We report the discovery of a long-sought-after phase of titanium nitride with stoichiometry ${\mathrm{Ti}}_3{\mathrm{N}}_4$ using diamond anvil cell experiments combined with in situ high-resolution x-ray diffraction and Raman spectroscopy techniques, supported by ab initio calculations. ${\mathrm{Ti}}_3{\mathrm{N}}_4$ crystallizes in the cubic ${\mathrm{Th}}_3{\mathrm{P}}_4$ structure [space group $I\overline{4}3d$ (220)] from a mixture of TiN and ${\mathrm{N}}_2$ above $\approx 75$ GPa and $\approx 2400$ K. The density ($\approx 5.22$ g/cc) and bulk modulus ($K_0=290$ GPa) of cubic-${\mathrm{Ti}}_3{\mathrm{N}}_4$ ($c-{\mathrm{Ti}}_3{\mathrm{N}}_4$) at 1 atm, estimated from the pressure-volume equation of state, are comparable to rocksalt TiN. Ab initio calculations based on the GW approximation and using hybrid functionals indicate that $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ is a semiconductor with a direct band gap between 0.8 and 0.9 eV, which is larger than the previously predicted values. The $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ phase is not recoverable to ambient pressure due to dynamic instabilities, but recovery of ${\mathrm{Ti}}_3{\mathrm{N}}_4$ in the defect rocksalt (or related) structure may be feasible.

We observe the magnetic proximity effect (MPE) in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$ bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e., anisotropic magnetoresistance) and the spin Hall effect (i.e., the spin Hall magnetoresistance) in the Pt layer. The strong evidence for induced ferromagnetism in Pt via the anisotropic magnetoresistance is supported further by density functional theory calculations and various control measurements including the insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk ${\mathrm{CoFe}}_2{\mathrm{O}}_4$. By demonstrating the MPE in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$, these results establish the spinel ferrite family as a promising material for the MPE and spin manipulation via proximity exchange fields.

Epitaxial thin films of the substitutionally alloyed half-Heusler series $\mathrm{CoT}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Sb}$ were grown by molecular beam epitaxy on InAlAs/InP(001) substrates for concentrations $0.0\le x\le 1.0$. The influence of Fe on the structural, electronic, and magnetic properties was studied and compared to that expected from density functional theory. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and x-ray diffraction. Using in situ x-ray photoelectron spectroscopy, only small changes in the valence band are detected for $x\le 0.5$. For films with $x\ge 0.05$, ferromagnetism is observed in SQUID magnetometry with a saturation magnetization that scales linearly with Fe content. A dramatic decrease in the magnetic moment per formula unit occurs when the Fe is substitutionally alloyed on the Co site indicating a strong dependence on the magnetic moment with site occupancy. A crossover from both in-plane and out-of-plane magnetic moments to only in-plane moment occurs for higher concentrations of Fe. Ferromagnetic resonance indicates a transition from weak to strong interaction with a reduction in inhomogeneous broadening as Fe content is increased. Temperature-dependent transport reveals a semiconductor to metal transition with thermally activated behavior for $x\le 0.5$. Anomalous Hall effect and large negative magnetoresistance (up to -18.5% at 100 kOe for $x=0.3)$ are observed for higher Fe content films. Evidence of superparamagnetism for $x=0.3$ and 0.2 suggests, for moderate levels of Fe, that demixing of the $\mathrm{CoT}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Sb}$ films into Fe-rich and Fe-deficient regions may be present. Atom probe tomography is used to examine the Fe distribution in an $x=0.3$ film. Statistical analysis reveals a nonhomogeneous distribution of Fe atoms throughout the film, which is used to explain the observed magnetic and electrical behavior.

Dynamic mechanical analysis (DMA) measurements as a function of temperature, frequency, and dynamic force amplitude are used to perform a detailed study of the domain wall motion in ${\mathrm{LaAlO}}_3$. In previous DMA measurements Harrison et al. [Phys. Rev. B 69, 144101 (2004)] found evidence for dynamic phase transitions of ferroelastic domain walls in ${\mathrm{LaAlO}}_3$. In the present work we focus on the creep-to-relaxation region of domain wall motion using two complementary methods. We determine, in addition to dynamic susceptibility data, waiting time distributions of strain jerks during slowly increasing stress. These strain jerks, which result from self-similar avalanches close to the depinning threshold, follow a power-law behavior with an energy exponent $\varepsilon =1.7\pm 0.1$. Also, the distribution of waiting times between events follows a power law $N\left(t_w\right)\propto t_w^{-(n+1)}$ with an exponent $n=0.9$, which transforms to a power law of susceptibility $S\left(\omega \right)\propto {\omega }^{-n}$. The present dynamic susceptibility data can be well fitted with a power law, with the same exponent ($n=0.9$) up to a characteristic frequency $\omega \approx {\omega }^{*}$, where a crossover from stochastic DW motion to the pinned regime is well described using the scaling function of Fedorenko et al. [Phys. Rev. B 70, 224104 (2004)].

