In a current-driven ferromagnetic resonance (FMR) system, heat generated by time-dependent magnetoresistance effects, caused by magnetization precession, cannot be overlooked. Here, we describe the generated heat by magnetization motion under electric current in a freestanding nanoelectromechanical resonator fashioned from a permalloy $(\mathrm{Py})/\mathrm{Pt}$ bilayer. By piezoresistive transduction of Pt, the mechanical mode is electrically detected at room temperature and the internal heat in Py excluding thermoelectric effects is quantified as a shift of the mechanical resonance. We find that the measured spectral shifts correspond to the FMR, which is further verified from the spin-torque FMR measurement. Furthermore, the angular dependence of the mechanical reaction on an applied magnetic field reveals that the full accounting of FMR heat dissipation requires the time-dependent magnetoresistance effect.

We report on magnetization, magnetostriction, and magnetocaloric-effect measurements of polycrystalline ${\mathrm{LaFe}}_{11.74}{\mathrm{Co}}_{0.13}{\mathrm{Si}}_{1.13}$ and ${\mathrm{LaFe}}_{11.21}{\mathrm{Co}}_{0.65}{\mathrm{Si}}_{1.11}$ performed in both pulsed and static magnetic fields. Although the two compounds behave rather differently at low fields ($\sim 5\mathrm{T}$), they show quite similar values of the magnetocaloric effect, namely a temperature increases of about 20 K at high fields (50–60 T). The magnetostriction and magnetization also reach very similar values here. We are able to quantify the magnetoelastic coupling and, based on that, apply the Bean-Rodbell criterion distinguishing first- and second-order transitions.

Circuit theory has been tremendously successful in translating physical equations into circuit elements in an organized form for further analysis and proposing creative designs for applications. With the advent of new materials and phenomena in the field of spintronics and nanomagnetics, it is imperative to construct the spin-circuit representations for different materials and phenomena. Spin pumping is a phenomenon by which a pure spin current can be injected into the adjacent layers. If the adjacent layer is a material with a high spin-orbit coupling, a considerable amount of charge voltage can be generated via the inverse spin Hall effect allowing spin detection. Here we develop the spin-circuit representation of spin pumping. We then combine it with the spin-circuit representation for the materials having spin Hall effect to show that it reproduces the standard results as in the literature. We further show how complex multilayers can be analyzed by simply writing a netlist.

The synthetic magnonic crystals (i.e., periodic composites consisting of different magnetic materials) form one fascinating class of emerging research field, which aims to command the process and flow of information by means of spin waves, such as in magnonic waveguides. One of the intriguing features of magnonic crystals is the presence and tunability of band gaps in the spin-wave spectrum, where the high attenuation of the frequency bands can be utilized for frequency-dependent control on the spin waves. However, to find a feasible way of band tuning in terms of a realistic integrated device is still a challenge. Here, we introduce an array of asymmetric saw-tooth-shaped width-modulated nanoscale ferromagnetic waveguides forming a pseudo-one-dimensional magnonic crystal. The frequency dispersion of collective modes measured by the Brillouin light-scattering technique is compared with the band diagram obtained by numerically solving the eigenvalue problem derived from the linearized Landau-Lifshitz magnetic torque equation. We find that the magnonic band-gap width, position, and the slope of dispersion curves are controllable by changing the angle between the spin-wave propagation channel and the magnetic field. The calculated profiles of the dynamic magnetization reveal that the corrugation at the lateral boundary of the waveguide effectively engineers the edge modes, which forms the basis of the interactive control in magnonic circuits. The results represent a prospective direction towards managing the internal field distribution as well as the dispersion properties, which find potential applications in dynamic spin-wave filters and magnonic waveguides in the gigahertz frequency range.

