Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of single-crystalline K_xFe_2-ySe_2-zS_z

We have studied the temperature dependence of the upper critical fields µ₀H_c2 of K_xFe_2-ySe_2-zS_z single crystals up to 60 T. The µ₀H_c2 for H parallel to ab and H parallel to c decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg theory including the Pauli spin-paramagnetic effect shows that µ₀H_c2 for H parallel to ab is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas µ₀H_c2 for H parallel to 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 µ₀H_c2 for high S content due to the different trends of the spin-paramagnetic and multiband effect for H parallel to ab and H parallel to c, respectively.