Constructing heterostructures of a topological insulator (TI) with an undoped magnetic insulator (MI) is a clean and versatile approach to break the time-reversible symmetry in the TI surface states. Despite a lot of efforts, the strength of the interfacial magnetic proximity effect (MPE) is still too weak to achieve the quantum anomalous Hall effect and many other topological quantum phenomena. Recently, a new approach, “magnetic extension,” was proposed to achieve strong MPE [Otrokov et al., 2D Mater. 4, 025082 (2017)]. This approach is demonstrated effective by intercalation of the MI layer to the TI [Hirahara et al., Nano Lett. 17, 3493 (2017)]. Motivated by this proposal, here we study a magnetic extension system prepared by molecular beam epitaxial growth of MnSe thin films on a topological insulator ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$. Direct evidence is obtained for intercalation of the MnSe atomic layer into a few quintuple layers of ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$, forming either a double magnetic septuple layer (SL) or an isolated single SL at the interface, where one SL denotes a van der Waals building block consisting of B-A-B-Mn-B-A-B ($\mathrm{A}={\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$, $\mathrm{B}={\mathrm{Te}}_{1-y}{\mathrm{Se}}_y$). The two types of interfaces (namely, TI/mono-SL and TI/bi-SL) have different MPE, which is manifested as distinctively different transport behaviors. Specifically, the mono-SL induces a spin-flip transition with a sharp change at a small magnetic field in the anomalous Hall effect of the TI layers, while the bi-SL induces a spin-flop transition with a slow change at large field. Our work provides a useful platform to realize the full potential of the magnetic extension approach for pursuing novel topological physics and related device applications.

The perovskite ${\mathrm{YRuO}}_3$ containing low-spin $d^5{\mathrm{Ru}}^{3+}$ has a Pnma orthorhombic structure with elongated ${\mathrm{RuO}}_6$ octahedra evidencing orbital order and is insulating to low temperatures with a band gap of 70 meV. Canted antiferromagnetic ${\mathrm{Ru}}^{3+}$ spin order is observed below $T_{\mathrm{C}}=97\mathrm{K}$, and magnetization plateaus that emerge below 62 K reveal a quantum weak ferromagnetic state where 1/8 of the spins are reversed, reflecting extreme single-ion anisotropy resulting from the strong spin-orbit coupling of the $d^5{\mathrm{Ru}}^{3+}$ ion. Dynamic effects of the reversed spins give rise to an unusual negative AC susceptibility response, and a partial freezing of this motion occurs at 27 K.

The last stage of material failure often shows a regime with power-law acceleration as the material lifetime is approached. We study this experimentally in tensile creep with paper samples using digital image correlation. The last, tertiary creep stage exhibits scale-free features in the sample response (strain rate) and its fluctuations. It is accompanied by an increasing localization of strain at the location of final failure. The main features are reproduced by a material model built on a viscoelastic fiber bundle model.

Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under ${\mathrm{N}}_2$ atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for $\mathrm{T}{\mathrm{i}}^{3+}$ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local ${\mathrm{N}}_2$ concentration during annealing.

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We perform hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm $3d$ core-level spectra, we estimate the valence of Sm to be $\sim 2.96$, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm $5d$-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.

We present the shape transformation of a single layer of Au nanoparticles (NPs) when embedded in, and at the interface of, amorphous $\mathrm{Si}{\mathrm{N}}_{\mathrm{x}}$ and $\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$ ($\mathrm{a}\text{−}\mathrm{Si}{\mathrm{N}}_{\mathrm{x}}$ and $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$) thin films upon irradiation with 185-MeV Au ions to fluences ranging from 0.3 to $30\times {10}^{13}\mathrm{c}{\mathrm{m}}^{-2}$. Transmission electron microscopy (TEM) and high angular annular dark field microscopy were used to study the ion-shaping process. The former allows us to follow the overall change in geometry, size, and structure, while the latter reveals information about the relative position with respect to the interface. For Au NPs embedded in a single material, a lower elongation rate for $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{N}}_{\mathrm{x}}$ was found in comparison to $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$. When at the interface of the two materials, TEM reveals a preferential elongation towards $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$. The latter demonstrates the use of $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{N}}_{\mathrm{x}}$ for confining the ion-shaping process within an intermediate $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$ layer. The simulation of the temperature evolution during a single-ion impact was used to understand the difference in elongation rates between $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{N}}_{\mathrm{x}}$ and $\mathrm{a}\text{−}\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$, as well as the asymmetric behavior when located at the interface using the three-dimensional inelastic thermal spike model with bulk thermophysical properties. The calculations show good agreement with the experimental observations and reveal a correlation between the thermal profile and the resulting NP geometry.

