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

Highlights: • The mechanism of grain growth in Nd_1−xCa_xMnO_3±δ nanopowders upon heating is established. • Grain growth in calcium-doped compositions is kinetically hindered. • Detected chemical modification of the surface, including segregation of Ca impurity. • The magnetic transition temperature in NdMnO3 significantly depend on the grinding time. -- Abstract: The nanograin polycrystalline Nd_1−xCa_xMnO_3±δ (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 NdMnO_3+δ monotonically decreased with decreasing grain size, while the Curie temperature in Nd_0.85Ca_0.15MnO_3-δ varied nonmonotonically. The spin-reorientation transition was found as strongly manifested in NdMnO_3+δ and completely suppressed in Nd_0.85Ca_0.15MnO_3-δ. 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.