Spin polarons in La1-x Srx CoO3 single crystals

Nanoscale inhomogeneity in La1-x Srx CoO3 has been investigated in single-crystal samples for 0.05≤x≤0.30 using L 139 a and C 59 o NMR to probe local magnetization. In contrast to polycrystalline samples, single crystals exhibit ferromagnetism only above the metal-insulator critical concentration xC =0.17. However, over the entire doping range, the single crystals exhibit an unusually broad and asymmetric distribution of hyperfine fields, evidencing (local) spin-polaron formation that persists to temperatures as high as 200 K, well above the glass transition reported previously from bulk magnetization. Above xC the asymmetry decreases rapidly with increasing doping as polarons overlap to give rise to long-range ferromagnetism. A modified phase diagram is presented. The key features of the data are reproduced by a simple model in which Sr dopants trigger spin-polaron formation, a physical picture first proposed by de Gennes [Phys. Rev. 118, 141 (1960)].