The half-doped perovskite manganites ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ and ${\text{La}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ in bulk and nanocrystalline form were structurally studied by x-ray and neutron-diffraction methods. The magnetic properties were probed by dc and ac susceptibilities and by isothermal magnetization measurements. The study shows that the room-temperature $Pbnm$ perovskite structure, as concerns the lattice distortion, Mn-O distances, and octahedral tilts, is practically unaffected by the particle size. Nonetheless, the low-temperature structural distortion, characteristic for (long- or short-range) charge and orbital ordering in bulk samples, is not observed for 25 nm particles. The absence of the charge-ordering transition is confirmed also by magnetic data. The different behavior compared to bulk is explained by effects of the particle surface. In the nanocrystalline ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$, an onset of ferromagnetic (FM) arrangement is observed at $\sim 100\text{K}$. At the lowest temperature, the magnetic state of the sample can be characterized as a mixture of particles in the metallic FM state with those in the insulating charge and orbitally disordered phase with frozen spins. There is a possibility to induce a global FM state by external field. The ${\text{La}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ nanocrystals develop FM ordering spontaneously below $T_{\text{C}}=260\text{K}$.

• Received 20 August 2009

DOI:https://doi.org/10.1103/PhysRevB.81.024403