Superparamagnetic behavior of ferromagnetic nanoclusters in RuSr2GdCu2O8 and RuSr2Gd1.6Ce0.4Cu2O10 samples observed by AC and DC magnetic measurements

AC and DC magnetic measurements have been performed on polycrystalline RuSr2GdCu2O8 and RuSr2Gd1.6Ce0.4Cu2O10 samples with the aim to study the complex magnetic structure of these compounds. Our results indicate that RuSr2GdCu2O8, in addition to the well-known antiferromagnetic ordering, exhibits another magnetic feature at temperature slightly higher than the antiferromagnetic ordering temperature, that we attribute to the presence of ferromagnetic nanoclusters that exhibit superparamagnetic behavior. This hypothesis is supported by magnetization decay measurements at various temperatures that show slow decays typical of superparamagnetic systems. The phase RuSr2Gd1.6Ce0.4Cu2O10 shows an even more complicated magnetic behavior, but also in it we observe slow magnetization decays. Therefore, also for this phase we hypothesize the presence of ferromagnetic nanoclusters with blocking temperature higher with respect to that of all the other magnetic structures. (C) 2007 Elsevier B.V. All rights reserved.