Magneto-optical study of ultrafast demagnetization, spin accumulation, and related torques

Ultrafast magnetization manipulation is a key challenge for developing faster, more energy- efficient information-storage devices. For this, ultrafast spintronics offers new possibilities by exploiting spin degrees of freedom, leveraging spin transport to manipulate and reverse magnetization. Such spin transport creates non-equilibrium spin accumulation; however, probing these accumulations or their induced torques is believed to be strenuous, and their role in ultrafast magnetization reversal remains the subject of debate.

This thesis investigates the ultrafast dynamics of magnetic systems under femtosecond optical and picosecond electrical excitation using time-resolved magneto-optical techniques. First, we examine the magneto-optical Kerr effect's ability to distinguish ultrafast demagnetization from spin accumulation in magnetic heterostructures under different conditions. We then apply this understanding to study ultrafast magnetization reversal in ferromagnetic spin valves and propose a reversal mechanism responsible for all-optical switching in these devices. We then explore dynamics in different systems induced by picosecond current pulses to investigate spin-orbit torques in heterostructures consisting of a ferromagnet paired with a light metal, which exhibit high charge-to-spin conversion efficiency. Keywords: Magneto-optics, ultrafast demagnetization, spin accumulation, ultrafast magneti- zation reversal, Terahertz spin-orbit torques.