Mössbauer spectroscopy measurements on the 35.5 K superconductor Rb1−δEuFe4As4

Abstract

The results of x-ray diffraction and $^{57}\mathrm{Fe}$ and $^{151}\mathrm{Eu}$ M\"ossbauer spectroscopy measurements, supplemented with ab initio hyperfine-interaction parameter calculations, on the new 35.5 K superconductor ${\mathrm{Rb}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{EuFe}}_{4}{\mathrm{As}}_{4}$ are presented. The superconductor crystallizes in the tetragonal space group $P4/mmm$ with the lattice parameters $a=3.8849(1)$ \AA{} and $c=13.3370(3)$ \AA{}. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature ${T}_{\mathrm{C}}=16.54(8)$ K is determined from the temperature dependence of both the hyperfine magnetic field at $^{151}\mathrm{Eu}$ nuclei and the transferred hyperfine magnetic field at $^{57}\mathrm{Fe}$ nuclei that is induced by the ferromagnetically ordered Eu sublattice. The Eu magnetic moments are demonstrated to be perpendicular to the crystallographic $c$ axis. The temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a ${T}^{3/2}$ power-law relation. Good agreement between the calculated and measured hyperfine-interaction parameters is observed. The Debye temperature of ${\mathrm{Rb}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{EuFe}}_{4}{\mathrm{As}}_{4}$ is found to be 391(8) K.