Superconductivity and ferromagnetism in hole-doped ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$

We discover a robust coexistence of superconductivity and ferromagnetism in an iron arsenide ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$. The new material crystallizes in an intergrowth structure of ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ and ${\mathrm{EuFe}}_2{\mathrm{As}}_2$, such that the Eu sublattice turns out to be primitive instead of being body-centered in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$. The FeAs layers, featured by asymmetric As coordinations, are hole doped due to charge homogenization. Our combined measurements of electrical transport, magnetization, and heat capacity unambiguously and consistently indicate bulk superconductivity at 36.5 K in the FeAs layers and ferromagnetism at 15 K in the Eu sublattice. Interestingly, the Eu-spin ferromagnetic ordering belongs to a rare third-order transition, according to the Ehrenfest classification of phase transitions. We also identify an additional anomaly at $\sim 5$ K, which is possibly associated with the interplay between superconductivity and ferromagnetism.

Figure 1

Rietveld refinement profile for the powder x-ray diffraction of ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ whose crystal structure is displayed in the inset. The relatively strong reflections in low diffraction angles are indexed.

Figure 2

(a) Temperature dependence of resistivity (left axis) and Hall coefficient (right axis) of the ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ polycrystalline sample. The inset zooms in the superconducting transition under zero field. (b) The superconducting resistive transitions under increased external magnetic fields. (c) Plot of the upper critical field as a function of temperature, derived from the data shown in (b). The dashed line is the linear fit.

Figure 3

Magnetic susceptibility of the ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ polycrystalline sample under low magnetic fields in field-cooling (FC) and zero-field-cooling (ZFC) modes, respectively. The superconducting transition at $T_{\text{sc}}^{\mathrm{onset}}=36.5$ K and the magnetic transition at $T_{\text{Curie}}=15$ K are marked by arrows, respectively. The inset magnifies low-temperature data from which a minor anomaly at $T^{*}=5$ K can be distinguished. Right axis: FC data measured in cooling (FCC) and heating (FCH) processes; left axis: ZFC data.

Figure 4

Isothermal magnetization of ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ at several representative temperatures. The top-left inset zooms in the data in the low-field region for $T=20$, 30, and 40 K.

Figure 5

Magnetic susceptibility ($M/H$) of ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ measured under $H=1$ kOe. The data from 50 to 300 K are fitted by the extended Curie law. The reciprocal of susceptibility ($H/M$) is shown using the right axis. The inset shows the temperature dependence of the magnetization in the low-$T$ region, in which a canonical ferromagnetic transition can be seen. The $T_{\text{Curie}}$ value can be estimated from the derivative of the magnetization (right axis).

Figure 6

Heat capacity measurements for ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$. The characteristic temperatures of superconducting transition ($T_{\text{sc}}$), ferromagnetic transition ($T_{\text{Curie}}$), and an unknown possible transition ($T^{*}$) are indicated by arrows, respectively. (a) Raw data of the specific heat, $C\left(T\right)$, at zero field. The inset zooms in the superconducting transition where a large specific-heat jump is determined. Panel (b) compares the $C\left(T\right)$ data at zero field and at 8 T. The inset magnifies the plot below 8 K, from which an anomaly can be distinguished at zero field. Panel(c) plots $C/T$ vs $T$ under various magnetic fields. The inserted plot shows $C/T$ vs $T^2$ for the low-$T$ data. (d) shows the derivative of specific heat (the data under fields are shifted downward for clarity).