We have studied the temperature dependence of the upper critical fields ${\mu }_0{H}_{c2}$ of $K_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{S}_z$ single crystals up to 60 T. The ${\mu }_0{H}_{c2}$ for $H\parallel ab$ and $H\parallel c$ decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg theory including the Pauli spin-paramagnetic effect shows that ${\mu }_0{H}_{c2}$ for $H\parallel ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas ${\mu }_0{H}_{c2}$ for $H\parallel c$ shows a linear temperature dependence with an upturn at high fields. The latter observation can be ascribed to multiband effects that become weaker for higher S content. This results in an enhanced anisotropy of ${\mu }_0{H}_{c2}$ for high S content due to the different trends of the spin-paramagnetic and multiband effect for $H\parallel ab$ and $H\parallel c$, respectively.

• Received 17 January 2014
• Revised 24 June 2014

DOI:https://doi.org/10.1103/PhysRevB.90.024505