We report a systematic study of complex pattern formation resulting from the driven dynamics of single-layer homoepitaxial islands on surfaces of face-centered-cubic (fcc) crystalline conducting substrates under the action of an externally applied electric field. The analysis is based on an experimentally validated nonlinear model of mass transport via island edge atomic diffusion, which also accounts for edge diffusional anisotropy. We analyze the morphological stability and simulate the field-driven evolution of rounded islands for an electric field oriented along the fast edge diffusion direction. For larger-than-critical island sizes on $\{110\}$ and $\{100\}$ fcc substrates, we show that multiple necking instabilities generate complex island patterns, including not-simply-connected void-containing islands mediated by sequences of breakup and coalescence events and distributed symmetrically with respect to the electric field direction. We analyze the dependence of the formed patterns on the original island size and on the duration of application of the external field. Starting from a single large rounded island, we characterize the evolution of the number of daughter islands and their average size and uniformity. The evolution of the average island size follows a universal power-law scaling relation, and the evolution of the total edge length of the islands in the complex pattern follows Kolmogorov-Johnson-Mehl-Avrami kinetics. Our study makes a strong case for the use of electric fields, as precisely controlled macroscopic forcing, toward surface patterning involving complex nanoscale features.

Recent experiments on spin-torque oscillators have revealed interactions between multiple magnetodynamic modes, including mode coexistence, mode hopping, and temperature-driven crossover between modes. The initial multimode theory indicates that a linear coupling between several dominant modes, arising from the interaction of the subdynamic system with a magnon bath, plays an essential role in the generation of various multimode behaviors, such as mode hopping and mode coexistence. In this work, we derive a set of rate equations to describe the dynamics of coupled magnetodynamic modes in a nanocontact spin-torque oscillator. Expressions for both linear and nonlinear coupling terms are obtained, which allow us to analyze the dependence of the coupled dynamic behaviors of modes on external experimental conditions as well as intrinsic magnetic properties. For a minimal two-mode system, we further map the energy and phase difference of the two modes onto a two-dimensional phase space and demonstrate in the phase portraits how the manifolds of periodic orbits and fixed points vary with an external magnetic field as well as with the temperature.

We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a $(111{)}_{\mathrm{pc}}\text{−}{\mathrm{BiFeO}}_3$ epitaxial-film device at room temperature by nonresonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed $(111{)}_{\mathrm{pc}}\text{−}{\mathrm{BiFeO}}_3$ epitaxial films consisting of a single ferroelectric domain are found to comprise a submicron-scale mosaic of magnetoelastic domains, all sharing a common direction of the magnetic polarity, which is found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magnetoelastic domain population. A real-space polarimetry map of our device clearly distinguishes between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in ${\mathrm{BiFeO}}_3$ is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90° at every cycle.

We investigate the radiative and nonradiative recombination processes in planar $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{GaN}(0001)$ quantum wells and (In,Ga)N quantum disks embedded in $\mathrm{GaN}(000\overline{1})$ nanowires using photoluminescence (PL) spectroscopy under both continuous-wave and pulsed excitation. The PL intensities of these two samples quench only slightly between 10 and 300 K, which is commonly taken as evidence for high internal quantum efficiencies. However, a side-by-side comparison shows that the absolute intensity of the Ga-polar quantum wells is two orders of magnitude higher than that of the N-polar quantum disks. A similar difference is observed for the initial decay time of PL transients obtained by time-resolved measurements, indicating the presence of a highly efficient nonradiative decay channel for the quantum disks. In apparent contradiction to this conjecture, the decay of both samples is observed to slow down dramatically after the initial rapid decay. Independent of temperature, the transients approach a power law for longer decay times, reflecting the fact that recombination occurs between individual electrons and holes with varying spatial separation. Employing a coupled system of stochastic integro-differential equations taking into account both radiative and nonradiative Shockley-Read-Hall recombination of spatially separate electrons and holes as well as their diffusion, we obtain simulated transients matching the experimentally obtained ones. The results reveal that even dominant nonradiative recombination conserves the power-law decay for $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{GaN}\{0001\}$ quantum wells and disks.

We report the design, analysis, and characterization of a three-inductor radio-frequency identification (RFID) and transceiver system for potential applications in individual cell tracking and monitoring. The RFID diameter is $22\mu \mathrm{m}$ and can be naturally internalized by living cells. Using magnetic resonance coupling, the system shows resonance shifts when the RFID is present and also when the RFID loading capacitance changes. It operates at 60 GHz with a high signal magnitude up to $-50\mathrm{dB}$ and a sensitivity of 0.2. This miniaturized RFID with a high signal magnitude is a promising step toward continuous, real-time monitoring of activities at cellular levels.

