Evidence for Magnetic Polarons in Hole-Doped Cobalt Perovskites

Abstract

A substitution of La 3+ by Sr 2+ in LaCoO 3 induces holes in the low-spin ground state of the Co ions, which behave like magnetic impurities with a very high spin value (13 B per hole). In this work, using single-crystal neutron spectroscopy, we prove that the charges introduced by strontium doping do not remain localized at the cobalt sites. Instead, each hole not only creates Co 4+ in low-spin state, but it also transforms the six nearest neighboring Co 3+ ions to the intermediate-spin state thereby forming a magnetic seven-site (heptamer) polaron. Spin-state polarons behave like magnetic nanoparticles embedded in an insulating nonmagnetic matrix. Therefore, lightly doped La 1-x Sr x CoO 3 is a natural analog to artificial structures composed of ferromagnetic particles in insulating matrices.