The recently discovered $(\mathrm{Rb},\mathrm{Cs}){\mathrm{EuFe}}_4{\mathrm{As}}_4$ compounds exhibit an unusual combination of superconductivity ($T_c\sim 35\mathrm{K}$) and ferromagnetism ($T_m\sim 15\mathrm{K}$). We have performed a series of x-ray diffraction, ac magnetic susceptibility, dc magnetization, and electrical resistivity measurements on both ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ and ${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$ to pressures as high as $\sim 30\mathrm{GPa}$. We find that the superconductivity onset is suppressed monotonically by pressure while the magnetic transition is enhanced at initial rates of $d{T}_m/dP\sim 1.7\mathrm{K}/\mathrm{GPa}$ and $1.5\mathrm{K}/\mathrm{GPa}$ for ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ and ${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$, respectively. Near 7 GPa, $T_c$ onset and $T_m$ become comparable. At higher pressures, signatures of bulk superconductivity gradually disappear. Room-temperature x-ray diffraction measurements suggest the onset of a transition from tetragonal (T) to a half-collapsed-tetragonal (hcT) phase at $\sim 10\mathrm{GPa}$ (${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$) and $\sim 12\mathrm{GPa}$ (${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$). The ability to tune $T_c$ and $T_m$ into coincidence with relatively modest pressures highlights $(\mathrm{Rb},\mathrm{Cs}){\mathrm{EuFe}}_4{\mathrm{As}}_4$ compounds as ideal systems to study the interplay of superconductivity and ferromagnetism.

• Received 23 May 2018
• Revised 28 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.014518