arXiv subjects

Axel Hoffmann

Publications and source records attributed to Axel Hoffmann.

At least 55 records · Page 3Linked to original sources

arXiv2020

Strong near-field light-matter interaction in plasmon-resonant tip-enhanced Raman scattering in indium nitride

We report a detailed study of the strong near-field Raman scattering enhancement which takes place in tip-enhanced Raman scattering (TERS) in indium nitride. In addition to the well-known first-order optical phonons of indium nitride, near-field Raman modes, not detectable in the far-field, appear when approaching the plasmonic probe. The frequencies of these modes coincide with calculated energies of second order combinational modes consisting of optical zone center phonons and acoustic phonons at the edge of the Brillouin zone. The appearance of strong combinational modes suggests that TERS in indium nitride represents a special case of Raman scattering in which a resonance condition on the nanometer scale is achieved between the localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs) of the probe with the surface charge oscillation of the material. We suggest that the surface charge accumulation (SCA) in InN, which can render the surface a degenerate semiconductor, is the dominating reason for the unusually large enhancement of the TERS signal as compared to other inorganic semiconductors. Thus, the plasmon-resonant TERS (PR-TERS) process in InN makes this technique an excellent tool for defect characterization of indium-rich semiconductor heterostructures and nanostructures with high carrier concentrations.

cond-mat.mtrl-sci↗

arXiv2020

Dynamic Excitations of Chiral Magnetic Textures

Spin eigenexcitations of skyrmions and related chiral magnetic textures have attracted considerable interest over the recent years owing to their strong potential for applications in information processing and microwave devices. The emergence of novel material systems, such as synthetic ferri- and antiferromagnets, the continuing progress in micro- and nanofabrication techniques, and the development of more sophisticated characterization methods will undoubtedly provide a further boost to this young particular line of research. This Perspective summarizes the most significant advances during the past years and indicates future directions of both theoretical and experimental work.

cond-mat.mes-hall↗

arXiv2020

Direct Imaging of Resonant Phonon-Magnon Coupling

Direct detection of phonons is important for the investigation of information interconversion between the resonantly coupled magnons and phonons. Here we report resonant coupling of magnons and phonons, which can be directly visualized by using micro focused Brillouin light scattering in Ni/LiNbO3 hybrid heterostructures. The patterns of surface acoustic wave phonons, originating from the interference between the original wave ψ0(A_0,k) and reflected wave ψ1(A_1,-k), can be modulated by magnetic field due to the magnon-phonon coupling. By analyzing the information of phonons obtained from Brillouin spectroscopy, the properties of the magnon system (Ni film), e.g., ferromagnetic resonance field and resonance linewidth can be determined. The results provide spatially resolved information about phonon manipulation and detection in a coupled magnon-phonon system.

physics.app-ph↗

arXiv2020

Coupled skyrmion breathing modes in synthetic ferri- and antiferromagnets

We present micromagnetic simulations of the dynamic GHz-range resonance modes of skyrmions excited by either out-of-plane ac magnetic fields or spin torques in prototypical synthetic ferri- and antiferromagnetic trilayer structures. The observed features in the calculated power spectra exhibit a systematic dependence on the coupling strength between the individual magnetic layers and are related to pure in-phase and anti-phase breathing modes as well as to hybridizations of breathing and spin-wave modes that are characteristic for the considered circular-shaped geometry. The experimental detection of these resonant oscillation modes may provide a means for skyrmion sensing applications and for the general characterization of skyrmion states in multilayer stacks with antiferromagnetic interlayer exchange coupling.

cond-mat.mes-hall↗

arXiv2020

Voltage control of magnon spin currents in antiferromagnetic Cr2O3

Voltage-controlled spintronic devices utilizing the spin degree of freedom are desirable for future applications, and may allow energy-efficient information processing. Pure spin current can be created by thermal excitations in magnetic systems via the spin Seebeck effect (SSE). However, controlling such spin currents, only by electrical means, has been a fundamental challenge. Here, we investigate voltage control of the SSE in the antiferromagnetic insulator Cr2O3. We demonstrate that the SSE response generated in this material can be effectively controlled by applying a bias voltage, owing to the sensitivity of the SSE to the orientation of the magnetic sublattices as well as the existence of magnetoelectric couplings in Cr2O3. Our experimental results are explained using a model based on the magnetoelectric effect in Cr2O3.

