ESR Study on λ-(BEDT-TSF)₂Fe_1-xGa _xCl₄

Electron spin resonance experiments with 15 GHz microwave have been performed on λ-(BEDT-TSF)₂Fe_1-xGa _xCl₄ (x = 0 and 0.4) single crystals at the various different temperatures down to 1.5 K. For λ-(BEDT-TSF) ₂FeCl₄, the g-value do not show the symmetric angular dependencies of the magnetic field direction with respect to the conducting ac-plane at several temperatures above 20 K. This means that the observed signals are not mainly due to π-electrons in BEDT-TSF molecules but to (d-electrons of Fe ions. The line width for H//a* and H//c increases with decreasing temperature from 200 to 70 K. This might be correlated to the anomaly in the dielectric response at 70 K reported earlier. For λ-(BEDT-TSF)₂Fe_0.6Ga_0.4Cl₄, the large enhancement of the g-value below 120 K and the peak of the line width at around 95 K are observed. These anomalies suggest the possible anomaly in the electronic states as in the case of λ-(BEDT-TSF)₂FeCl ₄. An enhancement of the line width associated with the antiferromagnetic transition is observed both for λ-(BEDT-TSF) ₂FeCl₄ and λ-(BEDT-TSF)₂Fe _0.6Ga_0.4Cl₄.