Superconductivity and Field-Induced Magnetism in ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_2$

Using muon-spin rotation, we studied the in-plane (${\lambda }_{ab}$) and the out of plane (${\lambda }_c$) magnetic field penetration depth in ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_2$ ($T_c\simeq 13.3\mathrm{K}$). The penetration depth anisotropy ${\gamma }_{\lambda }={\lambda }_c/{\lambda }_{ab}$ increases from ${\gamma }_{\lambda }\simeq 2.1$ at $T_c$ to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.

Figure 1

ZFC and FC magnetic susceptibility ($\chi$) of ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_2$ (the paramagnetic contribution measured at $T=22\mathrm{K}$ is subtracted). $T_c$ is obtained as the intersect of linearly extrapolated ${\chi }_{\mathrm{ZFC}}(T)$ with the zero line.

Figure 2

(a) ZF $\mu \mathrm{SR}$ time spectra of ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_2$. The lines are fits by Eq. (1). Insets show the temperature dependences of normalized asymmetries and the extended part of the ZF $\mu \mathrm{SR}$ data for $t\le 0.75\mu \mathrm{s}$. (b),(c) The magnetic field distributions $P(B)$ obtained in the $H\parallel c$ and $H\perp c$ set of TF $\mu \mathrm{SR}$ experiments. The lines are the fits by Eq. (4). The insets represent the contour plots of field variation.

Figure 3

(a) Dependence of ${\lambda }_{ab}^{-2}$ and ${\lambda }_c^{-2}$ on temperature of ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_2$. The solid and the dashed lines are the fits by Eq. (5) within the clean and the dirty limit with the parameters summarized in Table . The inset shows the temperature dependence of the penetration depth anisotropy ${\gamma }_{\lambda }={\lambda }_c/{\lambda }_{ab}$. (b) Temperature dependence of $⟨B⟩-⟨B{⟩}^{\mathrm{ns}}$ obtained in the $H\parallel c$ [red (light gray) symbols] and $H\perp c$ [blue (dark gray) symbols] set of experiments. The inset shows the dependence of $⟨B⟩(1.6\mathrm{K})-⟨B{⟩}^{\mathrm{ns}}$ on $⟨B{⟩}^{\mathrm{ns}}$ in $H\parallel c$ orientation.