  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 RuSr2RECu2O8 (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 RuSr2RE(2-x)Ce(x)Cu2O10 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 uSR and Mossbauer
spectroscopy. We find that the spin-spin correlation temperature is the same in
both families of ruthenocuprates.
