Thermodynamic properties of antiferromagnetic ordered states ofπ−dinteracting systems ofκ−(BETS)2FeX4(X=Br,Cl)

The results are presented for systematic heat capacity measurements of \ensuremath{\pi}-$d$ interacting systems of $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{Fe}{\mathrm{Br}}_{4}$ and $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{FeC}{\mathrm{l}}_{4}$ [BETS = bis(ethylenedithio)tetraselenafulvalene] performed under in-plane magnetic fields. We observed sharp thermal anomalies at 2.47 K for $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{FeB}{\mathrm{r}}_{4}$ and at 0.47 K for $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{FeC}{\mathrm{l}}_{4}$ at 0 T that are associated with antiferromagnetic transitions of the $3d$ electrons in the anion layers. From analyses of the magnetic heat capacity data, we indicate that the two compounds show unconventional thermodynamic behaviors inherent in the \ensuremath{\pi}-d interacting layered system. In the case of $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{FeB}{\mathrm{r}}_{4}$, a small hump structure was observed in the magnetic heat capacity below the transition temperature when a magnetic field was applied parallel to the $a$ axis. In the case of $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}\mathrm{FeC}{\mathrm{l}}_{4}$, a similar hump structure was observed at 0 T that remained in the data with magnetic fields applied parallel to the $a$ axis. We demonstrate that the temperature dependencies of the magnetic heat capacities scale well by normalizing the temperatures with dominant one-dimensional direct interactions $({J}_{\mathrm{dd}}/{k}_{\mathrm{B}})$ of each compound. The field dependencies of the transition temperatures and the hump structures are elucidated in one simple magnetic field vs temperature ($H\ensuremath{-}T$) phase diagram. These results indicate that the thermodynamic features of both \ensuremath{\kappa}-type BETS salts are essentially equivalent, and the observed hump structures are derived from the one-dimensional ${J}_{\mathrm{dd}}$ interaction characters, which are still influential for magnetic features even in the long-range magnetic ordered states.