  Many compounds based on CuO_2 planes (cuprates) super-conduct below a
critical temperature Tc. Some of them show a second phase where a spontaneous
static magnetic field appears below a critical temperature Tg, which is lower
then Tc. By comparing Tc and Tg in numerous superconducting families, each with
its own maximum Tc, we find that the same energy scale determines both critical
temperatures. This clearly indicates that superconductivity and magnetism are
intimately related in the cuprates.
