Mapping the energy-time landscape of spins with helical X-rays

Abstract: Unveiling the key mechanisms that determine optically driven spin dynamics is essential both to probe the fundamental nature of ultrafast light-matter interactions, but also to drive future technologies of smaller, faster, and more energy efficient devices. Essential to this task is the ability to use experimental spectroscopic tools to evidence the underlying energy- and spin-resolved dynamics of non-equilibrium electron occupations. In this joint theory and experimental work, we demonstrate that ultrafast helicity-dependent soft X-ray absorption spectroscopy (HXAS) allows access to spin-, time- and energy specific state occupation after optical excitation. We apply this method to the prototype transition metal ferromagnet cobalt and find convincing agreement between theory and experiment. The richly structured energy-resolved spin dynamics unveil the subtle interplay and characteristic time scales of optical excitation and spin-orbit induced spin-flip transitions in this material: the spin moment integrated in an energy window below the Fermi level first exhibits an ultrafast increase as minority carriers are excited by the laser pulse, before it is reduced as spin-flip process in highly localized, low energy states start to dominate. The results of this study demonstrate the power of element specific transient HXAS, placing it as a potential new tool for identifying and determining the role of fundamental processes in optically driven spin dynamics in magnetic materials.