${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathbf{P}}_x{)}_2$: Reentrant Spin Glass and Superconductivity

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of ${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ ($0\le x\le 1$), 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 $\sim 20\mathrm{K}$, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of ${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ coexists with superconductivity around $x\approx 0.2$.

Figure 1

$H\Vert ab$ magnetization of ${\mathrm{EuFe}}_2{\mathrm{P}}_2$. (a) The ZFC (black open squares) and FCC (orange dots) curves at 2 G show two consecutive magnetic transitions $T_1$ and $T_2$, with a strong hysteresis at $T<T_2$ (upper panel). The transition at $T_2$ is completely suppressed with ${\mu }_{0}H=500\mathrm{G}$ (lower panel). (b) The heat capacity for our single crystal (purple filled diamonds) and for polycrystalline ${\mathrm{EuFe}}_2{\mathrm{P}}_2$ [24] (open dark blue stars) shows a broad feature covering the two magnetic transitions. (c) ZFC magnetization for 500 G, 0.1 T, 0.3 T, 0.5 T, and 1 T. $T_1$ decreases with increasing external field, also depicted in the inset. (d) Magnetic hysteresis sets in at $T<T_2$, visible in ${\chi }_{\mathrm{dc},\mathrm{FCC}}-{\chi }_{\mathrm{dc},\mathrm{ZFC}}>0$ (filled green stars) (${\mu }_{0}H=2\mathrm{G}$). Time dependence for $T<T_2$ is revealed by FC cycling, visible in ${\chi }_{\mathrm{dc},\mathrm{FCH}}-{\chi }_{\mathrm{dc},\mathrm{FCC}}<0$ (open pink diamonds) and (e) time-dependent magnetization after ZFC cooling (${\mu }_{0}H=2\mathrm{G}$; open pink circles, 27.5 K; closed blue squares, 25 K; dark blue line, fit of 25 K data). (f) Frequency dependence in ac susceptibility ${\chi }_{\mathrm{ac}}^{\prime }(T)$ and ${\chi }_{\mathrm{ac}}^{\prime \prime }(T)$ (no dc field; ac drive amplitude 1 G; 0.1 Hz, dark grey open triangles; 100 Hz, filled brown dots) also sets in below $T_2$. (g) Vogel-Fulcher fit of the peak below $T_2$ in ${\chi }_{\mathrm{ac}}^{\prime \prime }(T)$.

Figure 2

${\mathrm{EuFe}}_2({\mathrm{As}}_{0.835}{\mathrm{P}}_{0.165}{)}_2$. In-plane resistivity ${\rho }_{ab}(T)$ shows (a) linear temperature dependence at high temperature and (b) reentrant superconductivity with onset $T_{c,\mathrm{on}}^{*}\approx 22\mathrm{K}$ and zero resistivity $T_{\rho =0}^{*}\approx 9\mathrm{K}$. ZFC (black open squares) as well as FCC (filled brown dots) magnetization shows for (c) $H\Vert ab$ (2 G) two magnetic transitions similar to $x=1.0$ ($T_2$ is indicated by a dashed blue line), with a steep drop in the ZFC curve below $\sim 15\mathrm{K}$ (dotted grey arrow), which also appears for (d) $H\Vert c$ (dotted grey line), where negative magnetization is reached. (e) FC cycling reveals two time-dependent glassy transitions, as $\mathrm{FCH}-\mathrm{FCC}$ $>$ at $T<T_{c,\mathrm{on}}$ and $\mathrm{FCH}-\mathrm{FCC}$ $<$ at $T<T_2$. (f) Combining ac susceptibility measurements (drive 1 G, frequency 7 Hz) with high dc fields (100 G, filled orange dots; 0.3 T, filled pink diamonds; 1 T, open purple circles), negative magnetization is revealed for $H\Vert ab$ below 15 K (dotted grey line).

Figure 3

Phase diagram of ${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$. $T_{c\mathrm{AFM}}$ (red dots) indicates a canted $A$-type antiferromagnetic transition, with a ferromagnetic net component of the ${\mathrm{Eu}}^{2+}$ spins along the $c$ direction, $T_{\mathrm{glass}}$ (blue triangles) a spin glass transition due to the freezing of the spins in the $ab$ plane, and $T_{c,\mathrm{on}}$ the onset of superconductivity (grey squares). Closed symbols indicate transition temperatures deduced from magnetization, open ones temperatures deduced from resistivity measurements. Shadowed lines are guides to the eye. The light grey area indicates the onset of superconductivity, while bulk superconductivity is fully developed in the dark grey regime [12, 36]. The right panel shows typical corresponding $M(T)$ curves. Nonsuperconducting $x=0.09$ (ZFC, open squares; FCC, pink dots), superconducting $x=0.17$ (ZFC, open squares; FCC, brown dots), as well as nonsuperconducting $x=0.35$ (ZFC, open squares; FCC, orange dots) samples are selected. Arrows indicate the transition temperatures for the antiferromagnetic (solid red lines), spin glass (dashed blue lines), and superconducting (dotted grey line) phases.