Evidence for nano-scale inhomogeneities in bilayer manganites in the Mn\sp{4+} rich region: $\bm{0.54 \leq x \leq 0.80}$

TL;DR

This study uses atomic pair distribution function analysis and neutron diffraction to reveal nanoscale inhomogeneities and local structural variations in bilayer manganites across different doping levels, linking these to magnetic and electronic behaviors.

Contribution

It provides direct evidence of nano-scale inhomogeneities and local structural responses in bilayer manganites, highlighting coexistence of different Jahn-Teller distortions without macroscopic phase separation.

Findings

Coexistence of two Jahn-Teller distorted MnO6 octahedra across doping levels.

Inhomogeneous electronic states with delocalized and localized electrons.

Nanoscale inhomogeneities may cause magnetic frustration in the spin disordered region.

Abstract

The atomic pair distribution function (PDF) technique is employed to probe the atomic local structural responses in naturally double layered manganites \lsmo in the doping range $0.54 \leq x \leq 0.80$. Our low temperature neutron powder diffraction measurements suggest the coexistence of two different Jahn-Teller (JT) distorted MnO$_6$ octahedra when its ordered magnetic structure crosses over from type A ($0.54 \leq x \leq 0.66$) to type C/C$^*$ ($0.74 \leq x \leq 0.90$) ordering. At all doping levels at low temperature the doped holes reside predominantly in the plane of the bilayer. In the type A magnetic ordering regime, the e$_g$ electrons appear to be significantly delocalized in the plane resulting in undistorted octahedra, while in type C/C$^*$ regime, elongated JT distorted octahedra are apparent. This is consistent with the presence of inhomogeneous coexisting delocalized and localized electronic states. No evidence of macroscopic phase separation has been observed. Such nanoscale inhomogeneities may explain the magnetically frustrated behavior observed in the spin disordered ``gap'' region ($0.66 \leq x \leq 0.74$).