Coexistence of Weak Ferromagnetism and Ferroelectricity in the High Pressure ${\mathrm{LiNbO}}_3$-Type Phase of ${\mathrm{FeTiO}}_3$

We report the magnetic and electrical characteristics of polycrystalline ${\mathrm{FeTiO}}_3$ synthesized at high pressure that is isostructural with acentric ${\mathrm{LiNbO}}_3$ (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate ferroelectricity at and below room temperature and weak ferromagnetism below $\sim 120\mathrm{K}$. These results validate symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure.

Figure 1

Surface topography (a), out-of-plane PFM amplitude (b) and phase (c) images in ${\mathrm{FeTiO}}_3$ bulk crystal sample. Bright contrasts in amplitude correspond to polarization vectors strongly aligned to the surface normal (either positive or negative normal direction). (d) Local piezoresponse hysteresis loops were collected inside a grain.

Figure 2

SHG mapping of polar regions at 296 K in (a) stabilized high pressure phase of ${\mathrm{FeTiO}}_3$ with ${\mathrm{LiNbO}}_3$ crystal structure (noncentrosymmetric), (b) pure ilmenite phase (centrosymmetric). (c) SHG “butterfly” hysteresis loops (blue) at a fixed point in (a) with applied electric fields at 296 K and the corresponding polarization hysteresis loop (red) (see text). The polarization of fundamental light was horizontal, and all the output SHG polarizations were detected without an exit analyzer.

Figure 3

(a) Magnetic susceptibility of ${\mathrm{FeTiO}}_3$ (ilmenite) and high-pressure ${\mathrm{FeTiO}}_3\mathrm{\text{−}}\mathrm{II}$. (b) Specific heat of high-pressure ${\mathrm{FeTiO}}_3\mathrm{\text{−}}\mathrm{II}$. (c) Isothermal magnetization of ${\mathrm{FeTiO}}_3\mathrm{\text{−}}\mathrm{II}$ following zero-field cooling. See text for details.