Observation of higher-harmonic helical spin-resonance modes in the chromium spinel CdCr2O4

High frequency ESR measurements on the chromium spinel compound CdCr2O4 have been performed. The observed ESR modes below H'(c) approximate to 5.7 T can be explained well by the calculated resonance modes based on a molecular field theory assuming a helical spin structure. Other than the fundamental ones, we have succeeded in observing the higher-harmonic modes for the first time. A large change of the ESR modes above H'(c) indicate that a variation of the spin structure from the helical to the four-sublattice canted one takes place around H'(c).