Direct visualization of dynamic magnetic coupling in a Co/Py bilayer with picosecond and nanometer resolution

We present a combination of ferromagnetic resonance (FMR) with spatially and time-resolved X-ray absorption spectroscopy in a scanning transmission X-ray microscope (STXM-FMR). The transverse high frequency component of the resonantly excited magnetization is measured with element-specifity in a Permalloy (Py) disk - Cobalt (Co) stripe bilayer microstructure. STXM-FMR mappings are snapshots of the local magnetization-precession with nm spatial resolution and ps temporal resolution. We directly observe the transfer of angular momentum from Py to Co and vice versa at their respective element-specific resonances. A third resonance could be observed in our experiments, which is identified as a coupled resonance of Py and Co.Comment: Version submitted to Physical Review Applied with updated author list and supplemental information (Ancillary file

Magnetization dynamic in prototype microstructures investigated with ultimate time and space resolution and element selectivity

In dieser Arbeit wird der dynamische magnetische Kontrast untersucht, welcher mittels Transmissions-Rasterröntgenmikroskop detektierter Ferromagnetischer Resonanz (STXM-FMR) gemessen wird. Durch einen zeitaufgelösten Detektionsmechanismus in transversaler röntgendetektierter Ferromagnetischer Resonanz (XFMR) ist es möglich das dynamische Verhalten von magnetischen Mikrostrukturen mittels Elementselektivität zu untersuchen. Zustätzlich besitzt diese Messtechnik eine nominelle Ortsauösung von 35 nm, begrenzt durch die verwendete Zonenplatte des STXM. Diese nominelle Ortsauösung wurde durch Messung an einer Eisen Nanopartikel Probe veriziert. Bei einer Anregungsfrequenz von f MW = 9.64 GHz kann eine Zeitauösung von 17.4 ps erreicht werden. Unterschiedliche Auswertemethoden der STXM-FMR Messungen werden vorgestellt. Diese Methoden ermöglichen es den reinen magnetischen Kontrast der Probe darzustellen. Der Önungswinkel der FMR und die relative Phase zwischen der Mikrowellenanregung und den Röntgenstrahlen können mittels der korrekten Auswertemethode bestimmt werden. Kontrollexperimente werden durchgeführt, um sicherzustellen, dass der dynamische Kontrast sich wie der klassiche magnetische Röntgenzirkulardichroismus Eekt verhält. Um dies zu gewährleisten, werden sowohl homogene, als auch inhomogene Anregungen verwendet. Diese unterschiedlichen Anregungen können mit größerer Genauigkeit betrachtet werden, wenn das Transmissionsprol der Probe verwendet wird. Laufende Spinwellen können in zueinander senkrecht stehenden Mikrostrukturen beobachtet werden. Werden diese Spinwellen genauer analysiert, kann eine Ausbreitungsgeschwindigkeit der Spinwellen von 42.5 km/s bestimmt werden.In this thesis the dynamic magnetic contrast observed with scanning transmission X-ray microscopy ferromagnetic resonance (STXM-FMR) is investigated. By using a time resolved detection scheme in transversal X-ray detected ferromagnetic resonance (XFMR) it is possible to investigate the dynamic behavior of magnetic micro-structures with element selectivity. The space resolution of nominally 35 nm due to the zone plate of the STXM-FMR was conrmed using a nano-particle sample. A time resolution of down to 17.4 ps corresponding to a excitation frequency of the FMR of f MW = 9.64 GHz can be achieved at the Stanford Synchrotron Radiation Lightsource (SSRL). Dierent methods to separate the background contribution from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) are presented. The opening angle of the FMR precession as well as the relative phase between the microwave excitation and the X-ray pulses can be quantied using the corresponding evaluation method. Control experiments using homogeneous and inhomogeneous magnetic excitations are used to verify that the dynamic magnetic contrast behaves like the classical XMCD eect under reversal of the X-ray photon helicity. Homogeneous as well as inhomogeneous FMR excitation modes can be analysed in greater detail using X-ray transmission proles obtained from the individual phases used to measure the precession of the magnetization during FMR excitation. Visualizing the dynamic magnetic contrast for non-homogeneous excited magnetic samples, travelling spin waves have been observed in two magnetic micro-stripes, perpendicular to each other. Analysing the spin wave movement enables to estimate that the spin wave velocity is of the order of 42.5 km/s .submitted by Taddäus SchaffersUniversität Linz, Dissertation, 2019OeBB(VLID)361740

Interaction of propagating spin waves with extended skyrmions

Funding Information: This work was supported by the Academy of Finland (Grant Nos. 295269, 306978, 321983, 325480, and 327804). We acknowledge the provision of computational resources provided by the Aalto Science-IT project. Publisher Copyright: © 2022 Author(s).Active control of propagating short-wavelength spin waves in perpendicularly magnetized materials is promising for designing nanoscale magnonic devices. One method of manipulating spin waves on the nanoscale is through their interaction with magnetic textures, an example of which is the magnetic skyrmion - a particle-like topological object stabilized in thin film heterostructures by the Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy. In this paper, the interaction between spin waves and skyrmions is studied using micromagnetic simulations. The magnetic parameters chosen are similar to those found experimentally, leading to a skyrmion with an extended core of reversed magnetization. The effect of a propagating spin wave on the skyrmion is to cause the emission of a secondary spin wave by the skyrmion. At low frequencies, where the incoming spin wave wavelength is much larger than the skyrmion, this leads to a nearly circular re-emitted spin wave. The pattern of emission becomes increasingly complex at higher frequencies as the wavelength becomes similar to the skyrmion size due to the complex excitation of the extended core. The emitted spin wave profile can be controlled by altering the size of the skyrmion through the magnitude of the DMI, providing a method of tuning the system.Peer reviewe

Evaluation protocol for revealing magnonic contrast in TR-STXM measurements

We present a statistically motivated method to extract magnonic contrast from time-resolved scanning transmission x-ray microscopy (TR-STXM) measurements. TR-STXM is an element-specific method for resolving spin-dynamics in space and time. It offers nanometer spatial resolution and picosecond temporal resolution. The presented method makes it possible to obtain phase and amplitude profiles of spin-waves from STXM measurements. Furthermore, it allows for a rigorous transformation to reciprocal magnon k⃗-space, revealing k⃗-dependent magnon properties such as the magnon dispersion in three dimensions and for all directions of the magnetic anisotropy. We demonstrate our method using X-band ferromagnetic resonance on a micrometer-sized permalloy assembly