  The recently discovered (Rb,Cs)EuFe4As4 compounds exhibit an unusual
combination of superconductivity (Tc = 35 K) and ferromagnetism (Tm = 15 K). We
have performed a series of x-ray diffraction, ac magnetic susceptibility, dc
magnetization, and electrical resistivity measurements on both RbEuFe4As4 and
CsEuFe4As4 to pressures as high as 30 GPa. We find that the superconductivity
onset is suppressed monotonically by pressure while the magnetic transition is
enhanced at initial rates of dTm/dP = 1.7 K/GPa and 1.5 K/GPa for RbEuFe4As4
and CsEuFe4As4, respectively. Near 7 GPa, Tc onset and Tm 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 10
GPa (RbEuFe4As4) and 12 GPa (CsEuFe4As4). The ability to tune Tc and Tm into
coincidence with relatively modest pressures highlights (Rb,Cs)EuFe4As4
compounds as ideal systems to study the interplay of superconductivity and
ferromagnetism.
