Evidence for Room-Temperature Multiferroicity in a Compound with a Giant Axial Ratio

In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio $c/a$) have been scrutinized intensely. However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics. Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of ${\mathrm{BiFeO}}_3$ with a giant axial ratio ($c/a=1.23$) that is both ferroelectric and magnetic at room temperature. Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with $c/a$. We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases.

Figure 1

(a) $2\theta –\omega$ scans for 7 and 56 nm single films and a 100 nm film grown on a LSMO buffer. (b) $\varphi$-scans of the $(0\overline{3}\overline{3})$ reflections for a 56 nm film (the $(30\overline{3})$ reflection is very similar). (c) reciprocal-space map for a $\mathrm{BFO}(100\mathrm{nm})/\mathrm{LSMO}\parallel \mathrm{LAO}$ sample. The line in (c) is a guide to the eye. S, B, and L are for LAO, BFO, and LSMO, respectively. (d) X-ray absorption spectra for two orthogonal direction of the incident linearly polarized beam and x-ray linear dichroism at the Fe $K$ edge for a 56 nm BFO film.

Figure 2

(a) High-resolution transmission electron microscopy image in the [100] zone axis of a 7 nm BFO film. (b) and (c) are images of the deformation perpendicular and parallel to the growth axis, respectively. Color scales in (b) and (c) correspond to a displacement relative to the LAO cell size.

Figure 3

(a) IP-PFM phase contrast of the as grown BFO (cantilever oriented along the [100] direction). The OP response is fully homogeneous. (b) OP-PFM phase after switching a $2\times 2\mu {\mathrm{m}}^2$ central square with $V=-8\mathrm{V}$ applied to the scanning tip (the initial state of polarization was down), and switching back a $1\times 1\mu {\mathrm{m}}^2$ central square with $V=+8\mathrm{V}$. (c) and (d) Current and polarization versus electric field for BFO (c) and Mn-BFO (d) films. (e) Neutron diffraction spectrum and (f) field dependence of the magnetization at 300 K.