  The recent discovery of iron ferropnictide superconductors has received
intensive concerns on magnetic involved superconductors. Prominent features of
ferropnictide superconductors are becoming apparent: the parent compounds
exhibit antiferromagnetic (AFM) ordered spin density wave (SDW) state; the
magnetic phase transition is always accompanied to a crystal structural
transition; superconductivity can be induced by suppressing the SDW phase via
either chemical doping or applied external pressure to the parent state. These
features generated considerable interests on the interplay between magnetism
and structure in chemical doped samples, showing crystal structure transitions
always precedes to or coincide with magnetic transition. Pressure tuned
transition on the other hand would be more straightforward to superconducting
mechanism studies since there are no disorder effects caused by chemical
doping; however, remarkably little is known about the interplay in the parent
compounds under controlled pressure due to the experimental challenge of in
situ measuring both of magnetic & crystal structure evolution at high pressure
& low temperatures. Here we show from combined synchrotron Mossbauer and x-ray
diffraction at high pressures that the magnetic ordering surprisingly precedes
the structural transition at high pressures in the parent compound BaFe2As2, in
sharp contrast to the chemical doping case. The results can be well understood
in terms of the spin fluctuations in the emerging nematic phase before the long
range magnetic order that sheds new light on understanding how parent compound
evolves from a SDW state to a superconducting phase, a key scientific inquiry
of iron based superconductors.
