Chiral Spin Pairing in Helical Magnets

A concept of chiral spin pairing is introduced to describe a vector-chiral liquid-crystal order in frustrated spin systems. It is found that the chiral spin pairing is induced by the coupling to phonons through the Dzyaloshinskii-Moriya interaction and the four-spin exchange interaction of the Coulomb origin under the edge-sharing network of magnetic and ligand ions. This produces two successive second-order phase transitions upon cooling: an O(2) chiral spin nematic, i.e., spin cholesteric, order appears with an either parity, and then the O(2) symmetry is broken to yield a helical magnetic order. Possible candidate materials are also discussed as new multiferroic systems.

Figure 1

Phase diagram of the Ginzburg-Landau Hamiltonian given by Eq. (2) and the ordering patterns. Note that ${\tilde{\mu }}_{*}=-8\tilde{u}$. For details, see the text.