Nichtreziproke magnetoakustische Oberflächenwellen in piezoelektrisch/ferromagnetischen Hybridstrukturen

Akustische Oberflächenwellen-Bandpassfilter werden seit vielen Jahren in Mobilfunkgeräten zur Trennung der Frequenzen in den Frontend-Modulen eingesetzt. Im einfachsten Fall besteht ein akustischer Oberflächenwellen-Bandpassfilter aus einem piezoelektrischen Kristall und zwei metallisierten Kammstrukturen. Auf sehr effiziente Weise können damit über den piezoelektrischen Effekt akustische Oberflächenwellen (engl. Surface Acoustic Waves, SAWs) in einem festgelegten Frequenzbereich angeregt und ausgelesen werden. Eine sehr nützliche Funktionalität - die Nichtreziprozität - hat sich bisher für akustische Oberflächenwellen-Bauelemente noch nicht erfolgreich in Anwendungen integrieren lassen. Somit ist es nicht möglich akustische Oberflächenwellen, die in entgegengesetzte Richtungen laufen, effizient zu trennen und beispielsweise akustische Dioden zu bauen. Im Gegensatz zu SAWs können Spinwellen (SWs) ein nichtreziprokes Verhalten aufweisen, da die Präzession der magnetischen Momente einen festgelegten rechtshändigen Drehsinn beschreibt und so die Zeitumkehr gebrochen wird. In diesem Sinne folgt diese Arbeit der Idee, reziproke SAWs mit nichtreziproken SWs zu koppeln, um eine nichtreziproke Transmissionscharakteristik für akustische Oberflächenwellen-Bauelemente zu erzielen. Hierzu werden unterschiedliche magnetische Dünnschichtsysteme zwischen den beiden metallisierten Kammstrukturen auf den piezoelektrischen Substraten LiNbO3 und LiTaO3 abgeschieden. Grundsätzlich können SAWs SWs in magnetischen Dünnschichten aufgrund von magnetoelastischer Kopplung, aber auch den erst vor kurzem beschriebenen Mechanismen der Magneto-Rotation Kopplung und Spin-Rotation Kopplung anregen. Resonante Wechselwirkung ist möglich, falls sich die Dispersionen von SAW und SW im ungekoppelten Zustand überlagern. Dabei wird zum einen ausgenutzt, dass die magnetoakustische Anregung von SWs durch SAWs bereits nichtreziprok ist. Grund dafür ist die mögliche Fehlanpassung der Helizitäten von magnetoakustischem Treibefeld und Magnetisierungsdynamik. Dieser Effekt ist aufgrund von Magneto-Rotation Kopplung in Dünnschichten aus CoFeB(d) besonders ausgeprägt und wird zusätzlich für unterschiedliche SAW-Moden in Dünnschichten aus Ni(10 nm) untersucht. Zum anderen werden das erste Mal in der Magnetoakustik gezielt ferromagnetische Dünnschichtsysteme eingesetzt, die eine nichtreziproke Spinwellendispersion aufweisen. Hierzu werden die Schichtsysteme CoFeB(d)/Pt(3 nm) und NiFe(20 nm)/Au(5 nm)/CoFeB(5 nm) untersucht, deren nichtreziproke Spinwellendispersionen aus der Grenzflächen-Dzyaloshinskii-Moriya-Wechselwirkung und magnetisch dipolaren Zwischenschichtkopplung resultieren. Aus der Kombination der nichtreziproken Anregung und nichtreziproken Spinwellendispersion ergibt sich eine stark nichtreziproke Transmissionscharakteristik für die gefertigten piezoelektrisch-ferromagnetischen Hybridstrukturen. Des Weiteren wird zur Interpretation der magnetoakustischen Transmissionsmessungen ein phänomenologisches Modell erarbeitet, das es erlaubt SWs und magnetische Parameter der magnetischen Dünnschichten zu charakterisieren. Mittels der neuen Methode der „magnetoakustischen SW-Spektroskopie" lassen sich beispielsweise gekoppelte SW-Moden in magnetischen Bilagen untersuchen und die effektiven Dzyaloshinskii-Moriya-Konstanten in magnetischen Dünnschichten bestimmen. Somit ist es künftig prinzipiell möglich magnetoakustische Dioden mit gewünschten Charakteristika zu designen.Surface acoustic wave bandpass filters have been used in mobile phones for many years to separate the frequencies in the front-end modules. In the simplest case, a surface acoustic wave bandpass filter consists of a piezoelectric crystal and two metalized comb structures. In a very efficient way, surface acoustic waves (SAWs) can be excited and detected in a defined frequency range via the piezoelectric effect. A very useful functionality - the non-reciprocity - has not yet been successfully integrated into applications for SAW devices. It is therefore not possible to efficiently separate SAWs traveling in opposite directions and to build acoustic diodes, for instance. In contrast to SAWs, spin waves (SWs) can show a non-reciprocal behavior, since the precession of the magnetic moments describes a fixed right-handed sense of rotation, thus breaking the time-reversal. In this sense, this work follows the idea of coupling reciprocal SAWs with non-reciprocal SWs to achieve non-reciprocal transmission characteristics for surface acoustic wave devices. For this purpose, different magnetic thin-film systems are deposited between the two metalized comb structures on the piezoelectric substrates LiNbO3 and LiTaO3. Surface acoustic waves can excite SWs in the magnetic thin films due to magnetoelastic coupling, but also magneto-rotational coupling and spin-rotational coupling. Resonant interaction is possible if the dispersions of SAW and SW intersect in the uncoupled state. On the one hand, it will be shown that the magnetoacoustic excitation of SWs by SAWs is already non-reciprocal. The reason for this is the possible mismatch of the helicities of the magnetoacoustic driving field and magnetization dynamics. This effect is particularly pronounced in CoFeB(d) thin films due to magneto-rotational coupling and is also investigated for different SAW modes in Ni(10 nm) thin films. On the other hand, for the first time in magnetoacoustics, ferromagnetic thin-film systems are used that have a non-reciprocal spin wave dispersion. The thin-film systems CoFeB(d)/Pt(3 nm) and NiFe(20 nm)/Au(5 nm)/CoFeB(5 nm) are studied whose non-reciprocal spin wave dispersion results from the interfacial Dzyaloshinskii-Moriya interaction and interlayer magnetic-dipolar coupling. The combination of non-reciprocal magnetoacoustic excitation and non-reciprocal spin wave dispersion results in a strongly non-reciprocal transmission characteristic for the fabricated piezoelectric-ferromagnetic hybrid structures. Furthermore, a phenomenological model is developed for the interpretation of the magnetoacoustic transmission measurements, which allows for the characterization of SWs and magnetic parameters of magnetic thin films. Using the new method of "magnetoacoustic SW spectroscopy" it is possible, for example, to investigate coupled SW modes in magnetic bilayers and to determine the effective Dzyaloshinskii-Moriya constant in magnetic thin films. In principle, it will therefore become possible to design magnetoacoustic diodes with desired characteristics in the future

