R-M INTERACTIONS IN R(2)BAMO(5) (R=Y OR GD, M=CU OR ZN)

R(2)BaMO(5) (R = Gd or Y; M = Cu or Zn) oxides have been studied using the specific heat, DC magnetic Susceptibility and electron paramagnet ic resonsance (EPR). For one member of the series without a magnetic m oment at M, namely Gd2BaZnO5, measurements reveal long-range antiferro magnetic order at T-N (Gd2BaZnO5) = 2.3 +/- 0.1 K, much lower than the Curie-Weiss temperature Theta of 15.9 +/- 0.3 K. This indicates the e xistence of competing interactions that introduce a large degree of ma gnetic frustration in the system. For Y2BaCuO5 the Cu-Cu interactions are responsible for the broad maximum in the magnetic contribution to the specific heat centred at 18.5 +/- 0.01 K that stretches beyond the instrumental limit of 25 K. The strong Cu-Cu interactions also presen t in Gd2BaCuO5, combined with the Gd-Cu interaction, polarize the Gd s ublattice, 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 ion s is raised to T-N (Gd2BaCuO5) = 12.0 +/- 0.1 K.