Surface Ferromagnetism in Pr0.5 Ca0.5 MnO3 Nanoparticles as a Consequence of Local Imbalance in Mn3+:Mn4+ Ratio

Half-doped praseodymium manganites, Pr0.5Ca0.5MnO3, synthesized by the sol-gel method produce highly crystalline nanoparticles with no structural disorder at the surface. As for most half-doped (Mn3+:Mn4+ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn3+ at the expense of Mn4+ in order to maintain electronic neutrality. In this work, we show that Mn3+ :Mn4+ = 1 ratio is preserved in the whole crystalline particle with the exception of the surface, where this ratio changes to Mn3+:Mn4+ > 1. This Mn3+ excess makes a double exchange Mn3+ -> O2- -> Mn4+ prevail over the superexchange, giving rise to the FM interactions. We show that a shell thickness of only one unit cell, with a(cell) = 0.38 nm, is enough to explain the onset of FM at the surface, whereas the volume remains AFM. Furthermore, the FM to AFM ratio fits to the increase of surface-to-volume ratio with decreasing particle size.