Nonswitchable polarization and magnetoelectric coupling in the high-pressure synthesized doubly ordered perovskites NaYMnWO6 and NaHoCoWO6

Abstract

The doubly ordered perovskites Na L n MnW O 6 ( L n = La , Nd, and Tb), Na L n FeW O 6 ( L n = La and Nd), and Na L n CoW O 6 ( L n = Sm − 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 NaYMnW O 6 and NaHoCoW O 6 , with smaller L n -ions synthesized by high pressure and high temperatures. These compounds crystallize in the polar structure ( P 2 1 ) and undergo antiferromagnetic ordering at T N ∼ 9 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 ( Δ P ) at T N , indicating a large energy barrier between the two polar states. The change in polarization at T < T N under magnetic field reveals magnetoelectric coupling.