Magnetic excitations of SrCu2(BO3)2 have been studied in very high fields including 1/4-plateau and 1/8-plateau phases by means of electron spin resonance. Below the 1/8-plateau, a bending of the SZ = -1 branch of the lowest excited triplet is observed, which is caused by the strong mixing between the ground state and the excited state. Besides the triplet, a strong main mode is observed which is apart from the resonance field expected for the conventional paramagnetic resonance. At magnetization plateaux, the main mode appears around the paramagnetic resonance position and several broad absorption peaks are observed at lower frequencies.
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