Magnetic Properties of a Coordination Polymer [Mn$_{3}$[(OH)$_{2}$Na$_{2}$(3-cnba)$_{6}]$_{n}$

Magnetic properties of [Mn$_{3}$(OH)$_{2}$Na$_{2}$(3-cnba)$_{6}$]$_{n}$ (3-Hcnba = 3-cyanobenzoic acid), a newly discovered three-dimensional coordination polymer, were investigated using magnetic susceptibility $M(T)$/$H$ and isothermal magnetization $M(H)$. The crystal structure of [Mn$_{3}$(OH)$_{2}$Na$_{2}$(3-cnba)$_{6}$]$_{n}$ is triclinic with a space group $P$-1. The lattice parameters are $a$ = 6.663 {\AA}, $b$ = 12.971 {\AA}, $c$ = 14.161 {\AA}, \textit{$\alpha $} = 70.13\r{ }, \textit{$\beta $} = 88.43\r{ }, and \textit{$\gamma $} = 76.47\r{ }. The results of $M(T)$/$H$ on powder samples show that the effective moment \textit{$\mu $}$_{eff}$ of Mn$^{2+}$ is 5.88 \textit{$\mu $}$_{B}$ at temperatures above 100 K, close to the expected value for a free Mn$^{2+}$ ion. Below 3 K, [Mn$_{3}$(OH)$_{2}$Na$_{2}$(3-cnba)$_{6}$]$_{n}$ orders antiferromagneticly. A sudden slope change in $M(H)$ measured at is observed at a very small critical field of H$_{c} \quad \approx $ 20 G, suggesting a metamagnetic transition. Above 20 kG, $M(H)$ starts to saturate, reaching a value equivalent to 1.7 \textit{$\mu $}$_{B}$ per Mn$^{2+}$ ion. The magnetic behavior of the complex is interpreted in terms of an effective ferrimagnetic Mn(II) chains in which spin moments are linked by interactions in an AF-F-AF (F = ferromagnetic and AF = antiferromagnetic) sequence in the triangular magnetic repeating unit.