Magnetic Reversal of the Ferroelectric Polarization in a Multiferroic Spinel Oxide

Ferroelectric transition has been detected in a ferrimagnetic spinel oxide of ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ upon the transition to the conical spin order below 25 K. The direction $[\overline{1}10]$ of the spontaneous polarization is normal to both the magnetization easy axis $[001]$ and to the propagation axis $[110]$ of the transverse spiral component, in accord with the prediction based on the spin-current model. The reversal of the spontaneous magnetization by a small magnetic field ($\sim 0.1\mathrm{T}$) induces the reversal of the spontaneous polarization, indicating the clamping of the ferromagnetic and ferroelectric domain walls.