Submillimeter and millimeter wave ESR measurements of (DMET) ₂FeBr₄ below T_N

Submillimeter and millimeter wave ESR measurements of (DMET) ₂FeBr₄, which has π-d interaction between quasi 1D π-electrons and localized d-electrons, have been performed at 2.1K. Frequency-field relation below the saturation field B_s=7T agrees with antiferromagnetic resonance (AFMR) modes with the uniaxial anisotropy. However, AFMR modes above B_s=7T for B//a and B//c do not follow the AFMR modes with the same uniaxial anisotropy. As the anomalies in the field dependence of g-value coincide with the previous magnetoresistance measurement, the ESR result also suggests the existence of strong π-d interaction.