the temperature-induced magnetization-reversal phenomenon. The neutron diffraction study infers a G-type antiferromagnetic ordering in BYNCO with ordered Cr moment of $2.44\left(3\right){\mu }_{\mathrm{B}}$ (30 K) along the $c$ direction. The dielectric properties were fitted using the Cole-Cole equation, and the obtained parameters were analyzed. Ferroelectricity seemed to be enhanced, with a higher maximum polarization observed for $\mathrm{B}{\mathrm{a}}_{0.05}{\mathrm{Y}}_{0.95}\mathrm{C}{\mathrm{r}}_{0.95}\mathrm{N}{\mathrm{i}}_{0.05}{\mathrm{O}}_3$ compared to ${\mathrm{YCrO}}_3$. Characterization of leakage current density ($J$) vs electric field ($E$) indicates a reduction in leakage current in BYNCO compared to the bare one. All these findings would be interesting for a detailed theoretical study for applications in future generation devices.

We use first-principle density functional theory (DFT) to predict properties for semicrystalline polyvinyl fluoride (PVF) and compare with polyvinylidiene fluoride. We note that the crystalline regions of PVF are complex in the sense that we lack a complete experimental characterization of the detailed atomic organization. We therefore turn to DFT to predict both the structure and associated materials properties, illustrating a possible work flow for complex soft-matter modeling. We rely on the nonempirical consistent-exchange van der Waals density functional version [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] and identify plausible ground-state and excited-state motifs. From there we predict the elastic response of the crystalline motifs, and an upper limit estimate of the PVF polarization at room temperature.

The discovery of two-dimensional (2D) materials derived from non-van der Waals (vdW) bulk counterparts has opened up a new era, drawing attention to the crystals composed of asymmetrically bonded vertical exotic layers. In this respect, $\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, a promising material utilized in various applications, built with coloring triangle layers, emerges as a suitable candidate. Through first-principles calculations, we show that a novel 2D structure, the $2\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer, consisting of a bilayer-like coloring triangle lattice, can be exfoliated from bulk $\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ with minimal external energy. The formation of this exotic 2D lattice is facilitated by ${\mathrm{sp}}^3$ hybrid bonds. Comprehensive phonon dispersion and finite-temperature molecular dynamics analyses confirm the thermodynamic stability of the $2\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer. The material exhibits an anisotropic mechanical response due to missing bonds at lattice sites, making it suitable for flexible nanoelectronic devices. It possesses semiconductor characteristics with an indirect band gap in the visible region. Analysis of band edge positions and charge carrier mobility suggests that the $2\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer is highly efficient for photocatalytic water-splitting applications.

We study two-dimensional (2D) magnetic materials using state-of-the-art machine learning models that use a graph-theory framework. We find that representing materials as graphs allows us to better learn structure-property relationships by leveraging both the chemical properties of the constituent atoms and the connectivity between those atoms. Graph neural network models are capable of predicting global properties of crystal structure (i.e., graphwise properties) and local properties of the constituent atoms (i.e., nodewise properties). We embed physical constraints into our model by simultaneously making predictions of local and global properties. In particular, we use the atomistic line graph neural network (ALIGNN) architecture. We train the ALIGNN model on data comprising local and global magnetic moments of 314 2D structures of the form $\mathrm{Cr}{A}^{\text{ii}}{B}^{\text{i}}{B}^{\text{ii}}{X}_6$, based on monolayer ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, calculated from first principles. By learning the relationships between both local and global magnetic properties, we demonstrate an improvement over models that only consider global magnetic properties.

In this paper, we study the synthesis, structure, magnetism, and electron spin resonance of a quantum spin liquid candidate ${\mathrm{LuCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Br}}_2\left[{\mathrm{Br}}_x{\left(\mathrm{OH}\right)}_{1-x}\right](x=0.45)$. The single-crystal x-ray diffraction results indicate that the $\mathrm{C}{\mathrm{u}}^{2+}$ ions constitute a perfect Kagome lattice along the ab plane, accompanying the partial disorder between $\mathrm{O}{\mathrm{H}}^{-}$ and $\mathrm{B}{\mathrm{r}}^{-}$ ions. The magnetic susceptibility and specific-heat measurements are employed to investigate the magnetism of this compound, revealing the absence of long-range magnetic order down to 80 mK with Weiss temperatures ${\theta }_{\parallel }=-26.81\mathrm{K}$ and ${\theta }_{\perp }=-26.79\mathrm{K}$, respectively. The estimated antiferromagnetic interaction between two neighboring $\mathrm{C}{\mathrm{u}}^{2+}$ ions is $J\sim -36\left(3\right)\mathrm{K}$. Below 0.7 K, the specific heat is proportional to $T^2$, indicating a possible Dirac quantum spin liquid state of this compound. At 2 K, the high-field electron spin resonance signals exhibit two gapped modes and one gapless mode, which may relate to the bond randomness arising from the partial site mixing between $\mathrm{O}{\mathrm{H}}^{-}$ and $\mathrm{B}{\mathrm{r}}^{-}$ ions.

We develop the phenomenological theory for uniaxial ferromagnets and derive explicit formulas for their magnetostriction (MS) and magnetic anisotropy energy (MAE) generated by the magnetoelastic (ME) effects. An analysis of the experimental results for the MS in hexagonal ${\mathrm{YCo}}_5$ using the formulas obtained reveals that (1) an elastic anomaly should appear in the elastic constant $c_{33}$, and (2) the MAE caused by the ME effects could be non-negligible. The calculated results of the electronic structure under lattice deformation indicate that the particular arrangement of Co atoms in the lattice is related to the anisotropy of MS in ${\mathrm{YCo}}_5$. It is suggested that the anisotropic MS observed for various Y compounds could be interpreted in a similar manner.

The traditional Landau-Lifshitz-Gilbert (LLG) equation has often delineated the linear and nonlinear magnetization dynamics, even at ultrashort timescales, e.g., femtoseconds. In contrast, several other nonrelativistic and relativistic spin torques have been reported as an extension of the LLG spin dynamics. Here, we explore the contribution of the relativistic field-derivative torque (FDT) in the nonlinear THz magnetization dynamics response applied to ferrimagnets with high Gilbert damping and exchange magnon frequency. Our findings suggest that the FDT plays a significant role in magnetization dynamics in both linear and nonlinear regimes, bridging the gap between the traditional LLG spin dynamics and experimental observations. We find that the coherent THz magnon excitation amplitude is enhanced with the field-derivative torque. Furthermore, a phase shift in the oscillation of the Néel vector is induced by the FDT term. This phase shift is almost ${90}^{\circ }$ for the antiferromagnet, while it is almost zero for the ferrimagnet under our investigation. Analyzing the dual THz excitation and their FDT, we find that the nonlinear signals can not be distinctly observed without the FDT terms. However, the inclusion of the FDT terms produces distinct nonlinear signals, which matches extremely well with the previously reported experimental results.