Using radio observations of thousands of distant quasars, we probe magnetic fields in the circumgalactic medium (CGM) of galaxies from z~0.4 to z~2.4. We find evidence that the diffuse CGM gas was already strongly magnetised when the Universe was roughly half its current age. This result provides new clues about how cosmic magnetic fields grow and evolve over billions of years, from galaxy halos to the large-scale structure of the Universe. These results are published as an A&A Letter: http://arxiv.org/abs/2605.16924
