In the recent years, the research fields of femtomagnetism and ultrafast spintronics has seen a recent boom in interest. Numerous scientific discoveries in these research field have spawned new concepts for next-generation magnetic recording technologies. The idea and study of all optical switching (AOS) has a large potential for technical advancement in the development of future high-density and fast operation photonic circuits. However, the comprehensive application and mass manufacturing of AOS technology have yet to be achieved, since there are still many unanswered questions on the physics of ultrafast spin dynamics. In this dissertation, we investigate the topics of optically induced ultrafast spin dynamics in multilayer ferromagnetic magnetic systems and its application in AOS. We focused our study particularly on the sample system of Co/Pt3 multilayer structure due to the deterministic switch-ability by ultrafast laser pulses, which has been demonstrated in many previous studies. In the work we have studied both the static and dynamic magnetic properties. For the investigation of static magnetic properties, various experimental technique are utilized, such as static magneto optical Kerr effect , superconducting quantum interference device , Kerr imaging, and x-ray magnetic circular dichroism. The static magnetic property of the Co/Pt3 sample have been thoroughly characterized as a function of ambient temperature. We have measured an enhancement of the orbital magnetic momentum of 64% for Cobalt due to the strong spin orbit coupling of the adjacent Platinum. The magnetic system with a strong spin orbit coupling is the crucial characteristic for the light driven magnetic switching. We studied the dynamic properties of the magnetic system intensively by using the optical pump-probe technique. Magnetic alteration and relaxation of Co/Pt3 ferromagnetic multilayer induced by ultrafast laser pulses has been studied in a wide time range (picoseconds to milliseconds). The results are analyzed by using various models, such as the extended microscopic three temperature model(eM3TM), extended three temperature model (E3TM), Landau-Lifschitz-Bloch model (LLB) as well as our improved phenomenological three temperature model. We show that the optically-induced manipulation and relaxation processes of the magnetic system are governed by multiple mechanisms, which involve longitudinal and transversal relaxation processes, as well as, domain wall motion. All these mechanisms dominate the magnetization dynamic in different time scales. Our experimental results reveal the geometrical evolution of magnetic domain distribution in the laser excited area. we have performed a quantitative analysis on the observed domain wall motion. Based on our results, a physical picture was provided to describe the formation process of the particular "ring-like" AOS pattern. Our results and analyses contribute to the fundamental understanding of spin dynamics in ferromagnetic multilayers at various timescales and are relevant to spintronic applications such as magnetoresistive random access memory (MRAM), AOS, and THz generation.
Fangzhou Wang (Tue,) studied this question.