Structural changes, clustering, and photoinduced phase segregation inPr0.7Ca0.3MnO3

Abstract

The structural properties of ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ were studied by x-ray synchrotron and neutron-powder diffraction as a function of temperature $(15<T<\mathrm{}300\mathrm{}$ K), and as a function of x-ray fluence at 15 and 20 K. The temperature evolution of the lattice parameters and of the superlattice reflections is consistent with the development of charge and orbital ordering below $T_{\mathrm{CO}}\sim$180 K, followed by antiferromagnetic ordering below $T_N$ $\sim$140 K, similar to what was previously observed for ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$. Below $T_C\sim$120 K, the magnetic structure develops a ferromagnetic component along the $a$ axis on the Mn ions. At low temperatures, a small ferromagnetic moment of 0.45(2)${\mu }_B$ oriented in the same direction appears on the Pr ions as well. The observation in ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ of significant lattice strain developing below $T_{\mathrm{CO}},$ as well as the development of a ferromagnetic component to the magnetic structure at $T_C\sim$120 K, can be interpreted in terms of the presence of ferromagnetic clusters with an associated lattice distortion from the average structure. At low temperatures, exposure to the x-ray beam produces a phase-segregation phenomenon, whereby the ferromagnetic droplets coalesce into larger aggregates. Further exposure results in a gradual melting of the charge-ordered phase and the formation of a second phase, recently shown to be a ferromagnetic metallic phase by Kiryukhin et al. [Nature (London) 386, 813 (1997)]. The ferromagnetic phase has a significantly smaller $a$ lattice parameter and unit-cell volume $(\Delta V/V\sim 0.4\%)$ than that of the charge-ordered phase.