Cation ordering and magnetic properties of neodymium-barium manganites

Nd$_{0.70}$Ba$_{0.30}$MnO$_{3+\delta}$ manganites with an ordered arrangement of Nd$^{3+}$ and Ba$^{2+}$ cations are fabricated. The initial stoichiometric Ba-disordered Nd$_{0.70}$Ba$_{0.30}$MnO$_3$ solid solution is synthesized in air using a traditional ceramic technology, is characterized by an orthorhombic unit cell (space group $Imma$, $Z$ = 4), and is ferromagnetic with Curie temperature $T_C\approx$ 151 K. The average crystallite size in the initial sample is $\langle D\rangle$ $\approx$ 2.169 $\mu$m. Annealing of the initial sample in a reducing atmosphere at a pressure $P$[O$_2$] $\approx$ 10$^{-4}$ Pa and then in air at $T$ = 800$^\circ$C leads to the formation of a material with a crystallite size $\langle D\rangle$ $\approx$ 440 nm. This material consists of the following two perovskite phases: a Ba-ordered stoichiometric NdBaMn$_2$O$_6$ phase with a tetragonal unit cell ($P4/mmm$, $Z$ = 2) and a Curie temperature of $\sim$301 K and a Ba-disordered hyperstoichiometric Nd$_{0.70}$Ba$_{0.30}$MnO$_{3+\delta}$ phase with an orthorhombic unit cell ($Imma$, $Z$ = 4) and $T_C\sim$ 121 K. Significant changes in the magnetic properties are explained in terms of chemical phase separation with allowance for cation ordering.