Muon spin rotation measurements on RbEuFe${}_4$As${}_4$ under pressure

We report muon spin rotation and magnetization measurements on the magnetic superconductor RbEuFe${}_4$As${}_4$ under hydrostatic pressures up to 3.8 GPa. At ambient pressure, RbEuFe${}_4$As${}_4$ exhibits a superconducting transition at $T_c\approx$ 36.5 K and a magnetic transition at $T_m\approx$ 15 K below which the magnetic and the superconducting order coexist. With increasing pressure, $T_c$ decreases while $T_m$ and the ordered Eu magnetic moment increase. In contrast to iron-based superconductors with ordering Fe moments, the size of the ordered Eu moment is not proportional to $T_m$. The muon spin rotation signal is dominated by the magnetic response impeding the determination of the superconducting properties.