High-pressure synthesis and characterization of BiCu3(Mn4−xFex)O12 (x=0, 1.0, 2.0) complex perovskites

Abstract

Abstract We have studied the series of nominal composition BiCu3(Mn4−xFex)O12 (x=0, 1.0, 2.0) where Mn is replaced by Fe cations in the ferrimagnetic perovskite BiCu3Mn4O12. These compounds have been prepared from citrate precursors under moderate pressure conditions (3.5 GPa) and 1000 °C in the presence of KClO4 as oxidizing agent. All the samples have been studied by x-ray and neutron powder diffraction (NPD) at room temperature and 4 K. The crystal structure has been defined in a cubic Im 3 ¯ (No. 204) space group with a 2a0×2a0×2a0 unit-cell. The doubling of the unit-cell occurs due to the ordering of Bi3+ and Cu2+ cations over A sites of the AA′3B4O12 structure. The A-site accommodates 12-fold coordinated Bi3+ ions and, at the A′-site, Jahn–Teller Cu2+ ions form pseudo-square planar units aligned perpendicular to each other. Mn4+/Fe3+ cations randomly occupy the centre of slightly distorted octahedra. These materials have also been characterized by magnetic and magnetotransport measurements. We found that all the samples are ferrimagnetic and show a progressive decrease of TC as the Fe content increases, since Fe ions disturb the ferromagnetic interactions within the B magnetic sublattice. In fact, the Curie temperature diminishes from TC=360 K (x=0) to TC=219 K (x=2). The magnetic structures, studied by low-temperature NPD data, correspond to an antiferromagnetic arrangement of spins at 8c and 6b sites; the ordered moments are in excellent agreement with those obtained from the saturation magnetization at 4 K. A significant magnetoresistant effect is determined for the x=1.0 oxide, with low-field values as high as 5% at 300 K and 1 T.