High field ESR study of the \textit{pi}-$d$ interaction effect in beta-(BDA-TTP)$_{2}$MCl$_{4 }$(M=Fe, Ga)

Novel magnetic organic conductors with \textit{pi}-$d$ interaction have commanded attention since the discovery of field induced superconductivity. One of them, beta-(BDA-TTP)$_{2}$FeCl$_{4}$, has alternating donor molecules and quasi 2$D$ electrical properties. Previous studies of electrical and magnetic properties show an M-I transition at 120K and an AF transition at $T_{N}$=8.5K, suggesting an exchange interaction between the conduction electrons and the Fe$^{3+} \quad d$-electrons. The properties of beta-(BDA-TTP)$_{2}$GaCl$_{4}$ are similar with exception of the absence of the AF transition, which is apparently due to the absence of \textit{pi}-$d$ exchange interaction. We report angular/temperature dependent 240GHz quasi optical ESR measurements on both compounds to probe the magnetic properties. The Ga compound signals follow the donor molecule structure, and show no magnetic order at any temperature. The Fe compound signals are quite different from the Ga compound, and exhibit AF behavior below $T_{N}$. The difference of Fe and Ga compounds will be discussed in terms of the interaction between localized and itinerant magnetic moments.