Mössbauer spectroscopy measurements on the 35.5 K superconductor Rb1-δEuFe4As4

Abstract

The results of x-ray diffraction and Fe57 and Eu151 Mössbauer spectroscopy measurements, supplemented with ab initio hyperfine-interaction parameter calculations, on the new 35.5 K superconductor Rb1-δEuFe4As4 are presented. The superconductor crystallizes in the tetragonal space group P4/mmm with the lattice parameters a=3.8849(1) Å and c=13.3370(3) Å. 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 TC=16.54(8) K is determined from the temperature dependence of both the hyperfine magnetic field at Eu151 nuclei and the transferred hyperfine magnetic field at Fe57 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 T3/2 power-law relation. Good agreement between the calculated and measured hyperfine-interaction parameters is observed. The Debye temperature of Rb1-δEuFe4As4 is found to be 391(8) K.