Single crystals K${}_{\mathrm{x}}$Fe${}_{2-y}$Se${}_{2-z}$S${}_{\mathrm{z}}$ ($z$ $=$ 0, 0.4) are successfully synthesized by self-flux method with the superconducting transition temperatures $T_c$${}^{\mathrm{onset}}$ of 31.3 K and 32.8 K, respectively. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress $T_c$, which suggests that chemical doping may mainly modulate the anion height from the Fe layer rather than compressing interlayer distance. In electron spin resonance (ESR) measurements, a resonance signal arising from the paramagnetic (PM) Fe ions is detected at room temperature for both K${}_{\mathrm{x}}$Fe${}_{2-y}$Se${}_2$ and K${}_{\mathrm{x}}$Fe${}_{2-y}$Se${}_{1.6}$S${}_{0.4}$ crystals. Upon cooling, the intensity of the spectrum weakens gradually and displays abrupt decrease below 140 K. The resonance signal disappears around 60 K (∼2 $T_c$), corresponding to the saturation temperature of antiferromagnetic (AFM) order revealed by recent neutron-diffraction study. The evolution of ESR spectrum is discussed by correlating with recent neutron-diffraction and Raman-scattering results, in support of the coexistence of superconductivity and AFM order in K${}_{\mathrm{x}}$Fe${}_{2-y}$Se${}_2$. Moreover, the variation of resonance field with decreasing temperature for K${}_{\mathrm{x}}$Fe${}_{2-y}$Se${}_{1.6}$S${}_{0.4}$ implies that the doping of S induces local weak ferromagnetism in the parent AFM background.

• Received 28 January 2011

DOI:https://doi.org/10.1103/PhysRevB.84.174501