Atypical multiferroicity of HoCrO₃ in bulk and film geometry

Abstract

We report ferroelectricity in antiferromagnetic HoCrO 3 with a reasonably large value of spontaneous electric polarization (B0.32 mC cm À2 at 10 K) from the measurement of pyroelectric current. The appearance of ferroelectricity is further confirmed in the film geometry. Intriguingly, the onset of polar order is observed at a significantly higher temperature (B240 K) than the Néel temperature (T N = 142 K).The survival of remanent polarization is confirmed from the electric polarization hysteresis loops above and below T N for polycrystalline and film specimens. Structural analysis suggests that polar oxygen octahedral rotations and Ho displacements in the non-centrosymmetric Pna2 1 space group engineer ferroelectricity. The results indicate that occurrence of polar order in HoCrO 3 resembles neither the typical case of proper ferroelectrics nor the improper ferroelectrics. † Electronic supplementary information (ESI) available: AFM image, schematic representation of electric connection for P-E loop measurements in film, fit of the X-ray diffraction pattern at 80 K using Rietveld refinement and details of the refined structural parameters. See Fig. 1 Powder X-ray diffraction data (black symbol) of HoCrO 3 with the Rietveld refinement (red line) for bulk (a) and for film (b) with indexing of (hkl). Raman spectra for bulk (c) and film (d). The three-dimensional AFM image (e) and a surface topography scan (f).

Hyperfine and crystal field interactions in multiferroic HoCrO3

“…The observed anomaly in CEF signal strengthens the importance of Ho-Cr exchange striction. The anomaly in 6(b) also implies the possible distortion of Ho ions and their surroundings since the pyroelectric current exhibits its maximal at the same temperature [8].…”

“…5(a) in the ref. [8]. Therefore, the change of CEF peak position at 60 K is related to the change of ferroelectric properties.…”

“…The non-Kramer's Ho Temperature dependence of integrated intensity of peak corresponding to CEF excitation ε 3 and (c) Temperature dependence of peak position, for the inelastic peak corresponding to crystal electric field excitation ε 3 . Vertical dashed line at 140 K indicates the magnetic ordering temperature and the dashed line at 60 K corresponds to the peak in pyroelectric current curve, reported in reference [8].…”

“…Recently, Ghosh et al, have studied the ferroelectric properties of polycrystalline HoCrO 3 by measuring the thermal variation of pyroelectric current [8]. It was found that pyroelectric current exhibits its maximum value around the antiferromagnetic transition temperature T N = 140 K, nevertheless, the ferroelectric order temperature which associated with the emergence of spontaneous electric polarization is observed at a higher temperature of T ≈ 240 K. The atypical multiferroic behavior observed in HoCrO 3 is argued to be a result of Ho displacements and oxygen octahedral rotations in the non-centrosymmetric P na2 1 space group [4,8,10,11].…”

“…Perovskite chromites RCrO 3 , where R is a rare earth element or yttrium are revisited in the recent years as possible multiferroic materials in which multiple ferroic orders such as ferroelectricity and antiferromagnetism coexist as discussed below [1][2][3][4][5][6][7][8]. Compared to perovskite manganites, which are well studied in light of multiferroicity, the microscopic physical properties of chromites are not explored in detail and the mechanism for multiferroicity in HoCrO 3 is still under debate.…”