The specific volumes and thermal expansion coefficients of 41 transition-metal based alloy liquids, which include both bulk and marginal glass-formers, are presented. Those parameters are compared with their values either in the corresponding crystal phases or in their constituent elemental liquids. The volume differences in both cases at the liquidus temperature, $T_l$, correlate well with the critical thicknesses, $d_{\mathrm{crit}}$ of the corresponding glass, taken either from literature or from estimates. The estimates are based on a recent study [Dai et al., J. Non-Cryst. Solids 525, 119673 (2019)], which requires knowledge of the liquid expansion coefficient and viscosity. The volume differences between the better glass-forming liquids and the crystal phases are smaller; they are larger when compared with the elemental liquids. The thermal expansion coefficients of the liquids correlate well with the cohesive energies and fragilities. Their differences with the crystal phases and estimates from elemental liquids correlate well with $d_{\mathrm{crit}}$. The differences are smaller for the former and larger for the latter for alloy liquids having larger values of $d_{\mathrm{crit}}$. This parameter then appears to be a prime indicator of glass formability. The results are explained in terms of changes in the anharmonicity and structural contributions to the expansion coefficients on alloying.

Due to the topological disorder, glass displays an anomalous vibrational density of states beyond the Debye model, i.e., formation of boson peaks, which is fundamental for understanding many glassy physical properties. However, the understanding of the boson peak remains notoriously complex and is a topic of hot debate. Here we report a universal quantitative relation between boson peak intensity and the Debye level of transverse phonons in different glasses, confirming the intrinsic link between boson peaks and transverse phonons. Moreover, an equation is derived for the boson peak intensity and Debye-Waller factor, indicating that boson peaks are fundamentally determined by the Debye-Waller factor. These findings could clarify some controversial issues and reveal a common basis for high-frequency boson peak dynamics $(\sim {10}^{12}\mathrm{Hz})$, short-time $\beta$ processes $({10}^3\sim {10}^6\mathrm{Hz})$, and long-time $\alpha$ processes $({10}^{-4}\sim {10}^3\mathrm{Hz})$ in disordered materials.

Ion irradiation experiments and quantitative x-ray diffraction analysis were used to demonstrate that the lattice and structure behave very differently in a complex oxide, $\delta \text{−}{\mathrm{Sc}}_4{\mathrm{Hf}}_3{\mathrm{O}}_{12}$, under irradiation. Analysis of the structure reveals that with increasing fluence, the characteristic $c/a$ ratio for the $\delta$ phase converges to that of an ideal fluorite phase, whereas locally it produces a metaphase consisting of frozen nanoscale structures having bixbyite-like flavor that increase the overall stability and radiation response of the apparent long-range oxygen-deficient average fluorite structure.

Two-dimensional (2D) ferromagnetic (FM) semiconductors with high Curie temperature have long been pursued for electronic and spintronic applications. Here we provide a general strategy to achieve robust FM state in bilayer $\mathrm{Cr}{\mathrm{I}}_3$ of the monoclinic stacking, which intrinsically has interlayer antiferromagnetic (AFM) order and weak in-plane FM coupling. We showed that the proximity effect from bulk semiconducting substrates induces electronic doping and significantly increases the FM nearest-neighbor exchange for bilayer $\mathrm{Cr}{\mathrm{I}}_3$, leading to the AFM-to-FM transition for the interlayer spin configuration as well as enhanced intralayer FM coupling. By first-principles calculations and Monte Carlo simulations, bulk and 2D semiconductors providing different interaction strengths from strong covalent bonding to weak van der Waals (vdW) interaction with $\mathrm{Cr}{\mathrm{I}}_3$ are compared to thoroughly address the substrate effect on magnetic behavior and Curie temperature of bilayer $\mathrm{Cr}{\mathrm{I}}_3$. These theoretical results offer a facile route for direct synthesis of 2D ferromagnets on proper semiconducting substrates to achieve high Curie temperature for device implementation.

