Multiferroicity in geometrically frustratedα-MCr2O4

We have successfully synthesized three quasi-two-dimensional geometrically frustrated magnetic compounds ($\ensuremath{\alpha}$-$M$Cr${}_{2}$O${}_{4}$, $M=$ Ca, Sr, Ba) using the spark-plasma-sintering technique. All these members of the $\ensuremath{\alpha}$-$M$Cr${}_{2}$O${}_{4}$ family consist of the stacking planar triangular lattices of Cr${}^{3+}$ spins ($S=3/2$), separated by nonmagnetic alkaline-earth ions. Their corresponding magnetic susceptibility, specific heat, dielectric permittivity, and ferroelectric polarization are systematically investigated. A long-range magnetic ordering arises below the N\'eel temperature (around 40 K) in each member of the $\ensuremath{\alpha}$-$M$Cr${}_{2}$O${}_{4}$ family, which changes to the quasi-${120}^{\ensuremath{\circ}}$ proper-screw-type helical spin structure at low temperature. A very small but confirmed spontaneous electric polarization emerges concomitantly with this magnetic ordering. The direction of electric polarization is found within the basal triangular plane. The multiferroicity in $\ensuremath{\alpha}$-$M$Cr${}_{2}$O${}_{4}$ can not be explained within the frameworks of the magnetic exchange striction or the inverse Dzyaloshinskii-Moriya interaction. The observed results are more compatible with the newly proposed Arima mechanism that is associated with the $d$-$p$ hybridization between the ligand and transition-metal ions, modified by the spin-orbit coupling. The evolution of multiferroic properties with the increasing interplanar spacing (as $M$ changes from Ca to Ba) reveals the importance of interlayer interaction in this new family of frustrated magnetic systems.