  Identifying the microscopic nature of non-equilibrium energy transfer
mechanisms among electronic, spin and lattice degrees of freedom is central for
understanding ultrafast phenomena such as manipulating magnetism on the
femtosecond timescale. Here we use time and angle-resolved photoemission
spectroscopy to go beyond the often-employed ensemble-averaged view of
non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We
show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics
display pronounced variations with electron momentum whereas the magnetic
exchange interaction remains isotropic. This highlights the influence of
lattice-mediated scattering processes and opens a pathway towards unraveling
the still elusive microscopic mechanism of spin-lattice angular momentum
transfer.
