Formation of Magnetic Microphases inCa3Co2O6

Abstract

We study a frustrated quantum Ising model relevant for ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ 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 $T\le T_{\mathrm{CI}}$. In contrast, long-wavelength spin-density-wave microphases are stabilized along the chains for $T_{\mathrm{CI}}<T<T_c$. 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 ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$. We also discuss implications of microphases for the related multiferroic compounds ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$ and ${\mathrm{Lu}}_2{\mathrm{MnCoO}}_6$.