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Magnetic frustration can produce exotic spin configurations and dynamics. Here, the authors explore the seemingly simple competition between ferromagnetic nearest- and antiferromagnetic next-nearest neighbor interactions along Ising spin chains, as realized in the magnetoelectric compound Lu${}_{2}$CoMnO${}_{6}$. Commonly, this situation is described by the axial next-nearest-neighbor Ising model. For the first time, the authors measure and calculate its correlated magnetic dynamics, resulting from a characteristic fractal set of first-order phase boundaries. Experiments and Monte Carlo simulations reveal a dynamics slowdown while approaching the phase transition regime.

4f spin to phonon coupling is crucial for ultrafast spin-wave generation and magnetization dynamics in rare-earth metals., Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin–lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

X-ray scattering and transmission electron microscopy carried out on unpurified soot recovered from open air detonations of a liquid explosive mixture of nitromethane (95 (v/v) % and diethylenetriamine 5 (v/v) % sensitizer determined primary particles possessing a hollow spherical core (davg. = 6.9 nm) coated in a sp2-hybridized carbon shell of∼4.2 nm thick. Similar nanophase carbons have been previously reported in studies on closed chamber detentions of composition B in air. Given the noted changes in charge size, explosive composition, and peak temperature and pressure between the two detonations, we hypothesize the formation of the nanocarbons are attributed to excess H2O (detonation byproduct and/or atmospherically-derived) and atmospherically-derived O2. The H2O is hypothesized to be the determining component leading to the formation of non-planar (curved), sp2-hybridized carbon frameworks, necessary for assembly into spherical particles. Excess O2 produces after-burn, an oxidizing environment sufficient to possibly create the interior voids.

Room‐temperature magnetoelectric (ME) coupling is developed in artificial multilayers and nanocomposites composed of magnetostrictive and electrostrictive materials. While the coupling mechanisms and strengths in multilayers are widely studied, they are largely unexplored in vertically aligned nanocomposites (VANs), even though theory has predicted that VANs exhibit much larger ME coupling coefficients than multilayer structures. Here, strong transverse and longitudinal ME coupling in epitaxial BaTiO3:CoFe2O4 VANs measured by both optical second harmonic generation and piezoresponse force microscopy under magnetic fields is reported. Phase field simulations have shown that the ME coupling strength strongly depends on the vertical interfacial area which is ultimately controlled by pillar size. The ME coupling in VANs is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. The revealed mechanisms shed light on the physical insights of vertical interface coupling in VANs in general, which can be applied to a variety of nanocomposites with different functionalities beyond the studied ME coupling effect.

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (${V}_{\mathrm{O}}$) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through ${V}_{\mathrm{O}}$ variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning both magnetism and ferroelectricity simultaneously by using ${V}_{\mathrm{O}}$. Combining experimental and density-functional theory studies of $\mathrm{E}{\mathrm{u}}_{0.5}\mathrm{B}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$, we demonstrate that oxygen vacancies create $\mathrm{T}{\mathrm{i}}^{3+}\phantom{\rule{4pt}{0ex}}3{d}^{1}$ defect states, mediating the ferromagnetic coupling between the localized Eu $4{f}^{7}$ spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the $\mathrm{E}{\mathrm{u}}_{0.5}\mathrm{B}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$.

In-service cables such as stay cables and suspenders of cable-stayed bridges and suspension bridges, are subjected to dynamic loads (e.g., the vehicle loads and wind excitation). Performing vibration measurements and subsequently identifying the dynamic properties and establishing a dynamic model of cable vibration are essential for their dynamic analysis, condition assessment, and performance prediction. For example, based on the taut-string theory, the cable tension, as a critical indicator of cable performance and health state, can be computed using its frequency that can be identified from the measured cable vibration responses. Traditional contact-type wired or wireless sensors, such as accelerometers and strain gauge sensors, require physically attaching to the structure for vibration measurements, which could induce the mass effect. In addition, installing these sensors on structures is costly, time-consuming, and allows instrumentations at a limited number of places. On the other hand, digital video cameras have emerged as a cost effective and agile non-contact vibration measurement method, offering high-resolution, simultaneous, measurements. Recently, digital video camera measurements processed by advanced computer vision and machine learning algorithms have been successfully used for experimental and operational full-field vibration measurement and modal analysis. This study develops a video measurement and processing based technique that can autonomously and blindly extract the full-field dynamic parameters of cable vibration from the video measurements. In addition, by exploiting the taut string theory, full-order (as many modes as possible) dynamic parameters are also extracted. Therefore, a full-field, full-order dynamic (modal) model of cable vibration is established. Laboratory experiments are conducted to validate the developed approach.