cond-mat.mtrl-sci↗

arXiv2020

Probing magnon-magnon coupling in exchange coupled Y$_3$Fe$_5$O$_{12}$/Permalloy bilayers with magneto-optical effects

We demonstrate the magnetically-induced transparency (MIT) effect in Y$_3$Fe$_5$O$_{12}$(YIG)/Permalloy(Py) coupled bilayers. The measurement is achieved via a heterodyne detection of the coupled magnetization dynamics using a single wavelength that probes the magneto-optical Kerr and Faraday effects of Py and YIG, respectively. Clear features of the MIT effect are evident from the deeply modulated ferromagnetic resonance of Py due to the perpendicular-standing-spin-wave of YIG. We develop a phenomenological model that nicely reproduces the experimental results including the induced amplitude and phase evolution caused by the magnon-magnon coupling. Our work offers a new route towards studying phase-resolved spin dynamics and hybrid magnonic systems.

cond-mat.mtrl-sci↗

arXiv2020

Hybrid magnonics: physics, circuits and applications for coherent information processing

Hybrid dynamic systems have recently gained interests with respect to both fundamental physics and device applications, particularly with their potential for coherent information processing. In this perspective, we will focus on the recent rapid developments of magnon-based hybrid systems, which seek to combine magnonic excitations with diverse excitations for transformative applications in devices, circuits and information processing. Key to their promising potentials is that magnons are highly tunable excitations and can be easily engineered to couple with various dynamic media and platforms. The capability of reaching strong coupling with many different excitations has positioned magnons well for studying solid-state coherent dynamics and exploiting unique functionality. In addition, with their gigahertz frequency bandwidth and the ease of fabrication and miniaturization, magnonic devices and systems can be conveniently integrated into microwave circuits for mimicking a broad range of device concepts that have been applied in microwave electronics, photonics and quantum information. We will discuss a few potential directions for advancing magnon hybrid systems, including on-chip geometry, novel coherent magnonic functionality, and coherent transduction between different platforms. As future outlook, we will discuss the opportunities and challenges of magnonic hybrid systems for their applications in quantum information and magnonic logic.

cond-mat.mes-hall↗

arXiv2020

Temperature-Dependent Anisotropic Magnetoresistance and Spin-Torque-Driven Vortex Dynamics in a Single Microdisk

Spin-orbit-torque-driven dynamics have recently gained interest in the field of magnetism due to the reduced requirement of current densities and an increase in efficiency, as well as the ease of implementation of different devices and materials. From a practical point of view, the low-frequency dynamics below 1 GHz is particularly interesting since dynamics associated with magnetic domains lie in this frequency range. While spin-torque excitation of high-frequency modes has been extensively studied, the intermediate low-frequency dynamics have received less attention, although spin torques could potentially be used for both manipulation of the spin texture, as well as the excitation of dynamics. In this work, we demonstrate that it is possible to drive magnetic vortex dynamics in a single microdisk by spin-Hall torque at varying temperatures, and relate the results to transport properties. We find that the gyrotropic mode of the core couples to the low-frequency microwave signal and produces a measurable voltage. The dynamic measurements are in agreement with magnetic transport measurements and are supported by micromagnetic simulations. Our results open the door for integrating magnetic vortex devices in spintronic applications.

cond-mat.mes-hall↗

arXiv2020

Perspective on Metallic Antiferromagnets

Antiferromagnet materials have recently gained renewed interest due to their possible use in spintronics technologies, where spin transport is the foundation of their functionalities. In that respect metallic antiferromagnets are of particular interest, since they enable complex interplays between electronic charge transport, spin, optical, and magnetization dynamics. Here we review phenomena where the metallic conductivity provides unique perspectives for the practical use and fundamental properties of antiferromagnetic materials.