Chiral magnetoacoustics

Nonreciprocal microwave devices are key components of communication platforms. Nonreciprocity can arise in chiral systems, where chirality refers to a fixed handedness that is preserved under time reversal. Chiral excitations (quasiparticles) provide opportunities for the realization of miniaturized microwave components with directional properties. In particular, surface acoustic waves that propagate in magnetic media are chiral and can display pronounced nonreciprocal character. Because surface acoustic waves are an established technological platform, hybrid surface acoustic wave/spin wave devices have great application potential. In this mini-review, we introduce the general concept of chiral and nonreciprocal magnetoacoustic waves. We discuss a widely employed phenomenological model based on magnetoelastic coupling and magneto-rotation that quantitatively accounts for many experimental findings and give a brief overview over selected experiments and advances in this emerging research field

Nonreciprocal magnetoacoustic waves in synthetic antiferromagnets with Dzyaloshinskii-Moriya interaction

The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a piezoelectric/magnetic thin film heterostructure yields potential for the realization of novel microwave devices and applications in magnonics. In the present work, we investigate the SAW-SW interaction in a Pt/Co(2nm)/Ru(0.85nm)/Co(4nm)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with different thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interfacial Dzyaloshinskii–Moriya interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode shows a large nondegenerate dispersion relation for oppositely propagating SWs. Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the piezoelectric/SAF hybrid device. The equilibrium magnetization directions of both Co layers are manipulated by an external magnetic field to set a ferromagnetic, canted, or antiferromagnetic configuration. This has a strong impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The experimental results are in agreement with a phenomenological SAW-SW interaction model, which considers the interlayer exchange coupling, iDMI, and interlayer dipolar coupling fields of the SWs

