Melting of the orbital order in LaMnO3 probed by NMR

TL;DR

This study investigates the melting of orbital order in LaMnO3 near the Jahn-Teller transition using NMR, revealing how spin correlations and orbital states evolve with temperature.

Contribution

It provides detailed NMR evidence of the temperature-dependent melting of orbital order and spin correlations in LaMnO3, highlighting anisotropic behavior and orbital mixing angles.

Findings

Spin correlations are ferromagnetic in the ab-plane up to T_{JT}.

Along the c-axis, antiferromagnetic correlations melt below T_{JT}.

Above T_{JT}, Mn-Mn exchange interactions become isotropic.

Abstract

The Mn spin correlations were studied near the O'-O phase transition at T{JT} = 750 K, up to 950 K with 17O and 139La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the eg- and t2g-orbitals to the 2s(O) and 6s(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the ab-plane and robust up to T{JT} whereas along the c-axis, they are antiferromagnetic and start to melt below T{JT}, at about 550 K. Above T{JT} the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, {\phi}nmr = 109+-1.5{\deg}, of the eg ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > T{JT} LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied.