Melting of the orbital order in LaMnO3probed by NMR

The Mn spin correlations were studied near the ${O}^{\ensuremath{'}}$-$O$ phase transition at ${T}_{\text{JT}}=750$ K up to 950 K with ${}^{17}\mathrm{O}$ and ${}^{139}\mathrm{La}$ NMR in a stoichiometric ${\mathrm{LaMnO}}_{3}$ crystalline sample. The measured local hyperfine fields originate from the electron density transferred from the ${e}_{g}^{}$ and ${t}_{2g}^{}$ orbitals to the 2$s$(O) and 6$s$(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the $\mathit{ab}$ plane and robust up to ${T}_{\text{JT}}$, whereas along the $c$ axis they are antiferromagnetic and start to melt below ${T}_{\text{JT}}$, at about 550 K. Above ${T}_{\text{JT}}$, the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room-temperature orbital mixing angle, ${\ensuremath{\varphi}}_{\mathrm{NMR}}=109\ifmmode\pm\else\textpm\fi{}1.5{}^{\ensuremath{\circ}}$, of the ${e}_{g}^{}$ ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted ${\mathrm{MnO}}_{6}$ octahedra. For $Tg{T}_{\text{JT}}$, ${\mathrm{LaMnO}}_{3}$ can be described in terms of nonpolarized ${e}_{g}$ orbitals since both ${e}_{g}$ orbitals are equally occupied.