Abstract
Anomalous superconducting properties are reported through transport, magnetic, and x-ray absorption near-edge spectroscopy (XANES) studies on the weak-ferromagnetic superconductors RuSr 2 GdCu 2 O 8 and RuCa 2 PrCu 2 O 8 . For RuSr 2 GdCu 2 O 8 [T m = 131 K, resistivity T c (onset) = 56 K, T c (zero) = T c (dia) = 39 K], a spontaneous vortex state above T 0 = 30 K and a weak Meissner state below T 0 with a lower critical field B c1 (0) = 12 G is observed due to the weak dipole field (∼ 4 G) of the weak-ferromagnetic order. In RuCa 2 PrCu 2 O 8 (T m = 47 K, possible T c = 37 K), an anomalously large field-cooled diamagnetic shielding signal is observed below T c . Both samples show mixed-valence Ru 4/5+ character from XANES data. © 2006 American Institute of Physics.