Field-Dependent Diamagnetic Transition in Magnetic Superconductor Sm_1.85Ce_0.15CuO_4-y

The magnetic penetration depth of single crystal Sm_1.85Ce_0.15CuO_4-y was measured down to 0.4 K in dc fields up to 7 kOe. For insulating Sm₂CuO₄, Sm³⁺ spins order at the Néel temperature, T_N=6  K, independent of the applied field. Superconducting Sm_1.85Ce_0.15CuO_4-y (T_c≈23  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  kOe) 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.