IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 19 (2007) 266214 (18pp) doi:10.1088/0953-8984/19/26/266214 Phase separation and size effects in Pr 0.70 Ba 0.30 MnO 3+δ perovskite manganites S V Trukhanov 1 , A V Trukhanov 2 , C E Botez 3 , A H Adair 3 , H Szymczak 4 and R Szymczak 4 1 Joint Institute of Solids and Semiconductor Physics of NAS of Belarus, P Brovka street 19, 220072 Minsk, Belarus 2 Chemistry Department, Vitebsk State University, Moscow Avenue 33, 210036 Vitebsk, Belarus 3 Physics Department, The University of Texas at El Paso, El Paso, TX 79968, USA 4 Institute of Physics, PAS, Lotnikov street 32/46, 02-668 Warsaw, Poland E-mail: truhanov@ifttp.bas-net.by (S V Trukhanov) Received 5 March 2007, in final form 24 May 2007 Published 14 June 2007 Online at stacks.iop.org/JPhysCM/19/266214 Abstract The crystal structure and magnetotransport properties of the A-site ionic ordered state in Pr 0.70 Ba 0.30 MnO 3+δ (δ = 0, 0.025) have been investigated. It is shown that such a state can be formed in complex manganites with cation ratios Pr 3+ /Ba 2+ ≫ 1 by using a ‘two-step’ reduction–reoxidization method. The parent A-site ionic disordered Pr 0.70 Ba 0.30 MnO 3+δ (δ = 0) compound is an orthorhombic (SG = Imma , Z = 4) ferromagnet with Curie temperature T C ≈ 173 K and ground-state spontaneous magnetic moment σ S ∼ 3.70 μ B /f.u. It exhibits two metal–insulator transitions, at T I ∼ 128 K and T II ∼ 173 K, as well as two peaks of magnetoresistance ∼74% and ∼79% in a field of 50 kOe. The parent A-site ionic disordered Pr 0.70 Ba 0.30 MnO 3+δ (δ = 0) sample used in our studies has an average grain size 〈 D〉≈ 10.213 μm. Successive annealing of this sample in vacuum P [O 2 ]≈ 10 −4 Pa and then in air at T = 800 ◦ C leads to the destruction of its initial grain structure and to its chemical separation into two phases: (i) oxygen stoichiometric A-site ordered PrBaMn 2 O 6 with a tetragonal (SG = P 4/ mmm, Z = 2) perovskite-like unit cell and Curie temperature T C ≈ 313 K and (ii) oxygen superstoichiometric A-site disordered Pr 0.90 Ba 0.10 MnO 3.05 with an orthorhombic (SG = Pnma , Z = 4) perovskite- like unit cell and Curie temperature T C ≈ 133 K. This processed sample has a spontaneous magnetic moment σ S ∼ 2.82 μ B /f.u. in its ground state, and σ S ∼ 0.59 μ B /f.u. at T ∼ 300 K. It also exhibits a magnetoresistance of ∼14% at ∼313 K in a field of 50 kOe. This processed sample has a reduced average grain size 〈 D〉≈ 491 nm. The two magnetic phases, Pr 0.90 Ba 0.10 MnO 3.05 and PrBaMn 2 O 6 , are exchange-coupled. For Pr 0.90 Ba 0.10 MnO 3.05 the temperature hysteresis is ∼22 K in a field of 10 Oe and ∼5 K in a field of 1 kOe. The observed magnetic properties are interpreted in terms of chemical phase separation, grain size, and A-site ionic ordering effects. 0953-8984/07/266214+18$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1