Antiferromagnetic resonance in La2CuO4+y

Our attempts to observe the antiferromagnetic resonance (AFMR) modes in La2CuO4+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 La2CuO4+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 La2CuO4+y transmission measurements versus magnetic field. Additional measurements on small particles show that the broad resonant feature observed around 8 cm-1 is produced by an electric-dipole-active superconducting sphere resonance of the La2CuO4+y particles.