Sulfur-induced magnetism in $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ revealed by muon spin rotation/relaxation

Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature-dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism at low temperatures in $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films. Although the behaviors of the magnetic and nematic phases in FeSe films toward chemical pressure by S substitution are similar to those for bulk FeSe toward hydrostatic pressure, the behavior of $T_{\mathrm{c}}$ is significantly different between these systems. Our results demonstrate that the detailed comparative investigation among physical and chemical pressure effects is essentially important to understand the interplay of the magnetism, the nematicity, and the superconductivity in iron chalcogenides.

Figure 1

(a), (b) Temperature dependence of the normalized dc electrical resistivity of the $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ films on LAO with (a) $x=0.3$ (S30-1 and S30-2) and (b) $x=0.4$ (S40-1 and S40-2). (c) SEM image for a film with $x=0.3$. (d) Cross-sectional TEM image for a film with $x=0.4$. This image was taken at a place where there were no precipitates at the surface.

Figure 2

Muon stopping profiles from Monte Carlo calculations for (a) 50-nm-thick $\mathrm{Fe}{\mathrm{Se}}_{0.7}{\mathrm{S}}_{0.3}$ film with $E_{\mathrm{imp}}=3$ keV and (b) 60-nm-thick $\mathrm{Fe}{\mathrm{Se}}_{0.6}{\mathrm{S}}_{0.4}$ film with $E_{\mathrm{imp}}=4$ keV.

Figure 3

Zero-field (ZF) muon time spectra of the $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films with (a) $x=0.3$ and (b) $x=0.4$. Solid lines are the fitted curves obtained by a two-component analysis.

Figure 4

Temperature dependence of (a) the initial asymmetry, $A_0$, (b) the relaxation rate of muon spins of the slow component, ${\lambda }_0$, and (c) the relaxation rate for the fast component, ${\lambda }_1$, in Eq. (1) obtained by the wTF $\mu \mathrm{SR}$ at 50 G for $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ films with $x=0.3$ and 0.4. Solid curves are guides for the eye.

Figure 5

(a), (b) LF muon time spectra for (a) $x=0.3$ at 5 K and (b) $x=0.4$ at 2.6 K. Solid lines are the fitted curves with a two-component analysis. (c) The normalized initial asymmetry as a function of applied longitudinal field. Solid lines are the fitting results by the theoretical curves for the case of homogeneous internal magnetic field in samples.