Magnetic transitions and site-disordered induced weak ferromagnetism in (1-<span class="aps-inline-formula"><math><mi>x</mi></math></span>)<span class="aps-inline-formula"><math><msub><mrow><mtext>BiFeO</mtext></mrow><mn>3</mn></msub></math></span>-<span class="aps-inline-formula"><math><mi>x</mi></math></span><span class="aps-inline-formula"><math><msub><mrow><mtext>BaTiO</mtext></mrow><mn>3</mn></msub></math></span>

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 $x$ \ensuremath{\sim} 0.50. We address the issue of weak ferromagnetism due to spin canting, which is allowed by the symmetry in the $R$3$c$ space group but not in the cubic $Pm\overline{3}m$ 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${}^{2\ensuremath{-}}$ along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O${}^{2\ensuremath{-}}$ disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe${}^{3+}$-O${}^{2\ensuremath{-}}$-Fe${}^{3+}$ bond angle from 180\ifmmode^\circ\else\textdegree\fi{} that allows spin canting due to Dzyaloshinskii-Moriya interaction ${\stackrel{P\vec}{D}}_{i,j}\ifmmode\cdot\else\textperiodcentered\fi{}({\stackrel{P\vec}{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\stackrel{P\vec}{S}}_{j})$, 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 $x$ \ensuremath{\gtrsim} 0.55.