Effects of A-site substitution on the structure and magnetic properties of Bi 0.15Sr 0.85− yAe yCo 1− xFe xO 3− δ perovskites

Abstract

The effects of partial substitution of Sr 2+ by Ca 2+ and Ba 2+ on the A-site of oxygen-deficient perovskites, Bi 0.15Sr 0.85− y Ae y Co 1− x Fe x O 3− δ , where y = 0.28 for Ae = Ba and y = 0.17 for Ae = Ca, and 0.0 ≤ x ≤ 1.0, have been investigated. The differing ionic size of the Ca 2+ and Ba 2+ cations influences both the crystal structure and the properties of the materials. The smaller Ca 2+ cation favoured formation of an oxygen vacancy ordered perovskite superstructure ( I4/ mmm, a = 2 a p, c = 4 a p), meanwhile the presence of the larger Ba 2+ cation promoted a disordered simple cubic structure ( P m 3 ¯ m , a = a p) that was also found for all Fe containing samples, i.e. x ≥ 0.25. The samples were studied with PXRD, NPD, TGA, electron microscopy and magnetic susceptibility measurements. All as-prepared samples exhibited long range G-type antiferromagnetic ordering. The effect of oxygen annealing was dramatic for the Bi 0.15Sr 0.68Ca 0.17Co 1− x Fe x O 3− δ series, with a disappearance of magnetic order for x ≥ 0.25 linked to increasing spin-glass properties. The oxygen content of the Bi 0.15Sr 0.57Ba 0.28Co 1− x Fe x O 3− δ as-prepared materials was generally higher than their Ca substituted counterparts, and the long range antiferromagnetic order was more resistant to oxygen annealing.