O17NMR evidence for vanishing of magnetic polarons in the paramagnetic phase of ceramicCaMnO3

An $^{17}\text{O}$ NMR study of the distribution of the spin and charge densities of a lightly electron-doped ${\text{CaMnO}}_{3\ensuremath{-}x}$ $(xl0.01,\text{ }{T}_{N}=123\text{ }\text{K})$ ceramic in the paramagnetic phase up to $T=670\text{ }\text{K}$ is reported. The isotropic and anisotropic components of the NMR line shift probe selectively the local spin susceptibility of the itinerant $({e}_{g})$ and the localized $({t}_{2g})$ electrons of the Mn neighbors whereas the nuclear quadrupole parameters look at the distribution of the charge density along the Mn-O bond. When approaching ${T}_{N}$, the spin density of the doped electrons becomes inhomogeneously distributed: a separation into slow carriers, forming magnetic polarons, and fast carriers develops few tens of degrees above ${T}_{N}$. The energy barrier corresponding to the change in carriers mobility was estimated to $\ensuremath{\sim}1100\text{ }\text{K}$. The spin and valence charge densities at the oxygen ions were also measured. From the comparison of these data, the ground state of ${\text{CaMnO}}_{3}$ appears as a charge-transfer insulator state.