Phase separation and size effects inPr0.70Ba0.30MnO3+δ perovskite manganites

Abstract

The crystal structure and magnetotransport properties of the A-site ionic ordered state inPr0.70Ba0.30MnO3+δ (δ = 0,0.025) have been investigated. It is shown that such a state can be formed incomplex manganites with cation ratios by using a ‘two-step’ reduction–reoxidization method. The parent A-site ionic disorderedPr0.70Ba0.30MnO3+δ (δ = 0) compound is anorthorhombic (SG = Imma,Z = 4) ferromagnet withCurie temperature TC≈173 K and ground-state spontaneous magnetic momentσS∼3.70 µB/f.u. It exhibits two metal–insulator transitions, atTI∼128 Kand TII∼173 K, as well as two peaks of magnetoresistance∼74% and ∼79% in a field of 50 kOe. The parent A-site ionic disorderedPr0.70Ba0.30MnO3+δ (δ = 0) sample used in our studies has an average grain size . Successive annealing of this sample in vacuumP[O2]≈10−4 Pa and thenin air at T = 800 °C leads to the destruction of its initial grain structure and to its chemicalseparation into two phases: (i) oxygen stoichiometric A-site orderedPrBaMn2O6 with atetragonal (SG = P4/mmm, Z = 2) perovskite-like unit cell and Curie temperatureTC≈313 K and (ii) oxygen superstoichiometric A-site disorderedPr0.90Ba0.10MnO3.05 with anorthorhombic (SG = Pnma, Z = 4) perovskite-like unit cell and Curie temperatureTC≈133 K. This processed sample has a spontaneous magnetic momentσS∼2.82 µB/f.u. in its groundstate, and σS∼0.59 μB/f.u. atT∼300 K. It also exhibits amagnetoresistance of ∼14% at ∼313 K in a field of 50 kOe. This processed sample has a reduced average grain size nm. The two magnetic phases, Pr0.90Ba0.10MnO3.05 and PrBaMn2O6, are exchange-coupled.For Pr0.90Ba0.10MnO3.05 the temperaturehysteresis is ∼22 K in afield of 10 Oe and ∼5 K in a field of 1 kOe. The observed magnetic properties are interpreted in terms of chemicalphase separation, grain size, and A-site ionic ordering effects.