  A hallmark of the iron-based superconductors is the strong coupling between
magnetic, structural and electronic degrees of freedom. However, a universal
picture of the normal state properties of these compounds has been confounded
by recent investigations of FeSe where the nematic (structural) and magnetic
transitions appear to be decoupled. Here, using synchrotron-based high-energy
x-ray diffraction and time-domain Moessbauer spectroscopy, we show that
nematicity and magnetism in FeSe under applied pressure are indeed strongly
coupled. Distinct structural and magnetic transitions are observed for
pressures, 1.0 GPa <= p <= 1.7 GPa, which merge into a single first-order phase
line for p >= 1.7 GPa, reminiscent of what has been observed, both
experimentally and theoretically, for the evolution of these transitions in the
prototypical doped system, Ba(Fe[1-x]Co[x])2As2. Our results support a
spin-driven mechanism for nematic order in FeSe and provide an important step
towards a universal description of the normal state properties of the
iron-based superconductors.
