Crystal Structure and Physical Properties of $\pi$--$d$ System $\kappa$-(BDH-TTP)2FeBr4

The structure and physical properties of metallic magnetic conductor $\kappa$-(BDH-TTP) 2 FeBr 4 (BDH-TTP: 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene) are investigated. It has a layered structure of BDH-TTP donor sheets extended along the $ac$-plane, which are sandwiched by FeBr 4 anion sheets. The $\pi$ electrons on the BDH-TTP sheets show simple metallic behavior down to 30 mK. The magnetic susceptibility obeys the Curie--Weiss law mainly associated with the Fe 3+ $d$ spins ($S = 5/2$), and indicates the presence of an anitiferromagnetic (AF) transition at $T_{\text{N}} = 3.9$ K. In the AF state, a steep S-shaped increase of the magnetization at 1.5 T ($H_{\text{SF}}$) in the field parallel to the $a$-axis ($H\parallel a$) is found, which is ascribed to a spin-flop transition. In addition, the magnetization curves for $H\parallel b$ and $H\parallel c$ show an inflection point at $H_{\text{c}} = 3.1$ T, suggesting the spin canting configuration in the $bc$-plane. A possible AF spin structure based on the magnetization data and molecular orbital calculations features triangular lattice consisting of the Fe $d$ electron spins and the donor $\pi$ electron spins. Possible origins responsible for the spin canting, Dzyaloshinskii--Moriya interaction, spin frustration effect, and anisotropy of the ligand field, are discussed. A steep decrease of the magnetoresistance in the AF state observed at $H_{\text{SF}}$ for $H\parallel a$ proves that the strong $\pi$--$d$ interaction seriously affects the electron transport in the donor system. An anomalous broadening of the ESR linewidth in the critical region above $T_{\text{N}}$ is suggestive of the development of a magnetic short range order, for which the low-dimensionality in the spin system is responsible.