${}^{17}\mathrm{O}$ NMR as a conclusive probe of charge-ordering models in half-doped manganites

We present a ${}^{17}\mathrm{O}$ NMR study of the half-doped manganite, ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ at $T⩽10\mathrm{K}$. The charge-ordered and the ferromagnetic states are studied at $H=0$ and $H=12T$, respectively. Since the ${}^{17}\mathrm{O}$ nucleus probes spin and orbital configuration of its two first Mn neighbors, local magnetic environments of oxygen in different structural sites could be analyzed in both states. Local fields expected in two models of the charge-ordered state, the site-centered and the bond-centered ones, are compared to the experimental data. It is shown that the hierarchy of local fields at different oxygen sites favors the site-centered charge ordering with two different Mn sites.

Figure 1

(Color online) CE-type magnetic structure in the (a) site-centered and in the (b) Zener polaron CO/OO models. The ferromagnetic zigzags are indicated by the solid lines, and the dotted rectangle indicates the low temperature unit cell; only the $ab$ plane is considered. The arrows show the Mn spins orientations. Schematic zero-field NMR spectrum expected for (c) the site-centered model—the two lines correspond to the local field experienced by the oxygen O5 and O4, inside and between the zigzags, respectively; (d) the Zener polaron model—the four lines correspond to the local field experienced by O5a, O5b, and O4a, O4b, inside and between the zigzags, respectively.

Figure 2

${}^{17}\mathrm{O}$ NMR spectra in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ measured at $T=10\mathrm{K}$ and $H_0=12\mathrm{T}$. (a) FM phase of the field-cooled sample: $t_{del}=160\mathrm{\mu }\mathrm{s}$, $t_{rep}=2\mathrm{s}$; (b) CO AF phase of the zero-field-cooled sample: $t_{del}=15\mathrm{\mu }\mathrm{s}$, $t_{rep}=100\mathrm{ms}$.

Figure 3

${}^{17}\mathrm{O}$ zero field NMR spectrum measured at $T=4.2\mathrm{K}$ in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (CO AF phase).