Intense, single-cycle terahertz (THz) pulses offer a promising approach for understanding and controlling the properties of a material on an ultrafast time scale. In particular, resonantly exciting phonons leads to a better understanding of how they couple to other degrees of freedom in the material (e.g., ferroelectricity, conductivity and magnetism) while enabling coherent control of lattice vibrations and the symmetry changes associated with them. However, an ultrafast method for observing the resulting structural changes at the atomic scale is essential for studying phonon dynamics. A simple approach for doing this is optical second harmonic generation (SHG), a technique with remarkable sensitivity to crystalline symmetry in the bulk of a material as well as at surfaces and interfaces. This makes SHG an ideal method for probing phonon dynamics in topological insulators (TI), materials with unique surface transport properties. Here, we resonantly excite a polar phonon mode in the canonical TI Bi$_2$Se$_3$ with intense THz pulses and probe the subsequent response with SHG. This enables us to separate the photoinduced lattice dynamics at the surface from transient inversion symmetry breaking in the bulk. Furthermore, we coherently control the phonon oscillations by varying the time delay between a pair of driving THz pulses. Our work thus demonstrates a versatile, table-top tool for probing and controlling ultrafast phonon dynamics in materials, particularly at surfaces and interfaces, such as that between a TI and a magnetic material, where exotic new states of matter are predicted to exist.

Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which have however been elusive due to the experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of h-LuFeO3 (001) films. The results indicate that a compressive biaxial strain significantly enhances the ferrodistortion, and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced ferrodistortion together, cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO3, according to our first principle calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO3. The experimental elucidation of the strain effect in h-LuFeO3 films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other epitaxial thin film materials.

Here, we demonstrate an approach for directly tracking antiferromagnetic (AFM) spin dynamics by measuring ultrafast changes in a magnon resonance. We also test this idea on the multiferroic HoMnO 3 by optically photoexciting electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced change in the magnon line shape that builds up over 5–12 picoseconds, which we show to be the spin-lattice thermalization time, indicating that electrons heat the spins via phonons. We compare our results to previous studies of spin-lattice thermalization in ferromagnetic manganites, giving insight into fundamental differences between the two systems. Finally, our work sheds light on the microscopic mechanism governing spin-phonon interactions in AFMs and demonstrates a powerful approach for directly monitoring ultrafast spin dynamics.

Lu${}_{2}$MnCoO${}_{6}$ is a rare example of a double-perovskite material exhibiting multiferroic behavior -- coupling of magnetic and electric order -- and a rare example of hysteretic coupling in a bulk material. Here, the authors lay to rest questions about whether it occurs in single crystals and verify that it is an intrinsic effect. The Mn and Co spins are arranged in a frustrated up-up-down-down ordering along the $c$ axis. Data from single crystals show that exchange striction produces electric polarization, albeit with a twist. Unlike other `up up down down' multiferroics, the ferroelectricity emerges orthogonal to the magnetic ordering easy axis. This may be explained by a model taking into account the oxygen bond distortion in Lu${}_{2}$MnCoO${}_{6}$. Consequently, the system has a rich phase diagram: a dielectric response emerges below the magnetic ordering temperature, whereas a hysteretic polarization emerges in the region of magnetic hysteresis, as shown in this paper.

