Large Nonreciprocal Propagation of Surface Acoustic Waves in Epitaxial Ferromagnetic/Semiconductor Hybrid Structures

Nonreciprocal propagation of sound, that is, the different transmission of acoustic waves traveling in opposite directions, is a challenging requirement for the realization of devices such as acoustic isolators and circulators. Here, we demonstrate efficient nonreciprocal transmission of surface acoustic waves (SAWs) propagating in opposite directions in a GaAs substrate coated with an epitaxial Fe3Si film. The nonreciprocity arises from the acoustic attenuation induced by the magnetoelastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic-transmission nonreciprocity, defined as the difference between the transmitted acoustic powers for forward and backward propagation at the ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest values of nonreciprocity reported for SAWs traveling in a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for the ME interaction, which also shows that the nonreciprocity can be further enhanced by optimization of the sample design. These results make Fe3Si/GaAs a promising platform for the realization of efficient nonreciprocal SAW devices

Qu­antification of propagating and standing surface acoustic waves by stroboscopic X-ray photoemission electron microscopy

The quantification of surface acoustic waves (SAWs) in LiNbO3 piezoelectric crystals by stroboscopic X-ray photoemission electron microscopy (XPEEM), with a temporal smearing below 80 ps and a spatial resolution below 100 nm, is reported. The contrast mechanism is the varying piezoelectric surface potential associated with the SAW phase. Thus, kinetic energy spectra of photoemitted secondary electrons measure directly the SAW electrical amplitude and allow for the quantification of the associated strain. The stroboscopic imaging combined with a deliberate detuning allows resolving and quantifying the respective standing and propagating components of SAWs from a superposition of waves. Furthermore, standing-wave components can also be imaged by low-energy electron microscopy (LEEM). Our method opens the door to studies that quantitatively correlate SAWs excitation with a variety of sample electronic, magnetic and chemical properties

Generation and Imaging of Magnetoacoustic Waves over Millimeter Distances

Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film.The authors acknowledge Jordi Prat of ALBA for his help during experiments. F. M. acknowledges support from the MINECO through Grant No. RYC-2014-16515. F. M., B. C., and R. C. acknowledge support from MINECO through Grants No. SEV-2015-0496 and No. MAT2017- 85232-R. F. M., J. M. H., and N. S. acknowledge funding from MINECO through Grant No. MAT2015-69144-P. We thank Werner Seidel and Sander Rauwerdink from PDI for assistance in the preparation of acoustic delay lines on LiNbO3. L. A. and M. F. acknowledge support from MINECO through RTI2018-095303-B-C53. This project was partially supported by the ALBA in-house research program through Projects No. ALBA-IH2015PEEM and ALBAIH2017PEEM.Peer reviewe

Generation and Imaging of Magnetoacoustic Waves over Millimeter Distances

Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film

Dynamic local strain in graphene generated by surface acoustic waves

We experimentally demonstrate that the Raman active optical phonon modes of single layer graphene can be modulated by the dynamic local strain created by surface acoustic waves (SAWs). In particular, the dynamic strain field of the SAW is shown to induce a Raman scattering intensity variation as large as 15% and a phonon frequency shift of up to 10 cm$^{-1}$ for the G band, for instance, for an effective hydrostatic strain of 0.24% generated in a single layer graphene atop a LiNbO$_{3}$ piezoelectric substrate with a SAW resonator operating at a frequency of $\sim$ 400 MHz. Thus, we demonstrate that SAWs are powerful tools to modulate the optical and vibrational properties of supported graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D systems.Comment: 10 pages, 7 figure

Resonant and Off-Resonant Magnetoacoustic Waves in Epitaxial Fe3Si/GaAs Hybrid Structures

Surface acoustic waves (SAWs) provide an efficient dynamical coupling between strain and magnetization in micro- and nanometric systems. Using a hybrid device composed of a piezoelectric, GaAs, and a ferromagnetic Heusler alloy thin film, Fe3Si, we are able to quantify the amplitude of magnetoacoustic waves generated with SAWs via magnetic imaging in an x-ray photoelectron microscope. The cubic anisotropy of the sample, together with a low damping coefficient, allows for the observation of resonant and nonresonant magnetoelastic coupling. Additionally, via micromagnetic simulation, we verify the experimental behavior and quantify the magnetoelastic shear strain component in Fe3Si, which appears to be large (b2=10±4MJm−3)

Polarized recombination of acoustically transported carriers in GaAs nanowires

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport

On the promotion of catalytic reactions by surface acoustic waves

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis,sensor and nanotechnology.SAWswere shown to cause astrong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However,the promotion mechanism has not been unambiguously identified. Using stroboscopic X-ray photoelectron spectro-microscopy, we were able to evidence asub-nano-second work function change during propagation of 500 MHz SAWs on a9nm thick platinum film. We quantify the work function change to 455 meV.Such asmall variation rules out that electronic effects due to elastic deformation (strain) play amajor role in the SAW-induced promotion of catalysis.In asecond set of experiments,SAW-induced intermixing of afive monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be amajor cause for catalytic promotion