Understanding the mechanism of ultrafast all optical switching – UMAMI

The scientific goal of this project is to understand the mechanism underlying the recently discovered all optical magnetic switching phenomenon. The proposed approach presents a series of novelties, which are expected to yield unprecedented insight into this process of tremendous potential for future data storage devices. Firstly, we plan to investigate this phenomenon in a variety of novel transition metal and rare earth materials, which have recently been developed by the participating research group from the University of Nancy and have shown to be suitable for all-optical switching. Most notably, these are artificially fabricated anitferromagnetically coupled magnetic multilayer structures. The strength of this approach is that these materials enable deterministic tuning of material properties like spatial inhomogeneity and magnetization of the respective ferromagnetic sublattices, properties which are currently discussed to be of key relevance for enabling the all-optical switching process. These studies will be complemented by the investigation of TM-RE alloys and multilayers, which naturally exhibit antiferromagnetically coupled magnetic sublattices.
Secondly, we will employ novel experimental techniques at femtosecond pulsed high harmonic generation (HHG) based X-ray sources to follow the magnetization dynamics. The X-rays' short wavelength will enable us to follow the time evolution of the magnetization with combined femtosecond time and nanometer spatial resolution. The inherent element sensitivity of resonant X-ray techniques will thereby reveal independently the evolution of the magnetization within each of the magnetic sublattices, thus revealing any potential exchange between these two systems. Of particular importance is thereby that the dynamics of both sublattices can be followed at the same time due to the multicolor emission of HHG sources. This will reveal unambiguously, if present, any temporal sequence and interplay between the dynamics of the sublattices. To resolve these dynamics quantitatively we will employ the powerful X-ray magnetic circular dichroism effect in absorption. This will be possible for the first time at a HHG source due to our recent development of intense, highly circularly polarized harmonics. We note that the proposed energy dispersive intensity monitor will be crucial for these experiments, since correcting for source fluctuations will yield at HHG sources unprecedented signal-to-noise ratios.
Thirdly, due to the recent development of phase-matched high harmonic generation in a hollow waveguide at the LOA, we will be able to access for the first time in a laboratory based experiment the magnetically dichroic N4,5 edges of the rare earth elements, thus giving us access to the evolution of the magnetization carried by their localized, tightly bound 4f electrons.
We finally note that should the laboratory based experiments indicate the need for it we will also perform complementary experiments on X-ray free electron laser (XFEL) sources. A potential motivation for this could be to explore more extreme pumping conditions or non-deterministic dynamics, which may be possible to realize only in single X-ray pulse based snapshot experiments. Also, X-ray-pump X-ray probe experiments using a few femftosecond short X-ray pulses originating, e.g., from a split-and-delay unit can be employed to reach unprecedented time resolution.
We expect that the proposed research program, involving novel experimental techniques and innovative sample structures will yield unprecedented novel insight into the mechanism underlying the all-optical switching phenomenon and, more generally, the closely related ultrafast demagnetization phenomenon itself. Ultimately, we expect that this understanding will guide us to the development of materials optimized for all-optical switching.