Magnetic Ground State Crossover in a Series of Glaserite Systems with Triangular Magnetic Lattices

The magnetic properties are reported for three members of the glaserite series of compounds, Na₂BaM­(VO₄)₂, M = Mn, Mn_0.6Co_0.4, and Co. Large single crystals are grown using a high-temperature hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M²⁺O₆ octahedra are interconnected with nonmagnetic (VO₄)^3– groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group <i>P</i>3̅<i>m</i>1); however, at lower temperatures both Na₂BaMn­(VO₄)₂ and Na₂BaMn_0.6Co_0.4(VO₄)₂ undergo a structural transition to lower symmetry (monoclinic, <i>C</i>2/<i>c</i>). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na₂BaMn_0.6Co_0.4(VO₄)₂ does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na₂BaMn­(VO₄)₂ and Na₂BaCo­(VO₄)₂ were determined using neutron powder diffraction. At zero magnetic field, Na₂BaMn­(VO₄)₂ possesses an antiferromagnetic structure with the moments ordered in a Néel-type arrangement and aligned along the C₄ axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na₂BaCo­(VO₄)₂ represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the <i>c</i>-axis direction. The relationships between these structures and magnetic properties are discussed.