  We study the gravitational-wave background produced by f-mode oscillations of
neutron stars triggered by magnetar giant flares. For the gravitational-wave
energy, we use analytic formulae obtained via general relativistic
magnetohydrodynamic simulations of strongly magnetized neutron stars. Assuming
the magnetar giant flare rate is proportional to the star-formation rate, we
show the gravitational-wave signal is likely undetectable by third-generation
detectors such as the Einstein Telescope and Cosmic Explorer. We calculate the
minimum value of the magnetic field and the magnetar giant flare rate necessary
for such a signal to be detectable, and discuss these in the context of our
current understanding of magnetar flares throughout the Universe.