Nominal InN monolayers grown by molecular beam epitaxy on GaN(0001) are investigated combining in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and density functional theory (DFT). TEM reveals a chemical intraplane ordering never observed before. Employing DFT, we identify a novel surface stabilization mechanism elastically frustrated rehybridization, which is responsible for the observed chemical ordering. The mechanism also sets an incorporation barrier for indium concentrations above 25% and thus fundamentally limits the indium content in coherently strained layers.

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$ and ${\mathrm{NaSrCo}}_2{\mathrm{F}}_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously proposed model of short-range XY-like correlations with a length scale of 10–15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing an opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

The full comprehension of magnetic phenomena at the femtosecond (fs) time scale is of high demand for current material science and technology. Here we report the observation of coherent collective modes in the antiferromagnetic insulator nickel oxide (NiO) identified by a frequency of 0.86 THz, which matches the expected out-of-plane single-mode magnon resonance. Such collective excitations are inelastically stimulated by extreme ultraviolet (EUV) pulses delivered by a seeded free-electron laser (FEL) and subsequently revealed probing the transient optical activity of NiO looking at the Faraday effect. Moreover, the unique capability of the employed FEL source to deliver circularly polarized pulses allows us to demonstrate optomagnetic control of such collective modes at EUV photon energies. These results may set a starting point for future investigations of magnetic materials at time scales comparable or faster than those typical of exchange interactions.

We report the fabrication of epitaxial ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) thin films on $\mathrm{G}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$ (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of $\mathrm{F}{\mathrm{e}}^{3+}$ ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications.

Ionic-liquid/gel-based transistors have emerged as a potentially ideal means to accumulate high charge-carrier densities at the surfaces of materials such as oxides, enabling control over electronic phase transitions. Substantial gaps remain in the understanding of gating mechanisms, however, particularly with respect to charge carrier vs oxygen defect creation, one contributing factor being the dearth of experimental probes beyond electronic transport. Here we demonstrate the use of synchrotron hard x-ray diffraction and polarized neutron reflectometry as in operando probes of ion-gel transistors based on ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$. An asymmetric gate-bias response is confirmed to derive from electrostatic hole accumulation at negative gate bias vs oxygen vacancy formation at positive bias. The latter is detected via a large gate-induced lattice expansion (up to 1%), complementary bulk measurements and density functional calculations enabling quantification of the bias-dependent oxygen vacancy density. Remarkably, the gate-induced oxygen vacancies proliferate through the entire thickness of 30–40-unit-cell-thick films, quantitatively accounting for changes in the magnetization depth profile. These results directly elucidate the issue of electrostatic vs redox-based response in electrolyte-gated oxides, also demonstrating powerful approaches to their in operando investigation.

Recent experimental work has revealed that the unusually strong, isotropic structural negative thermal expansion in cubic perovskite ionic insulator ${\mathrm{ScF}}_3$ occurs in excited states above a ground state tuned very near a structural quantum phase transition, posing a question of fundamental interest as to whether this special circumstance is related to the anomalous behavior. To test this hypothesis, we report an elastic and inelastic x-ray scattering study of a second system ${\mathrm{Hg}}_2{\mathrm{I}}_2$ also tuned near a structural quantum phase transition while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant negative thermal expansion below 100 K for dimensions along the body-centered-tetragonal $c$ axis, bolstering the connection between negative thermal expansion and zero-temperature structural transitions. We identify the common traits between these systems and propose a set of materials design principles that can guide discovery of new materials exhibiting negative thermal expansion.

We report that in a $\beta$-Mn-type chiral magnet ${\mathrm{Co}}_9{\mathrm{Zn}}_9{\mathrm{Mn}}_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_{\mathrm{c}}\sim 400$ K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology and provide a significant step toward technological applications of skyrmions in bulk chiral magnets.

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

High-throughput first-principles computational methods, such as density functional theory (DFT), offer the ability to predict the properties of materials, provided their crystal structures are known. However, there are many compounds for which the structure is unknown and, consequently, many potentially useful materials cannot be assessed by high-throughput DFT searches. Here, we demonstrate an automated tool to solve the structures of materials from powder diffraction patterns based on the first-principles-assisted structure solution (FPASS) method. We validate this tool by using it to solve 95 already-known crystal structures and find that FPASS can determine the correct structure in all cases. We then tuned FPASS to improve its performance on the most-difficult test cases, which include structures with larger numbers of symmetrically unique atoms. We also used FPASS to solve the structures of 10 materials and found, using DFT, several are interesting candidates for semiconductor applications.

