Recent progress with the thermally driven spin-state crossover in $\mathrm{LaCo}{\mathrm{O}}_3$ has made it increasingly apparent that the nominally nonmagnetic low-spin ground state of this material actually hosts defect-based magnetism. This is investigated here via a small-angle neutron scattering (SANS) study of $\mathrm{LaCo}{\mathrm{O}}_{3-\delta }$ crystals. The results provide (i) the surprising finding that the spin-state crossover is clearly reflected in SANS via quasieleastic and inelastic scattering from paramagnetic spin fluctuations and excitations, and (ii) evidence for the formation, likely around oxygen defects, of local entities known as magnetic excitons. The latter generate distinct magnetic scattering below 60 K, providing valuable quantitative information on exciton densities and interactions. Potential relevance to the unexpected ferromagnetism recently discovered in epitaxial $\mathrm{LaCo}{\mathrm{O}}_3$ films is discussed.

• Received 24 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.060404