The energy and momentum selectivity of time- and angle-resolved photoemission spectroscopy is exploited to address the ultrafast dynamics of the antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial thin films of chromium. We are able to quantitatively extract the evolution of the SDW order parameter $\Delta$ through the ultrafast phase transition. $\Delta$ is defined by the transient temperature of the thermalized electron gas. The complete destruction of SDW order on a sub-100~fs time scale is observed, much faster than for conventional charge density wave materials. Our results reveal that equilibrium concepts for phase transitions such as the order parameter may be utilized even in the strongly non-adiabatic regime of ultrafast photo-excitation., Comment: 7 pages, 7 figures

We investigated ultrafast strain dynamics at the BiFeO3 (BFO) / La0.3Sr0.7MnO3 (LSMO) interface using time resolved second harmonic generation (trSHG). The dynamics are driven by the strain in a small boundary layer at the interface.

Ultrafast optical spectroscopy was used to study carrier dynamics in the large magnetoresistance material WTe2. Our study reveals relaxation processes due to electron-phonon thermalization and phonon-assisted electron-hole recombination, providing insights into the large magnetoresistance mechanism.

We demonstrate a powerful, table-top approach for directly visualizing crystal lattice dynamics using optical second harmonic generation after intense terahertz photoexcitation of a specific phonon mode in the topological insulator Bi 2 Se 3 .

We demonstrate a powerful, table-top approach for directly visualizing crystal lattice dynamics using optical second harmonic generation after intense terahertz photoexcitation of a specific phonon mode in the topological insulator Bi2Se3.

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

Cryogenic molecular beam experiments show that the ferroelectric dipole moments of small niobium clusters with an even number of valence electrons n are typically greater than those with odd n. This is verified in alloy clusters NbNXM where X=Al, Au, O, Mn, Fe, and Co; N≤100;M≤3. Like in superconducting alloys, Mn doping quenches the effect while Al and Au enhance it, suggesting a relation between cluster ferroelectricity and bulk superconductivity. A correlated ground state is proposed where the even-odd effect is caused by the depolarizing effect of a single unpaired electron.

Ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large magnetoresistance material WTe$_{2}$. Our experiments reveal a fast relaxation process occurring on a sub-picosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of $\sim$5-15 picoseconds, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the timescale governing this process increases due to the reduction of the phonon population. However, below $\sim$50 K, an unusual decrease of the recombination time sets in, most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material., 6 pages, 4 figures

Hexagonal LuFeO 3 is a multiferroic showing ferroelectricity and antiferromagnetism at room temperature. Below T R =130 K, it becomes ferrimagnetic. Ultrafast optical pump-probe measurements reveal that the dynamics are correlated with the structural changes responsible for ferrimagnetism.

Rare-earth metals in their bulk form possess rather similar crystallographic structures, which is due to the very similar features of their outer electronic states. On the other hand, their magnetic properties are of rich variety, which is related to the specific form of the indirect magnetic exchange interaction between the inner electronic shells. In cluster form, this interplay may lead to very unusual magnetic structures. Here we show how the magnetic moments vary with size and temperature in Tm and Pr clusters. While in Pr clusters clear atom-by-atom oscillations indicate antiferromagnetic ordering, smooth variation and anomalous temperature behavior in Tm is representative for an essentially non-collinear spin arrangement. Their electric behavior is also very different, with a metallic-like behavior of Pr and localized electronic states in Tm.

The Heisenberg–Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale., Due the strength of the intra-atomic exchange interaction, it is generally assumed that alignment of spin moments in intra-atomic orbitals is quasi-instantaneous. Here, the authors demonstrate the breakdown of this relation between the 4f and 5d electrons in gadolinium following ultrashort optical excitation.

We have studied the response of the exchange split valence bands of ferromagnetic gadolinium tofemtosecond laser excitation. We observe a drop of the exchange splitting with a time constant of 0.9 ps but different response times of minority and majority spin bands. Furthermore, even above the Curie temperature there is a finite exchange splitting, which also decreases with laser excitation.

Simultaneous magnetic and electric deflection measurements of rhodium clusters (Rh(N), 6 ≤ N ≤ 40) reveal ferromagnetism and ferroelectricity at low temperatures, while neither property exists in the bulk metal. Temperature-independent magnetic moments (up to 1 μ(B) per atom) are observed, and superparamagnetic blocking temperatures up to 20 K. Ferroelectric dipole moments on the order of 1D with transition temperatures up to 30 K are observed. Ferromagnetism and ferroelectricity coexist in rhodium clusters in the measured size range, with size-dependent variations in the transition temperatures that tend to be anticorrelated in the range n = 6-25. Both effects diminish with size and essentially vanish at N = 40. The ferroelectric properties suggest a Jahn-Teller ground state. These experiments represent the first example of multiferroic behavior in pure metal clusters.

