Grain growth kinetics, microstructure and magnetic properties of mechanically activated Nd1-xCaxMnO3±δ manganites

The nanograin polycrystalline Nd1-xCaxMnO3 +/-delta (x = 0; 0.15; 0.25) with orthorhombic structure were obtained by mechanical activation. X-ray diffraction analysis and transmission electron micrographs showed that the average crystallite size is in the range of 10-30 nm. Grain growth kinetics was studied using three different models using data from high temperature X-ray diffraction analysis. Two models - normal grain growth and grain growth in a two-phase system - were compatible with experimental data up to higher temperatures. Grain growth was found to be kinetically hindered in Ca-doped compositions despite the lower activation energy. That was accounted for by the surface modification involving formation of oxide phases trapping grain boundaries and limiting their mobility. It was found that the Neel temperature in NdMnO3+delta monotonically decreased with decreasing grain size, while the Curie temperature in Nd0.85Ca0.15MnO3-delta varied nonmonotonically. The spin-reorientation transition was found as strongly manifested in NdMnO3+delta and completely suppressed in Nd0.85Ca0.15MnO3-delta. The magnitude of the spin-reorientation effect decreased with decreasing grain size. The peculiarities of the magnetic behaviour were accounted for by the reduction of Jahn-Teller distortion with decreasing grain size, the emergence and increase of local structural and magnetic inhomogeneities, competing d-d superexchange, double-exchange and f-d exchange. (C) 2021 Published by Elsevier B.V.

Automated summary

Nanograin polycrystalline Nd1-xCaxMnO3 +/-delta (x = 0; 0.15; 0.25) with orthorhombic structure were obtained by mechanical activation with an average crystallite size of 10-30 nm. Grain growth was found to be kinetically hindered in Ca-doped compositions despite the lower activation energy.