  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.
