Hyperfine magnetic interactions of $^{57}$Fe nuclei in NaFeAs arsenide

The results of the Mцssbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at $T>T_{\text{N}}$ demonstrate that the main part (${\sim}\,90\%$) of iron atoms are in the low-spin state Fe$^{2+}$. The other atoms can be attributed to the impurity NaFe$_2$As$_2$ phase or to the extended defects in NaFeAs. The structural phase transition (at $T_S\approx55\,$K) does not produce any effect on hyperfine parameters $(\delta,\Delta)$ of iron atoms. At $T<T_{\text{N}}$, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field ($H_{\text{Fe}}$) at $^{57}$Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function $p(H_{\text{Fe}})$ allows us to determine the temperature of the magnetic phase transition ($T_{\text{N}}=46\pm2\,$K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the $H_{\text{Fe}}(T)$ 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 $(V_{ZZ})$ and the asymmetry parameter $(\eta)$ of the tensor describing the electric field gradient at $^{57}$Fe nuclei.