Tuning of multiferroic order with Co doping in CuCr2O4 : Interplay between structure and orbital order

Abstract

We observe a ferroelectric (FE) order in an unexplored CuCr 2 O 4 with a reasonably high value of the FE Curie temperature (T FE) at 170 K, which is also much higher than the magnetic ordering temperature. The systematic substitution of Jahn-Teller (J-T) active divalent Cu ion by a non-Jahn-Teller active divalent Co ion causes a systematic shift of T FE from 170 K for x = 0 to 146 K for x = 0.8 in Cu 1−x Co x Cr 2 O 4. The values of electric polarization vary from 0.0665 μC cm −2 to 0.1704 μC cm −2 , which is maximum for x = 0.6, associated with the highest value of the coercivity. The synchrotron diffraction studies of the compounds with x = 0.2 and 0.8 confirm that a structural transition to a polar Ima2 space group from the tetragonally distorted I4 1 /amd structure gives rise to the ferroelectricity. In all the members of Cu 1−x Co x Cr 2 O 4 series, the T FE is observed at much higher temperature than the corresponding magnetic ordering temperatures (T N). These results are in contrast to that of the reported results of T FE < T N for the end member with x = 1 or CoCr 2 O 4 , where the J-T active Cu 2+ is absent. We propose that the J-T distortion in the entire series with 0 x 0.8 holds the key, where interplay between the J-T distortion driven orbital order and the structural distortion correlates tuning of the T FE in Cu 1−x Co x Cr 2 O 4 .

Rhombohedral distortion-driven ferroelectric order and exchange bias effect in geometrically frustrated ZnFe2O4

“…This intensity change around T FE1 is remarkably high as ∼ 73%, which is much larger than the results observed for the multiferroics, involving the steplike isostructural changes [13,14] and the structural transitions to the lower symmetries [7,8,45,64]. We note that the refinement is not satisfactory below T FE1 with the F d3m structure.…”

“…In the figure, T pole (1), T pole (2), T pole (3), and T pole (4) represent the poling temperatures at 150, 95, 20, and 8 K, respectively. In all the cases, a peak at 110 K is always observed, pointing to a genuine occurrence of the peak [7,58,59], which is shown in the figure. For T pole (1) another high-temperature peak is observed, which disappears with T pole close to or below the polar order.…”

“…For T pole (1) another high-temperature peak is observed, which disappears with T pole close to or below the polar order. The high-T peak appears due to the extrinsic thermally stimulated depolarization currents (TSDC) [7,[58][59][60]. For the poling at 95 K, an additional peak close to 95 K appears below the peak at 110 K. The peak around 95 K disappears for the poling at 150, 20, and 8 K, although the peak at 110 K is always evident, which is described as T FE1 .…”

“…For example, the solely Asite magnetic atom in the spinel structure gives rise to exotic effects such as spin dimerization [1], spin liquid ground states [2][3][4], and suppressed antiferromagnetic order [5], which are often regulated by either orbital ordering or geometric magnetic frustrations, or both. Transition elements in both the Aand B-sites reduce the magnetic frustration, but this produces interesting consequences such as structural instabilities [6][7][8], intricate magnetic structures [9,10], ferroelectricity [11][12][13][14], etc. The solely B-site magnetic atom in AB 2 X 4 is found to have potential for the pyrochlore structure, producing strong geometric magnetic frustration [15,16].…”

Spin reorientation by Ni doping in Cu1−Ni Cr2O4 spinels with x = 0 and 0.1, and evidence for canted magnetic Cr order above the onset of a ferromagnetic Cu

“…Beside the unusual magnetic behavior of CuCr2O4 it has been reported recently that a ferroelectric order with a polarization of 0.15 µC cm −2 occurs at a temperature around 170 K [28]. A transition to a polar structure with a space group Ima2 becomes evident for Co-doped copper chromites.…”