Magnetic resonance of collective states in spin-gap magnets

Abstract Various types of magnetic resonance signals observed in quantum-disordered magnets of the Haldane and dimer types are reported. The collective states of correlated spins near the breaks of the S = 1 spin chains in the Haldane magnet PbNi 2 V 2 O 8 demonstrate magnetic response of an effective spin S = 1 / 2 , while the excited states of long chains reveal the ESR spectrum of an effective spin S = 1 in a crystal field. A magnetic resonance spectrum of the latter type is also observed for the collective excited states in the system of coupled S = 1 2 dimers in crystals of TlCuCl 3 . Further, the interaction of triplet excitations and defects in PbNi 2 V 2 O 8 is shown to result in a hybrid spin resonance mode.