Magnetization and ferromagnetic resonance (FMR) studies on RuSr₂(Eu_1.5Ce_0.5)Cu₂O₁₀ magneto-superconductor

Temperature dependence of magnetic susceptibility χ(T) of a pure polycrystalline RuSr₂(Eu_1.5Ce_0.5)Cu₂O₁₀ compound in the zero-field-cooled (ZFC) and the field-cooled branches deviate below 150 K with a sharp upward turn around 100 K. At lower temperatures the ZFC branch shows a cusp at T_cusp = 75 K, a superconducting transition temperature at T_c = 25 K. χ(T) becomes negative around 20 K. Isothermal ferromagnetic M(H) loops are observed up to 80 K. At 5 K, the remanent magnetization and the coercive fields (H_c) are 0.6 Bohr magnetons and 300 Oe, respectively. With increasing T, the H_c does not decrease monotonically as expected for normal FM domains, but in an anomalous way with a peak around 80 K before collapsing above 150 K. Electron spin resonance (ESR) measurements show clear ferromagnetic resonances (FMR) at applied fields of 5 mT. Interestingly enough the change in FMR intensity also exhibits a peak around 80 K, the same as for H_c.