Study on the oxygen isotope effect in A-site ordered manganite $R$BaMn$_{2}$O$_{6}$ ($R$ = La, Pr, Nd, Sm)

TL;DR

This study investigates how substituting oxygen isotopes affects magnetic and charge ordering temperatures in A-site ordered manganites, revealing weaker electron-phonon coupling compared to disordered variants and highlighting complex phase interactions.

Contribution

It provides new insights into isotope effects in A-site ordered manganites, showing different behaviors from disordered ones and suggesting weaker electron-phonon coupling influences.

Findings

Oxygen isotope substitution increases $T_N$ and $T_{CO}$, decreases $T_C$.

Isotope exponent is small for $T_C$ and $T_{CO}$ compared to disordered manganites.

Large isotope exponent observed for $T_N$ in La-based compound.

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.