New antiferromagnetic order with pressure-induced superconductivity in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$

The interplay between magnetism and superconductivity is one of the important subjects to investigate the pairing mechanism in novel superconductors. We have found a new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ superconductor in the pressure range from 2.4 to 3.0 GPa by ${}^{57}\mathrm{Fe}$ nuclear forward scattering (NFS) using a single-crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in the normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of ${}^{57}\mathrm{Fe}$ NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This nontrivial coupling of two ordered states in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.

Figure 1

Selected ${}^{57}\mathrm{Fe}$ (left) and ${}^{151}\mathrm{Eu}$ (right) NFS spectra of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ under pressure at 4 and 3 K, respectively. Black circles with error bars are the observed spectra. Red lines represent the best-fit curves obtained by motif, respectively [30].

Figure 2

Pressure dependences of (a) $H_{\mathrm{hf}}^{\mathrm{Fe}}$, (b) $H_{\mathrm{hf}}^{\mathrm{Eu}}$, and (c) $T_{\mathrm{o}}^{\mathrm{Fe}}$(red squares) and $T_{\mathrm{o}}^{\mathrm{Eu}}$(green circles) in the Fe and Eu sublattices of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$, respectively, evaluated by the ${}^{57}\mathrm{Fe}$ and ${}^{151}\mathrm{Eu}$ NFS experiments. Green triangles and the blue area in (c) represent $T_{\mathrm{o}}^{\mathrm{Eu}}$ and the superconductivity region, respectively, obtained by $4\pi M/H$ under pressure. The ambient pressures $T_{\mathrm{o}}^{\mathrm{Fe}}$ and $T_{\mathrm{o}}^{\mathrm{Eu}}$ were evaluated by magnetization and resistivity measurements [22]. The red and green lines are visual guides. Dashed lines indicate $p_{\mathrm{c}}^{\mathrm{L}}$ and $p_{\mathrm{c}}^{\mathrm{H}}$. The cartoon in (a) illustrates the directions of $H_{\mathrm{hf}}^{\mathrm{Fe}}$'s in the mAFM state along ${\mathit{k}}_0$ extracted from the analysis of the ${}^{57}\mathrm{Fe}$ NFS spectrum at 4 K and 2.7 GPa. The blue circles represent the Fe atoms and the solid (open) arrows indicate the spectroscopy nonequivalent $H_{\mathrm{hf}}^{\mathrm{Fe}}$'s.

Figure 3

Temperature dependences of (a) $4\pi M/H$ along the [001] axis, (b) $R_{\mathrm{mAFM}}$ (black squares), $V_{\mathrm{sc}}$ (black circles), and (c) the ${}^{57}\mathrm{Fe}$ NFS spectra of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 2.7 GPa. The inset in (b) shows $H_{\mathrm{hf}}^{\mathrm{Fe}}$ at 2.7 GPa as a function of temperature. The red solid lines in (b) and (c) are visual guides and the best-fit curves obtained by motif [30], respectively.

Figure 4

Pressure dependence of total energy $E$ in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ calculated using DFT. Blue and red circles indicate the NM and sAFM state, respectively. The inset shows the pressure dependence of the magnetic moment $m_{\mathrm{Fe}}$ in Fe sublattice.