  We investigate the hydrodynamic stability and transport of magnetic
microswimmers in an external field using a kinetic theory framework. Combining
linear stability analysis and nonlinear 3D continuum simulations, we show that
for sufficiently large activity and magnetic field strengths, a homogeneous
polar steady state is unstable for both puller and pusher swimmers. This
instability is caused by the amplification of anisotropic hydrodynamic
interactions due to the external alignment and leads to a partial
depolarization and a reduction of the average transport speed of the swimmers
in the field direction. Notably, at higher field strengths a reentrant
hydrodynamic stability emerges where the homogeneous polar state becomes stable
and a transport efficiency identical to that of active particles without
hydrodynamic interactions is restored.
