The magnon diffusion length (MDL) is understood to play a major role in the thickness dependence of spin Seebeck effect (SSE) voltages in ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{Pt}$ thin films. Here we extract the MDL in an ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single crystal using inelastic neutron scattering (INS) and in ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{Pt}$ thin films using accurate heat flux SSE and static magnetization measurements. The INS MDLs were $34\pm 8\mathrm{nm}$ at $300\mathrm{K}$ and $27\pm 6\mathrm{nm}$ at $50\mathrm{K}$. The SSE MDLs decreased with temperature ($19\pm 2\mathrm{nm}$ at $300\mathrm{K}$ and $13\pm 4\mathrm{nm}$ at $50\mathrm{K}$), but were markedly smaller. Whilst the bulk MDL is expected to be an upper limit of the MDL in thin films, we show that the film magnetization must be considered to obtain MDLs from SSE measurements. This study highlights the importance of disentangling the role of various effects in SSE measurements which is crucial in increasing the efficiencies of thermomagnetic energy harvesting devices.

Monolayer iron-oxide films are attractive for both spintronic application and surface chemical catalysis. However, the detailed atomic structure of the FeO monolayer which is crucial for understanding the physical and chemical properties of the films is not well understood. In this work, we fabricate high-quality monolayer FeO films on the Au(111) substrate by molecular-beam epitaxy. High-resolution scanning tunneling microscopy measurements simultaneously reveal the close-packed atomic arrangement and hexagonal moiré pattern of the monolayer FeO and were combined with hexagonal commensurate mode calculations to establish the atomic structure of the monolayer FeO(111) with respect to the Au(111) substrate. The identification of this stable commensurate arrangement of FeO(111) monolayer on Au(111) paves the way for the exploration of its fundamental science and its applications in industry.

We demonstrate that the cluster-glass state emerges as ferromagnetic quantum criticality is avoided in the itinerant ferromagnet ${\mathrm{Sr}}_{1-x}{\left({\mathrm{La}}_{0.5}{\mathrm{K}}_{0.5}\right)}_x{\mathrm{RuO}}_3$. In this compound, the ferromagnetic order is suppressed by increasing $x$ and then disappears at the critical concentration: $x=0.5$. In this $x$ range, the present study reveals that no prominent feature is ascribed to the quantum critical fluctuations in specific heat. Instead, ac magnetic susceptibility exhibits a broad peak due to spontaneous spin freezing, and the peak temperature depends significantly on the frequency of the applied ac magnetic field. Furthermore, specific heat is enhanced within a wide temperature range, whereas specific heat shows no salient anomaly associated with spin freezing. These features are characteristics of the formation of cluster glass; in particular, the observed frequency variations in ac magnetic susceptibility are well described by the Vogel-Fulcher law. We compare the features concerning the suppression of the ferromagnetic order in this doped compound with those in isostructural Ca- and La-doped ${\mathrm{SrRuO}}_3$ and suggest that a local correlated disorder effect and the very small coherence of itinerant Ru $4d$ electrons are responsible for the cluster-glass formation instead of the quantum phase transition in ${\mathrm{Sr}}_{1-x}{\left({\mathrm{La}}_{0.5}{\mathrm{K}}_{0.5}\right)}_x{\mathrm{RuO}}_3$.

Antiferromagnets generally have a quite high magnetic resonance, as known as the antiferromagnetic resonance, whose frequency goes up to the THz range. They are one of the few materials that can magnetically couple to THz electromagnetic waves and are therefore important materials for THz technologies. In this work, we demonstrate that the THz resonance properties, such as resonant frequency and the $Q$ factor, of antiferromagnetic NiO can be controlled over a wide range by substituting the $\mathrm{N}{\mathrm{i}}^{2+}$ cation with various different cations. Our discussion extends to the trends of how different substitute cations $\mathrm{M}{\mathrm{n}}^{2+}, \mathrm{L}{\mathrm{i}}^{+}$, and $\mathrm{M}{\mathrm{g}}^{2+}$ impact the resonant properties, which suggests a guideline for designing antiferromagnetic THz materials.

