Unusual Magnetic Behaviors with Large Anisotropy and 1/3 Magnetization Plateau in the CoV$_{2}$O$_{6}$

The pseudo-one dimensional brannerite type CoV$_{2}$O$_{6}$ has been studied by magnetization measurement, heat capacity and neutron powder diffraction between 300 and 1.5K. CoV$_{2}$O$_{6} $ crystallizes in two modifications; a low-temperature $\gamma$- form of triclinic structure and high-temperature $\alpha$-form of monoclinic structure. $\alpha$-CoV$_{2}$O$_{6}$ of monoclinic structure shows a large magnetic anisotropy and an antiferromagnetic transition at T$_{N}$=14K. Furthermore, the magnetization measurement up to 9T shows 1/3 magnetization plateau below 10K and saturated magnetic moment ~4.5$\mu_{B} $/Co$^{2+}$, which is much larger than the spin-only value of 3.0$\mu_{B}$/Co$^{2+}$ for the full moments of Co$^{2+}$ ions. On the contrary, $\gamma$-CoV$_{2}$O$_{6}$ of triclinic structure shows saturated magnetic moment 3.0$\mu_{B}$/Co$^{2+} $ with antiferromagnetic trantion at T$_{N}$=6.2K. We discuss the different magnetic behaviors of the $\alpha$- and $\gamma$- phase of CoV$_{2}$O$_{6}$.