The magnetic penetration depth of single crystal ${\mathrm{S}\mathrm{m}}_{1.85}{\mathrm{C}\mathrm{e}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-y}$ was measured down to 0.4 K in dc fields up to 7 kOe. For insulating ${\mathrm{S}\mathrm{m}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$, ${\mathrm{S}\mathrm{m}}^{3+}$ spins order at the Néel temperature, $T_N=6\mathrm{K}$, independent of the applied field. Superconducting ${\mathrm{S}\mathrm{m}}_{1.85}{\mathrm{C}\mathrm{e}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-y}$ ($T_c\approx 23\mathrm{K}$) shows a sharp increase in diamagnetic screening below $T^{*}(H)$ which varied from 4.0 K ($H=0$) to 0.5 K ($H=7\mathrm{k}\mathrm{O}\mathrm{e}$) for a field along the $c$ axis. If the field was aligned parallel to the conducting planes, $T^{*}$ remained unchanged. The unusual field dependence of $T^{*}$ indicates a spin-freezing transition that dramatically increases the superfluid density.

• Received 2 June 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.147001