cond-mat.str-el↗

arXiv2020

Coherent spin pumping in a strongly coupled magnon-magnon hybrid system

We experimentally identify coherent spin pumping in the magnon-magnon hybrid modes of permalloy/yttrium iron garnet (Py/YIG) bilayers. Using broadband ferromagnetic resonance, an "avoided crossing" is observed between the uniform mode of Py and the spin wave mode of YIG due to the fieldlike interfacial exchange coupling. We also identify additional linewidth suppression and enhancement for the in-phase and out-of-phase hybrid modes, respectively, \textcolor{black}{which can be interpreted as concerted dampinglike torque from spin pumping}. Our analysis predicts inverse proportionality of both fieldlike and dampinglike torques to the square root of the Py thickness, which quantitatively agrees with experiments.

cond-mat.mes-hall↗

arXiv2020

Control of spin dynamics in artificial honeycomb spin-ice-based nanodisks

We report the experimental and theoretical characterization of the angular-dependent spin dynamics in arrays of ferromagnetic nanodisks arranged on a honeycomb lattice. The magnetic field and microwave frequency dependence, measured by broadband ferromagnetic resonance, reveal a rich spectrum of modes that is strongly affected by the microstate of the network. Based on symmetry arguments with respect to the external field, we show that certain parts of the ferromagnetic network contribute to the detected signal. A comparison of the experimental data with micromagnetic simulations reveals that different subsections of the lattice predominantly contribute to the high-frequency response of the array. This is confirmed by optical characterizations using microfocused Brillouin light scattering. Furthermore, we find indications that nucleation and annihilation of vortex-like magnetization configurations in the low-field range affect the dynamics, which is different from clusters of ferromagnetic nanoellipses. Our work opens up new perspectives for designing magnonic devices that combine geometric frustration in gyrotropic vortex crystals at low frequencies with magnonic crystals at high frequencies.

cond-mat.mes-hall↗

arXiv2019

Magnetic damping modulation in $IrMn_{3}/Ni_{80}Fe_{20}$ via the magnetic spin Hall effect

Non-collinear antiferromagnets can have additional spin Hall effects due to the net chirality of their magnetic spin structure, which provides for more complex spin-transport phenomena compared to ordinary non-magnetic materials. Here we investigated how ferromagnetic resonance of permalloy ($Ni_{80}Fe_{20}$) is modulated by spin Hall effects in adjacent epitaxial $IrMn_{3}$ films. We observe a large dc modulation of the ferromagnetic resonance linewidth for currents applied along the [001] $IrMn_{3}$ direction. This very strong angular dependence of spin-orbit torques from dc currents through the bilayers can be explained by the magnetic spin Hall effect where $IrMn_{3}$ provides novel pathways for modulating magnetization dynamics electrically.

cond-mat.mtrl-sci↗

arXiv2019

Distinguishing antiferromagnetic spin sublattices via the spin Seebeck effect

Antiferromagnets are beneficial for future spintronic applications due to their zero magnetic moment and ultrafast dynamics. But gaining direct access to their antiferromagnetic order and identifying the properties of individual magnetic sublattices, especially in thin films and small-scale devices, remains a formidable challenge. So far, the existing read-out techniques such as anisotropic magnetoresistance, tunneling anisotropic magnetoresistance, and spin-Hall magnetoresistance, are even functions of sublattice magnetization and thus allow us to detect different orientations of the Néel order for antiferromagnets with multiple easy axes. In contrast direct electrical detection of oppositely oriented spin states along the same easy axes (e.g., in uniaxial antiferromagnets) requires sensitivity to the direction of individual sublattices and thus is more difficult. In this study, using spin Seebeck effect, we report the electrical detection of the two sublattices in a uniaxial antiferromagnet Cr2O3. We find the rotational symmetry and hysteresis behavior of the spin Seebeck signals measured at the top and bottom surface reflect the dierction of the surface sublattice moments, but not the Néel order or the net moment in the bulk. Our results demonstrate the important role of interface spin sublattices in generating the spin Seebeck voltages, which provide a way to access each sublattice independently, enables us to track the full rotation of the magnetic sublattice, and distinguish different and antiparallel antiferromagnetic states in uniaxial antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2019

