The last decade of research has revealed that ultrafast spin dynamics of a particular material can ultimately only be understood when the dynamics of every single atomic species’ magnetic moment, comprising orbital and spin moment, can be analyzed on a microscopic level. Using the element specific soft x-ray experimental techniques of X-ray Magnetic Circular Dichroism (XMCD) and X-ray Resonant Magnetic Reflection (XRMR), allowing probing individual spin and orbital components on an ultrafast time-scale, our project addresses this particular point.
We focus our studies on metallic magnetic systems consisting of the elements of the 3d transition metal (TM) and the 4f rare earth (RE) periods, i.e. elemental magnetic metals, alloys, and layered sample structures. By varying the elemental and material properties through the choice of the particular element, the sample composition and morphology, and the excitation conditions we study which parameters determine the dynamic evolution of spin and orbital momenta from the initial via the intermediate to the final magnetic state on the relevant sub-ps time scale.
So far, we have been carrying out time-resolved studies on the magnetization dynamics on a selection of pure elemental magnets of the TMs and the heavy REs. Our work-plan extends this work towards increasing the involvement of the experimental aspects. In the first step we intend to investigate the yet unknown individual atomic spin and orbital moment dynamics on selected elemental magnets of the heavy REs. Then we move on to RE alloys. Here, we will focus on how the changed local environment affects the spin and orbital moment dynamics of the REs sublattices; moreover, the inclusion of light REs in the selected alloys allows exploring their individual spin and orbital momentum dynamics. In contrast to the heavy REs investigated in the first step, here the atomic spin and orbital momentum are coupled antiparallel. Implications of this antiparallel coupling on the dynamic behavior will be assessed. Finally we turn toward layered sample system of TMs and REs. In the focus of these studies is the role of the interfaces for the processes involved in ultrafast magnetization dynamics.
For all these steps, we additionally strive to gain depth-resolved information by using the XRMR and X-Ray Resonant Diffraction (XRD) method. Depth resolution is an indispensable aspect for the comprehension of the entire magnetic dynamics since most experiments are associated with intrinsically inhomogeneous excitation scenarios. Furthermore, analyzing magnetic dynamics at surfaces and interfaces is intimately associated with depth-dependent information.
For this reason and others, an important aspect of our project is the further development of the time-resolved XRMR technique towards a powerful probe of microscopic observables together with support from theory on the one hand. This aim is also motivated and supported by the excellent experimental experiences on reflectivity measurements of the recent years demonstrating that our experimental setup is very well adapted to slicing source of the HZB facility. Along with the depth sensitivity, this further development will open up unique possibilities for probing spin dynamics in an extended class of sample systems allowing for more quantitative and fundamental analysis.
A long-term goal is to establish a more sophisticated control of the magnetic dynamic pathway, on the one hand through time-dependent pumping schemes such as double pulse pumping or time-shaped pulses. On the other hand, we additionally plan to implement THz pumping schemes to extend the sample excitation capabilities.