Dynamic motion and freezing of polaronic local structures in a colossal-magnetoresistive perovskite manganite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ detected with radioactive nuclei

The magnetic hyperfine field and electric field gradients at a radioactive impurity probe of ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ occupying the La/Ca $A$ site in a perovskite manganaite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ ($T_C\sim 250\mathrm{K}$) were measured by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. In the paramagnetic-insulator phase at room temperature, the ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ probes are distributed in two different environments: distorted and less distorted sites; whereas in the ferromagnetic-metal phase below $T_C$, the oscillatory structure of the distorted component vanishes from the TDPAC function. The vanishing of the oscillation is ascribable to dynamic motion of polarons dragged by conduction electrons induced by the double exchange interaction. In liquid helium, the dynamic motion freezes, leaving averaged local distortion. The dynamic motion and freezing of local structures associated with the colossal-magnetoresistance phase transition to the ferromagnetic phase is discussed on the basis of temperature-dependent electromagnetic fields at the probe nuclei of the $A$ site ions.

Figure 1

Simplified decay schemes (a) of the ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ and (b) of the ${}^{140}\mathrm{Ce}(\leftarrow {}^{140}\mathrm{La})$ probes. The latter is displayed for reference for the discussion in 3b2.

Figure 2

(a) Powder x-ray diffraction pattern of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ at room temperature, and (b) temperature dependence of the diffraction angle for the peak at $2\theta \sim 58.55$ degrees. The solid line is a guide to the eye.

Figure 3

Magnetization curves for ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ taken zero field cooled (ZFC) and field cooled (FC) at 100 Oe.

Figure 4

(a) Powder XRD pattern, (b) magnetization curves taken zero field cooled (ZFC) and field cooled (FC) at 100 Oe and (c) resistivity curves observed for 2 at. % Cd-doped ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$.

Figure 5

TDPAC functions of ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ in polycrystalline ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ measured (a) at room temperature, (b) at 201 K, (c) at 77 K, and (d) at 4.2 K.