Spin freezing in Sm$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$

Insulating Sm$_{2}$CuO$_{4}$ shows antiferromagnetic ordering of Sm spins at T$_{Neel}$ = 6 K. T$_{Neel }$ is insensitive to external fields. Penetration depth measurements in superconducting Sm$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ (T$_{C}$ = 23 K) reveal a sharp increase of diamagnetism below T$^{\ast }$= 4.2 K. A c-axis magnetic field rapidly suppresses T*. The strong field dependence of T* suggests a spin-freezing transition below which spin-flip scattering is reduced, the density of states becomes sharper and Meissner screening is enhanced.[1] We discuss the role of Cu $^{2+}$ spins in the observed behavior. Work at UIUC supported through NSF DMR 01-01872. Work at USC supported by the NSF/EPSCoR, Grant No. EPS-0296165. PF acknowledges the support of CIAR, CFI, NSERC, FQRNT and the Universite de Sherbrooke.[1] R. Prozorov et. al., Phys. Rev. Lett. \textbf{93}, 147001 (2004)