Density-functional analysis of spin exchange and ferroelectric polarization inAgCrO2

Density-functional calculations were carried out for ${\text{AgCrO}}_2$ to examine its spin exchange and ferroelectric polarization. In the multiferroic state of ${\text{AgCrO}}_2$ the triangular spin lattice of each ${\text{CrO}}_2$ layer forms parallel chains with helical-spin order. The spin exchange interactions of ${\text{AgCrO}}_2$ are strongly frustrated both within and between adjacent ${\text{CuO}}_2$ layers, which gives rise to the observed helical-spin order. The observed ferroelectric polarization of ${\text{AgCrO}}_2$ is not caused by individual helical-spin chains, but by the spiral-spin chain structures that propagate between the helical-spin chains.