R-site Randomness Effect in Double-Perovskite RBaMn₂O₆

We have investigated the $R$-site randomness effect of $R$/Ba-ordered $R$BaMn$_{2}$O$_{6}$ ($R$ = rare earth) by using Y$_{1-y}$La$_y$BaMn$_2$O$_6$ (0 (\leq) $y$ (\leq) 1) in which $R$ (Y,La) and Ba are regularly arranged while Y and La randomly occupy the $R$-site. YBaMn$_2$O$_6$ ($y$ = 0) undergoes charge/orbital ordering (CO) transition at $T_{\rm CO}$ = 500 K while LaBaMn$_{2}$O$_{6}$ ($y$ = 1) shows ferromagnetic metallic (FM) behavior below 350 K\@. In 0 (\leq) $y$ (\leq) 0.6, $T_{\rm CO}$ decreases with an increase in $y$. In 0.6 (\leq) $y$ (\leq) 0.8, $R$-site randomness causes strong phase separation tendencies among the FM, antiferromagnetic (AFM), and CO insulating (COI) states. The large magnetoresistance is observed in the phase separated region.