FERROELECTRICITY IN AN ISING CHAIN MAGNET

V. Kiryukhin, Rutgers University

Magnetically-driven ferroelectrics exhibit intrinsically large magnetoelectric coupling, which gives rise to dramatic magnetoelectric effects. This opens up a possibility for controlling electric properties (e.g. electric polarization) by a magnetic field, and vice versa. Two microscopic mechanisms of magnetoelectric coupling have been discussed in application to the recently-discovered magnetoelectrics: inverse Dzyaloshinskii-Moriya (DM) interaction in spiral magnets, and exchange striction in (nearly) collinear chain magnets. Many of the actual compounds were found to belong to the first class. However, unlike the DM interaction, exchange striction is associated with symmetric superexchange, and therefore is, generally, a much stronger effect. Thus, magnetic ferroelectrics driven by exchange striction hold significant promise as candidate materials with enhanced functional properties. In this talk, we discuss a conceptually simple experimental system of the exchange-striction type. The compound in question, Ca3(Co,Mn)2O6, consists of frustrated quasi-one-dimensional Ising chains assembled on a triangular lattice [1]. Due to in-chain magnetic frustration, the up-up-down-down magnetic order is realized. Combined with the Co/Mn ionic order, it breaks inversion symmetry and gives rise to a ferroelectric moment. Thus, Ca3(Co,Mn)2O6 is a clean realization of a simple exchange-driven model magnetoelectric that was previously discussed theoretically. In addition, this compound is a good 1D frustrated Ising system in which the well known ANNNI model can be studied. Effects of a magnetic field are discussed in the context of this model, and strong magnetoelectric effects are presented. We also discuss interesting effects associated with interchain geometrical frustration (magnetoelecric freezing), and effects of impurities on the magnetic order.

[1] Y.J. Choi, H.T. Yi, S. Lee, Q. Huang, V. Kiryukhin, and S.-W. Cheong, Phys,. Rev. Lett. 100, 047601 (2008).

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