Field-dependent diamagnetic transition in magnetic superconductor $S{m}_{1.85}C{e}_{0.15}Cu{O}_{4-y}$

Abstract

The magnetic penetration depth of single crystal $\mathrm{S}{\mathrm{m}}_{1.85}\mathrm{C}{\mathrm{e}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-\mathrm{y}}$ was measured down to 0.4 K in dc fields up to 7 kOe. For insulating $\mathrm{S}{\mathrm{m}}_2\mathrm{Cu}{\mathrm{O}}_4$, Sm${}^{3+}$ spins order at the Néel temperature, $T_N=6$ K, independent of the applied field. Superconducting $\mathrm{S}{\mathrm{m}}_{1.85}\mathrm{C}{\mathrm{e}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-\mathrm{y}}$ ($T_c\approx 23$ 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.

4 pages, RevTex 4