Abstract
Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO3 with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty in determining the P of polycrystalline specimens, namely 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 positive-up negative-down (PUND) method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO3 specimens below their Néel temperatures. We found that all the HoMnO3 samples had similar remnant polarization Pr values at each temperature, regardless of their variation in resistivity, dielectric constant and pyroelectric current levels. Moreover, the Pr value 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 the intrinsic P of polycrystalline HoMnO3 can be determined through the PUND method and P at 0 K may reach ∼0.24 μC cm−2 in a single crystalline specimen. This P value is still much smaller than the theoretically predicted one but is one of the largest observed in magnetism induced ferroelectrics.
Highlights
Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO3 with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics
Depending on whether the electric fields are applied above or below a ferroelectric ordering temperature, the space charges are known as the P2 or –P2 type [23], respectively, and these two types can be a source of erroneous charges that may interfere with intrinsic pyroelectric currents
We attempt to determine the intrinsic P of o-HoMnO3 by employing a commonly well-established technique for the P–E loop measurement, namely the positive-up negative-down (PUND) technique [25], which overcomes the experimental difficulties existing in the previous methods
Summary
Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO3 with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty in determining the P of polycrystalline specimens, namely 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. Our findings suggest that the intrinsic P of polycrystalline HoMnO3 can be determined through the PUND method and P at 0 K may reach ∼0.24 μC cm−2 in a single crystalline specimen This P value is still much smaller than the theoretically predicted one but is one of the largest observed in magnetism induced ferroelectrics. The most common method of using the pyroelectric current measurements is subject to significant errors in the case of polycrystalline ferroelectrics This is due to imperfect sample quality, such as porosity, oxygen deficiency or large leakage current, which result in incomplete electric poling and space charge effects [23]. The As-grown samples were further post-annealed at 350 ◦C for 24 h in O2 and N2 atmospheres to obtain the O2-annealed and N2-annealed o-HoMnO3 samples, respectively
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