Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO 2

Type-II multiferroics \char21{}- materials where magnetic order induces ferroelectricty -\char21{} offer great potential for novel electronic devices due to the strong intrinsic coupling between order parameters. Such coupling is found in CuFeO${}_{2}$, where ferroelectricity is driven by proper-screw ``helical'' magnetic ordering for applied magnetic fields between 7.5 and 13 T at low temperatures. Here, the authors report a polarization memory effect in CuFeO${}_{2}$, whereby the nonpolar antiferromagnetic ground state retains a strong memory of the high-magnetic-field ferroelectric phase. The dependence of the polarization on the magnetothermal history is investigated, and it is found that the direction and almost the full polarization magnitude is ``remembered'' after cycling through the zero-field nonpolar phase. This polarization memory persists in samples in which multiple crystallographic domains are suppressed by applied stress, ruling out crystallographic boundaries as the cause of the observed memory. Instead, the authors propose that the memory is carried by Bloch-type antiferromagnetic domain walls with definite helicity. As similar memory effects have been also been reported for other multiferroics, such as MnWO${}_{4}$ and CuO, the proposed mechanism could be a general feature of phase transitions between collinear-antiferromagnetic and incommensurate magnetic phases.