Determination of Intrinsic Ferroelectric Polarization in Orthorhombic   Manganites with E-type Spin Order
  By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we accurately determined 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 uC/cm^2 at 6 K in the series, indicating that its single-crystal form can produce a Pr of at least 0.6 \muuC/cm^2 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 variations in the in-plane Mn-O-Mn bond angle and Mn-O distances. Our experimental results suggest that the contribution of the Mn t2g orbitals dominates the ferroelectric polarization. 
Y. S. Chai, Y. S. Oh, L. J. Wang, N. Manivannan, S. M. Feng, Y. S.   Yang, L. Q. Yan, C. Q. Jin, and Kee Hoon Kim
cond-mat.mtrl-sci cond-mat.str-el