  Magnetic fields are expected to be efficiently amplified during the formation
of the first massive black holes via the small-scale dynamo and in the presence
of strong accretion shocks occurring during gravitational collapse. Here, we
analyze high-resolution cosmological magneto-hydrodynamical simulations of
gravitational collapse in atomic cooling halos, exploring the dynamical role of
magnetic fields, particularly concerning the effect of magnetic braking and
angular momentum transport. We find that after the initial amplification,
magnetic fields contribute to the transport of angular momentum and reduce it
compared to pure hydrodynamical simulations. However, the magnetic and Reynolds
torques do not fully compensate for the inward advection of angular momentum,
which still accumulates over timescales of $\sim1$~Myr. A Jeans analysis
further shows that magnetic pressure strongly contributes to suppressing
fragmentation on scales of $0.1-10$~pc. Overall, the presence of magnetic
fields thus aids in the transport of angular momentum and favors the formation
of massive objects.
