Ferromagnetic nanoclusters observed by ac and dc magnetic measurements in RuSr2GdCu2O8 samples.

Both ac and dc magnetic measurements have been performed on polycrystalline RuSr2GdCu2O8 samples, aimed at studying the magnetic ordering of such a phase. Linear ac susceptibility measurements give clear evidence of the antiferromagnetic transition at a temperature T=TN, which has been already widely observed in such system with different techniques. In addition, nonlinear (third-order) susceptibility shows that another magnetic structure exists at a temperature near but slightly higher than TN. Shape and sign of this small signal are compatible with the presence of ferromagnetic nanoparticles exhibiting superparamagnetic behavior. Such possibility has been confirmed by magnetization decay measurements performed at temperatures lower than the blocking temperature TB of the superparamagnetic system. A fit of magnetization decay measurements with a model describing the slow dynamic of magnetic nanoparticle systems gives the size distribution of the nanoparticles and a mean volume of the order of V=2.5×10−19 cm3, in accord with previous estimates. The scenario of RuSr2GdCu2O8 as a phase separated material, already proposed to explain some peculiarities of its magnetic behavior, is here confirmed.