The ability to tune the competition between the in-plane and out-of-plane orientation of magnetization provides a means to construct thermal sensors with a sharp spin reorientation transition at specific temperatures. We have observed such a tuneable, temperature-driven spin reorientation in structurally amorphous, ferrimagnetic rare-earth transition-metal alloy thin films using scanning transmission x-ray microscopy and magnetic measurements. The nature of the spin reorientation transition in FeGd can be fully explained by a nonequilibrium, nanoscale modulation of the chemical composition of the films. This modulation leads to a magnetic domain pattern of nanoscale speckles superimposed on a background of in-plane domains that form Landau configurations in µm-scale patterned elements. It is this speckle magnetic structure that gives rise to a sharp two-step reversal mechanism that is temperature dependent. The possibility to balance competing anisotropies through the temperature opens opportunities to create and manipulate topological spin textures.

Understanding formation and distribution of defects in GaN substrates and device layers is needed to improve device performance in rf and power electronics. Here we utilize conductive atomic force microscopy (c-AFM) for studying defect-related leakage paths in an unintentionally doped GaN film formed by nanowire reformation. A nanoscopic Schottky contact is formed between the c-AFM probe and the GaN surface, which, under reverse-bias conditions, reveals local leakage currents at the positions of the nanowires. Cathodoluminescence shows these areas to be dominated by yellow-band luminescence, in contrast to the surrounding GaN matrix, which mainly shows near-band-gap luminescence. These results are attributed to a high density of native and residual defects, confined to the nanowires. In addition, we use anodic oxidation to map defect-related conductive paths through locally induced growth of gallium oxide. The oxide yield, which is known to depend on the local electric field strength between the AFM tip and the sample, correlates well with the level of reverse-bias leakage current. Local irregularities in oxide height reveal extended oxidation attributed to defect-related deep-level states. This is confirmed by controlled dissolution of the oxide in NaOH, showing that a deeper oxide film is grown over areas where defect-related conductive paths are formed. Finally, we demonstrate how this approach can be used as a quick and easy diagnostic tool for evaluating the influence of specific growth conditions and process steps on defect-induced leakage current levels and defect distribution in GaN structures, demonstrating its potential for accelerated test of leakage degradation at critical positions in GaN-based devices.

Inert gas condensation (IGC) is a versatile method for the synthesis of ultrafine nanoparticles. While it has been mostly used in the synthesis of nanocrystalline particles, it has also been employed in the synthesis of metallic glass nanoparticles, which can be further used in the generation of nanostructured metallic glasses, known as nanoglasses. In this work, we use molecular dynamics simulations to investigate the IGC processes leading to the synthesis of $\mathrm{C}{\mathrm{u}}_{64}\mathrm{Z}{\mathrm{r}}_{36}$ amorphous nanoparticles. We investigate the growth of nanoparticles up to 7.3 nm diameter using long simulations of up to 130 ns, under relatively low pressures in the range of 4–10 bars. Results indicate a significant effect of pressure on the final structure of the amorphous nanoparticles, yet no clear influence on the segregation of Cu atoms to its surface. Cu segregation occurring at all pressures results in a thin Cu-rich layer, about 2.5 Å thick, that coats all nanoparticles. Statistics of atomic Voronoi polyhedra shows that the fraction of full icosahedra in the nanoparticles produced at 4 bars is equivalent to that of bulk metallic glasses produced by melt and quench at ${10}^9\mathrm{K}/\mathrm{s}$. Higher pressures result in a lower fraction of full icosahedra.

Deformation twinning is one of the major deformation mechanisms in crystals, which plays an important role in determining the mechanical properties of metals and alloys. One of the important issues to understand twinning mechanisms is the determination of the deformation path. However, due to a lack of theoretical tools, a fundamental relationship between symmetry breaking and the deformation path has not been established in materials science, which conceals the physical origin of deformation twinning. Utilizing a graph approach for deformation pathways, we show that twinning modes in hexagonal close-packed (hcp) titanium and zirconium are dictated by both the symmetry of hcp and the symmetry breaking associated with the bcc to hcp transformation. Our work not only opens another avenue to investigate the symmetry and symmetry breaking in hcp crystals, but also provides insight into the physical origin of crystalline defects.