The importance of considering near-field effects on photon recycling and spontaneous emission in a thermophotovoltaic device is investigated. Fluctuational electrodynamics is used to calculate external luminescence from a photovoltaic cell as a function of emitter type, vacuum gap thickness between the emitter and cell, and cell thickness. The observed changes in external luminescence suggest strong modifications of photon recycling caused by the presence of the emitter. Photon recycling for propagating modes is affected by reflection at the vacuum-emitter interface and is substantially decreased by the leakage towards the emitter through the tunneling of frustrated modes. In addition, spontaneous emission by the cell can be strongly enhanced by the presence of an emitter supporting surface-polariton modes. It follows that using a radiative recombination model with a spatially uniform radiative lifetime, even corrected by a photon-recycling factor, is inappropriate. Applying the principles of detailed balance and accounting for nonradiative recombination mechanisms, the impact of external luminescence enhancement in the near field on thermophotovoltaic performance is investigated. It is shown that unlike isolated cells, the external luminescence efficiency is not solely dependent on cell quality but significantly increases as the vacuum gap thickness decreases below 400 nm for the case of an intrinsic silicon emitter. In turn, the open-circuit voltage and power density benefit from this enhanced external luminescence toward the emitter. This benefit is larger as cell quality, characterized by the contribution of nonradiative recombination, decreases.

The influence of losses in double-negative metamaterial slabs recently introduced by Graciá-Salgado et al. [Phys. Rev. B 88, 224305 (2013)] is comprehensively studied. Viscous and thermal losses are considered in the linearized Navier-Stokes equations with no flow. Despite the extremely low thicknesses of boundary layers associated with each type of losses, the double-negative behavior is totally suppressed for the rigid structures under analysis. In other words, almost 100% of the energy transmitted into the slab is dissipated by viscothermal effects, in agreement with experimental data. Simulations undertaken for larger structures, using scale factors of up to 20 times, show that double-negative behavior is never recovered. The huge dissipation obtained by these structures leads us to propose them as interesting alternatives to conventional absorbers for specific situations, e.g., when treating low frequencies or when the excitation is narrow banded.

We investigate atom-based electric-field calibration and polarization measurement of a 100-MHz linearly polarized radio-frequency (rf) field using cesium Rydberg-atom electromagnetically induced transparency in a room-temperature vapor cell. The calibration method is based on matching experimental data with the results of a theoretical Floquet model. The utilized $60D_J$ fine-structure Floquet levels exhibit $J$- and $m_j$-dependent ac Stark shifts and splittings, and they develop even-order rf-modulation sidebands. The Floquet map of cesium $60D_J$ fine-structure states exhibits a series of exact crossings between states of different $m_j$’s which are not rf coupled. These exact level crossings are employed to perform a rapid and precise ($\pm 0.5\%$) calibration of the rf electric field. We also map out three series of narrow avoided crossings between fine-structure Floquet levels of equal $m_j$’s and different $J$’s, which are weakly coupled by the rf field via a Raman process. The coupling leads to narrow avoided crossings that can also be applied as spectroscopic markers for rf-field calibration. We further find that the line-strength ratio of intersecting Floquet levels with different $m_j$’s provides a fast and robust measurement of the rf field’s polarization.

We explore a photonic-integrated-circuit platform that implements optical-frequency measurements and timekeeping with a perspective towards next-generation portable and spaceborne frequency references and optical-clock networks. The stoichiometric-silicon-nitride waveguides we create provide an efficient and low-noise medium for nonlinear spectral broadening and supercontinuum generation with fiber-based optical-frequency combs. In particular, we demonstrate detailed control over supercontinuum emission to target specific atomic-transition wavelengths and perform an optical-clock comparison using on-chip supercontinuum sources. We report a clock-limited relative frequency instability of $3.8\times {10}^{-15}$ at $\tau =2\mathrm{s}$ between a 1550-nm cavity-stabilized reference laser and NIST’s calcium atomic-clock laser at 657 nm using a two-octave waveguide-supercontinuum frequency comb.

The band offsets between crystalline and hydrogenated amorphous silicon ($a\text{−}\mathrm{Si}:\mathrm{H}$) are key parameters governing the charge transport in modern silicon heterojunction solar cells. They are an important input for macroscopic simulators that are used to further optimize the solar cell. Past experimental studies, using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurements, have yielded conflicting results on the band offset. Here, we present a computational study on the band offsets. It is based on atomistic models and density-functional theory (DFT). The amorphous part of the interface is obtained by relatively long DFT first-principles molecular-dynamics runs at an elevated temperature on 30 statistically independent samples. In order to obtain a realistic conduction-band position the electronic structure of the interface is calculated with a hybrid functional. We find a slight asymmetry in the band offsets, where the offset in the valence band (0.29 eV) is larger than in the conduction band (0.17 eV). Our results are in agreement with the latest XPS measurements that report a valence-band offset of 0.3 eV [M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015)].

