Separation between antiferromagnetic and ferromagnetic transitions in Ru_1−xCu_xSr₂EuCu₂O_8+δ

The macroscopic magnetizations of Ru_1−xCu_xSr₂EuCu₂O_8+δ with x between 0 and 0.15 were investigated. A ferromagnetlike transition as well as an antiferromagnetlike transition appear around T_M in the low-field magnetization and around T_AM in the high-field differential susceptibility, respectively. The separation between them, which is accompanied by a flat plateau in the magnetic C_p, increases with x. Superparamagnetic M(H) and slow spin dynamics, i.e., characteristics of nanomagnetic clusters, were observed far above T_M. A comparison with RuSr₂(Eu_1−yCe_y)Cu₂O_10+δ and some manganites further suggests that a phase separation occurs, which can describe well the conflicting magnetic-superconductivity data previously reported.