${}^{57}\mathrm{Fe}$ Mössbauer study of magnetic ordering in superconducting ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ single crystals

The magnetic ordering of superconducting single crystals of ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ has been studied between 10 and 550 K using ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy. Despite being superconducting below $T$${}_{\mathrm{sc}}~30$ K, the iron sublattice in ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ clearly exhibits magnetic order from well below $T$${}_{\mathrm{sc}}$ to its Néel temperature of $T$${}_N=532\pm 2$ K. The iron moments are ordered almost parallel to the crystal $c$ axis. The order collapses rapidly above 500 K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.

Figure 1

${}^{57}\mathrm{Fe}$ Mössbauer spectra of ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ showing the evolution of the magnetic ordering on heating from 10 K (well below $T$${}_{\mathrm{sc}}~30$ K) to 533 K where the material is paramagnetic. The extreme weakness of the $\Delta m_I=0$ transitions in the ordered state indicates that the moments are almost parallel to the crystal $c$ axis (see text), while the growth of a central paramagnetic component above 500 K is characteristic of a first-order magnetic transition. Solid lines are fits as described in the text.

Figure 2

Backscattered electron analysis image of a cleaved crystal surface of ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ taken at an accelerating voltage of 20 kV. The lighter regions have lower potassium concentrations than the darker background area.

Figure 3

Temperature dependence of the magnetic hyperfine field ($B$${}_{\mathrm{hf}}$) in ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$. The solid line is a fit to a $J=\frac{1}{2}$ Brillouin function between 200 and 500 K that yields an expected transition of $600\pm 30$ K, well above the observed value of $532\pm 2$ K. Fitted errors on $B$${}_{\mathrm{hf}}$ are less than 0.1 T, much smaller than the plotting symbols. The rapid collapse above 500 K is accompanied by the growth of a paramagnetic component (see Fig. 4).

Figure 4

Temperature dependence of the magnetic fraction in ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$. The rapid collapse above 500 K indicates that the magnetic transition has first-order character and may be associated with a structural transition.