title: Phase separation and size effects in Pr(0.70)Ba(0.30)MnO(3+δ) perovskite manganites.
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 [Formula: see text] 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 [Formula: see text]. 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 = P4/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 [Formula: see text] 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.
authors.affiliation: Joint Institute of Solids and Semiconductor Physics of NAS of Belarus, P Brovka street 19, 220072 Minsk, Belarus.
authors.forename: S V
authors.initials: SV
authors.lastname: Trukhanov
authors.forename: A V
authors.initials: AV
authors.lastname: Trukhanov
authors.forename: C E
authors.initials: CE
authors.lastname: Botez
authors.forename: A H
authors.initials: AH
authors.lastname: Adair
authors.forename: H
authors.initials: H
authors.lastname: Szymczak
authors.forename: R
authors.initials: R
authors.lastname: Szymczak