Nonlinear magnetic response in ruthenocuprates

We have performed an investigation of the nonlinear magnetic response in ruthenocuprates. A negative, diverging-like peak at the main magnetic transition TN in RuSr2RECu2O8 (RE = Gd, Y) indicates a possible canted antiferromagnetic order. Another well defined feature above TN points to a blocking of superparamagnetic particles through the T-3 dependence of the third harmonic at higher temperatures. Below TN a nondiverging peak appears, which is strongly affected by the addition of 10% of Cu ions in the RuO2 planes. In RuSr2RE2-xCexCu2O10 the main magnetic transition TM is accompanied by two characteristic temperatures in the third harmonic of the ac susceptibility, in agreement with recent studies from μSR and Mössbauer spectroscopy. We find that the spin-spin correlation temperature is the same in both families of ruthenocuprates