  Heterostructures composed of the intrinsic magnetic topological insulator
MnBi$_2$Te$_4$ and its non-magnetic counterpart Bi$_2$Te$_3$ host distinct
surface band structures depending on the stacking order and exposed
termination, allowing fine control of their magnetic, electronic and optical
properties. Here, we probe the ultrafast dynamical response of MnBi$_2$Te$_4$
and MnBi$_4$Te$_7$ following near-infrared optical excitation using time- and
angle-resolved photoemission spectroscopy. We gain access to the
out-of-equilibrium surface electronic structure of both MnBi$_2$Te$_4$ and
Bi$_2$Te$_3$ surface terminations of MnBi$_4$Te$_7$, revealing an instantaneous
occupation of states that are resonant with the optical excitation in the
Bi$_2$Te$_3$ layer followed by carrier extraction into the adjacent
MnBi$_2$Te$_4$ layers with a laser fluence-tunable delay of up to 350 fs. The
ensuing thermal relaxation processes are driven by in-plane phonon scattering
with significantly slower relaxation times in the magnetic MnBi$_2$Te$_4$
septuple layers. The competition of interlayer charge transfer and intralayer
phonon scattering establishes MnBi$_2$Te$_4$-based compounds as a platform for
controlling ultrafast charge transfer processes in combination with magnetism
and topology in van der Waals heterostructures.
