Pressure-Induced Ferromagnetic Interactions in the Molecule-based Magnet Mn(dca)$_2$

Using SQUID magnetometry, we have studied the pressure dependence of the magnetization of the three-dimensional antiferromagnetic coordination polymer Mn(N(CN)$_2$)$_2$, referred to as Mn(dca)$_2$, up to 1.2 GPa and down to 5~K. The isostructural compounds M(dca)$_2$, where M = Fe, Co, and Ni, have been previously studied by others and are known to show variations in their transition temperatures of up to 26\% for pressures as large as \mbox{1.7 GPa.}\footnote{C. J. Nuttall \textit{et al.}, Mol. Cryst. Liq. Cryst. \textbf{343} (2000) 227.} Our results on Mn(dca)$_2$ indicate a linear dependence of the transition temperature on the applied pressure, where a change of $48\%$ is measured at 1.2 GPa. In addition, a marked difference in the behavior of the magnetization is observed above and below 0.8 GPa. Specifically, for P $<$ 0.8 GPa, the magnetization decreases with increasing pressure, and for P $>$ 0.8 GPa, the behavior is inverted. These results indicate that external pressure changes the angle along the \mbox{Mn-[N(1)-C-N(2)]-Mn} superexchange path, thereby favoring ferromagnetic interactions.\footnote{C. R. Kmety \textit{et al.}, Phys. Rev. B \textbf{62} (2000) 5576.}