A-site ordered manganites $R$BaMn$_{ 2}$O$_{ 6}$ ($R$ = La, Pr, Nd, Sm) were synthesized and the oxygen isotope effect in them was studied. It was found that the substitution of $^{ 16}$O by $^{ 18}$O has caused increases in both the Neel temperature ($T$$_{ N}$) and the charge ordering temperature ($T$$_{ CO}$) and an decrease in the Curie temperature ($T$$_{ C}$). The isotope exponent is small on $T$$_{ C}$ and $T$$_{ CO}$ compared with that in A-site disordered manganites such as La$_{ 1-x}$Ca$_{ x}$MnO$_{ 3}$, which may be caused by weaker electron-phonon coupling in A-site ordered ones. But the isotope exponent of $T$$_{ N}$ is large for $R$ = La. We didn't observe the large oxygen-isotope shift that occurs in NdBaMn$_{ 2}$O$_{ 6}$, where a multicritical point is believed to appear near $R$ = Nd, and strong fluctuation due to the competition between ferromagnetic metal and charge ordering phase should be present.
Categories: cond-mat.str-el cond-mat.mtrl-sci