The superconducting and ferromagnetic phase boundaries in the (${\mathrm{Er}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ho}}_{\mathrm{x}}$)${\mathrm{Rh}}_4$${\mathrm{B}}_4$ mixed ternary alloy system meet in a multicritical point at $x_{\mathrm{cr}\mathrm{\approxeq }0.9}$. For x<$x_{\mathrm{cr}}$, the compounds first become superconducting as the temperature is lowered, and then lose superconductivity in a transition to ferromagnetism. The coexistence of superconductivity and ferromagnetism for alloys near the erbium-rich end of the phase diagram is well established. It has also been suggested that ferromagnetism and superconductivity coexist in alloys with x just below $x_{\mathrm{cr}}$. We have carried out neutron-diffraction, ac magnetic susceptibility, and heat-capacity measurements on a sample of (${\mathrm{Er}}_{0.16}$${\mathrm{Ho}}_{0.84}$)${\mathrm{Rh}}_4$${\mathrm{B}}_4$ to investigate the possibility of coexistence of ferromagnetism and superconductivity for x≊$x_{\mathrm{cr}}$. We find that there are minor discrepancies in the superconducting and magnetic transition temperatures reported for different samples of (${\mathrm{Er}}_{0.16}$${\mathrm{Ho}}_{0.84}$)${\mathrm{Rh}}_4$${\mathrm{B}}_4$, but that ferromagnetism and superconductivity do occur simultaneously over a narrow temperature range in this sample. We suggest that an inhomogeneous state occurs, consisting of separate ferromagnetic and superconducting regions, rather than microscopic coexistence.

• Received 3 June 1988

DOI:https://doi.org/10.1103/PhysRevB.38.8724