Ga-substitution-induced single ferroelectric phase in multiferroic ${\text{CuFeO}}_2$

We have succeeded in realizing a single ferroelectric phase in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ by substituting nonmagnetic ${\text{Ga}}^{3+}$ for ${\text{Fe}}^{3+}$ sites in ${\text{CuFeO}}_2$. Ferroelectric polarization $P$ in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ is observed below 7.5 K, and has the relatively large value of $\sim 250\mu \text{C}/{\text{m}}^2$, which is comparable to $P=300\sim 400\mu \text{C}/{\text{m}}^2$ in the magnetic-field-induced ferroelectric phase of ${\text{CuFeO}}_2$. In neutron-diffraction measurements, a single magnetic diffraction peak with an incommensurate wave number was observed below 7.5 K in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$, indicating that the ferroelectric-incommensurate (FEIC) phase is realized as a single phase. Therefore, ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ with a single FEIC phase is strongly expected to provide the best opportunity to investigate unresolved problems regarding the ferroelectric mechanism in ${\text{CuFeO}}_2$. In this paper, we report measurements of magnetic susceptibility, specific heat, pyroelectric, dielectric constant, and neutron diffraction of a single crystal of ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$.

Figure 1

(Color online) (a) Crystal structure of ${\text{CuFeO}}_2$. (b) $x\text{−}T$ magnetic phase diagram of ${\text{CuFe}}_{1-x}{\text{Al}}_x{\text{O}}_2$. The data were taken from Ref. 22. The phase-transition temperatures in the Al concentration are denoted by the dotted line with the arrow corresponds to those in ${\text{CuFe}}_{1-y}{\text{Ga}}_y{\text{O}}_2$ with $y=0.037$ (see the main text for details).

Figure 2

(Color online) Temperature dependence of (a) magnetic susceptibility $\chi$ (b) specific heat $C$ (c) electric polarization $P_{110}$ (d) real part of dielectric constant ${\epsilon }_{100}^{\prime }$ (e) imaginary part of dielectric constant along the [110] axis ${\epsilon }_{110}^{″}$ (f) propagation wave number $q$ of magnetic modulations (g) integrated intensity of the magnetic Bragg reflections and (h) peak intensity of the nuclear Bragg reflections at ${\left(110\right)}_{\lambda /2}$ (Ref. 27) and (003). Open and closed symbols denote the data for decreasing and increasing temperature processes, respectively. (c) $P_{110}$ was measured after cooling with several poling electric fields of 407, 325, 163, 81, and 41 kV/m. The inset in (c) shows the poling electric-field dependence of $P_{110}$ at 2 K. Solid and dotted lines denote the magnetic phase-transition temperatures for cooling and warming processes, respectively.

Figure 3

(Color online) (a) Schematic of the reciprocal $\left(HHL\right)$ zone. Squares and circles denote the reciprocal-lattice positions of the nuclear and magnetic reflections, respectively. (b) Typical neutron-diffraction profiles along $\left(HH\frac{3}{2}\right)$ at several temperatures in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$. The inset shows the diffraction profiles at 4 K for ${\text{CuFe}}_{1-x}{\text{Al}}_x{\text{O}}_2$ with $x=0.0155$, which was taken from Ref. 22.

Figure 4

(Color online) (a) The reciprocal-lattice (HK0) plane. Open and closed circles denote the reciprocal-lattice points for the rhombohedral structure and the distorted monoclinic structure, respectively. The subscripts $h$ and $m$ denote the unit vectors with hexagonal and monoclinic notations [see Fig. 1(b) in Ref. 8], respectively. The arrow on ${\left(110\right)}_h$ denotes the direction of the reciprocal-lattice scan in (c). (b) Temperature dependence of half width at half maximum of the nuclear scattering peak for the $\left(HH0\right)$ scan around ${\left(110\right)}_{\lambda /2}$. (c) Typical diffraction profiles at ${\left(110\right)}_{\lambda /2}$ at several temperatures.

Figure 5

(Color online) Comparison of the temperature dependence of the electric polarization along the [110] axis $P_{110}\left(T\right)$ and the integrated intensity of the magnetic Bragg reflection at $\left(qq\frac{3}{2}\right)$.