Magnetic excitations at magnetization plateaux: ESR study on SrCU2 (BO3)(2)

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 res onance. Below the 1/8-plateau, a bending of the S-z = - 1 branch of the low est excited triplet is observed, which is caused by the strong mixing betwe en the ground state and the excited state. Besides the triplet, a strong ma in mode is observed which is apart from the resonance field expected for th e conventional paramagnetic resonance. At magnetization plateaux, the main mode appears around the paramagnetic resonance position and several broad a bsorption peaks are observed at lower frequencies. (C) 2001 Elsevier Scienc e B.V. All rights reserved.