Phonon Transport Controlled by Ferromagnetic Resonance

The resonant coupling of phonons and magnons is important for the interconversion of phononic and spin degrees of freedom. We studied the phonon transmission in LiNbO3 manipulated by the dynamic magnetization in a Ni thin film. It was observed that the phonons could be absorbed strongly through resonant magnon-phonon coupling, which was realized by optimizing the interfacial coupling between Ni and LiNbO3. The line shapes of phonon transmission were further investigated considering the magnon-phonon interconversion in the elastically driven ferromagnetic resonance process. The results promote unique routes for phonon manipulation and detection in the presence of magnetization dynamics.

physics.app-ph↗

arXiv2019

Spin-Wave frequency division multiplexing in an yttrium iron garnet microstripe magnetized by inhomogeneous field

Spin waves are promising candidates for information processing and transmission in a broad frequency range. In the realization of magnonic devices, the frequency depended division of the spin wave frequencies is a critical function for parallel information processing. In this work, we demonstrate a proof-of-concept spin-wave frequency division multiplexing method by magnetizing a homogenous magnetic microstripe with an inhomogeneous field. The symmetry breaking additional field is introduced by a permalloy stripe simply placed in lateral proximity to the waveguide. Spin waves with different frequencies can propagate independently, simultaneously and separately in space along the shared waveguide. This work brings new potentials for parallel information transmission and processing in magnonics.

physics.app-ph↗

arXiv2019

Enhancement of the UV emission from gold/ZnO nanorods exhibiting no green luminescence

Large reflection losses at interfaces in light emitting semiconductor devices cause a significant reduction in their light emission and energy efficiencies. Metal nanoparticle (NP) surface coatings have been demonstrated to increase the light extraction efficiency from planar high refractive index semiconductor surfaces. This emission enhancement in Au NP-coated ZnO is widely attributed to involvement of a green (~ 2.5 eV) deep level ZnO defect exciting localized surface plasmons in the NPs that non-radiatively decay into hot electrons, which return to ZnO and radiatively recombine. In this work, we achieve a 6 times enhancement of the ultra-violet excitonic emission in ZnO nanorods that are coated with 5 nm Au NPs without the aid of ZnO defects. Cathodoluminescence (CL), photoluminescence (PL) and a novel concurrent CL-PL technique as well as time resolved PL spectroscopy revealed that the increase in UV emission is due to the formation of an additional fast excitonic relaxation pathway that increases the exciton spontaneous emission rate. Concurrent CL-PL measurements ruled out the presence of charge transfer mechanism in the emission enhancement process. While time resolved PL results confirmed the existence of a new excitonic recombination channel that is attributed to exciton relaxation via the plasmon-assisted excitation of rapid non-radiative Au interband transitions. Our results establish that ZnO defect levels ~ 2.5 eV are not required to facilitate Au NP induced enhancement of the ZnO UV emission.

physics.optics↗

arXiv2019

Correlative Study of Enhanced Excitonic Emission in ZnO Coated with Al Nanoparticles using Electron and Laser Excitation

Metal nanoparticle (NP) surface coatings are known to significantly enhance the ultra-violet luminescence intensity of ZnO. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal NPs can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in a ZnO single crystal and ZnO nanorods coated with a 2 nm Al layer has been investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show enhancement factors up to 12 times of the free exciton (FX) emission at 80 K. The FX couple more efficiently to the LSPs in Al compared to the localized donor-bound excitons. Furthermore, a strong polarization dependence of the LSPs with respect to the FX was observed with higher enhanced FX transitions polarized in the same direction as the electric field of the incident excitation. These results indicate that selective enhancement of the ultra-violet excitonic PL in ZnO can be achieved by careful alignment of the crystallographic axes of the ZnO relative to the electric vector of the excitation source.

physics.optics↗

arXiv2019

Tuning edge localized spin waves in magnetic microstripes by proximate magnetic structures

The propagation of edge localized spin waves (E-SWs) in yttrium iron garnet (YIG) microstripes with/without the proximate magnetic microstructures is investigated by micromagnetic simulations. A splitting of the dispersion curve with the presence of permalloy (Py) stripe is also observed. The E-SWs on the two edges of YIG stripe have different wavelengths, group velocities, and decay lengths at the same frequencies. The role of the Py stripe was found to be the source of the inhomogeneous static dipolar field without dynamic coupling with YIG. This work opens new perspectives for the design of innovative SW interference-based logic devices.

cond-mat.mtrl-sci↗