Kink in Stoner Factor as a Signature of Changing Magnetic Fluctuations in the Organic Conductor <i>λ</i> -(BETS) ₂ GaCl ₄

In this study, we theoretically investigated the magnetic properties of the quasi-two-dimensional organic conductor $λ$-(BETS)$_2$GaCl$_4$ using a multi-band Hubbard model and the two-particle self-consistent method. We employed a four-band model in which each BETS molecule is considered as a site and a two-band model that considers each BETS dimer as a site. Our results for the temperature dependence of the Stoner factor reveal a kink around $T_\mathrm{kink} \approx 5 \mathrm{meV}$, which indicates a change in the dominant magnetic fluctuations. A broad structure indicating smeared antiferromagnetic (AFM) fluctuations was observed above $T_\mathrm{kink}$, whereas the spin susceptibility peaked at a wavevector corresponding to spin-density-wave (SDW)-like fluctuations below $T_\mathrm{kink}$. The kink disappears as the intra-dimer transfer integral increased and the AFM fluctuations were enhanced. Our findings are consistent with those of previous experimental observations, which have also reported a change in magnetic properties from AFM to SDW-like fluctuations upon cooling.