Ferroelectricity in anS=1/2Chain Cuprate

Abstract

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S=1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P(c) increases with H(c), and P(a) appears with H(a). LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the "1D spiral-magnetic ferroelectrics." However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S=1/2 magnet of LiCu2O2.

Thermodynamic properties of LiCu2O2 multiferroic compound

“…When the system is polarized by an opposite electric field, P c is reversed as well, indicating the ferroelectric nature of spiral order phase [19]. However, the reported remarkable field effects on the electric polarization [7] have not been exhibited in our results.…”

“…However, based on this ab-spiral picture, the fact that the polarization emerges along the c axis can not be interpreted by the commonly accepted microscopic mechanisms: the inverse Dzyaloshinskii-Moriya interaction or spin-current model. Later, Park et al suggested another spin picture that the transverse spiral spin component was in the bc-plane [7], which is partially confirmed by Seki et al, but the quantitative calculation on the intensity of polarized neutron reflections shows a prominent discrepancy [19]. Recently, based on their own NMR and neutron diffraction data, Sato et al proposed a new noncollinear modulated magnetic structure.…”

“…For a classical spin chain, when |J 4 /J 2 | is larger than a critical value 1/4, an incommensurate spiral spin structure with pitching angle 2πξ = arccos(1/|4J 4 /J 2 |) can be expected at ground state [7]. And therefore the changes of J 4 will evidently modify the magnitude of ξ and are macroscopically reflected in the variations of magnetic phase patterns.…”

“…Actually, a class of low-dimensional cuprate oxides such as LiCuVO 4 , Li 2 CuO 2 , Li 2 ZrCuO 4 etc. were reported to have such spiral magnetic orders [4][5][6][7][8][9]. And it is believed that the research on the ferroelectricity of magnetic origin in these low-dimensional compounds will be helpful in understanding the multiferroic nature [10,11].…”

The influence of magnetic anisotropy on magnetoelectric behavior in conical spin ordered multiferroic state

“…In these frustrated materials, the ferroelectricity is always induced by some special magnetic orders with the collapse of inversion symmetry, such as the collinear up-up-down-down magnetic order [3][4][5] and noncollinear spiral spin state. [6][7][8] It is noted that the anisotropy is an important factor to affect the magnetic structure, and thus plays a key role on the magnetism-driven ferroelectric behavior. Therefore, the investigation on the anisotropy is very helpful to understand the ME behavior of magnetic origin.…”