Room-Temperature Multiferroic Hexagonal ${\mathrm{LuFeO}}_3$ Films

The crystal and magnetic structures of single-crystalline hexagonal ${\mathrm{LuFeO}}_3$ films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal ${\mathrm{LuFeO}}_3$ films are room-temperature multiferroics.

Figure 1

The structural characterizations of the h-${\mathrm{LuFeO}}_3$ films using RHEED and XRD. (a) A typical $\theta$-$2\theta$ scan of an h-${\mathrm{LuFeO}}_3$ film. The inset shows the close-up view of the h-${\mathrm{LuFeO}}_3$ (002) peak. (b) A RHEED image of the h-${\mathrm{LuFeO}}_3$ film with an electron beam along the ${\mathrm{Al}}_2{\mathrm{O}}_3$ and h-${\mathrm{LuFeO}}_3$ [100] direction. (c) Intensities of the RHEED (100) peak and XRD (102) peak as functions of temperature. (d) PFM response displaying a square-shaped hysteresis loop. The amplitude and phase are shown in the insets. (e) Schematic of a written domain pattern with dc voltage ($V=20{\mathrm{V}}_{\mathrm{dc}}$). (f) PFM image of the same region without dc voltage.

Figure 2

The schematic representation of $P{6}_3/mmc$ and $P{6}_{3}cm$ structures and the possible transition routes. (a) and (b) are the $P{6}_3/mmc$ structure viewed from the [120] and [001] directions, respectively. (c) and (d) are the $P{6}_{3}cm$ structure viewed from the [100] and [001] directions, respectively. The bold lines in (b) and (d) indicate the unit cells in the $a\mathrm{\text{−}}b$ plane. (e) Possible routes from $P{6}_3/mmc$ to $P{6}_{3}cm$ structures.

Figure 3

Magnetic Bragg peaks and the temperature dependence of their intensities from neutron diffraction. (a) and (c) are the diffraction intensity profile near the (102) and (100) magnetic Bragg points, respectively. The lines are the Gaussian fits. (b) and (d) are the temperature dependence of the (102) and (100) magnetic Bragg peak intensities, respectively. The curves in (b) and (d) are fits to the data points (see text). The inset of (d) shows the temperature dependence of the magnetization in ZFC and FC processes with the magnetic field along the $c$ axis.

Figure 4

The possible magnetic structures of ${\mathrm{Fe}}^{3+}$ sites in $P{6}_{3}cm$ crystallographic structure and the assignments of magnetic structures for h-${\mathrm{LuFeO}}_3$. The darker (lighter) balls represent ${\mathrm{Fe}}^{3+}$ sites at $z/c=0$ ($z/c=1/2$) and the arrows indicate the magnetic moment orientation. ${\Gamma }_1$ (${\Gamma }_3$) and ${\Gamma }_2$ (${\Gamma }_4$) are related by the in-plane rotation angle $\varphi$. The ${\Gamma }_1$ (${\Gamma }_2$) and ${\Gamma }_3$ (${\Gamma }_4$) are a homometric pair, meaning indistinguishable in our neutron scattering measurement. The magnetic structure of h-${\mathrm{LuFeO}}_3$ takes the form ${\Gamma }_2$, mixed with ${\Gamma }_1$ at $T<T_R$ and ${\Gamma }_4$ at $T_R<T<T_N$.