The magnetic penetration depth of single crystal $\rm{ Sm_{ 1.85}Ce_{ 0.15}CuO_{ 4-y}}$ was measured down to 0.4 K in dc fields up to 7 kOe. For insulating $\rm{ Sm_2CuO_4}$, Sm$^{ 3+}$ spins order at the N\'{ e}el temperature, $T_N = 6$ K, independent of the applied field. Superconducting $\rm{ Sm_{ 1.85}Ce_{ 0.15}CuO_{ 4-y}}$ ($T_c \approx 23$ K) shows a sharp increase in diamagnetic screening below $T^{ \ast}(H)$ which varied from 4.0 K ($H = 0$) to 0.5 K ($H =$ 7 kOe) for a field along the c-axis. If the field was aligned parallel to the conducting planes, $T^{ \ast}$ remained unchanged. The unusual field dependence of $T^{ \ast}$ indicates a spin freezing transition that dramatically increases the superfluid density.
Categories: cond-mat.supr-con cond-mat.str-el