Mossbauer studies of helimagnetic FeAs

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 [Formula: see text] Å and [Formula: see text] 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 [Formula: see text] 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.

Mössbauer study of superconducting NdFeAsO(0.88)F(0.12) and its parent compound NdFeAsO

(57)Fe Mössbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO (T(N) = 140 K) and superconducting NdFeAsO(0.88)F(0.12) (T(c) = 45 K). A magnetic hyperfine field B(hf) was observed at the (57)Fe nucleus below T(N)∼140 K for NdFeAsO. Below ∼2 K an increase of B(hf) relative to the saturation value was attributed to the transferred B(hf) at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Nd moments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No  B(hf) is observed in superconducting NdFeAsO(0.88)F(0.12) down to 1.5 K.

Phonon density of states of LaFeAsO(1-x)Fx

We have studied the phonon density of states (PDOS) in LaFeAsO(1-x)Fx with inelastic neutron scattering methods. The PDOS of the parent compound (x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x approximately 0.1). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.