Formation of Magnetic Microphases in Ca3Co2O6

We study a frustrated quantum Ising model relevant for Ca3Co2O6 that consists of a triangular lattice of weakly coupled ferromagnetic chains. According to our quantum Monte Carlo simulations, the chains become ferromagnetic and form a three-sublattice up-up-down structure for T <= T-CI. In contrast, long-wavelength spin-density-wave microphases are stabilized along the chains for T-CI < T < T-c. Our mean field solutions reveal a quasicontinuous temperature dependence of the modulation wavelength, implying the existence of metastable states that explain the very slow dynamics observed in Ca3Co2O6. We also discuss implications of microphases for the related multiferroic compounds Ca3CoMnO6 and Lu2MnCoO6.