Magnetic structures of LiMBO₃ (M = Mn, Fe, Co) lithiated transition metal borates

The magnetic ordering within LiMBO₃ compounds (M = Mn, Fe, and Co) has been explored by magnetization measurements and neutron powder diffraction. For all M, an incommensurately ordered magnetic phase is established on cooling, followed by a change to a commensurate long-range antiferromagnetic state below T_N2 = 12(1) K for LiMnBO₃, T_N2 = 25(1) K for LiFeBO₃, and T_N2 = 12(1) K for LiCoBO₃. For LiMnBO₃, the magnetic ordering at T = 2 K exhibits a propagation vector k = (1, 0, 0) and consists of antiferromagnetic chains that are coupled antiferromagnetically to each other, the magnetic moments being oriented along the [001] direction. In contrast, the magnetic order at T = 2 K in LiFeBO₃ and LiCoBO₃ exhibits a propagation vector of k = (¹/₂, ¹/₂, ¹/₂) and consists of ferromagnetic chains that are antiferromagnetically coupled. The magnetic moments lie roughly along the [023̄] direction within the bc plane for LiFeBO₃, and along the [301̄] direction within the ac plane for LiCoBO₃. The moment orientations in both LiMnBO₃ and LiFeBO₃ suggest an Ising character arising from unquenched orbital momentum due to unusual trigonal bipyrimidal coordination environments. No evidence of Ising behavior is found in the case of LiCoBO₃.