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Magnetism at the time and length scale of the Exchange interaction

Final Report Summary - EXCHANGE (Magnetism at the time and length scale of the Exchange interaction)

Excitation by femtosecond lasers has revealed extraordinary spin dynamics in magnetic materials in recent years, that cannot be explained by equilibrium descriptions of magnetism. The main reason for this is the fact that all theories of magnetic phenomena substantially rely on the adiabatic approximation, assuming that different parts of a magnetic system are in equilibrium with each other. But, when a magnet is excited with a stimulus, like a femtosecond laser pulse, much shorter than the time of thermal equilibration in solids (~1-100 ps) and pertinent to the characteristic time of the exchange interaction, such an excitation brings the medium into a strongly non-equilibrium state, where conventional thermodynamic descriptions of magnetic phenomena are no longer valid.
The ultimate goal of the project was to study the physics of ultrafast magnetic phenomena in such a strongly non-equilibrium state, in particular by employing recent developments of a new generation of femtosecond X-ray and picosecond THz free electron lasers, which offer the opportunity to investigate how the properties of matter emerge out of the microscopic interactions between its building blocks on the relevant nanometer (nm) length and femtosecond (fs) time scales.
The results of the project have fully answered our high expectations: we have been able to demonstrate and theoretically describe the emergence of magnetic order after femtosecond laser demagnetization, using fsXray scattering and multiscale modelling. Stable, all-optical switching of magnetization at the nanoscale was achieved by using nanoscale plasmonic antennas. In addition, we have been able to demonstrate and model the creation of nanoscale topological spin structures (skyrmions), the excitation and detection of spin-polarized currents using THz spectroscopy in magnetic multilayers and the photoinduced macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength in antiferromagnetic structures. Finally, we have been able to contribute to the recent development of optical control of magnetization in Co/Pt and Fe/Pt multilayers, unknown yet at the start of the project, and have not only found a theoretical understanding of these unexpected results but also found a method to dramatically reduce the number of required pulses from hundreds to less than ten. In addition we have shown that these materials may succesfuly be applied as neuromorphic materials, in the quest of developing novel, more energy efficient methods of Information and Communication Technologies beyond Moore.
In the course of this project we have (co-)developed several novel methodologies that have helped us and others to unveil the details of this exciting regime of ultrafast, strongly non-equilibrium magnetiztion dynamics, to wit: femtosecond Xray spectroscopy and scattering, multiscale modelling, THz emission spectroscopy, femtosecond pump-probe spectroscopy in high magnetic fields up to 38Tesla, spin wave tomography and all-optical switching of magnetization with vortex beams.