Magnetization and ⁵⁷Fe and ¹¹⁹Sn Mössbauer studies of magnetism in RuR_2-xCe_xSr₂Cu ₂O₁₀ (R = Eu, Gd)

Magnetization and ⁵⁷Fe and ¹¹⁹Sn Mössbauer effect studies of the magneto-superconducting Ru_1-xAl _xEu_1.5Ce_0.5Sr₂Cu₂O _10-δ, Ru_0.99⁵⁷Fe_0.01Gd _1.5Ce_0.5Sr₂Cu₂O _10-δ, and Ru_1-x¹¹⁹Sn_xGd _1.5Ce_0.5Sr₂Cu₂O _10-δ systems, have been performed. The studies show that at T_M ∼ 160 K, a sharp rise in magnetization occurs, not changing much down to T_M2 ∼ 90 K and displaying all along hysteresis loops. The coercive field increases when the temperature decreases, reaches a maximum at about 120 K and decreases again almost to zero around 90 K. At this temperature, a second, much larger rise in magnetization takes place, displaying typical ferromagnetic hysteresis loops. At T_c ∼ 30-35 K the system becomes superconducting. The ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectra display at room temperature more than three inequivalent Fe or Sn sites, in accordance to their local environment. At low temperatures the spectra also display two to three magnetic inequivalent sites. Below T _M the spectra display magnetic hyperfine structure only for about 10% of the sample, the rest display magnetic order only below T_M2. While the magnetization studies tend to indicate that the material is homogeneous and around T_M2 it undergoes some kind of spin or structural rearrangement; the ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectra tend to prove the occurrence of phase separation in these compounds. The two contradicting possibilities and an alternative approach are discussed in detail.