  Low-frequency polarisation observations of pulsars, facilitated by
next-generation radio telescopes, provide powerful probes of astrophysical
plasmas that span many orders of magnitude in magnetic field strength and
scale: from pulsar magnetospheres to intervening magneto-ionic plasmas
including the ISM and the ionosphere. Pulsar magnetospheres with teragauss
field strengths can be explored through their numerous emission phenomena
across multiple frequencies, the mechanism behind which remains elusive.
Precise dispersion and Faraday rotation measurements towards a large number of
pulsars probe the three-dimensional large-scale (and eventually small-scale)
structure of the Galactic magnetic field, which plays a role in many
astrophysical processes, but is not yet well understood, especially towards the
Galactic halo. We describe some results and ongoing work from the Low Frequency
Array (LOFAR) and the Murchison Widefield Array (MWA) radio telescopes in these
areas. These and other pathfinder and precursor telescopes have reinvigorated
low-frequency science and build towards the Square Kilometre Array (SKA), which
will make significant advancements in studies of astrophysical magnetic fields
in the next 50 years.
