Structural and magnetic properties of A-site ordered manganites RBaMn$_2$O$_6$ (R=Pr, Nd, Pr$_{1/2}$Nd$_{1/2}$)

Temperature and magnetic-field dependent structural and physical properties of A-site ordered manganites RBaMn$_{2}$O$_{6}$ (R = Pr, Nd, Pr$_{1/2}$Nd$_{1/2})$ were studied using high-resolution high-energy X-ray powder diffraction and magnetic and transport measurements. The ferromagnetic (FM) to antiferromagnetic (AF) phase transitions of all three materials are accompanied by first-order structural changes. Both the FM and AF phases of PrBaMn$_{2}$O$_{6}$ and Pr$_{1/2}$Nd$_{1/2}$BaMn$_{2}$O$_{6}$ have tetragonal structures, though the FM phase of the latter shows significant broadening of the (200) peak, suggesting a slight in-plane orthorhombic distortion. NdBaMn$_{2}$O$_{6}$ is tetragonal in the AF phase and orthorhombic in the FM phase. The FM-AF transition temperature T$_{c}$ increases with decreasing R$^{3+}$ ionic radius, while decreases with applied magnetic fields. The T$_{c}$ can be shifted by 15$\sim $25 K for H=6 T. Use of the Advanced Photon Source was supported by the~U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38 and work at NIU by NSF- DMR-0302617.