Spontaneous magnetic order in ${\mathrm{Ce}}_{0.73}$${\mathrm{Ho}}_{0.27}$${\mathrm{Ru}}_2$ below the superconducting transition temperature

Mössbauer-effect studies on the Laves-phase alloy ${\mathrm{Ce}}_{0.73}$ ${\mathrm{Ho}}_{0.27}$ ${\mathrm{Ru}}_2$ have been carried out as a function of temperature to investigate the possible coexistence of spontaneous magnetic order and superconductivity. The alloy was dilutely doped with ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$, which substitutes for the ruthenium atoms. At a temperature of about 2 K, the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectra showed the onset of line broadening that increased with decreasing temperature. The broadening is attributed to magnetic ordering of the holmium atoms, which produces magnetic interactions at some of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ impurity nuclei, and is interpreted in terms of a model which yields temperature-dependent hyperfine fields $H_i$. Saturation of $H_i$ is suggested below 0.5 K. A molecular-field model was fit to $H_i$, giving a saturation hyperfine field of 3.47 ± 0.13 T and a magnetic ordering temperature of 1.72 ± 0.03 K. Low-field-susceptibility measurements determined the superconducting transition temperature to be 1.57 K with a half-width of 0.18 K. The paramagnetic data were extrapolated to a Curie temperature of 1.3 ± 0.2 K. There is no evidence for reentrant behavior down to 35 mK. Magnetization measurements gave a magnetic moment of 5.08 ± 0.2${\mathrm{\mu }}_{\mathrm{B}}$ per formula unit, and the derived high-field-susceptibility data gave a Curie temperature of 1.5 ± 0.7 K. The reported results are consistent with the recent neutron scattering measurements on ${\mathrm{Ce}}_{0.73}$ ${\mathrm{Ho}}_{0.27}$ ${\mathrm{Ru}}_2$. The coexistence of superconductivity with some type of short-range ferromagnetism is supported.