Anomalies of ESR spectra of the La2NiO4+δ system

X- and Q-band ESR experiments on ceramics and single crystals of La2NiO4+δ (0.03 ≤ δ ≤ 0.17) are reported. The observed intrinsic ESR signals were interpreted in terms of ferromagnetic resonance of isolated oxidized nickel species in the lattice. The origin of the ferromagnetic component differs according to the δ value, consistent with the phase diagram and the phenomenon of charge and spin order. The ESR spectra on powders exhibit an anisotropic signal attributed to the presence of a magnetic local field, induced by the external magnetic field applied during the first ESR measurement. For specific value of δ, the ESR spectra show a “multilines signal” from the origin which could be due to the presence of magnetic polarons which are pinned on the surface. An ESR study vs. temperature for a powder sample with δ = 0.16 shows that the linewidth evolution is in relation with the transition from a dynamic stripe to a static stripe state at 160 K. At high temperature (T > 600 K), a polaronic liquid is observed.

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