Using aluminum nitride photonic-chip waveguides, we generate optical-frequency-comb supercontinuum spanning from 500 to 4000 nm with a 0.8-nJ seed pulse, and we show that the spectrum can be tailored by changing the waveguide geometry. Since aluminum nitride exhibits both quadratic and cubic nonlinearities, the spectra feature simultaneous contributions from numerous nonlinear mechanisms: supercontinuum generation, difference-frequency generation, second-harmonic generation, and third-harmonic generation. As one application of integrating multiple nonlinear processes, we measure and stabilize the carrier-envelope-offset frequency of a laser comb by direct photodetection of the output light. Additionally, we generate approximately 0.3 mW of broadband light in the 3000- and 4000-nm spectral region, which is potentially useful for molecular spectroscopy. The combination of broadband light generation from the visible through the midinfrared, combined with simplified self-referencing, provides a path towards robust comb systems for spectroscopy and metrology in the field.

We report on a comprehensive study of the interlayer exchange coupling in $\mathrm{Co}\text{−}\mathrm{Fe}(5\mathrm{nm})/\mathrm{Ru}(t)/\mathrm{Co}\text{−}\mathrm{Fe}(8\mathrm{nm})$ trilayers $(t=0.8,\dots ,2.8\mathrm{nm})$ using broadband ferromagnetic resonance. A systematic frequency dependence of the field separation between the acoustic and optic modes is found, which is caused by different effective magnetizations of the two ferromagnetic layers. Hence, it is shown that the broadband measurements are vital for reducing the systematic error margins in the determination of interlayer exchange coupling using ferromagnetic resonance. We also investigate the temperature dependence of the interlayer exchange coupling and compare our results with existing theories. It is shown that models which take into account the temperature dependence due to thermal excitations of spin waves within the ferromagnetic layers have a considerably better agreement with the experiment than models solely based on spacer and interface contributions to the temperature dependence.

We extend a previously developed ultrafast ultrasonic technique [T. Gallot et al., Rev. Sci. Instrum. 84, 045107 (2013)] to concentration-field measurements in non-Brownian particle suspensions under shear. The technique provides access to time-resolved concentration maps within the gap of a Taylor-Couette cell simultaneously to local velocity measurements and standard rheological characterization. Benchmark experiments in homogeneous particle suspensions are used to calibrate the system. We then image heterogeneous concentration fields that result from centrifugation effects, from the classical Taylor-Couette instability, and from sedimentation or shear-induced resuspension.

The voltage signal driven by a microwave at the nonresonant condition has been systematically studied in well-designed single-crystalline Fe and $\mathrm{Fe}/\mathrm{Pt}$ bilayers. The voltage signals at the nonresonant condition in Fe and $\mathrm{Fe}/\mathrm{Pt}$ show different angular dependence on the magnetization orientation. Through theoretical simulation and experiments of varying sample structures and field strength, it can be concluded that the voltage signal at the nonresonant condition consists of the thermal effects including the anomalous Nernst effect and the spin Seebeck effect, and the inverse spin Hall effect induced by spin pumping. Our results indicate that the microwave-induced signal at the nonresonant conditions may enable the device operation in the absence of any external magnetic field, which is beneficial for practical applications.

Square arrays of submicrometer columnar defects in thin ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films with spacings down to 300 nm are fabricated by a He ion-beam projection technique. Pronounced peaks in the critical current and corresponding minima in the resistance demonstrate the commensurate arrangement of flux quanta with the artificial pinning landscape, despite the strong intrinsic pinning in epitaxial YBCO films. While these vortex-matching signatures are exactly at the predicted values in field-cooled experiments, they are displaced in zero-field-cooled, magnetic-field-ramped experiments, conserving the equidistance of the matching peaks and minima. These observations reveal an unconventional critical state in a cuprate superconductor with an artificial, periodic pinning array. The long-term stability of such out-of-equilibrium vortex arrangements paves the way for electronic applications employing fluxons.