We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications., Comment: 4+ pages, 3 figures. Submitted

Small lanthanide clusters have interesting magnetic properties, but their structures are unknown. We have identified the structures of small terbium cluster cations Tbn+ (n = 5-9) in the gas phase, by analysis of their vibrational spectra. The spectra have been measured via IR multiple photon dissociation of their complexes with Ar atoms in the 50-250 1/cm range with an infrared free electron laser. Density functional theory calculations using a 4f-in-core effective core potential (ECP) accurately reproduce the experimental far-IR spectra. The ECP corresponds to a 4f85d16s2 trivalent configuration of terbium. The assigned structures are similar to those observed in several other transition metal systems. From this, we conclude that the bonding in Tb clusters is through the interactions between the 5d and 6s electrons, and that the 4f electrons have only an indirect effect on the cluster structures.

We investigate the fluence-dependent dynamics of the exchange-split 5d6s valence bands of Gd metal after femtosecond, near-infrared (IR) laser excitation. Time- and angle-resolved photoelectron spectroscopy (tr-ARPES) with extreme ultraviolet (XUV) probe pulses is used to simultaneously map the transient binding energies of the minority and majority spin valence bands. The decay constant of the exchange splitting increases with fluence. This reflects the slower response of the occupied majority-spin component, which we attribute to Elliot–Yafet spin-flip scattering in accordance with the microscopic three-temperature model (M3TM). In contrast, the time constant of the partly unoccupied minority-spin band stays unaffected by a change in pump fluence. Here, we introduce as an alternative to superdiffusive spin transport exchange scattering, which is an ultrafast electronic mechanism explaining the observed dynamics. Exchange scattering can reduce the spin polarization in the partially unoccupied minority-spin band and thus its energetic position without effective demagnetization.

Cryogenic cluster beam experiments have provided crucial insights into the evolution of the metallic state from the atom to the bulk. Surprisingly, one of the most fundamental metallic properties, the ability of a metal to efficiently screen electric fields, is still poorly understood in small clusters. Theory has predicted that many small Na clusters are unable to screen charge inhomogeneities and thus have permanent dipole moments. High precision electric deflection experiments on cryogenically cooled Na$_N$ ($N, 4 pages, 3 figures

Stern–Gerlach deflection measurements have been performed on rare earth clusters TbN, HoN, and TmN (N≤40) at cryogenic temperatures (T≤77 K). TbN and HoN share a common size dependence in their magnetic moments. They both exhibit common “magic number” sizes which show reduced net magnetic moments, similar to previous observations for Gd and Dy clusters. TmN have smaller magnetic moments that do not differ significantly between cluster sizes. The reduced net magnetic moments are evidence that the atomic moments are canceled by a canted or antiferromagnetic alignment. Electric deflection experiments reveal that TmN have electric dipole moments and show an enhanced response to an electric field compared to TbN and HoN.

The magnetic moments and electric dipoles of Tb and Pr clusters are investigated using the Stern–Gerlach deflection technique. The addition of a single oxygen atom induces an increase in the electric dipole of TbN clusters, however the magnetic moment is largely not affected. In Pr neither the magnetic moment nor the electric dipole is affected. This raises questions as to the role of conduction electrons in the exchange interaction of rare earth clusters, and puts into doubt the validity of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism in small systems.

Electric deflections of niobium clusters in molecular beams show that they have permanent electric dipole moments at cryogenic temperatures but not higher temperatures, indicating that they are ferroelectric. Detailed analysis shows that the deflections cannot be explained in terms of a rotating classical dipole, as claimed by Anderson et al. The shapes of the deflected beam profiles and their field and temperature dependences indicates that the clusters can exist in two states, one with a dipole and the other without. Cluster with dipoles occupy lower energy states. Excitations from the lower states to the higher states can be induced by low fluence laser excitation. This causes the dipole to vanish., 25 pages, 7 figures