Element specific spin dynamics of nano- and heterostructues

The rapid progress in miniaturization of computational devices over the past decades has pushed the limits towards ever-smaller structures and ever-faster operation at the same time. Current technology is already close to the fundamental limit of the atomic length-scale. Thus, various novel concepts have been put forward to further increase performance not by miniaturization but via increased functionality. This includes the fields of spintronics and magnonics which have in common that they involve the quantum-mechanical spin degree of freedom. Consequently, also in the field of magnetism this demands investigating ever-smaller spatial dimensions and increasingly faster time-scales to be able to study spin dynamics, which is typically in the GHz regime, at the nanometer scale. The aim of this proposal is to carry out investigations of the dynamic magnetic properties of individual magnetic micro- and nano-objects in a complex magnetic environment. For that a unique novel instrument will be used which allows element specific investigations of magnetism using a combination of a scanning transmission x-ray microscopy (STXM, lateral resolution down to 35 nm) with time-resolved (down to 17 ps), x-ray detected ferromagnetic resonance (FMR). In particular, STXM-FMR enables to directly map the intrinsic magnetic properties of individual micro- and nano-objects as well as of individual magnetic nanoparticles under the influence of a tailored surrounding ensemble of other magnetic particles or structures. Those typically lead to complex inhomogeneous local fields originating from dipolar or indirect magnetic coupling. On the other hand, the induced dynamic spin polarization generated in a non-magnetic material by spin-pumping from an adjacent ferromagnet at magnetic resonance shall be imaged directly for the first time to study its spatial distribution and phase correlation with the resonating ferromagnet. Comparing the findings of STXM-FMR with micromagnetic simulations the validity of the common theory of spin-wave excitations can be tested. In summary, new paths will open up to tailor magnetic properties and interactions in complex ensembles of magnetic mirco-, hetero-, and nanostructures directly studied with element specificity and ultimate spatial and temporal resolution.

Element specific spin dynamics of nano- and heterostructues In recent years novel concepts are developed which shall enable to base the processing of information not on the electric charge of the electron but on its intrinsic angular momentum, the so-called spin. In that context the spin has to be has to be excited, transferred, manipulated and detected, a field known nowadays as "magnonics". It was a central aim of this project to investigate the spin-dynamics in model systems with an unprecedented spatial and temporal resolution based on time-resolved x-ray microscopy. Such experimental instrumentation was just getting to become available at the beginning of the project based on scanning x-ray microscopy with high temporal resolution down to the GHz regime, where resonant spin-dynamics typically occur. In the course of the project it was found out, that the propagation of inhomogeneous spin-excitations, so-called spin-waves can not only be influenced by external magnetic fields but by the geometry of the nano- or microstructure itself even in the presence of homogeneous excitations. This was contrary to the common expectation that in confined structures only standing spin-waves should exist. In addition, the use of x-rays enables to separately study the driving spin-dynamics is a ferromagnet from the driven, or pumped, spin-dynamics in an adjacent non-ferromagnet inside a heterostructure due to the element selectivity of the technique. Remarkably, the spin dynamics in the non-ferromagnetic was transferred without any charge current, because the material was highly insulating. Both results provide valuable new insight in the fundamental properties of magnetic nano- and heterostructures for potential future applications in magnonic devices.