ESR evidence for partial melting of the orbital order in LaMnO$_3$ below the Jahn-Teller transition

TL;DR

This study uses high-temperature ESR to investigate the orbital order in LaMnO$_3$, revealing partial melting of orbital order below the Jahn-Teller transition and emphasizing the role of elastic strain interactions.

Contribution

It provides new evidence for partial orbital order melting at T* and highlights the importance of elastic strain fields in orbital ordering phenomena.

Findings

Partial melting of orbital order occurs at T* = 550 K.

Jahn-Teller transition at T_JT = 750 K is first order.

Elastic strain interactions influence orbital order melting.

Abstract

We report on high-temperature electron spin resonance studies of a detwinned LaMnO$_3$ single crystal across the Jahn-Teller transition at $T_{\rm JT}$ = 750 K. The anisotropy of the linewidth and g-factor reflects the local Jahn-Teller distortions in the orbitally ordered phase. A clear jump in the linewidth accompanies the Jahn-Teller transition at $T_{\rm JT}$ = 750 K confirming that the transition is of first order. Already at $T^*$ = 550 K a significant decrease of the reduced linewidth is observed. This temperature scale is discussed with respect to the interaction of the $e_g$-electrons of the Mn$^{3+}$-ions and the elastic field of the cooperative distortions. Our results support a partial melting of the orbital order along the antiferromagnetically coupled $b$-axis at $T^*$. The remaining two-dimensional orbital ordering within the ferromagnetically coupled $ac$-plane finally disappears together with the cooperative distortion at $T_{\rm JT}$. Moreover in our discussion we show that elastic strain field interactions can explain the melting of the orbital order and, thus, has to be taken into account to explain the orbital ordering in LaMnO$_3$.