Density-functional analysis of spin exchange and ferroelectric polarization in AgCrO2

Density-functional calculations were carried out for AgCrO2 to examine its spin exchange and ferroelectric polarization. In the multiferroic state of AgCrO2 the triangular spin lattice of each CrO2 layer forms parallel chains with helical-spin order. The spin exchange interactions of AgCrO2 are strongly frustrated both within and between adjacent CuO2 layers, which gives rise to the observed helical-spin order. The observed ferroelectric polarization of AgCrO2 is not caused by individual helical-spin chains, but by the spiral-spin chain structures that propagate between the helical-spin chains.