R-M interactions in R2BaMO5 (R=Y or Gd and M=Cu or Zn)

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.