Antiferromagnetic resonance inLa2CuO4+y

Our attempts to observe the antiferromagnetic resonance (AFMR) modes in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{y}}$ in both single crystal and small particle forms have been unsuccessful. An argument based on a magnetic-susceptibility sum rule shows that the resonance previously identified with the AFMR in polycrystalline ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{y}}$ [R. T. Collins, et al., Phys. Rev. B 37, 5817 (1988)] is too strong to be compatible with a magnetic spin-wave mode. No evidence of an AFMR mode is found in our single-crystal ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{y}}$ transmission measurements versus magnetic field. Additional measurements on small particles show that the broad resonant feature observed around 8 ${\mathrm{cm}}^{\mathrm{-}1}$ is produced by an electric-dipole-active superconducting sphere resonance of the ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{y}}$ particles.