  We report comprehensive high pressure studies on correlation between
intercalated magnetic layers and superconductivity in EuFe2(As0.81P0.19)2
single crystal through in-situ high pressure resistance, specific heat, X-ray
diffraction and X-ray absorption measurements. We find that an unconfirmed
magnetic order of the intercalated layers coexists with superconductivity in a
narrow pressure range 0-0.5GPa, and then it converts to a ferromagnetic (FM)
order at pressure above 0.5 GPa, where its superconductivity is absent. The
obtained temperature-pressure phase diagram clearly demonstrates that the
unconfirmed magnetic order can emerge from the superconducting state. In stark
contrast, the superconductivity cannot develop from the FM state that is
evolved from the unconfirmed magnetic state. High pressure X-ray absorption
(XAS) measurements reveal that the pressure-induced enhancement of Eu's mean
valence plays an important role in suppressing the superconductivity and tuning
the transition from the unconfirmed magnetic state to a FM state. The unusual
interplay among valence state of Eu ions, magnetism and superconductivity under
pressure may shed new light on understanding the role of the intercalated
magnetic layers in Fe-based superconductors.
