  Correlated magnetic noise in the form of Schumann resonances could introduce
limitations to the gravitational-wave background searches of future Earth-based
gravitational-wave detectors. We consider recorded magnetic activity at a
candidate site for the Einstein Telescope, and forecast the necessary measures
to ensure that magnetic contamination will not pose a threat to the science
goals of this third-generation detector. In addition to global magnetic
effects, we study local magnetic noise and the impact it might have on
co-located interferometers. We express our results as upper limits on the
coupling function of magnetic fields to the interferometer arms, implying that
any larger values of magnetic coupling into the strain channel would lead to a
reduction in the detectors' sensitivity. For gravitational-wave background
searches below $\sim 30$ Hz it will be necessary for the Einstein Telescope
magnetic isolation coupling to be two to four orders of magnitude better than
that measured in the current Advanced LIGO and Virgo detectors.
