The charge ordered La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3-\delta }$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, Mössbauer, and polarized neutron studies. A complex scenario of short-range charge and magnetic ordering is realized from the polarized neutron studies in nanocrystalline specimen. This short-range ordering does not involve any change in spin state and modification in the charge disproportion between Fe${}^{3+}$ and Fe${}^{5+}$ compared to bulk counterpart as evident in the Mössbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe${}^{3+}$ and Fe${}^{5+}$ are about 3.15 ${\mu }_B$ and 1.57 ${\mu }_B$ for bulk, and 2.7 ${\mu }_B$ and 0.53 ${\mu }_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\sim$70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass-like transition around $\sim$65 K, below which EB appears. Overall results propose that finite-size effect directs the complex glassy magnetic behavior driven by unconventional short-range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.

• Received 2 March 2012

DOI:https://doi.org/10.1103/PhysRevB.86.104416