Ordered aeschynite-type polar magnets $R\mathrm{FeW}{\mathrm{O}}_6$ ($R=\mathrm{Dy}$, Eu, Tb, and Y): A new family of type-II multiferroics

We report the discovery of magnetoelectric multiferroicity in a family of oxides, $R\mathrm{FeW}{\mathrm{O}}_6$ ($R=\mathrm{Dy}$, Eu, Tb, and Y) that crystallize in a polar aeschynite-type structure ($Pna{2}_1$) with an ordered arrangement of $\mathrm{F}{\mathrm{e}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions. Magnetization and analysis of neutron-diffraction data of $\mathrm{DyFeW}{\mathrm{O}}_6$ reveal a commensurate noncollinear antiferromagnetic ordering of $\mathrm{F}{\mathrm{e}}^{3+}$ spins ($T_{\mathrm{N}}^{\mathrm{Fe}}\sim 18\mathrm{K}$), which induce Dy spins to order at the same temperature. A sudden change in electric polarization $\left(\mathrm{\Delta }P\right)$ appears in all the compounds at $T_{\mathrm{N}}^{\mathrm{Fe}}=15–18\mathrm{K}$. The electric polarization is sensitive to an applied magnetic field, and the coupling between different magnetic $R$-ion and Fe-ion moments suppresses the polarization to a different extent. Although the measured polarization in polycrystalline $\mathrm{DyFeW}{\mathrm{O}}_6$ at 3.5 K is about $3\mu \mathrm{C}/{\mathrm{m}}^2$, a simple calculation of the ionic contribution to polarization with formal charges is $75560\mu \mathrm{C}/{\mathrm{m}}^2$ and it is of the form $\mathbf{P}=(p_x,0,p_z)$ where, $p_x$ comes from magnetic ordering and $p_z$ is associated with the polar structure in the paramagnetic state. These findings open up an avenue to explore further new polar magnets with rare-earth/transition-metal ions in the ordered aeschynite-type structure.

Figure 1

Results of combined Rietveld refinement of room-temperature (a) synchrotron x-ray- and (b) neutron-diffraction data of $\mathrm{DyFeW}{\mathrm{O}}_6$. (c) The aeschynite-type crystal structure of $\mathrm{CaT}{\mathrm{a}}_2{\mathrm{O}}_6$. (d) The ordered aeschynite-type crystal structure of $\mathrm{DyFeW}{\mathrm{O}}_6$.

Figure 2

(a) Temperature dependence of field-cooled magnetization of $\mathrm{DyFeW}{\mathrm{O}}_6$ under a magnetic field of 100 Oe. The inset shows magnetic-field-dependent magnetization at different temperatures. (b) Temperature-dependent heat capacity $\left(C_{\mathrm{P}}\right)$ of $\mathrm{DyFeW}{\mathrm{O}}_6$.

Figure 3

(a) Results of Rietveld refinement of neutron-diffraction data of $\mathrm{DyFeW}{\mathrm{O}}_6$ collected at 3.5 K. The first row of vertical bars represents the nuclear Bragg positions, and the second row represents magnetic Bragg peak positions. (b) The magnetic structure of $\mathrm{DyFeW}{\mathrm{O}}_6$.

Figure 4

(a) Temperature variation of the real part of dielectric constant ${\varepsilon }^{\prime }\left(T\right)$ of $\mathrm{DyFeW}{\mathrm{O}}_6$ measured with different frequencies. The inset shows the corresponding imaginary part of the dielectric constant ${\varepsilon }^{\prime \prime }\left(T\right)$. (b) Temperature dependence of the real part of dielectric constant ${\varepsilon }^{\prime }\left(T\right)$ at 50 kHz measured in the presence of different applied magnetic fields.

Figure 5

(a) Temperature evolution of electric polarization $\mathrm{\Delta }P\left(T\right)$ of $\mathrm{DyFeW}{\mathrm{O}}_6$ below $T_{\mathrm{N}}^{\mathrm{Fe}}=18\mathrm{K}$ obtained from pyroelectric current measurements under an electric field of $E=\pm 9.2\mathrm{kV}/\mathrm{cm}$ and different magnetic fields. The inset shows a magnetic-field-dependent change in electric polarization measured at 10 K with a ramping magnetic field with a rate of 120 Oe/s after poling the sample with an electric field of $E=+9.2\mathrm{kV}/\mathrm{cm}$. (b) Temperature dependence of a dc-biased current of $\mathrm{DyFeW}{\mathrm{O}}_6$ measured with an electric field of $E_{\mathrm{dc}}=+9.2\mathrm{kV}/\mathrm{cm}$ and magnetic fields of 0 and 40 kOe.

Figure 6

(a) Variation of FC magnetization with temperature $M\left(T\right)$ under a magnetic field of 100 Oe in $\mathrm{EuFeW}{\mathrm{O}}_6$. The inset shows magnetization vs magnetic-field data measured at 2 K. (b) Temperature-dependent heat capacity $\left(C_{\mathrm{P}}\right)$ of $\mathrm{EuFeW}{\mathrm{O}}_6$. (c) Temperature variation of the real part of dielectric constant ${\varepsilon }^{\prime }\left(T\right)$ of $\mathrm{EuFeW}{\mathrm{O}}_6$ measured with 50 kHz under different magnetic fields. (d) Evolution of electric polarization $\mathrm{\Delta }P\left(T\right)$ with temperature below $T_{\mathrm{N}}^{\mathrm{Fe}}$ in $\mathrm{EuFeW}{\mathrm{O}}_6$ obtained from pyroelectric current measurements with an electric field of $E=+7.7\mathrm{kV}/\mathrm{cm}$ and different magnetic fields.

Figure 7

(a) Temperature-dependent FC magnetization $M\left(T\right)$ of $\mathrm{TbFeW}{\mathrm{O}}_6$ measured under a magnetic field of 100 Oe. The inset shows the magnetic-field dependence of magnetization measured at different temperatures. (b) Temperature variation of heat-capacity $C_{\mathrm{P}}\left(T\right)$ of $\mathrm{TbFeW}{\mathrm{O}}_6$. (c) Temperature dependence of the real part of the dielectric constant ${\varepsilon }^{\prime }\left(T\right)$ measured with 50 kHz under different magnetic fields. (d) Electric polarization $\mathrm{\Delta }P\left(T\right)$ measured with an electric field of $E=+5.8\mathrm{kV}/\mathrm{cm}$ and different applied magnetic fields.

Figure 8

(a) Temperature variation of magnetization of $\mathrm{YFeW}{\mathrm{O}}_6$ measured with a magnetic field of 100 Oe under a field-cooled condition. The $M\left(H\right)$ loop measured at 2 K is shown in the inset. (b) Temperature dependence of $C_{\mathrm{P}}\left(T\right)$. (c) and (d) Temperature evolution of dielectric constant ${\varepsilon }^{\prime }\left(T\right)$ and electric polarization $\mathrm{\Delta }P\left(T\right)$ measured with different magnetic fields in $\mathrm{YFeW}{\mathrm{O}}_6$.