Magnetic structure of thespin−12frustrated quasi-one-dimensional antiferromagnetCu3Mo2O9: Appearance of a partially disordered state

Abstract

We investigated the crystal and magnetic structures of the $\text{spin}\ensuremath{-}\frac{1}{2}$ frustrated antiferromagnet ${\mathrm{Cu}}_{3}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic $Pnma$ (No. 62). We infer that the space group above ${T}_{\mathrm{N}}=7.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is monoclinic $P{2}_{1}/m$ (No. 11) from the observation of reflections forbidden in $Pnma$ in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of ${\mathrm{Cu}}_{3}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the $ac$ planes. The magnitudes are $(0.50\ensuremath{\sim}0.74){\ensuremath{\mu}}_{\mathrm{B}}$. Moments on chain sites may exist but the magnitudes are very small. The magnetic structure indicates that a partially disordered state is realized. We consider the origin of the magnetic structure, weak ferromagnetism, and electric polarization.