$\beta -{\mathrm{LiMnBO}}_3$ crystallizes in a hexagonal lattice and is built up of ${\mathrm{MnO}}_5$ chains linked by ${\mathrm{BO}}_3$ groups. Magnetic susceptibility shows a broad maximum at 47 K, above which the behavior is typical for a one-dimensional Heisenberg antiferromagnet with $J=5.4\mathrm{}\hspace{0.5em}\mathrm{K}.$ Magnetic neutron diffraction reveals a much lower ordering temperature (28.5 K) to give a novel chiral ground state with a magnetic unit cell of $\sqrt{3}{a}_{\mathrm{N}},$ ${2c}_{\mathrm{N}}$ consisting of a mixture of one ferromagnetic ${\mathrm{Mn}}_3$ triangle and two normal 120° antiferromagnetic ${\mathrm{Mn}}_3$ triangles.

• Received 19 April 2002

DOI:https://doi.org/10.1103/PhysRevB.66.052405