Structural and magnetic disorder inLa1-xSrxMnO3

Abstract

Total structure factors, that is both Bragg and diffuse scattering,have been measured for La1-xSrxMnO3(x = 0.2 and 0.4) at temperatures between 15 and 1000 K bypowder neutron diffraction using the SLAD diffractometer at theStudsvik Neutron Research Laboratory. The data have been used tosimultaneously model the atomic and magnetic structures usingthe reverse Monte Carlo (RMC) method. The RMC models confirmthat there are local lattice distortions, involving the O octahedrasurrounding Mn ions, that disappear as T decreases throughTc (here defined as the point of inflexion of the net magneticmoment determined from Rietveld refinement, representing theonset of long range ferromagnetic order). The distortion referredto is away from the `normal' Jahn-Teller distortedoctahedron associated with Mn3+ and towardsthe more uniform octahedron associated with Mn4+. Itinvolves more than a single octahedron, so the correlation lengthis larger than ~5 Å. The models are consistent with theproposal that the distortions occur around Mn4+ ions,though it is not possible to say whether the extra charge islocalized on a single Mn. They may therefore be considered aslattice polarons. Both long range magnetic order (LRMO) and shortrange magnetic order (SRMO) increase rapidly as T decreasesthrough Tc, but SRMO also persists above Tc. This isevidence for the existence of magnetic polarons above Tc. Forthe first time we show that the SRMO is correlated with the locallattice distortion, a higher ferromagnetic correlation beingassociated with shorter Mn-Mn distances. Lattice and magneticpolarons are therefore one and the same.