Magnetism of the 35 K superconductor CsEuFe₄As₄

The results of ab initio hyperfine-interaction parameters calculations, and of x-ray diffraction and ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy study of the new 35 K superconductor CsEuFe₄As₄ are reported. The superconductor crystallizes in the tetragonal space group P4/mmm with the lattice parameters $a=3.8956\left(1\right)$ Å and $c=13.6628\left(5\right)$ Å. It is demonstrated unequivocally that there is no magnetic order of the Fe magnetic moments down to 2.1 K and that the ferromagnetic order is associated with the Eu magnetic moments. The Curie temperature $T_C=15.97\left(8\right)$ K determined from the temperature dependence of the hyperfine magnetic field at ¹⁵¹Eu nuclei is shown to be compatible with the temperature dependence of the transferred hyperfine magnetic field at ⁵⁷Fe nuclei that is induced by the ferromagnetically ordered Eu sublattice. The Eu magnetic moments are shown to be perpendicular to the crystallographic c-axis. The temperature dependence of the principal component of the electric field gradient tensor, both at 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 CsEuFe₄As₄ is found to be 295(3) K. (paper)