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

We report on high-temperature electron spin resonance studies of a detwinned ${\mathrm{LaMnO}}_3$ single crystal across the Jahn-Teller transition at $T_{\mathrm{JT}}=750$ 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 $T_{\mathrm{JT}}=750$ 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 ${\mathrm{Mn}}^{3+}$ 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 $T_{\mathrm{JT}}$. Moreover, in our discussion we show that elastic strain field interactions can explain the melting of the orbital order and, thus, have to be taken into account to explain the orbital ordering in ${\mathrm{LaMnO}}_3$.

Figure 1

ESR spectra of ${\mathrm{LaMnO}}_3$. The solid lines refer to the corresponding fit curves using a Dyson line shape. (a) $X$-band ESR spectra at different temperatures below and above $T_{\mathrm{JT}}=750$ K. The magnetic field is aligned parallel to the $b$ axis of the crystal. (b) $Q$-band ESR spectra at $T=200$ K with the magnetic field aligned parallel to the crystallographic axes.

Figure 2

Temperature dependence of the $g$ factor for different orientations measured at 34 GHz. Solid lines correspond to fit curves following Eq. (1). Inset: temperature dependence of the ESR linewidth measured at 34 GHz.

Figure 3

(a) ESR linewidth for different orientations of the single crystal and for the powder sample below and above $T_{\mathrm{JT}}=750$ K. The lines correspond to fits following Eq. (2). (b) Temperature dependence of the relative anisotropy ratio $\Delta H_a/\Delta H_b$. The solid line was obtained by simulating the relative anisotropy with Eq. (2) and values obtained from fits to the linewidth.

Figure 4

Temperature dependence of the reduced ESR linewidth $\Delta H\chi T$ for the powdered sample. Lines are to guide the eyes; solid lines indicate equal slope. Light blue (checkered) and orange areas show DM and CF contributions to the linewidth.