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, Mn0.6Co0.4, 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 P (3) over bar m1); however, at lower temperatures both Na2BaMn(VO4)(2) and Na2BaMn0.6Co0.4(VO4)(2) undergo a structural transition to lower symmetry (monoclinic, C2/c). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na2BaMn0.6Co0.4(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 Neel-type arrangement and aligned along the C-4 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 c-axis direction. The relationships between these structures and magnetic properties are discussed.