Transient grating investigation of magnetization dynamics in thin films

This PhD thesis aims to investigate magnetization dynamics in thin films by employing the transient grating (TG) technique. The uniqueness of this technique is to have precise control over the length scale of the induced dynamics. A laser interference pattern triggers spatially modulating excitation of materials with a length scale equal to the TG period and the following dynamics is monitored using a time-delayed probe laser. Length scales of the excited dynamics can be varied by working in optical, extreme ultraviolet (EUV) and x-ray regimes. This key feature enables us to study magnetic excitation-relaxation dynamics at different length scales. The two main reported studies in this thesis are based on ferrimagnetic thin film samples. The broad primary goal is to understand the dynamics of magnetization coupled to the dynamics of elastic and electronic systems. In addition, the capabilities of the TG technique were explored in optical and EUV regimes for studying magnetic samples. The first experiment involves the investigation of magnetization dynamics in thin metallic films of Co78TbxGd22−x family of alloy samples. The optical TG tabletop setup developed at the SPRINT laboratory, a part of Istituto Officina dei Materiali of Consiglio Nazionale delle Ricerche (IOM-CNR), within the framework of Nanoscale Foundries and Fine Analysis (NFFA) consortium, was used to study the magneto-elastic coupling dynamics. Magnetization dynamics in Co78Gd22 was found to be dominated by a response that is similar to the previously reported behaviour of all-optical switching, whereas the Co78Tb12Gd10 sample shows a completely different behaviour. It exhibits a magnetization relaxation driven by coherent acoustic waves, clearly indicating the magneto-elastic coupling dynamics. The second experiment was performed at the TIMER beamline established at the FERMI (FEL) facility, located in Trieste. This setup is a dedicated user facility for EUV TG-based investigations. The capabilities at TIMER were successfully used to study ultrafast magnetization dynamics in a CoNi thin film sample at the nanoscale. Interference of circularly polarized photons with the same helicities forms intensity gratings with circular polarization of the same helicity. This excites magnetization non-thermally by the inverse Faraday effect and also the core electrons by resonant dichroic photon absorptions. In a degenerate TG experiment, where the wavelength used for the pump and probe is the same, a unique phase-matching condition exists during the time window equal to the experimental setup resolution. This results in a novel helicity-dependent ultrafast response. Moreover, the results are reproducible with opposite helicities for a fixed magnetic field direction and also with opposite magnetic field directions for a fixed photon helicity, supporting the dichroic nature of the ultrafast dynamics. In addition, this thesis briefly describes additional collaborative FEL-based projects. One of the major studies was based on the excitation and detection of nanoscale magnons in ferrimagnetic materials by employing the EUV TGs at FERMI. Under this investigation, a magnon dispersion diagram was constructed inducing excitations at different wavelengths. In the second major collaboration, an approach for performing x-ray TG (XTG) experiments was developed by a team of scientists and a protocol was designed to help the scientific community be able to perform XTG experiments. This experiment was one of the five grated beamtimes at the EuXFEL facility, that aim to establish the XTG technique and demonstrate its flexibility characterizing diverse material science research. These projects open up further directions for follow-up investigations. The results obtained from the optical TG experiment lead to ideas exploring new materials with enhanced magneto-elasticity. Furthermore, this study can be extended to EUV and xray regimes where one could selectively excite and probe a magnetic element within the material and see the differences in the following dynamics. Also, these open up directions towards studying the dynamics of the inverse effect called the magnetostriction in these materials. The EUV TG investigation sheds light on the magnetization dynamics at play during the ultrafast timescale where electronic excitations and relaxations take place. The results demonstrate the feasibility of the technique to understand magnetization dynamics associated with dichroic absorptions and also unlock the doors to using such configurations for studying non-thermally excited spin waves at the nanoscale.