Weak ferromagnetic order breaking the threefold rotational symmetry of the underlying kagome lattice in CdCu3(OH)6(NO3)2·H2O

Abstract

Novel magnetic phases are expected to occur in highly frustrated spin systems. Here, we study the structurally perfect kagome antiferromagnet $\mathrm{CdC}{\mathrm{u}}_{3}{(\mathrm{OH})}_{6}{({\mathrm{NO}}_{3})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$ by magnetization, magnetic torque, and heat capacity measurements using single crystals. An antiferromagnetic order accompanied by a small spontaneous magnetization that surprisingly is confined in the kagome plane sets in at ${T}_{\mathrm{N}}\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, well below the nearest-neighbor exchange interaction $J/{k}_{\mathrm{B}}=45\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This suggests that a unique ``$\mathbf{q}=0$'' type ${120}^{\ensuremath{\circ}}$ spin structure with ``negative'' (downward) vector chirality, which breaks the underlying threefold rotational symmetry of the kagome lattice and thus allows a spin canting within the plane, is exceptionally realized in this compound rather than a common one with ``positive'' (upward) vector chirality. The origin is discussed in terms of the Dzyaloshinskii-Moriya interaction.