$\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ is widely used for high-temperature structural materials in the form of polycrystals, and the physical and chemical properties are significantly influenced by the impurity types and their concentrations. Although the concentrations of some impurities in $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ have been experimentally investigated, there are no detailed reports on the point-defect structures and their charge states. In this study, we systematically investigate the point-defect structures of native defects, group IIA (Be, Mg, Ca, Sr, and Ba) and group IV (C, Si, Ge, Sn, and Pb) impurities by using Heyd-Scuseria-Ernzerhof hybrid functional combined with a finite-size supercell correction. We found that although the favorable defect types in the group IIA impurities are independent of the atmosphere (or oxygen partial pressure), the impurity defects in group IV show a strong dependence on the atmosphere and the impurity types. On the basis of the defect energetics, we calculate the impurity concentrations as a function of temperature for the respective extreme conditions. This could be useful for controlling the properties of $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, especially sintering behaviors.

Particular combinations of solute atoms segregated to the interface of the Al and ${\theta }^{\prime }\text{−}{\mathrm{Al}}_2\mathrm{Cu}$ in Al-Cu alloys can help stabilize ${\theta }^{\prime }$ precipitates at high temperatures. Stabilization of such precipitates is determined by a combination of thermodynamics (including driving forces for coarsening and transformation and solute segregation tendencies) and kinetic effects (including solute diffusion and interfacial mobility in the presence of interfacial solute segregation). For some alloys such as recent Al-Cu-based alloys, multiple solutes segregate in significant quantities to interfaces, and solute-solute interactions at the interface are important, with multiple types of solutes competing for similar interfacial sites. To treat this situation, we develop and apply a statistical mechanics approach to calculate the temperature-dependent equilibrium solute atoms distribution near the coherent and semicoherent interfaces between the Al matrix and the ${\theta }^{\prime }\text{−}{\mathrm{Al}}_2\mathrm{Cu}$ precipitates. The developed approach is applied to the investigation of Si, Mn, and Zr segregation at the interface, as particular combinations of these elements affect the thermal stability of the ${\theta }^{\prime }$ precipitates. We demonstrate that because Si and Mn atoms segregate on the same semicoherent interface, the presence of Si reduces the concentration of Mn solutes at the interface. Si atoms preferably occupy the first layer of the interface and compete with Mn atoms for one type of particular sites in the layer. Mn atoms preferably occupy the second layer of the semicoherent interface, and the Mn-Mn interaction plays an important role in their distribution. Zr atoms mostly segregate on one of the two nonequivalent sites of the second layer of the coherent interface. Due to symmetry properties of the coherent interface, the calculations show that the segregation Zr of atoms to this interface will likely lead to the formation of ${\mathrm{L}1}_2$ ordered ${\mathrm{Al}}_3\mathrm{Zr}$ layer.

Identification of two-dimensional (2D) materials with magnetic properties has received strong research attention in the development of advanced spin-based devices. By means of first-principles calculations, we investigate the stability, electronic properties, and the hole-doping-induced magnetic properties of metal oxide ($M\mathrm{O}, M=\mathrm{Ga},\mathrm{In}$) monolayers. They are intrinsically nonmagnetic stable semiconductors, with high energetic, vibrational, and thermal stability. Hole doping can switch them from nonmagnetic to ferromagnetic and turn them into half-metals over a wide range of hole densities. Monte Carlo simulations predict that the highest Curie temperature of the GaO monolayer can reach ∼125 K. Our results indicate that monolayer $M\mathrm{O}$ could be eligible candidate materials for 2D spintronic devices.

The structural, electronic, and magnetic properties of VSSe, VSeTe, and VSTe monolayers in both 2H and 1T phases are investigated via first-principles calculations. The 2H phase is energetically favorable in VSSe and VSeTe, whereas the 1T phase is lower in energy in VSTe. For V-based Janus monolayers in the 2H phase, calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. As such, they should not exhibit a ferromagnetic phase transition, but instead, a Berezinskii-Kosterlitz-Thouless (BKT) transition. A classical XY model with nearest-neighbor coupling estimates critical temperatures (${\mathrm{T}}_{\text{BKT}}$) ranging from 106 K for VSSe to 46 K for VSTe.

