Detection of low energy spin loop excitations in rare earth kagom\'{e} systems

Collective spin excitation spectra in frustrated antiferromagnets have been detected using high field electron magnetic resonance (EMR). At low temperatures the langasite kagom\'{e} systems R$_{3}$Ga$_{5}$SiO$_{14}$(R = Pr and Nd) exhibit short range spin correlation effects. Neutron scattering has shown that these systems do not exhibit long-range magnetic order at temperatures down to 30 mK. Field-sweep EMR measurements made on single crystals of Pr$_{3}$Ga$_{5}$SiO$_{14}$ and Nd$_{3}$Ga$_{5}$SiO$_{14}$ in the temperature range 1.3 - 20 K, and in fields up to 22 T, give a series of absorption peaks which are quite different to conventional EMR spectra. The resonances are interpreted using a model which involves spin-wave excitations in short range antiferromagnetically correlated spin loops or clusters.