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 T-N in RuSr2 RECu2O8 (RE = Gd, Y) indicates a possible canted antiferromagnetic order. Another well defined feature above T-N points to a blocking of superparamagnetic particles through the T-3 dependence of the third harmonic at higher temperatures. Below T-N a nondiverging peak appears, which is strongly affected by the addition of 10% of Cu ions in the RuO2 planes. In RuSr2 RE 2-xCexCu2O10 the main magnetic transition T-M is accompanied by two characteristic temperatures in the third harmonic of the ac susceptibility, in agreement with recent studies from mu SR and Mossbauer spectroscopy. We find that the spin-spin correlation temperature is the same in both families of ruthenocuprates.