We investigated the structural, magnetic, transport, and high-temperature thermoelectric properties of single crystals of the pseudobrookite ${\mathrm{Al}}_{1-x}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_5$ for $0\le x\le 1$ grown using a floating zone. We found a correlation of spin-singlet ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ dimers coupled with the lattice even in the conductive $\alpha$ and $\lambda$ phases which develops with increasing $x$. This developing dimer correlation reduces the number of unpaired ${\mathrm{Ti}}^{3+}$ ions, which makes the compound more conductive owing to the suppression of disorder for $x$ up to about 0.75. The dimer fluctuation causes a critical enhancement of the magnetic susceptibility at around 150 K in the $\lambda$ phase near the boundary ($x\sim 0.9$) between the $\lambda$ and the $\beta$ phases. Such a correlation of the spin-singlet ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ dimers may produce a high Seebeck coefficient in the conductive $\alpha$ and $\lambda$ phases leading to a large thermoelectric power factor at high temperatures.

Weak van der Waals interactions have a notorious influence on the stability of molecular solids and layered materials. In light of this fact, accurate models have been designed to account for such long-range interactions. Here, we explore the influence of the dispersion on the structural properties of dense rutile-type ${\mathrm{SiO}}_2$, ${\mathrm{GeO}}_2$, ${\mathrm{SnO}}_2$, and ${\mathrm{PbO}}_2$ structures. Several dispersion methods are discussed including pairwise force field and more physically grounded methods such as the so-called many body dispersion and nonlocal correlation functionals. The results clearly show that such methods and functionals have influence on the predicted crystal structure of the investigated solids, usually improving the structural properties with a reduction of the errors with respect to experiment. In particular, Perdew-Burke-Ernzerhof (PBE) coupled with many body dispersion (MBD)@rsSCS emerges as the most suitable method showing the lowest relative errors. Interestingly, this method reproduces similar results to those obtained by the PBEsol generalized gradient approach (GGA) functional designed to reproduce crystal structures without considering dispersion terms. Finally, we want to point out that the present study suggests that the proper inclusion of dispersion allows one to improve the description of the crystal structure and, consequently, due to the sensibility of either electronic and magnetic properties to it, methods coupling a suitable description of dispersion to a standard PBE GGA functional would lead to an improved description of crystalline solids.

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{–}\text{decyl}\text{–}7\text{-phenyl-}\left[1\right]\text{benzothieno}[3,2\text{–}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}\left[1\right]\text{benzothieno}[3,2\text{–}b]\text{naphtho}[2,3\text{–}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

The effects of Sn $5s$ lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn $5s$ orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn $5s$ state contribution at the top of the valence band in the different Sn-sulphides, SnS, ${\mathrm{Sn}}_2{\mathrm{S}}_3$, and ${\mathrm{SnS}}_2$, to be clearly identified. SnS and ${\mathrm{Sn}}_2{\mathrm{S}}_3$ contain Sn(II) cations and the corresponding Sn $5s$ lone pairs are at the valence band maximum (VBM), leading to $\sim 1.0$–1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, ${\mathrm{SnS}}_2$ only contains Sn(IV) cations, no filled lone pairs, and therefore has a $\sim 2.3$ eV room-temperature band gap and much lower VBM compared with SnS and ${\mathrm{Sn}}_2{\mathrm{S}}_3$. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS, ${\mathrm{Sn}}_2{\mathrm{S}}_3$, and ${\mathrm{SnS}}_2$, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn $5s$ lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with $\mathrm{n}{s}^2$ configuration.

The electronic, magnetic, thermoelectric, and topological properties of Heusler compounds (composition $XYZ$ or $X_{2}YZ$) are highly sensitive to stoichiometry and defects. Here we establish the existence and experimentally map the bounds of a semi-adsorption-controlled growth window for semiconducting half-Heusler FeVSb films, grown by molecular beam epitaxy (MBE). We show that due to the high volatility of Sb, the Sb stoichiometry is self-limiting for a finite range of growth temperatures and Sb fluxes, similar to the growth of III-V semiconductors such as GaSb and GaAs. Films grown within this window are nearly structurally indistinguishable by x-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED). The highest electron mobility and lowest background carrier density are obtained towards the Sb-rich bound of the window, suggesting that Sb vacancies may be a common defect. Similar semi-adsorption-controlled bounds are expected for other ternary intermetallics that contain a volatile species $Z=${Sb, As, Bi}, e.g., CoTiSb, LuPtSb, GdPtBi, and NiMnSb. However, outstanding challenges remain in controlling the remaining Fe/V ($X/Y$) transition metal stoichiometry.