Intrinsic ferroelectric polarization of orthorhombic manganites with E -type spin order

By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we determined accurately the remanent ferroelectric polarization Pᵣ of orthorhombic RMnO₃ (R $=$ Ho, Tm, Yb, and Lu) compounds below their E-type spin ordering temperatures. We found that LuMnO₃ has the largest Pᵣ of 0.17 μC/cm² at 6 K in the series, the value of which allows us to predict that its single-crystal form can produce a Pᵣ of at least 0.6 μC/cm² at 0 K. Furthermore, at a fixed temperature, Pᵣ decreases systematically with increasing rare earth ion radius from R $=$ Lu to Ho, exhibiting a strong correlation with the variation of the in-plane Mn--O--Mn bond angle and Mn--O distances. Our experimental results suggest that the contribution of the Mn t2g orbitals may dominate the ferroelectric polarization.