Magnetic transitions and site-disordered induced weak ferromagnetism in (1-x )BiFeO 3 -x BaTiO 3

Abstract

We present evidence for weak ferromagnetism in both the rhombohedral and cubic compositions of BF-x BT solid solutions for x < 0.55. Rietveld refinement of nuclear and magnetic structures reveals that the G -type antiferromagnetic ordering of the Fe 3+ 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 R 3c space group but not in the cubic Pm3 ¯ m space group. It is shown that the local symmetry of the average cubic compositions of BF-x BT for 0.35 < x < 0.55 is broken due to off-centering of Bi 3+ in the (1-10) plane and O 2− along the ⟨110⟩ direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O 2− disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe 3+ -O 2− -Fe 3+ bond angle from 180° that allows spin canting due to Dzyaloshinskii-Moriya interaction D ⃗ i,j ⋅(S ⃗ i ×S ⃗ j ) , 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 ≳ 0.55. ©2014 American Physical Society