Electron spin resonance experiments with 15.4 GHz microwave have been performed on λ-(BEDT-TSF)2Fe1-xGa xCl4 (x = 0:0, 0.4, and 1.0) single crystals at various temperatures down to 1.5 K, where BEDT-TSF denotes bis(ethylenedithio) tetraselena-fulvalene. For λ-(BEDT-TSF)2GaCl4, the resonance signal due to π-electrons at the BEDT-TSF molecule layer is observed. For λ-(BEDT-TSF)2Fe0.6Ga 0.4Cl4, signals which originate both from π-electrons at the BEDT-TSF molecule layer and from d-electrons at Fe ions are observed. For λ-(BEDT-TSF)2FeCl4, only one resonance signal is observed. The magnetic field direction dependencies of the g-value do not show the symmetry of the conducting ac-plane above 20 K. This means that the observed signals are not mainly due to the π-electrons at the BEDT-TSF molecule layer but due to the d-electrons at Fe ions. The resonance due to the π-electrons is expected to be renormalized by the strong correlation between the π- and d-electrons. The anomaly has been observed in the g-value as well as the line width both for λ-(BEDT-TSF)2FeCl4 at around 70K and for λ-(BEDT-TSF)2Fe0.6Ga0.4Cl 4 at around 95 K. This might be correlated to the dielectric and structural anomaly observed for λ-(BEDT-TSF)2FeCl4.
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