I have worked at the Department of Physics, Stockholm University as a post-doc in the lab of Stefano Bonetti from March, 2017-December, 2019. I studied ultrafast magnetization dynamics using THz and NIR pulses using MOKE technique. I have built a THz spectroscopy setup and measured THz conductivity in magnetic thin films in the frequency range of 0.1-3.0 THz. We recently bought a commercial THz spectroscopy setup from TOPTICA which provides THz radiation up to 5.5 THz with a maximum signal-to-noise ratio over 90 dB. I have also studied de-magnetization dynamics of LSMO thin films and current induced THz conductivity of antiferromagnetic PtMn devices. I extensively used a commercial FDTD software COMSOL (Wave Optics Module) to study the THz field enhancement in metamaterial structure both in time and frequency domain.
THz-induced Demagnetization in Co/Pt Thin Film
Single-cycle THz pulse can demagnetize magnetic thin films although the reason is quite different than laser-induced demagnetization. THz-induced demagnetization increases quadratically with the THz field strength, but increasing THz magnetic field in a table-top geometry has its own limitations. In this work, we have shown that if we use out-of-plane magnetized films, the THz-indued demagnetization can be enhanced by 5 times than in-plane films at similar THz field strength. We did a simulation study of a THz MM to increase the THz magnetic field at the center of the structure by ten times at a resonance frequency of 1.4 THz. As demagnetization increases quadratically with the THz field; combining this MM and an out-of-plane magnetized film we hope to observe THz-induced magnetic switching with table-top THz sources.
''THz-driven demagnetization with perpendicular magnetic anisotropy: towards ultrafast ballistic switching'' D. Polley, M. Pancaldi, M. Hudl, P. Vavassorri, S. Urazhdin, and S. Bonetti; Journal of Physics D: Applied Physics 51, 084001 (2018).
THz Meta-material
We did a simulation study of a unique THz MM both in the time and frequency domain to study the THz field enhancement at the center of the spiral structure. The resonance frequency of such an MM can be tuned by varying the structure parameters of the MM and can be modeled in analog with a coupled LCR system. For a resonant excitation, THz magnetic field can be enhanced by 40 times and for a broadband THz excitation the magnetic field can be enhanced up to 6 times at the resonant frequency (1 THz). Such an MM can be used as a table-top tunable and resonant THz source with a magnetic field as high as 2 T when coupled with a broadband table-top THz system.
''Terahertz magnetic field enhancement in an asymmetric spiral metamaterial'' D. Polley, N. Zhöu Hagström, C. von Kroff Schmising, S. Eisebitt, and S. Bonetti; Journal of Physics B: Atomic Molecular and Optical Physics 51, 224001 (2018).
Inertial Spin Dynamics in Ferromagnets
The understanding of how spins move and can be manipulated at pico- and femtosecond timescales has implications for ultrafast and energy-efficient data-processing and storage applications. However, the possibility of realizing commercial technologies based on ultrafast spin dynamics has been hampered by our limited knowledge of the physics behind processes on this timescale. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast timescales, but a clear observation of such effects in ferromagnets is still lacking. Here, we report direct experimental evidence of intrinsic inertial spin dynamics in ferromagnetic thin films in the form of nutation of the magnetization at a frequency of ~0.5 THz. This allows us to reveal that the angular momentum relaxation time in ferromagnets is on the order of 10 ps.
"Inertial spin dynamics in ferromagnets'' K. Neeraj, N. Awari, S. Kovalev, D. Polley, N. Zhou Hagström, S. S. P. K. Arekapudi, A. Semisalova, K. Lenz, B. Green, J. -C. Deinert, I. Ilyakov, M. Chen, M. Bawatna, V. Scalera, M. d’Aquino, C. Serpico, O. Hellwig, J.-E. Wegrowe, M. Gensch & S. Bonetti; Nature Physics 17, 245 (2020).
Megahertz-Rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL
The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid-state samples were chosen as representatives, providing an ideal test bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
“Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL” N. Z. Hagström et al., Journal of Synchrotron Radiation 29, 1454 (2022).
