Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

Abstract

The results of the Mossbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > TN demonstrate that the main part (∼90%) of iron atoms are in the low-spin state Fe2+. The other atoms can be attributed to the impurity NaFe2As2 phase or to the extended defects in NaFeAs. The structural phase transition (at TS ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < TN, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (HFe) at 57Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(HFe) allows us to determine the temperature of the magnetic phase transition (TN = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the HFe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (VZZ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at 57Fe nuclei.