EuFe2(As(1-x)P(x))2: reentrant spin glass and superconductivity.

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}1$), we explore the complex interplay of superconductivity and ${\mathrm{Eu}}^{2+}$ magnetism. Below 30 K, two magnetic transitions are observed for all P substituted crystals, suggesting a revision of the phase diagram. In addition to the canted $A$-type antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ at $\ensuremath{\sim}20\text{ }\text{ }\mathrm{K}$, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ coexists with superconductivity around $x\ensuremath{\approx}0.2$.