Study on the oxygen isotope effect in A-site ordered manganite $R$BaMn${}_2$O${}_6$ ($R$ = La, Pr, Nd, Sm)

Abstract

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.

5 pages, 4 figures