Surface ferromagnetism in Pr0.5Ca0.5MnO3 nanoparticles as a consequence of local imbalance Mn3+:Mn4+ ratio

Abstract

Half-doped praseodymium manganites, Pr0.5Ca0.5MnO3, nanoparticles synthesized by sol-gel method are annealed at different temperatures in order to obtain nanoparticles sizes ranging from 15 to 100 nm. HRTEM shows that the nanoparticles are very crystalline with no structural disorder at the surface. Rietveld powder diffraction profile determines that the samples have orthorhombic structure with space group Pnma and cell volume decreases with increasing particle size. The thermodiffraction patterns from 50 to 300 K indicate that the charge ordering (CO) is suppressed for the smallest particles, whereas the largest one still resembles the bulk. Hysteresis loops and ZFC-FC curves evince that, as the most of half-doped (Mn3+:Mn4+ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. Hysteresis loops at FC procedure show a coercivity increase and a shift to negative field, confirming that both AFM and FM phases not only coexist but are also coupled. In addition, a spin glass behaviour is observed with glass temperature around 50 K. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn3+ at expenses of Mn4+ in order to maintain the 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 double exchange Mn3+→ O2-→Mn4+ prevails over the super-exchange giving place to the FM interactions. We show that a “shell” thickness of only one-unit cell, with acell = 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.[1].