Glassy magnetic phase driven by short-range charge and magnetic ordering in nanocrystalline La_(1/3)Sr_(2/3)FeO_(3-δ): Magnetization, Mossbauer, and polarized neutron studies

摘要

The charge ordered La_(1/3)Sr_(2/3)FeO_(3-δ) (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, Mossbauer, 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 Mossbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe~(3+) and Fe~(5+) are about 3.15 μB and 1.57 μb for bulk, and 2.7 μ-b and 0.53 μb 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 ～70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass-like transition around ～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.