Weak ferromagnetic order breaking the threefold rotational symmetry of the underlying kagome lattice in CdCu^sub 3^(OH)^sub 6^(NO^sub 3^)^sub 2^ ⋅ H^sub 2^O

Novel magnetic phases are expected to occur in highly frustrated spin systems. Here, we study the structurally perfect kagome antiferromagnet CdCu3(OH)6(NO3)2 ⋅ H2O 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 TN ∼ 4 K , well below the nearest-neighbor exchange interaction J/kB = 45 K . This suggests that a unique “q = 0” type 120 ∘ 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.