Crystal Structure and Physical Properties of π–dSystem κ-(BDH-TTP)2FeBr4

Abstract

The structure and physical properties of metallic magnetic conductor κ-(BDH-TTP)2FeBr4 (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 FeBr4 anion sheets. The π 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 Fe3+ \(d\) 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 π 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 π–\(d\) 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.