High Field ESR Study of the pi-d Correlated Organic Conductor lambda-(BETS)2Fe0.6Ga0.4Cl4

Submillimeter and millimeter wave electron spin resonance (ESR) measurements of the pi-d correlated organic conductor lambda-(BETS)2Fe0.6Ga0.4Cl4 have been performed. Antiferromagnetic resonance (AFMR) has been observed in the insulating antiferromagnetic phase, and its frequency-field dependence can be reproduced by the biaxial anisotropic AFMR theory. We find that in this alloy system, the easy-axis is near the b-axis, unlike previous results for the pure lambda-(BETS)2FeCl4 salts where it is closer to the c*-axis. We have also observed electron paramagnetic resonance (EPR) in the metallic phase at higher fields where the g-value is shown to be temperature and frequency dependent for field applied along the c*-axis. This behavior indicates the existence of strong pi-d interaction. Our measurements further show the magnetic anisotropy associated with the anions (the D term in the spin Hamiltonian) is |D|~0.11 cm-1.