Nonswitchable polarization and magnetoelectric coupling in the high-pressure synthesized doubly ordered perovskites $\mathrm{NaYMnW}{\mathrm{O}}_6$ and $\mathrm{NaHoCoW}{\mathrm{O}}_6$

The doubly ordered perovskites $\mathrm{Na}Ln\mathrm{MnW}{\mathrm{O}}_6$ ($Ln=\mathrm{La}$, Nd, and Tb), $\mathrm{Na}Ln\mathrm{FeW}{\mathrm{O}}_6$ ($Ln=\mathrm{La}$ and Nd), and $\mathrm{Na}Ln\mathrm{CoW}{\mathrm{O}}_6$ ($Ln=\mathrm{Sm}-\mathrm{Er}$, Yb, and Y) have been reported to be potential multiferroics because of their polar crystal structure ($P{2}_1$) and antiferromagnetic ordering of the transition-metal ions at low temperatures. In these materials, the octahedral rotation resulting from the layered ordering of cations at the $A$-site and the rock-salt ordering of cations at the $B$-site is suggested to be responsible for ferroelectric polarization. However, the ferroelectricity of these compounds has never been confirmed experimentally. Here we report an experimental demonstration of nonswitchable electric polarization below the magnetic ordering temperatures in two representative compounds, namely $\mathrm{NaYMnW}{\mathrm{O}}_6$ and $\mathrm{NaHoCoW}{\mathrm{O}}_6,$ with smaller $Ln$-ions synthesized by high pressure and high temperatures. These compounds crystallize in the polar structure ($P{2}_1$) and undergo antiferromagnetic ordering at $T_{\mathrm{N}}\sim 9\mathrm{K}$, where a dielectric anomaly occurs in zero magnetic field. While the conventional $P-E$ loop measurements suggest the absence of ferroelectric polarization in the paramagnetic state, the pyroelectric current measurements across the magnetic ordering temperature show a nonswitchable change in polarization ($\mathrm{\Delta }P$) at $T_{\mathrm{N}}$, indicating a large energy barrier between the two polar states. The change in polarization at $T<T_{\mathrm{N}}$ under magnetic field reveals magnetoelectric coupling.

Figure 1

Powder x-ray diffraction pattern recorded at room temperature and the Rietveld refined pattern using $P{2}_1$ space group in (a) $\mathrm{NaYMnW}{\mathrm{O}}_6$ and (b) $\mathrm{NaHoCoW}{\mathrm{O}}_6$. The second Bragg lines in $\mathrm{NaYMnW}{\mathrm{O}}_6$ correspond to the impurity phase ${\mathrm{Y}}_2\mathrm{W}{\mathrm{O}}_6$.

Figure 2

(a) Crystal structure of $\mathrm{NaYMnW}{\mathrm{O}}_6$ showing layered ordering of Na and Y and rock-salt ordering of Mn and W with rotated and tilted octahedra of the Mn and W atom. (b) Coordination of oxygen around Mn and W ions with different Mn-O and W-O bond lengths.

Figure 3

(a) and (c) dc magnetization on the left axis measured at zero-field-cooled and field-cooled condition with 1 kOe and heat capacity divided by temperature on the right axis measured at 0 T for $\mathrm{NaYMnW}{\mathrm{O}}_6$ and $\mathrm{NaHoCoW}{\mathrm{O}}_6$. Parts (b) and (d) are $M$($H$) curves at various temperatures along with dM/dH in the inset, respectively.

Figure 4

Polarization vs electric-field loop measured at different temperatures with an $f=1\mathrm{Hz}$ pulse for (a) $\mathrm{NaYMnW}{\mathrm{O}}_6$ and (b) $\mathrm{NaHoCoW}{\mathrm{O}}_6$.

Figure 5

(a) Frequency-dependent dielectric constant and loss (inset) in $\mathrm{NaYMnW}{\mathrm{O}}_6$ measured from 2 to 240 K at zero field. (b) Dielectric constant measured at various magnetic fields from 2 to 30 K at 100 kHz. The inset shows the magnetodielectric effect at 9.4 K. (c) Frequency-dependent dielectric constant. (d) The corresponding loss data recorded from 2 to 240 K in $\mathrm{NaHoCoW}{\mathrm{O}}_6$. (e) Magnetic-field-dependent dielectric constant measured from 2 to 30 K at 100 kHz. (f) Magnetodielectric effect at various temperatures in $\mathrm{NaHoCoW}{\mathrm{O}}_6$.

Figure 6

(a) Change in polarization with temperature across magnetic ordering obtained through pyrocurrent in $\mathrm{NaYMnW}{\mathrm{O}}_6$ under various poling processes. (b) Effect of magnetic field on the change of polarization with constant electric field poling in $\mathrm{NaHoCoW}{\mathrm{O}}_6$.