Multiferroicity in Geometrically Frustrated α-MCr2O4 systems (M=Ca, Sr, Ba)
Abstract
We have successfully synthesized three quasi-2D geometrically frustrated magnetic compounds (α-MCr2O4, M=Ca, Sr, Ba) using the spark-plasma-sintering technique. All these members of the α-MCr2O4 family consist of the stacking planar triangular lattices of Cr3+ spins ( S=3/2), separated by non-magnetic 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 40K) in each member of the α-MCr2O4 family, which changes to the quasi-120 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 α-MCr2O4 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 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 inter-planar spacing (as M changes from Ca to Ba) reveals the importance of interlayer interaction in this new family of frustrated magnetic systems.
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