Spatial Distribution Of The Doped Electrons In Cubic Sr1-Xlaxmno3 (X <= 0.04) Oxides Probed By Sr-87 Nmr

The spin density of doped electrons was investigated by Sr-87 NMR in the paramagnetic (PM) and antiferromagnetic (AF) G-type phases of electron-doped Sr1-xLaxMnO3 (x = 0.00, 0.02, 0.04; T-N = 236-200 K) ceramics with the cubic structure. It is shown that the Sr-87 NMR shift is proportional to the local density of the itinerant doped electrons surrounding the Sr sites; these electrons have mainly the e(g) character. In the PM phase, all the doped electrons are itinerant; however, they are inhomogeneously distributed in the La-containing oxides, creating electron-doped regions (EDRs) with a number of eg electrons perMn larger than in the rest of the oxide. At room temperature, the network of the overlapping EDRs does not cover all Sr sites. Nevertheless, the number of the Sr sites inside an EDR exceeds the site percolation threshold even for x = 0.02, so that the eg electrons can move on large distances. In the AF phase, below 80 K the EDRs cover the entire crystal. In this T range the doped electrons separate into two species: some of them slow down their motion and form below 50 K static FM domains, which are considered as bound magnetic polarons (MPs) of small size with the effective moment p(eff) = 23(10) mu(B) and a MP formation energy similar to 40 meV. The second species concerns the electrons which remain itinerant at low temperature participating in the fast hopping in the AF G-type ordered lattice of the Mn4+ ions. Nevertheless, their motion is slower than what is expected in an AF metal phase without cation disorder; this is probably due to the imperfect shielding of the (La3+/Sr2+) charge disorder.