Hematite $\left(\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)$ is known for poor electronic transport properties, which are the main drawback of this material for optoelectronic applications. In this study, we investigate the concept of enhancing electrical conductivity by the introduction of oxygen vacancies during temperature treatment under low oxygen partial pressure. We demonstrate the possibility of tuning the conductivity continuously by more than five orders of magnitude during stepwise annealing in a moderate temperature range between 300 and 620 K. With thermoelectric power measurements, we are able to attribute the improvement of the electrical conductivity to an enhanced charge-carrier density by more than three orders of magnitude. We compare the oxygen vacancy doping of hematite thin films with hematite nanoparticle layers. Thereby we show that the dominant potential barrier that limits charge transport is either due to grain boundaries in hematite thin films or due to potential barriers that occur at the contact area between the nanoparticles, rather than the potential barrier within the small polaron hopping model, which is usually applied for hematite. Furthermore, we discuss the transition from oxygen-deficient hematite $\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{3-x}$ towards the magnetite $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ phase of iron oxide at high density of vacancies.

The metal/semiconductor hybrid system Tl/Si(111)-$\left(1\times 1\right)$ exhibits a unique Tl-derived surface state with remarkable properties. It lies within the silicon band gap and forms spin-momentum-locked valleys close to the Fermi energy at the $\overline{K}$ and ${\overline{K}}^{\prime }$ points. These valleys are completely spin polarized with opposite spin orientation at $\overline{K}$ and ${\overline{K}}^{\prime }$ and show a giant spin splitting of more than 0.5 eV. We present a detailed preparation study of the surface system and demonstrate that the electronic valleys are extremely robust, surviving exposure to 100 L hydrogen and 500 L oxygen. We investigate the influence of additional Tl atoms on the spin-polarized valleys. By combining photoemission and inverse photoemission, we prove the existence of fully spin-polarized valleys crossing the Fermi level. Moreover, these metallic valleys carry opposite Berry curvature at $\overline{K}$ and ${\overline{K}}^{\prime }$, very similar to ${\mathrm{WSe}}_2$, promising a large spin Hall effect. Thus, Tl/Si(111)-$\left(1\times 1\right)$ possesses all necessary key properties for spintronic applications.

Single-layer (SL) phosphorus (phosphorene) has drawn considerable research attention recently as a new two-dimensional (2D) material with great application promises. Blue phosphorus (blueP), an allotrope of black phosphorus, has been suggested to exist in the SL form on some substrates. Here, we report the growth of blueP by molecular-beam epitaxy and reveal an explicit sequential growth behavior that involves a dewetting process of the first adsorbed P atoms in the $\left(\sqrt{3}\times \sqrt{3}\right)R30^{\circ }$ patches for the nucleation and growth of SL blueP-like islands. We also reveal a one-dimensional (1D) chain structure at low coverage. Interestingly, over a large coverage range, a composite surface containing locally high-P coverage blueP islands surrounded by locally low-coverage 1D chains prevails over the homogeneous $\left(\sqrt{3}\times \sqrt{3}\right)R30^{\circ }$ or P-trimer covered surfaces. Such a phase-separated composite surface is found to be favorable from an energy-minimization point of view and can be general for the growth of other systems.

Enhanced van der Waals (vdW) epitaxy of semiconductors on a layered vdW substrate is identified as the formation of dative bonds. For example, despite that ${\mathrm{NbSe}}_2$ is a vdW layered material, first-principles calculations reveal that the bond strength at a CdTe-${\mathrm{NbSe}}_2$ interface is five times as large as that of vdW interactions at a CdTe-graphene interface. The unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at a CdTe surface to metallic nonbonding ${\mathrm{NbSe}}_2$ states, which is a necessary condition to activate the Cd for enhanced binding with Se.

The crystallographic and magnetic properties of the cleavable $4d^3$ transition metal compound $\alpha \text{−}{\mathrm{MoCl}}_3$ are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

Electronic instabilities in transition-metal compounds may lead to ground states containing orbital molecules when direct metal-metal orbital interactions occur. The spinel $\mathrm{Al}{\mathrm{V}}_2{\mathrm{O}}_4$ was reported to contain ${\mathrm{V}}_7^{17+}$ orbital heptamers that emerge below a 700 K charge ordering transition. Our x-ray total scattering analysis of $\mathrm{Al}{\mathrm{V}}_2{\mathrm{O}}_4$ between 300 and 1100 K reveals a very different picture as the postulated heptamers are found to be pairs of spin-singlet ${\mathrm{V}}_3^{9+}$ trimers and ${\mathrm{V}}_4^{8+}$ tetramers, and these orbital molecules persist to at least 1100 K in a disordered high-temperature cubic phase.