99Ru Mössbauer effect measurements in superconducting and magnetic RuSr2GdCu2O8

99Ru Mossbauer effect measurements have been carried out at a temperature of 4.2 K on a sample RuSr 2 GdCu 2 O 8 with a Neel temperature of T N =132 K and superconducting transition temperature T c = 45 K. The hyperfine magnetic field was determined to be 59.1(1.0) T at 4.2 K and the isomer shift was measured to be +0.05(0.02) mm/s, determining a valence state for the single Ru site to be close to +5. The Mossbauer measurements also show an electric quadrupole interaction eQV z z of 0.37(.04) mm/s. These results differ from the NMR results in RuSr 2 GdCu 2 O 5 and RuSr 2 YCu 2 O 8 , where two different hyperfine magnetic fields (∼60 T, ∼29 T) and charge states (+5, +4) were found for Ru.