Antiferromagnetically coupled magnetic skyrmions are considered ideal candidates for high-density information carriers. This is due to the suppressed skyrmion Hall effect compared to conventional skyrmions and a smaller size due to the cancellation of some contributions to the magnetostatic dipolar fields. By means of systematic first-principles calculations based on density functional theory we search for suitable materials that can host antiferromagnetically coupled skyrmions. We concentrate on fcc-stacked (111)-oriented metallic $Z/\mathrm{Co}/\mathrm{Pt}$ ($Z=4d$ series: Y–Pd, the noble metals: Cu, Ag, Au, post-noble metals: Zn and Cd) magnetic multilayers of films of monatomic thickness. We present quantitative trends of magnetic properties: magnetic moments, interlayer exchange coupling, spin stiffness, Dzyaloshinskii-Moriya interaction, magnetic anisotropy, and the critical temperature. We show that some of the $Z$ elements (Zn, Y, Zr, Nb, Tc, Ru, Rh, and Cd) can induce antiferromagnetic interlayer coupling between the magnetic Co layers, and that they influence the easy magnetization axis. Employing a multiscale approach, we transfer the micromagnetic parameters determined from ab initio to a micromagnetic energy functional and search for one-dimensional spin-spiral solutions and two-dimensional skyrmions. We determine the skyrmion radius by numerically solving the equation of the skyrmion profile. We found an analytical expression for the skyrmion radius that covers our numerical results and is valid for a large regime of micromagnetic parameters. Based on this expression we have proposed a model that allows to extrapolate from the ab initio results of monatomic films to multilayers with Co films consisting of several atomic layers containing $10\text{−}\mathrm{nm}$ skyrmions. We found thickness regimes where tiny changes of the film thickness may alter the skyrmion radius by orders of magnitude. We estimated the skyrmion size as function of temperature and found that the size can easily double going from cryogenic to room temperature. We suggest promising material systems for ferromagnetically and antiferromagnetically coupled spin-spiral and skyrmion systems.

Kitaev-type interactions between neighboring magnetic moments emerge in the honeycomb material $\alpha -\mathrm{Ru}{\mathrm{Cl}}_3$. It is debated, however, whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a $d^4$ triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperatures the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for $\alpha -\mathrm{Ru}{\mathrm{Cl}}_3$ with ferromagnetic Kitaev coupling.

We have studied a nearly stoichiometric insulating ${\mathrm{Y}}_{0.97\left(2\right)}{\mathrm{Cr}}_{0.98\left(2\right)}{\mathrm{O}}_{3.00\left(2\right)}$ single crystal by performing measurements of magnetization, heat capacity, and neutron diffraction. Albeit that the $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound behaves like a soft ferromagnet with a coersive force of $\sim 0.05$ T, there exist strong antiferromagnetic (AFM) interactions between ${\mathrm{Cr}}^{3+}$ spins due to a strongly negative paramagnetic Curie-Weiss temperature, i.e., $-433.2$(6) K. The coexistence of ferromagnetism and antiferromagnetism may indicate a canted AFM structure. The AFM phase transition occurs at $T_{\text{N}}=141.5\left(1\right)\mathrm{K}$, which increases to $T_{\text{N}}$(5 T) = 144.5(1) K at 5 T. Within the accuracy of the present neutron-diffraction studies, we determined a G-type AFM structure with a propagation vector k = (1 1 0) and ${\mathrm{Cr}}^{3+}$ spin directions along the crystallographic $c$ axis of the orthorhombic structure with space group $Pnma$ below $T_{\text{N}}$. At 12 K, the refined moment size is 2.45(6) ${\mu }_{\text{B}}, \sim 82\%$ of the theoretical saturation value $3{\mu }_{\text{B}}$. The ${\mathrm{Cr}}^{3+}$ spin interactions are probably two-dimensional Ising like within the reciprocal (1 1 0) scattering plane. Below $T_{\text{N}}$, the lattice configuration ($a$, $b$ , $c$ , and $V$ ) deviates largely downward from the Grüneisen law, displaying an anisotropic magnetostriction effect and a magnetoelastic effect. Especially, the sample contraction upon cooling is enhanced below the AFM transition temperature. There is evidence to suggest that the actual crystalline symmetry of $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound is probably lower than the currently assumed one. Additionally, we compared the $t_{2\text{g}}\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ and the $e_{\text{g}}{\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_3$ single crystals for a further understanding of the reason for the possible symmetry lowering.

