  The launch of IXPE telescope in late 2021 finally made polarization
measurements in the 2-8 keV band a reality, more than 40 years after the
pioneering observations of the OSO-8 satellite. In the first two years of
operations IXPE targeted more than 60 sources, including four magnetars,
neutron stars with magnetic fields in the petaGauss range. In this paper we
summarize IXPE main findings and discuss their implications for the physics of
ultra-magnetized neutron stars. Polarimetric observations confirmed theoretical
predictions according to which X-ray radiation from magnetar sources is highly
polarized, up to $\approx 80\%$, the highest value detected so far. This
provides an independent confirmation that magnetars are indeed endowed with a
super-strong magnetic field and that the twisted magnetosphere scenario is the
most likely explanation for their soft X-ray emission. Polarization
measurements allowed us to probe the physical conditions of the star's
outermost layers, showing that the cooler surface regions are in a condensed
state, with no atmosphere on top. Although no smoking-gun of vacuum QED effects
was found, the phase-dependent behaviour of the polarization angle strongly
hints that vacuum birefringence is indeed at work in magnetar magnetospheres.
