Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of $La_{1-x}Ca_xMnO_3$ at high temperatures

TL;DR

This study investigates how oxygen isotope mass influences the exchange-energy in La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ at high temperatures, revealing an unusual temperature-dependent isotope effect that challenges existing theories.

Contribution

It presents the first high-temperature measurements of isotope effects on exchange-energy in manganites, showing a non-monotonic temperature dependence unexplained by current models.

Findings

Isotope mass significantly affects the ferromagnetic exchange-energy J.

The isotope effect on J decreases up to 400 K and then increases above 400 K.

Results impose new constraints on theoretical models of manganite physics.

Abstract

We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence of the isotope effect cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. The present results thus provide essential constraints on the physics of manganites.