Optically induced magnetization reversal in Co/Pt multilayers: Role of domain wall dynamics

All-optical switching (AOS) of magnetization has been attracting an increasing attention due to the promising application prospects in magnetic data recording technology. Since the first demonstration in 2007, AOS has been observed in a limited range of materials. Among them, the ferromagnetic Co/Pt multilayer system comes to the forefront owing to its unique magnetic and magneto-optical properties. In the present project, we investigated optically induced magnetization reversal mechanisms in [Co/Pt]$_{N}$ multilayers depending on the number of bilayers N and the laser beam properties. The multilayers were grown using magnetron sputtering technique at precisely controlled conditions to maintain sub-nanometer thickness precision and reduced interface roughness. Imaging of optically induced magnetic domains was performed, using optical microscopy based on magneto-optical Kerr effect (MOKE), during laser illumination. We also employed photoemission electron microscope for imaging with enhanced lateral resolution and element selectivity. Our detailed investigation of optically induced domains revealed that AOS in [Co/Pt]$_{N}$ is only possible in a well defined laser influence interval associated with the Curie temperature of the sample. Moreover, we confirmed that helicity-dependent AOS requires multiple laser pulses as well as a fine tuning of the laser beam parameters. In order to further investigate the effect of laser-induced heating on AOS, we illuminated the samples using different laser repetition rates at different temperatures. These experiments allowed us to develop a model based on domain wall dynamics induced by thermal gradient due to the Gaussian intensity profile of the laser beam. We discuss the AOS mechanisms within the framework of this model. In order to gain a detailed insight in AOS, we studied ultrafast demagnetization dynamics of [Co/Pt]$_{3}$ multilayers using a time-resolved MOKE system. We measured characteristic demagnetization and recovery times as a function of the laser influence. Experiments showed that the magnetization quenches within $\sim$300 fs and relaxation occurs in two different timescales pointing towards multiple processes governing the relaxation. Our extensive study of ferromagnetic [Co/Pt]$_{N}$ multilayers and their magnetic response to the femtosecond laser pulses contribute to a clearer physical picture of laser-induced AOS in ferromagnetic multilayers