Low-temperature interactions of magnetic excitons in ${\text{LaCoO}}_3$

The low-temperature magnetic behavior of ${\text{LaCoO}}_3$, containing oxygen vacancies, is reported. Magnetic-susceptibility measurements made in the temperature range of 0.5–35 K on a single crystal and a polycrystalline sample provide strong evidence for the existence of magnetic excitons as fundamental entities within the bulk of the material system. Specifically, two distinct types of excitons form, isolated and interacting excitons, both of which are associated with oxygen vacancies. Isolated magnetic excitons act as high-spin paramagnetic particles while the interacting excitons appear to be coupled antiferromagnetically. It is proposed that the interaction arises from the overlap of magnetic excitons as a consequence of the statistical clustering of oxygen vacancies. The striking similarity of these results with those of the lightly doped ${\text{La}}_{0.97}{\text{Sr}}_{0.03}{\text{CoO}}_3$ suggests that the observed excitons are a precursor to magneto-electronic phase separation and supports the idea that phase separation is initiated by disorder in the material system.

Figure 1

(Color online) Mass magnetization curves for ${\text{LaCoO}}_{2.98}$ as a function of temperature up to 5 T, demonstrating scaling behavior from 5–25 K. Details of the fit to the 5 K data (solid line) are described in the text. The inset shows the temperature dependence of the susceptibility measured in 5 T.

Figure 2

(Color online) Temperature dependence of magnetic susceptibility of ${\text{LaCoO}}_{2.98\pm 0.02}$ cooled and measured in a residual field of $10\mu \text{T}$. The raw data along with the subtraction of the paramagnetic fit are shown.

Figure 3

(Color online) Temperature dependence of magnetic susceptibility of ${\text{LaCoO}}_{2.98\pm 0.02}$ cooled in a field of 4 and 10 mT and measured in a residual field of $10\mu \text{T}$.

Figure 4

(Color online) Temperature dependence of magnetic susceptibility of the polycrystalline ${\text{LaCoO}}_{2.96\pm 0.02}$ (circles) and the derivative of the susceptibility of ${\text{La}}_{0.97}{\text{Sr}}_{0.03}{\text{CoO}}_3$ (line). Both samples were cooled in a field of 10 mT and measured in a residual field of $10\mu \text{T}$. The inset shows the temperature dependence of the susceptibility for ${\text{La}}_{0.97}{\text{Sr}}_{0.03}{\text{CoO}}_3$.