Magnetic Properties of Porous Metal-Organic Frameworks: Ni$_{2}$(BODC)$_{2}$(TED) and Ni$_{2}$(BDC)$_{2}$(TED)

Results of \textit{$\chi $}($T)$, $M(H)$, and heat capacity $C(T)$ measurements on two Ni dimer based porous materials Ni$_{2}$(BODC)$_{2}$(TED) and Ni$_{2}$(BDC)$_{2}$(TED) are reported. These materials form a tetragonal crystal structure of space group P4/ncc with $a=b$ = 14.9 {\AA} and $c$ = 19.4 {\AA} and Ni-Ni separation of 2.61{\AA} within the dimer. Magnetic data of Ni$_{2}$(BODC)$_{2}$(TED) revealed a ferromagnetic-like transition at about 17 K with \textit{$\theta $} = 8 K, and a coercivity field of 1700 G was observed in the hysteresis curve. Though isostructural to Ni$_{2}$(BODC)$_{2}$(TED), \textit{$\chi $}($T)$ and $M(H)$ results of Ni$_{2}$(BDC)$_{2}$(TED) showed an antiferromagnetic transition at 10 K with \textit{$\theta $} = - 132 K, and no hysteresis was observed. Although specific heat data $C(T)$ showed no clear transition in both compounds, nonlinear behavior is clearly seen in $C/T$ vs. $T$ plots, and a fit to the electron and phonon contributions to $C(T)$ gives a large heavy-fermion-like $\gamma $ in both cases. A model for the magnetic interactions is proposed and a comparison to the Cu and Co analogues is also made.