139LaNMR in lanthanum manganites: Indication of the presence of magnetic polarons from spectra and nuclear relaxations

Abstract

We present ${}^{139}\mathrm{La}$ NMR experiments on five powder samples of lanthanum manganites, with a ${\mathrm{Mn}}^{4+}$ concentration ranging from the antiferromagnetic-insulator (AFM) to the ferromagnetic-conducting (FM) region of the phase diagram. We measure a positive hyperfine coupling $\mathcal{C}=0.13\mathrm{}\hspace{0.5em}\mathrm{T}/{\mu }_B.$ A signal from nuclei in a FM environment is present at all compositions, as evidenced by a hyperfine frequency in zero-field experiments, by a positive hyperfine shift in NMR experiments below $T_c,$ and by a paramagnetic frequency shift following Curie-Weiss law. A signal from nuclei in an AFM environment is identified by a similar negative intercept Curie-Weiss law. The NMR spectra reveal a large temperature dependent fraction of static spin defects below $T_c$ in the FM domains. Nuclear relaxation indicates that the FM regions are influenced by diffusing, AFM-correlated excitations, while the AFM regions probe spin fluctuations from diffusing, FM correlated excitations. These results are interpreted in terms of electronic inhomogeneities due to the presence of a magnetic small polaron.