Field-dependent diamagnetic transition in magnetic superconductor Sm₁.₈₅ Ce₀.₁₅ Cu O₄₋y

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The magnetic penetration depth of single crystal Sm₁.₈₅Ce₀.₁₅CuO₄₋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, Tₙ = 6 K, independent of the applied field. Superconducting Sm₁.₈₅Ce₀.₁₅CuO₄₋y (Tc ≈ 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.