Nonreciprocal transmission of magnetoacoustic waves in compensated synthetic antiferromagnets

We investigate the interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a Pt/Co(2nm)/Ru(0.85nm)/Co(2nm)/Pt compensated synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with equal thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interlayer dipolar coupling fields and interfacial Dzyaloshinskii–Moriya interaction (iDMI), the optical SW mode shows a large nondegenerate dispersion relation for counter-propagating SWs. Due to resonant SAW-SW interaction, we observe a nonreciprocal SAW transmission in the prepared piezoelectric/SAF hybrid device. We demonstrate that the nonreciprocity of the SAW transmission in symmetric SAFs with equal thicknesses of the magnetic layers can show a substantially different characteristic behavior in comparison to asymmetric SAFs or magnetic single layers with iDMI. For the prepared SAF, the nonreciprocal shift of the magnetoacoustic resonance fields and the magnetoacoustic SW excitation efficiency depend on the external magnetic field sweep direction. For one magnetic field sweep direction and angle of the magnetic field, the resonance fields of the waves propagating in one direction are larger than for the waves propagating in the opposite direction. In addition, the magnitude of the nonreciprocal field shift is at minimum if the external magnetic field is aligned perpendicular to the SW propagation direction. The experimental results are in agreement with a phenomenological SAW-SW interaction model

Coherent phonon-magnon interactions detected by micro-focused Brillouin light scattering spectroscopy

We investigated the interaction of surface acoustic waves and spin waves with spatial resolution by micro-focused Brillouin light scattering spectroscopy in a Co$_{40}$Fe$_{40}$B$_{20}$ ferromagnetic layer on a LiNbO$_{3}$-piezoelectric substrate. We experimentally demonstrate that the magnetoelastic excitation of magnons by phonons is coherent by studying the interfering BLS-signals of the phonons and magnons during their conversion process.We find a pronounced spatial dependence of the phonon annihilation and magnon excitation which we map as a function of the magnetic field. The coupling efficiency of the surface acoustic waves (SAWs) and the spin waves (SWs) is characterized by a magnetic field dependent decay of the SAWs amplitude

GaN heterostructures as innovative x-ray imaging sensors — change of paradigm

Direct conversion of X-ray irradiation using a semiconductor material is an emerging technology in medical and material sciences. Existing technologies face problems, such as sensitivity or resilience. Here, we describe a novel class of X-ray sensors based on GaN thin film and GaN/AlGaN high-electron-mobility transistors (HEMTs), a promising enabling technology in the modern world of GaN devices for high power, high temperature, high frequency, optoelectronic, and military/space applications. The GaN/AlGaN HEMT-based X-ray sensors offer superior performance, as evidenced by higher sensitivity due to intensification of electrons in the two-dimensional electron gas (2DEG), by ionizing radiation. This increase in detector sensitivity, by a factor of 104 compared to GaN thin film, now offers the opportunity to reduce health risks associated with the steady increase in CT scans in today’s medicine, and the associated increase in exposure to harmful ionizing radiation, by introducing GaN/AlGaN sensors into X-ray imaging devices, for the benefit of the patient

Detection of x rays by a surface acoustic delay line in contact with a diamond crystal

In this study, we present proof of concept for an x-ray detector. The hybrid device consists of a synthetic single crystal diamond in mechanical contact with a piezoelectric lithium niobate surface acoustic wave (SAW) delay line. Upon x-ray irradiation, the diamond crystal experiences a change in conductivity, which, in turn, very sensitively influences the SAW transmission on the delay line. This change in SAW attenuation is directly used to monitor the x-ray beam intensity. The SAW attenuation shows a monotonic variation with dose rate D in the studied range between 100 and 1800 μGy/s. While the response time leaves room for further improvement, the SAW detection principle offers the unique possibility for wireless remote powering and sensing