Magnetism in a novel quasi-2D honeycomb lattice S=3 antiferromagnet.*

Honeycomb lattice antiferromagnets have been of tremendous research interests due to their complex magnetic structures ranging from Neél, zigzag, stripy, and spin liquid depending on details of competing exchange interactions. To study these interesting magnetic states, we have synthesized single crystals of a novel spin-3 honeycomb lattice antiferromagnet, Tb₂Ir₃Ga₉, which is a member of a large family of compounds with chemical formula R₂T₃X₉ (R = rare-earth element, T = d-element, and X = p-block element). Structural analysis showed the crystals have formed in orthorhombic symmetry with space group 63 (Cmcm). From the magnetization measurements, we find characteristics of low-dimensional magnetism followed by a long range ordering at 12 K. As determined both by magnetometry and powder neutron diffraction, the Tb spins lie strictly along the a-axis, parallel to the Tb-Tb contact. Two spin-flop transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding to M=M_s/2. Magnetoresistance data reflect the anisotropy and contains features that correlate with the spin flop transitions. We propose a model for the magnetic ground state and a phenomenological theory for the magnetic interactions.