Magnetic transitions and site-disordered induced weak ferromagnetism in (1- x) BiFeO3-x BaTiO3

Abstract

We present evidence for weak ferromagnetism in both the rhombohedral and cubic compositions of BF-xBT solid solutions for x < 0.55. Rietveld refinement of nuclear and magnetic structures reveals that the G-type antiferromagnetic ordering of the Fe3+ magnetic sublattice survives up to x ∼ 0.50. We address the issue of weak ferromagnetism due to spin canting, which is allowed by the symmetry in the R3c space group but not in the cubic Pm3̄m space group. It is shown that the local symmetry of the average cubic compositions of BF-xBT for 0.35 < x < 0.55 is broken due to off-centering of Bi3+ in the (1-10) plane and O2- along the 110 direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O2- disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe3+-O2 - Fe3+ bond angle from 180 that allows spin canting due to Dzyaloshinskii-Moriya interaction Dai,j(S-i), which is otherwise irreconcilable with the ideal cubic symmetry. The magnetization and neutron powder diffraction measurements confirm the absence of magnetic ordering at room temperature for x 3 0.55. © 2014 American Physical Society.