  R2BaMO5 (R = Gd, Y and M = Cu, Zn) oxides have been studied by specific heat,
dc magnetic susceptibility, and electron paramagnetic resonance (EPR). For one
member of the series without magnetic moment at M, namely Gd2BaZnO5,
measurements reveal long range antiferromagnetic order at TN(Gd2BaZnO5) =
2.3(0.1)K, much lower than the Curie Weiss temperature of {\Theta}=15.9(0.3)K.
This indicates the existence of competing interactions that introduce a large
degree of magnetic frustration in the system. For Y2BaCuO5 the Cu Cu
interactions are responsible for the broad maximum in the magnetic contribution
to the specific heat centered at 18.5(0.01)K that stretches beyond the
instrumental limit of 25K. The strong Cu Cu interactions also present in
Gd2BaCuO5, combined with the Gd-Cu interaction, polarize the Gd sublattice
giving measurable contributions to the specific heat in the same temperature
range. In addition, they broaden the Gd EPR line and saturate its integrated
intensity. The ordering temperature of Gd ions is raised to
TN(Gd2BaCuO5)=12.0(0.1)K.