We investigate acoustic emission (AE) that arises during the martensitic transition in a polycrystalline specimen of the prototypical superelastic/elastocaloric alloy ${\mathrm{Ni}}_{50.4}{\mathrm{Ti}}_{49.6}$ (at. %) driven using tensile strain. We use two independent AE sensors in order to locate AE events, and focus on contributions to the AE that arise away from the grips of the mechanical testing machine. Significant AE activity is present during the first mechanical loading primarily due to nucleation and growth of wide Lüders-like bands during the forward martensitic transition (imaged using visible light and infrared radiation) that lead to persistent changes in intergranular interactions. AE activity is suppressed during the subsequent reverse martensitic transition on unloading, and in successive loading/unloading cycles, for which the Lüders-like bands narrow and modify intergranular interactions to a much smaller extent. After the first loading, we find that the AE activity associated with the martensitic transition is weak, and we suggest that this is because the elastic anisotropy and strain incompatibility in Ni-Ti are low. We also find that the AE activity becomes weaker on mechanical cycling due to increased retained martensite.

We report the synthesis and characterization of polycrystalline ${\mathrm{Na}}_2{\mathrm{RuO}}_3$, a layered material in which the ${\mathrm{Ru}}^{4+}$ ($4d^4$ configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered ${\mathrm{Na}}_2{\mathrm{RuO}}_3$ ($C2/c$ phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism $[{\chi }_0\sim 1.42\left(2\right)\times {10}^{-3}$ emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (${\chi }_0$) together with the Sommerfeld coefficient of $\gamma =11.7\left(2\right)$ mJ/Ru mol ${\mathrm{K}}^2$ estimated from heat capacity measurements gives an enhanced Wilson ratio of $R_W\approx 8.9\left(1\right)$, suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that ${\mathrm{Na}}_2{\mathrm{RuO}}_3$ is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that ${\mathrm{Na}}_2{\mathrm{RuO}}_3$ is an antiferromagnetic insulator at low temperatures.

We report the experimental and theoretical studies of a magnetic topological nodal line semimetal candidate HoSbTe. Single crystals of HoSbTe are grown from Sb flux, crystallizing in a tetragonal layered structure (space group: $P4/nmm$, no. 129), in which the Ho-Te bilayer is separated by the square-net Sb layer. The magnetization and specific heat present distinct anomalies at ∼4 K related to an antiferromagnetic (AFM) phase transition. Meanwhile, with applying magnetic field perpendicular and parallel to the crystallographic $c$ axis, an obvious magnetic anisotropy is observed. Electrical resistivity undergoes a bad-metal-like state below 200 K and reveals a plateau at about 8 K followed by a drop due to the AFM transition. In addition, with the first-principle calculations of band structure, we find that HoSbTe is a topological nodal line semimetal or a weak topological insulator with or without taking the spin-orbit coupling into account, providing a platform to investigate the interplay between magnetic and topological fermionic properties.

Although crystallographic properties are considered to be intrinsic, several studies have reported significant different values for the lattice constants of the $\mathrm{N}{\mathrm{d}}_2\mathrm{F}{\mathrm{e}}_{14}\mathrm{B}$ phase. In this study, by analyzing rod-shaped isotropic (randomly oriented) and anisotropic ($c$-axis oriented) Nd-Fe-B bulk-sintered magnets, their powdered counterparts, and powder made from grinding a single crystal, we experimentally demonstrate that the lattice constants of the $\mathrm{N}{\mathrm{d}}_2\mathrm{F}{\mathrm{e}}_{14}\mathrm{B}$ phase in the ferromagnetic ordering state depend on the sample texture. The temperature dependence of the lattice constants was investigated by focusing on their expansion associated with ferromagnetic ordering, that is, the spontaneous linear expansion along the $a$-axis (${\lambda }_a$) and $c$-axis (${\lambda }_c$). The ${\lambda }_a$ value of the isotropic (anisotropic) bulk-sintered magnets at 105 K is 10% (2%) smaller while the ${\lambda }_c$ value is 130% (40%) greater than that of the powdered single crystal. These results indicate the presence of an internal stress perpendicular to the $c$-axis in the isotropic and anisotropic bulk-sintered magnets. In addition, we observed that the ratios of the length along the $a$-axis to that along the $c$-axis ($c/a$) for the various Nd-Fe-B magnets as well as those reported previously are correlated to the sample texture. In other words, the $c/a$ values of the sintered magnets are greater than those of the powdered samples. This result suggests that the sample texture and hence the internal stress has a determining effect on the lattice constants.