Determination of the intrinsic ferroelectric polarization in orthorhombic HoMnO₃

Abstract

Whether large ferroelectric polarization P exists in the orthorhombic HoMnO 3 with the E-type antiferromagnetic spin ordering or not remains as one of unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty for determining P of polycrystalline specimens that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the PUND method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO 3 specimens below their Néel temperatures. We found that all the * These authors equally contributed to this work. HoMnO 3 samples had similar remnant polarization P r values at each temperature, regardless of their variations in resistivity, dielectric constant, and pyroelectric current levels. Moreover, P r of~0.07 µC/cm 2 at 6 K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that intrinsic P of polycrystalline HoMnO 3 can be determined through the PUND method and P at 0 K may reach~0.24 µC/cm 2 in a single crystalline specimen.

Multiferroic properties of uniaxially compressed orthorhombic HoMnO3 thin films

“…2 (b)). These dielectric properties of the strained o-HMO film are different from any of the reported bulk results [16][17][18] but are close to those of o-REMnO3 with smaller RE ions such as Tm 36 and Lu. 37 The 200 nm film exhibited different dielectric properties compared to the strained 20 nm film, as shown in Fig.…”

Intrinsic ferroelectric polarization of orthorhombic manganites withE-type spin order

“…In Ref. 21 we also observed that the P r values were almost the same even after annealing in the O 2 atmosphere. Therefore, we conclude that the P r values as obtained from the hysteresis loops measured by the PUND method are not sensitive to the leakage level of the specimen, thus indicating that the PUND method provides the intrinsic electric polarization of the specimen.…”

“…The onset temperatures of ferroelectric polarization were found to be 26, 35, 37, and 38 K for R = Ho, Tm, Yb, and Lu, respectively, and they are consistent with the reported lock-in transition temperature, T L , in each compound. 17,21,22 It is noteworthy that all the hysteresis loops appearing in Figs. 3 and 4 show saturating behavior at high electric fields except in very low temperature regions below ∼10-15 K. Related to this observation, the coercive electric fields at temperatures above 15 K seem clearly lower than the maximum applied electric field in all the compounds.…”

“…During the next two pulses, P1 and P2 (N1 and N2), two curves representing effective polarization changes are recorded in the Sawyer-Tower circuit, and they are subtracted to obtain the pure hysteretic parts of the hysteresis loop without being obscured by resistive or capacitive components. 21 Moreover, by employing a short pulse, the maximum peak field for electric poling can be increased to better align the ferroelectric domains without inducing electrical break-down effects. In particular, the space charge effect is minimized as the sample is poled in isothermal conditions.…”

“…These experimental difficulties in determining the P value in polycrystalline ferroelectrics can be greatly reduced by employing the so-called Positive-Up Negative-Down (PUND) method, 19 which has been extensively employed in ferroelectric thin film researches and recently has been applied to multiferroic single crystals. 20 In particular, we have recently used this technique to prove that a polycrystalline o-HoMnO 3 sample has an intrinsic P of approximately 0.24 μC/cm 2 , 21 which is much smaller than that predicted by band calculations. 10,12,15 In this paper we report our systematic efforts to determine accurately the r R -dependent ferroelectric polarization of o-RMnO 3 for R = Ho, Tm, Yb, and Lu with E-type spin order.…”