ESR Investigations of the Magnetic Anisotropy in $κ$-(BETS)$_2$Mn[N(CN)$_{2}$]$_3$

The two-dimensional molecular conductor $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ has been studied because of the intriguing magnetic coupling of the molecular $π$-electrons to the Mn$^{2+}$ ions. Utilizing X-band electron spin resonance spectroscopy we have performed comprehensive investigations of the magnetic properties, in particular on the temperature and angular dependences of the spin susceptibility, the $g$-factor and the linewidth. Due to the $π$-$d$-coupling, a rearrangement of the $π$-spins occurs: At low temperatures the $g$-factor shifts enormously with a pronounced in-plane anisotropy that flips as the temperature decreases; the lines broaden significantly; and the spin susceptibility increases upon cooling with a kink at the phase transition. By carefully analyzing the angular dependence of $g(θ)$ and $ΔH(θ)$ we reveal the influence of anisotropic Zeeman interaction in addition to spin-phonon coupling. We conclude the presence of two magnetically distinct BETS chains and discuss the possibility of altermagnetic order.