  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.
